Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
The Tropical Asian-Pacific Climate Simulated in a Hybrid
Coupled General Circulation Model (HcGCM)
Xiouhua Fu*a, Bin Wanga,b, Tim Lia,b , and Fei-fei Jinc
aIPRC, SOEST, University of Hawaii, Honolulu, Hawaii
bDepartment of Meteorology, SOEST, University of Hawaii, Honolulu, Hawaii
cDepartment of Meteorology, Florida State University, 404 Love Building, Tallahassee,
Florida
Journal of Climate
Revised on February 20, 2005
Corresponding author address: Dr. Xiouhua Joshua Fu (E-mail: [email protected]),
International Pacific Research Center (IPRC), SOEST, University of Hawaii at Manoa,
1680 East West Road, POST Bldg., 4th Floor, Honolulu, HI 96822
ABSTRACT
The first hybrid coupled general circulation model (HcGCM) that exercises full
atmosphere-ocean coupling is described and evaluated. This coupled model comprises an
ECHAM-4 AGCM with an intermediate tropical upper ocean model. In this study, the
first 40-year (after 5-year spin-up) model simulation has been analyzed and compared
with available long-term observations and analysis products.
Overall, the model simulations of the climatology and variability in the tropical
Asian-Pacific sector (including Indo-Pacific mean SST and its annual cycle, Asian-
Australian monsoon, tropical intraseasonal oscillations (TISO) and ENSO) are fairly
realistic and comparable with those state-of-the-art coupled GCMs. The model bias in
mean SST is within 1oC in most of the ocean domain except in the southeast Pacific,
where the model suffers a warm-bias syndrome present in many coupled GCMs. The
simulated TISO exhibits significant seasonality as in the observations with eastward-
propagating MJO favored in boreal winter and northward-propagating mode over the
Indian and western Pacific regions in boreal summer. The model ENSO has dominant
periods about 2 years and 5 years. It also shows significant annual phase locking with the
minimum and maximum variance, respectively, in boreal spring and late fall.
The intention to develop a hybrid coupled GCM, in which the ocean component
only considers the upper ocean dynamics and thermodynamics, is to capture the major
intraseasonal-to-interannual climate variability in the tropical Asian-Pacific sector. The
encouraging results presented here suggest that this framework is a pragmatic approach in
representing the present-day tropical climatology and its climate variability.
2
1. Introduction
In the past two decades, under the auspices of the TOGA (Tropical Ocean-Global
Atmosphere) program and the follow-on CLIVAR (Climate Variability and
Predictability) program, many atmosphere-ocean coupled models have been developed to
improve our understanding of the nature and the predictability of the tropical Pacific and
global climate variability (Meehl et al. 2000; Latif et al. 2001; Davey et al. 2002). In the
TOGA decade (1985-1994), the coupled models are primarily designed to simulate and
predict the El Nino-Southern Oscillation (ENSO) and the associated tropical and extra-
tropical climate variability. The early inter-comparisons of coupled models (Neelin et al.
1992; Mechoso et al. 1995) found that many coupled GCMs (CGCMs) are able to
produce a realistic warm-pool/cold tongue configuration and reasonable zonal SST
gradient along the equatorial central Pacific. However, the interannual variability in these
models ranged from very weak to moderate. Among those coupled models, ENSO
simulated by the Cane-Zebiak intermediate model looks most realistic (Zebiak and Cane
1987). Even today, the Cane-Zebiak model is still one of the best coupled models
(including CGCMs) in terms of the ENSO simulation and prediction (Latif et al. 2001;
Chen 2004). Nevertheless, the CGCMs still had substantial biases from the observed
state. The simulated equatorial cold tongue generally tends to be too strong, too narrow,
and extends too far west. SSTs in the southeast Pacific are generally too warm, which is
accompanied by a fictitious double inter-tropical convergence zone (ITCZ). The CGCMs
also have a variety of problems in simulating the seasonal cycle of the equatorial SST in
the eastern Pacific (e. g., a too-weak annual harmonic but a too-strong semiannual
harmonic). To summarize the development of coupled models during the TOGA decade,
3
Delecluse et al. (1998) concluded that substantial progress was made in representation of
the tropical mean state and climate variability through the synergic efforts of the
observational, theoretical, and hierarchal modeling studies (McPhaden et al. 1998; Neelin
et al. 1998; Latif et al. 1998). However, several general systematic errors in both mean
state and seasonal cycle have yet to be eliminated, especially in the east Pacific.
Two recent CGCM inter-comparison projects, ENSIP (the El Nino simulation
intercomparison project, Latif et al. 2001) and STOIC (a study of coupled model
climatology and variability in tropical ocean regions, Davey et al. 2002), also revealed
that there remains substantial potential for further model improvement. Latif et al. (2001),
through comparing the performance of 24 CGCMs in the tropical Pacific, indicated that
almost all models (even those employing flux correction) still have considerable
problems in simulating the SST climatology. Only a few of the coupled models
realistically simulate the ENSO in terms of gross equatorial SST anomalies (e.g.,
amplitude and annual phase locking). In particular, many models overestimate the
variability in the western equatorial Pacific and underestimate the SST variability in the
east, which may be associated with the extension of the model cold tongue too far
westward. Davey et al. (2002) further found that the interannual variability (both SST
and zonal wind stress) is commonly too weak in the models. Most models have difficulty
in reproducing the observed Pacific ‘horseshoe’ pattern of negative SST correlations with
interannual Nino-3 SST anomalies, and the observed Indo-Pacific lag correlations. Both
inter-comparison projects confirm that improving the simulations of the tropical Pacific
climatology and ENSO remains a continuing challenge for the coupled-model
community.
4
In this paper, we present a Hybrid coupled GCM (HcGCM) newly developed at
the International Pacific Research Center (IPRC), University of Hawaii. This model
couples the ECHAM-4 AGCM (Roeckner et al. 1996) with an intermediate ocean model
(Wang et al. 1995; Fu and Wang 2001). Active air-sea coupling is in the tropical Indo-
Pacific region only1. In contrast to other anomalous hybrid coupled GCMs (e.g.,
Alexander 1992; Kirtman and Zebiak 1997; Yeh et al. 2004), this hybrid coupled model
exercises full coupling. The model was designed to simulate the annual mean, annual
cycle, and interannual variability within one framework. The model simulations of the
climate and variability in the tropical Asian-Pacific sector are very encouraging even
compared to those well-known, state-of-the-art coupled GCMs (e.g., SINTEX CGCM,
Gualdi et al. 2003). The main objective of this study is to validate the model simulations
of the climatology and intraseasonal-to-interannual variability in the tropical Asian-
Pacific sector with available observations, thus establishing a baseline for evaluating
future improvements and for comparison with other models.
Because our focus is the tropical Asian-Pacific climate, we will validate the
model simulations of tropical Pacific climate along with the Asian-Australian monsoon
and tropical intraseasonal oscillations (TISO). In literature, a few AGCM inter-
comparison projects have been conducted to focus on the Asian summer monsoon
(Gadgil and Sajani 1998; Kang et al. 2002) and the TISO (Slingo et al. 1996; Waliser et
al. 2003). We will also refer to some results from those inter-comparison projects when
we validate the relevant simulations from this hybrid-coupled model.
1 IPRC’s mission is to understand the nature and predictability of climate variability and regional aspects of global environmental change in the Asian-Pacific sector (http://iprc.soest.hawaii.edu).
5
The remaining parts of the paper are organized as follows. The model and the data
used to validate the model are described in section 2. In section 3, we validate the model
climatology (both the annual mean and annual cycle) in the tropical Asian-Pacific sector.
In section 4, the simulated TISO, both the northward-propagating ISO in boreal summer
and eastward-propagating MJO in boreal winter, are compared to the available
observations. The interannual variability of the tropical Pacific is evaluated in section 5.
Finally, we summarize our main results and discuss the pathways to further improve the
model in section 6.
2. The Hybrid coupled GCM (HcGCM)
a. The atmospheric component ECHAM-4
The atmospheric model used in this study is the ECHAM-4 general circulation
model, which has been documented in detail by Roeckner et al. (1996). A brief
description is given here for the convenience of readers. We used the T30 version (the
corresponding horizontal resolution is about 3.75o) in this study instead of the standard
T42 version, because it produces similar results but requires fewer computational
resources (Stendel and Roeckner 1998). The model has 19 layers extending from the
surface to 10 hPa. Its land surface scheme is a modified bucket model with improved
parameterization of rainfall-runoff (Dumenil and Todini 1992). The surface fluxes of
momentum, heat, water vapor, and cloud water are based on the Monin-Obukhov
similarity theory. The vertical diffusion in the model is computed with a high-order
closure scheme depending on the turbulent kinetic energy. Gravity wave drag associated
with orographic gravity waves is simulated after Miller et al. (1989). The mass flux
6
scheme of Tiedtke (1989) for deep, shallow, and mid-level convection has been used with
CAPE closure (Nordeng 1994). The radiation scheme is a modified version of the
European Center for Medium-Range Weather Forecasts (ECMWF) scheme. Two- and
six-band intervals are used in the solar and terrestrial part of the spectrum, respectively.
b. The updated intermediate ocean model
The ocean component of this hybrid coupled model is a tropical upper ocean
model with intermediate complexity. It was originally developed by Wang et al. (1995)
and later improved by Fu and Wang (2001). The ocean model comprises a mixed layer,
in which the temperature and velocity are vertically uniform, and a thermocline layer in
which temperature decreases linearly from the mixed layer base to the thermocline base.
Both layers have variable depths. The deep ocean beneath the thermocline base is
motionless with a constant reference temperature. This ocean model combines the mixed-
layer physics of Gaspar (1988) and the upper ocean dynamics of McCreary and Yu
(1992). It reasonably simulates the annual cycles of sea surface temperature, upper ocean
currents, and mixed layer depth in the tropical Pacific (Wang and Fu 2001; Fu and Wang
2001).
In this study, the parameterization of the entrained water temperature has been
updated. In our previous studies, the entrained water temperature is parameterized in
terms of the vertical temperature gradient between the mixed layer and the deep inert
layer. The weakness of this parameterization is that the mixed-layer temperature is not
sensitive to the changes of thermocline depth. Therefore, the SST interannual variability
that is strongly associated with thermocline feedback (Zebiak and Cane 1987) is very
weak in our early versions of coupled models (Fu and Wang 2001; Fu et al. 2002).
7
Following the footsteps of other intermediate ocean models (Zebiak and Cane 1987;
Seager et al. 1988; McCreary et al. 1993; Jin 1996, 1998), we have parameterized the
entrained water temperature as an explicit function of thermocline depth, very similar to
the one used in Zebiak and Cane (1987) and Jin (1996). In this study, the ocean model is
active in the tropical Indian and Pacific Oceans (from 30oS to 30oN) with realistic but
simplified coastal boundaries of the Oceans. It is feasible to further extend the ocean
domain to the tropical Atlantic Ocean and mid-latitude region (Lu et al. 1998). The
horizontal resolution of the model is 0.5o longitude by 0.5o latitude, which requires an
approximate time interval of 3 h. No-flux conditions for temperature and free-slip
conditions for velocities are applied at the coastal boundaries.
c. The coupling procedure
The ECHAM-4 was coupled with the intermediate ocean model in the tropical
Indian and Pacific Oceans without heat flux correction (except that the SSTs in the north-
south open boundaries have been relaxed to the observations as in Fu and Wang 2001).
Beyond the coupling regions, the underlying sea surface temperature is specified as the
climatological monthly mean SST from the AMIP II experiment (Taylor et al. 2000). In
the active air-sea coupling domain, atmospheric component exchanges information with
ocean component once per day. The atmosphere provides daily mean surface winds and
heat fluxes to the ocean model. The latter send daily mean SST back to the former. The
coupled model is integrated with seasonally varying solar radiation forcing. The initial
atmospheric field is a restart file on January 1 from previous long-term integration. The
initial ocean field is the steady state in January after a ten-year integration of the ocean
model forced with observed climatological surface winds and heat fluxes. The spin-up
8
period for the coupled model is 5 years. The output from the next 40 years’ integration
was used in the following analyses.
d. The data used to validate the model
Several long-term datasets either from the observations or from the analysis and
reanalysis have been used in this study to validate the model simulations. The datasets
include the Hadley Center monthly-mean SST from 1901 to 2000 (GISST, Rayner et al.
1998); Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP)
pentad-mean rainfall from 1979 to 2000 (Xie and Arkin 1997); daily–mean winds of
ECMWF analysis from 1991 to 2000; and monthly-mean sea level pressure from NCEP
reanalysis from 1961 to 2000 (Kalnay et al. 1996).
e. Other ECHAM4 family coupled GCMs
It is worthwhile to mention that there are three other CGCMs that also used
ECHAM-4 as their atmospheric component. Two coupled versions were developed at
Max Planck Institute, Germany: 1) ECHO-G, which coupled ECHAM-4 with HOPE
ocean general circulation model (Frey et al. 1997; Guilyardi et al. 2004); 2) ECHAM-4 is
also coupled to OPYC3 general circulation model with annual-mean heat flux correction
(Bacher et al. 1998); 3) SINTEX CGCM (Gualdi et al. 2003) couples the ECHAM-4 with
the ORCA ocean general circulation model. Through comparing the results from different
atmospheric GCMs (or from the same GCM with different resolutions) coupled to one
ocean model, Guilyardi et al. (2004) have suggested the important role of the atmospheric
component in setting up ENSO characteristics. On the other hand, coupling one AGCM
to different ocean models may give us a clue about how the ocean component will affect
the behaviors of coupled systems. In our following analyses, we will briefly refer to the
9
papers describing other ECHAM-4 family CGCMs on relevant simulations. More detail
comparisons of those models are deferred to future studies2.
3. The Model Climatology
a. In the tropical Asian-Pacific sector
During the 40-year integration, the coupled model doesn’t exhibit apparent
climate drift though no explicit heat flux correction is applied. The simulated annual-
mean SST (averaged in 40-years) agrees with the observations (averaged in 100-years)
quite well (Fig. 1). The model bias is within 1oC in most tropical Indo-Pacific regions
except in the southeast Pacific, particularly close to the Peruvian coast, where the model
SST is too high (Fig. 1c). This warm bias is associated with the fictitious eastward
extension of the south Pacific convergence zone (SPCZ) warm water. This problem is a
common syndrome for most state-of-the-art CGCMs (Davey et al. 2002; Frey et al. 1997;
Gordon et al. 2000; Meehl et al. 2001; Guilyardi et al. 2003; Wang et al. 2004, among
others). It is believed that this problem originates primarily from deficiencies in
atmospheric models due to underestimate of stratocumulus and/or along-shore winds
(Philander et al. 1996; Schneider et al. 1997; Mechoso et al. 2000), and probably
amplified by local air-sea coupling (Li and Philander 1996).
Figure 2a-d shows the climatological zonal wind shear (850 hPa-200 hPa) and
precipitation from the observations and the coupled model. We assessed the model wind
shear here, because it is suggested to be an important factor to initiate and steer the
intraseasonal variability in the Indo-western Pacific region (Wang and Xie 1996; Jiang et
2 We are now collaborating with SINTEX-F group at Frontier Research Center of Japan to diagnose their model output and search the ways to further improve the model.
10
al. 2004). In boreal summer, the major observed rainfall systems (Fig. 2a) are associated
with the Asian summer monsoon, ITCZ and the eastern North Pacific summer monsoon
(ENPSM, Murakami et al. 1992). Strong easterly shear is observed in the northern Indian
Ocean and the western North Pacific (WNP) associated with the monsoon rainbelt around
15oN (Fig. 2a). The coupled model well captures this easterly shear (Fig. 2b) with a
maximum of about 30 m s-1 located in the northwest Indian Ocean. In the eastern Pacific,
the model tends to overestimate the easterly shear associated with the ENPSM even
though the rainfall is a little underestimated. The ITCZ rainfall in the central Pacific
(between 160oW and 120oW) is weaker than the observed. This bias also exists in the
stand-alone ECHAM-4 GCM (Roeckner et al. 1996). This rainfall bias may be associated
with the westerly bias in the upper troposphere (Fig. 4b). The erroneous westerly duct in
the model favors intrusion of the subtropical dry (and cold) air into the equatorial region
and tends to suppress the model ITCZ convection in the central Pacific (Mapes and
Zuidema 1996; Yoneyama and Parsons, 1999; Tompkins 2001).
In the Asian-western Pacific region, the model captures the major rainfall
systems, for example, strong rainfall at the eastern Arabian Sea and the Bay of Bengal
(Figs. 2a-b). The simulated maximum rainfall in the Arabian Sea shifts too far away from
the Indian western coast probably owing to the coarse resolution (see also Gualdi et al.
2003 and Rajendran et al. 2004). The observed rainbelt just south of the equatorial
Indian Ocean is reproduced but with a weaker intensity. This rainbelt may play an
important role in initiating the dominant northward propagating ISO in the Indian Ocean
(Waliser et al. 2003; Fu and Wang 2004b), but was missed by many AGCMs (Kang et al.
2002). The observed rainbelt in the South China Sea and the WNP is captured but with a
11
different orientation compared to the observations (Figs. 2a-b). A rainy center is observed
in the SPCZ but the model only shows a hint there. Overall, the simulated rainfall pattern
associated with Asian summer monsoon is very similar with that in the SINTEX CGCM
(Gualdi et al. 2003) and is much better than the simulations with stand-alone ECHAM-4
GCM (Roeckner et al. 1996) and many other AGCMs (Kang et al. 2002). This result
supports our previous finding that warm-pool air-sea coupling significantly improves the
simulation of mean Asian summer monsoon (Fu et al. 2002).
In boreal winter, major convective zones move to the Southern Hemisphere (Figs.
2c-d) following the seasonal migration of solar radiation. The major rainbelt extends
from the southern Indian Ocean to the SPCZ, with a tail (the Pacific ITCZ) remaining in
the Northern Hemisphere. The observed easterly shear centers over the maritime
continent. The overall rainfall and wind shear patterns are well simulated by the model
(Fig. 2d). However, there are two systematic errors in the simulation: the northern ITCZ
is too weak and the SPCZ extends too far eastward. Most likely, both errors are
associated with the warm bias in the southeast Pacific (Fig. 1c).
Figures 3a-d compare the climatology of 1000-hPa winds from the ECMWF
analysis and the model. In boreal summer (Figs. 3a-b), the monsoonal flows associated
with the Asian summer monsoon in the Indian Ocean are reasonably captured. In the
tropical Pacific, the simulated northeast trades are more realistic than the southeast trades.
The latter are too strong in the western South Pacific (between 150oE and 150oW), which
may be related to the weaker SPCZ rainfall in the model (Fig. 2a). In boreal winter (Figs.
3c-d), the overall flow patterns are similar between the model and the observations except
12
that the strong northeast trades in the model penetrate too far equatorward and the south
Pacific convergence zone extends too far eastward (Fig. 3d).
The upper troposphere (200-hPa) zonal winds from the ECMWF analysis and
coupled model are compared in Figs. 4a-d. In boreal summer, the easterlies associated
with the Asian-western Pacific summer monsoon are reproduced but with smaller
amplitude, particularly in the northern Indian Ocean (Fig. 4b). On the other hand, the
easterlies in the eastern equatorial Pacific are slightly too strong. As mentioned before, an
erroneous westerly duct is produced in the equatorial central Pacific. In boreal winter, the
observed easterlies associated with the Australian monsoon (Fig. 4c) are much weaker
than their summer counterparts (Fig. 4a). The simulated easterly center locates over the
maritime continent as in the observations but with slightly smaller amplitude (Fig. 4d).
b. In the equatorial Indo-Pacific region
Davey et al. (2002) evaluated the global equatorial SSTs and zonal wind stresses
simulated in 15 CGCMs (without heat flux correction). They found that most CGCMs
(11 of 15) have significant cold bias in the western-central equatorial Pacific. At the same
time, easterly winds are considerably overestimated in the western Pacific, but
underestimated in the central Pacific (figures 1 and 2 in Davey et al. 2002). The causes of
these systematic errors are probably associated with the misrepresentations of oceanic
mixing (Li et al. 2001), adjacent monsoon systems (Fu 2005), and local atmosphere-
ocean feedback (Li and Philander 1996).
We have compared the mean SSTs and zonal winds in the equatorial Indo-Pacific
Oceans from this hybrid coupled model with the observations (Figs. 5a-b). The model
13
SST bias (Fig. 5a) is very small in most equatorial regions except in the eastern end of the
Pacific (east of 120oW), where simulated SST is too warm with a bias about 2oC. For this
hybrid coupled model, the warm bias primarily originates from the underestimated
stratocumulus in the atmospheric component (figure not shown). The simulated zonal
winds (Fig. 5b) also show a reasonable agreement with the observations except in the
western Pacific, where the model easterlies are slightly too strong.
In the deep tropics, the downward solar radiation at the top of atmosphere has a
dominant semiannual cycle. However, the observed SST in the equatorial eastern-central
Pacific shows a peculiar annual cycle (Fig. 6a) with highest and lowest SST, respectively,
in spring and fall. Many state-of-the-art CGCMs have various problems in reasonably
simulating this feature (Mechoso et al. 1995; Latif et al. 2001). Some models tend to
produce a dominant semiannual cycle in the equatorial eastern Pacific. The failure to
simulate a reasonable annual cycle may result in an unrealistic annual phase locking of
the model El Nino (Guilyardi et al. 2004). This hybrid coupled model produces a
significant SST annual cycle in the eastern Pacific (Fig. 6b) even though the simulated
phase lagged the observations by about 1 month and the amplitude is somewhat reduced.
Compared to other ECHAM-4 family CGCMs (Frey et al. 1997; Gualdi et al. 2003;
Guilyardi et al. 2004), the annual cycle seems better represented in this hybrid coupled
model. The reason is likely related to the introduction of an explicit mixed-layer in the
intermediate ocean model (Wang et al. 1995; Fu and Wang 2001). In the western Pacific,
the observed semiannual cycle is also well reproduced by the model. In the Indian Ocean,
the model captures the annual cycle in the eastern basin and the semiannual cycle in the
western basin. However, the observed strong summer cooling near the Somali coast is
14
underestimated, suggesting the coastal upwelling is not well represented. Higher
horizontal resolutions in both the atmosphere and ocean models are probably needed to
mitigate this bias.
Figure 7 compares the annual cycles of the equatorial zonal winds between the
observations and the model. The major observed features are captured in the model: the
annual cycle in the eastern Pacific, the westerly in both winter and spring along with the
easterly in summer over the western Pacific, and semiannual cycle in the Indian Ocean.
The simulation, however, shows a few obvious biases, particularly in the western Pacific,
such as a stronger westerly in boreal spring and a weaker westerly in boreal winter.
4. The Tropical Intraseasonal Oscillations (TISO)
The atmosphere-ocean variability (e.g., precipitation, low-level winds, surface
heat fluxes and SST) associated with the TISO has its strongest signal in the tropical
Indo-Pacific sector even though its impacts could spread around the world. The TISO
significantly regulates the onset and retreat of Asian-Australian monsoons (Yasunari
1980), tropical storm activity (Maloney and Hartmann 2000), and ENSO evolution
(McPhaden and Yu 1999), even the subseasonal rainfall variability over Americas (Mo
2000; Jones 2000). This intraseasonal mode is first revealed by Madden and Julian (1971)
as an eastward propagating planetary-scale zonal wind oscillation with a period of about
40-50 days (popularly termed as Madden-Julian Oscillation or MJO). Many follow-up
studies have found that the eastward propagating MJO prevails primarily in boreal
winter. In boreal summer, the dominant intraseasonal mode propagates northeastward
15
from the equatorial Indian Ocean to East Asia (Yasunari 1979; Lau and Chan 1986;
Wang and Rui 1990).
As revealed by several GCM inter-comparison projects (Slingo et al. 1996;
Sperber et al. 1997; Waliser et al. 2003), most current GCMs have considerable problems
in realistically representing the TISO. Many recent studies have suggested that air-sea
coupling significantly improves the simulations of the TISO in terms of its intensity,
convection-SST phase relationship, propagation and seasonality (Flatau et al. 1997;
Waliser et al. 1999; Inness and Slingo 2003; Fu et al. 2003). Fu and Wang (2004a, b)
further demonstrated that two different TISO solutions, respectively, exist in an air-sea
coupled system and a forced atmosphere-only system. The solution from the coupled
system resembles the observations more than that from the atmosphere-only system. In
this section, the TISO simulated in this hybrid coupled GCM is assessed briefly. We
focus on the seasonal variations of the TISO, such as the spatial patterns of rainfall
variability, eastward-propagating MJO in boreal winter and northward-propagating ISO
in boreal summer.
Figures 8a-d show the standard deviations of filtered rainfall (with period of 20-
90 days retained), which are used to represent the intensity of the TISO, from the CMAP
observations and the coupled model. In boreal winter (Fig. 8a), major rainfall variances
associated with TISO activities are observed in the southern Indian Ocean, SPCZ, ITCZs
and South America. The model captures almost all these activities with the intensity
overestimated in the maritime continent, southern Indian Ocean and SPCZ (Fig. 8b). The
simulation in the ITCZ is weaker than the observations, probably a consequence of the
underestimated mean rainfall in this area (Figs. 2c-d). During boreal summer (Fig. 8c),
16
major rainfall variability shifts to the Northern Hemisphere following the seasonal march
of mean rainfall. The action centers appear in the equatorial and northern Indian Ocean,
South China Sea, the WNP and ITCZs. They are well collocated with the mean summer
rainfall (Fig. 2a). As in the model mean (Fig. 2b), the orientation of maximum TISO
intensity in the WNP is not in line with the observations (Figs. 8c-d). In the eastern
Pacific ITCZ, a relatively isolated TISO center occurs in association with the ENPSM
(Maloney and Esbensen 2003). In general, the coupled model captures all the action
centers that appeared in the observations but with slightly larger amplitude (Fig. 8d).
Compared to the simulations of all 10 AGCMs that participated in the CLIVAR/Asian-
Australian monsoon inter-comparison project (Waliser et al. 2003), the simulations of
this hybrid coupled model are relatively better in terms of both the spatial pattern and
amplitude of the TISO rainfall variability. The space-time evolutions of rainfall, surface
winds and SST associated with boreal-summer ISO (Fu and Wang 2004b) and MJO
(figure not shown) are also quite similar with the observations.
A limited-domain wavenumber-frequency spectral analysis is used to summarize
the spatio-temporal characteristics of the TISO rainfall in the Asian-Pacific sector. The
advantages and usefulness of this method have been substantiated by several previous
studies (Teng and Wang 2003; Fu et al. 2003). In boreal winter (NDJFMA), this
wavenumber-frequency analysis is applied in zonal direction between 40oE and 140oW to
extract the eastward-westward propagating modes. In boreal summer (MJJASO), the
analysis is applied in meridional direction between 10oS and 30oN to extract the
northward-southward propagating modes.
17
Figures 9a-b compare the wavenumber-frequency spectra of rainfall (averaged
between 10oS and 5oN) associated with the eastward-westward propagating intraseasonal
modes in boreal winter from the CMAP observations and the coupled model. The results
from the observations and the model are, respectively, 22-year (1979-2000) mean and 39-
year mean. In the observations (Fig. 9a), eastward propagating disturbances
overwhelmingly dominate their westward counterparts. The maximum spectrum
corresponds to the MJO mode discovered by Madden and Julian (1971) with a period of
50 days and a wavelength of 200 degrees in longitude. The associated eastward
propagating speed is about 5 m s-1. The major characteristics of the observed MJO (e.g.,
period, wavelength (or speed), and intensity) seem well simulated by this model (Fig.
9b). On the other hand, there are also several biases in the simulation, for example, too
strong westward propagating disturbances and too much variance with shorter periods
and smaller spatial scales.
During boreal summer, northward propagating TISO dominates in the Indian
and western Pacific Oceans (Lau and Chan 1986; Wang and Rui 1990; Fu and Wang
2004b). Our previous studies have focused over the Indian Ocean (Fu et al. 2003; Fu and
Wang 2004a). Here, we shift our attention to the western Pacific. Figures 10a, b compare
the wavenumber-frequency spectra of rainfall variability averaged between 120oE and
150oE from the CMAP observations and the coupled model. The TISO characteristics in
the simulation resemble those in the observations. The observed maximum spectrum
corresponds to an oscillation of a period about 40-50 days and a wavelength of 40
degrees in latitude. The corresponding northward propagating speed is about 1 m s-1. In
the simulation (Fig. 10b), the dominant period is about 30-50 days with both northward
18
and southward variances slightly larger than the observations. The model also tends to
produce more variability in shorter time scales and smaller spatial scales.
The above analyses indicate that the observed seasonal variations of the TISO
have been largely captured by this hybrid coupled model. The spatial patterns of the
TISO intensity (Fig. 8) and mean rainfall (Fig. 2) are highly correlated with each other in
both the observations and the simulation. This coincidence probably suggests that the
better simulation of mean rainfall is the pre-requirement for the better simulation of
TISO. Additional analyses of 10 AGCMs’ outputs from the CLIVAR/monsoon inter-
comparison project (Kang et al. 2002) showed that none of them are able to reasonably
capture both the dominant northward propagation in summer and eastward propagation in
winter (figure not shown). It is very encouraging to see that this hybrid coupled model is
capable of representing this significant seasonality of TISO with some realism.
5. The ENSO Variability
The most significant interannual variability in the tropical Asian-Pacific region is
the variability associated with ENSO. Though considerable improvements of the
simulation and prediction of ENSO have been made during the past twenty years (Latif et
al. 1998; Chen et al. 2004), current coupled models still need to be improved with regard
to realistically representing ENSO (Latif et al. 2001; Davey et al. 2002; Wang et al.
2004). In this section, the ENSO variability simulated by this hybrid coupled GCM is
evaluated.
First, we compare the spatial patterns of SST standard deviation in the tropical
Asian-Pacific sector between the observations and the model (Figs. 11a-b). The coupled
19
model produces significant SST variability in the central-eastern equatorial Pacific as in
the observations. The observations have two maximum variance centers: one near the
Peruvian coast and the other in the eastern equatorial Pacific (~110oW). The model well
locates the first maximum center, but the second center is shifted 40 degrees west of the
observed one. This is a common error presented in many other coupled GCMs (e.g., Frey
et al. 1997; Knutson and Manabe 1998), probably associated with too much westward
extension of the cold tongue in mean state.
The simulated time series of SST anomaly in the Nino-3.4 region (Fig. 12b) has a
somewhat similar evolution as in the observations during the period of 1912-1951 (Fig.
12a). As in the observations, the simulated SST time series indicates considerable
irregularity. For example, before the warm event that peaks in 1930 in the observations
(Fig. 12a), there is no significant cold event preceding it. Similar warm events (years 34
and 37) appear in the simulation. The model also produces significant biennial variations
during years 16-20 as in the observations during the period of 1922-1926. The elongated
warm event (years 27-30) with embedded annual variability in the simulation also finds a
resemblance in the observations from 1939 to 1941. With a longer simulation (~ 85
years), a power spectrum analysis indicates that the time series of SST anomaly in the
Nino-3.4 region has two peaks with periods of about 2 years and 5 years, respectively
(figure not shown).
The model ENSO shows reasonable phase locking with annual cycle (Fig. 13). The
observed SST standard deviation in the Nino-3.4 region is minimum (~ 0.45oC) in late
spring and maximum (~ 0.65oC) in winter. The simulated SST standard deviation has
considerable annual variations as in the observations. The minimum (maximum) in the
20
model occurs one month (two months) earlier than that in the observations, with the
annual range slightly larger. Compared to the results from those coupled GCMs
participating in the ENSIP project (Latif et al. 2001), the simulated ENSO annual phase
locking in this model is better than most of them. The reason is probably related to the
reasonable simulation of the climatology in the central-eastern Pacific. For example, the
ENSO simulated in SINTEX CGCM (Guilyardi et al. 2004), which also used ECHAM-4
as its atmospheric component, has no apparent annual phase locking possibly due to the
systematic errors in the simulated climatology. After improving the model climatology,
the SINTEX-F at Frontier Research Center of Japan shows much better simulation of
ENSO (Luo et al. 2005).
Although the strongest SST signal associated with ENSO is in the equatorial
eastern Pacific, its impacts actually spread around the world. Figures 14a-b compare the
global sea-level-pressure (SLP) teleconnection patterns correlated with the Nino-3.4
indices from the NCEP reanalysis (surrogate of the observations) and the coupled model.
The model captures almost all the large-scale teleconnection features presented in the
observations, particularly the see-saw patterns between the tropical Pacific Ocean and
Indian Ocean. Compared to the observations (Fig. 14a), the simulated pattern shifts
slightly westward due to the flaw in SST anomaly pattern (Fig. 11). The teleconnection
with the North American continent is also reproduced with the simulated positive SLP
center slightly locating westward of the reanalysis. El Nino in the Pacific also acts to
increase the SLP in the equatorial Atlantic (Fig. 14a). The model tends to exaggerate this
connection.
21
6. Discussion and Summary
b. Discussion
Many previous theoretical studies have suggested that the tropical Pacific
climatology (e.g., Dijkstra and Neelin 1995; Jin 1996) and annual-to-interannual
variability (e.g., Zebiak and Cane 1987; Chang and Philander 1994; Xie 1994; Li and
Philander 1996) can be reasonably explained with shallow-water-type atmosphere-ocean
coupled models. In these intermediate coupled systems, the major processes involved are
the stationary atmospheric response to underlying SST forcing (or related atmospheric
heating) and upper ocean dynamics and thermodynamics. In this study, we further tested
the above hypothesis through coupling an AGCM with an intermediate ocean model
without heat flux correction or using anomaly coupling. The results presented here tend
to support the notion that the present-day climatology and its major climate variability in
the tropical Asian-Pacific sector can be reasonably simulated with a hybrid coupled
GCM, in which the ocean component only considers the upper ocean dynamics and
thermodynamics. Although encouraging results have been obtained with this model, we
believe that there is still room to improve it for both the atmosphere component and
ocean component. Further model improvements will focus on the following two aspects.
First, we will try to improve the stratocumulus’ simulation in the atmospheric
model, which probably leads to significant mitigation of the warm bias near the Peruvian
coast in the coupled model (Fig. 1c). Recent numerical studies with regional atmospheric
models (Wang et al. 2004; McCaa and Bretherton 2004) have shown that the
stratocumulus in the southeastern Pacific is very sensitive to the cumulus
parameterization schemes and model resolutions. Because both studies have proven that
22
the stratocumulus can be reasonably represented with a mass flux scheme, it is optimistic
that ECHAM-4 AGCM (also using a mass flux scheme) can be improved through better
validation of the cumulus parameterization scheme and increase of model resolution.
Second, the parameterization of entrained water temperature in the ocean model can be
further improved. Because of the coarse vertical resolution of intermediate ocean models,
they cannot produce an entrainment temperature as do ocean general circulation models
(Gent and Cane 1989). The success of intermediate ocean models largely depends on the
parameterization of entrained water temperature. The usefulness of this approach has
been supported by many previous studies (Zebiak and Cane 1987; Seager et al. 1988;
McCreary et al. 1993; Wang et al. 1995; Jin 1998). The results presented in this study
further support that this framework is a pragmatic approach to represent the
thermodynamics and dynamics of upper tropical oceans. As suggested by previous
studies (Perigaud and Dewitte 1996; Zhang and Zebiak 2003), the parameterization of
entrained water temperature can be further improved through validating it with available
ocean observational data or reanalysis products. We are also aware that ultimate
improvement of fully coupled GCMs needs to better represent various mixing processes
in the ocean GCMs. Our current effort to develop a hybrid coupled GCM is an approach
complementary to full CGCMs.
b. Summary
In this study, A hybrid coupled GCM (HcGCM) has been developed. It showed
reasonable skill to reproduce both the climatology and intraseasonal-to-interannual
variability in the tropical Asian-Pacific sector. This hybrid coupled GCM combined the
23
ECHAM-4 GCM (Roeckner et al. 1996) with an intermediate ocean model developed at
University of Hawaii (Wang et al. 1995; McCreary and Yu 1992; Fu and Wang 2001).
In this study, the first 40-year (after 5-year spin-up) integration has been analyzed
and validated with available observations and compared to other state-of-the-art coupled
GCMs. Overall, the model simulations of the climatology and variability in the tropical
Asian-Pacific sector (e.g., Indo-Pacific mean SST and its annual cycle, Asian-Australian
monsoon, tropical intraseasonal oscillations and ENSO) are quite reasonable and
comparable with many sophisticated CGCMs (Latif et al. 2001; Davey et al. 2002;
Gualdi, et al. 2003).
The mean SST difference between the model and observations is within 1oC in
most of the tropical Indo-Pacific Oceans (Fig. 1c). The cold bias problem of the
equatorial western-central Pacific that troubled many CGCMs is only minor in this
model. It suggests that the ‘climatological Bjerknes atmosphere-ocean feedback’
mechanism (Dijkstra and Neelin 1995; Jin 1996), which is critical to configure the warm-
pool/cold tongue along the equatorial Pacific, has been reasonably represented. However,
the model also suffers a warm-bias syndrome in the southeast Pacific as do many other
CGCMs (Davey et al. 2002). The primary reason is that the atmospheric model
considerably underestimates the stratocumulus in this region. The simulated seasonal
cycle in the eastern Pacific resembles the observations with a dominant annual harmonic
(Fig. 6). However, the amplitude is slightly underestimated with a phase delayed about 1-
2 months.
The Asian summer monsoon and tropical intraseasonal variability are also well
simulated in this hybrid coupled model as in most other ECHAM-4 family coupled
24
models (e.g., Gualdi et al. 2003; Sperber et al. 2003). The spatial patterns of summer-
mean rainfall (Fig. 2) and its intraseasonal standard deviation (Fig. 8) in the model are
much closer to the observations than all 10 AGCMs participating in the CLIVAR/Asian-
Australian monsoon inter-comparison project (Kang et al. 2002; Waliser et al. 2003). The
simulated tropical intraseasonal oscillations (TISO) exhibit significant seasonality as in
the observations. In boreal winter (NDJFMA), the TISO action centers are primarily
located in the southeast Indian Ocean and Pacific SPCZ region (Fig. 8b). The dominant
ISO mode is the eastward-propagating MJO (Fig. 9). In boreal summer (MJJASO), major
TISO activities shift to the Northern Hemisphere (Fig. 8d), with dominant northward-
propagating mode in the Indian and western Pacific regions (Fig. 10).
The simulated ENSO is comparable to the observed one in terms of the variance
and frequency. The model ENSO has two dominant periods about 2 years and 5 years.
The ENSO variance shows enough meridional expansion, but the location of the
maximum shifts slightly too westward. The simulated ENSO indicates reasonable annual
phase locking with minimum and maximum variances, respectively, in boreal spring and
late fall. This hybrid coupled model also reasonably captures the global teleconnection of
ENSO, including the remote impacts to the Indian Ocean sector and North America (Fig.
15).
We have conducted a two-hundred-year integration with this hybrid coupled
model. Preliminary analysis of this long integration shows almost same results (e.g., SST
climatology and annual cycle; ENSO pattern, period, and annual phase locking et al.) as
obtained with 40-year integration. We also validated model ENSO with SODA POP v1.2
reanalysis (provided by Benjamin Giese, personal communication). The associated space-
25
time evolutions of surface winds and thermocline depth between the model and SODA
reanalysis are very similar. This result will be reported elsewhere.
Acknowledgments. This work was supported by NASA Earth Science Program, NOAA
PACS Program, and NSF Climate Dynamics Program and by the Japan Agency for
Marine-Earth Science and Technology (JAMSTEC) through its sponsorship of the IPRC.
XF would like to thank Julian P. McCreary for helpful discussions and Diane Henderson
for editing the manuscript. This paper is SOEST contribution number xxxx and IPRC
contribution number yyyy.
26
References
Alexander, M. A., 1992: Midlatitude atmosphere-ocean interaction during El Nino. Part I:
the North Pacific Ocean. J. Climate, 5, 944-958.
Bacher, A., J. M. Oberhuber, and E. Roeckner, 1998: ENSO dynamics and seasonal cycle
in the tropical Pacific as simulated by the ECHAM4/OPYC3 coupled general
circulation model, Clim. Dyn., 14, 431-450.
Chang, P., and S. G. Philander, 1994: A coupled ocean-atmosphere instability of
relevance to seasonal cycle. J. Atmos. Sci., 51, 3627-3648.
Chen, D., M. A. Cane, A. Kaplan, S. E. Zebiak, and D. Huang, 2004: Predictability of El
Nino over the past 148 years. Nature, 428, 733-736.
Davey, M., and Coauthors, 2002: STOIC: A study of coupled model climatology and
variability in tropical ocean regions. Clim. Dyn., 18, 403-420.
Delecluse, P., M. K. Davey, Y. Kitamura, S. G. H. Philander, M. Suarez, and L.
Bengstsson, 1998: Coupled general circulation modeling of the tropical Pacific. J.
Geophys. Res., 103, C7, 14357-14374.
Dijkstra, H. A. and J. D. Neelin, 1995: Ocean-atmosphere interaction and the tropical
climatology. Part II: Why the Pacific cold tongue is in the east. J .Climate, 8,
1343-1359.
Dumenil, L., and E. Todini, 1992: A rainfall-runoff scheme for use in the Hamburg
climate model. Advances in Theoretical Hydrology, A Tribute to James Dooge,
27
European Geophysical Society Series on Hydrological Sciences, Vol. 1, Elsevier
Press, 129-157.
Flatau, M., P. Flatau, P. Phoebus, and P. Niller, 1997: The feedback between equatorial
convection and local radiative and evaporative processes: The implications for
intraseasonal oscillations. J. Atmos. Sci., 54, 2373-2386.
Frey, H., M. Latif, and T. Stockdale, 1997: The coupled GCM ECHO-2, Part I: the
tropical Pacific. Mon. Wea. Rev., 125, 703-720.
Fu, X., and B. Wang, 2001: A coupled modeling study of the seasonal cycle of the
Pacific cold tongue. Part I: Simulation and sensitivity experiments. J. Climate, 14,
765-779.
Fu, X., B. Wang, and T. Li, 2002: Impacts of air-sea coupling on the simulation of mean
Asian summer monsoon in the ECHAM4 model. Mon. Wea. Rev., 130, 2889-
2904.
Fu, X., and B. Wang, T. Li and J. P. McCreary, 2003: Coupling between northward
propagating, intraseasonal oscillations and sea-surface temperature in the Indian
Ocean. J. Atmos. Sci., 60, 1733-1753.
Fu, X., and B. Wang, 2004a: Differences of boreal-summer intraseasonal oscillations
simulated in an atmosphere-ocean coupled model and an atmosphere-only model.
J. Climate, 17, 1263-1271.
Fu, X., and B. Wang, 2004b: The boreal-summer intraseasonal oscillations simulated in a
hybrid coupled atmosphere-ocean model. Mon Wea. Rev., in press.
28
Fu, X., 2005: Monsoonal impacts on Pacific cold-tongue strength and its implications for
cold bias in coupled GCMs. To be submitted.
Gadgil, S., and S. Sajani, 1998: Monsoon precipitation in the AMIP runs. Clim. Dyn., 14,
659-689.
Gaspar, P., 1988: Modeling the seasonal cycle of the upper ocean. J. Phys. Oceanogr.,
18, 161-180.
Gent, P. R., and M. A. Cane, 1989: A reduced gravity, primitive equation model of the
upper equatorial ocean. J. Comput. Phys., 81, 444-480.
Gordon, C. T., A. Rosati, and R. Gudgel, 2000: Tropical sensitivity of a coupled model to
specified ISCCP low clouds. J. Climate, 13, 2239-2260.
Gualdi, S., A. Navarra, E. Guilyardi, and P. Delecluse, 2003: Assessment of the tropical
Indo-Pacific climate in the SINTEX CGCM. Annals of Geophysics, 46, 1-26.
Guilyardi, E., P. Delecluse, S. Gualdi, and A. Navarra, 2003: Mechanisms for ENSO
phase change in a coupled GCM. J. Climate, 16, 1141-1158.
Guilyardi, E., S. Gualdi, J. Slingo, A. Navarra, P. Delecluse, J. Cole, G. Madec, M.
Roberts, M. Latif, and L. Terray, 2004: Representing El Nino in coupled ocean-
atmosphere GCMs: the dominant role of the atmosphere. J. Climate, 17, 4623-
4629.
Inness, P. M., and J. M. Slingo, 2003: Simulation of the Madden-Julian Oscillation in a
coupled general circulation model I: Comparison with observations and an
atmosphere-only GCM. J. Climate, 16, 345-364.
Jiang, X., T. Li, and B. Wang, 2004: Structures and mechanisms of the northward
propagating boreal summer intraseasonal oscillations. J. Climate, 17, 1022-1039.
29
Jin, F. F., 1996: Tropical ocean-atmosphere interaction, the Pacific cold tongue, and El
Nino-Southern Oscillation. Science, 274, 76-78.
Jin, F. F., 1998: A simple model for the Pacific cold tongue and ENSO. J. Atmos. Sci.,
55, 2458-2469.
Jones, C., 2000: Occurrence of extreme precipitation events in California and
relationships with the Madden-Julian Oscillation. J. Climate, 13, 3576-3587.
Kalnay, E., and Co-authors, 1996: The NCEP/NCAR 40-year reanalysis project. Bull.
Amer. Meteor. Soc., 77, 437-471.
Kang, I.-S., and Co-authors, 2002: Inter-comparison of the climatological variations of
Asian summer monsoon precipitation simulated by 10 GCMs. Clim. Dyn., 19,
383-395.
Kirtman, B. P., and S. E. Zebiak, 1997: ENSO simulation and prediction with a hybrid
coupled model. Mon. Wea. Rev., 125, 2620-2641.
Knutson, T. R., and S. Manabe, 1998: Model assessment of decadal variability and trends
in the tropical Pacific Ocean. J. Climate, 11, 2273-2296.
Latif, M., D. Anderson, T. Barnett, M. Cane, R. Kleeman, A. Leetmaa, J. OBrien, A.
Rosati, and E. Schneider, 1998: A review of the predictability and prediction of
ENSO. J. Geophys. Res., 103, C7, 14375-14394.
Latif, M., and Coauthors, 2001: ENSIP: the El Nino simulation intercomparison project.
Clim. Dyn., 18, 255-276.
Lau, K. M., and P. H. Chan, 1986: Aspects of the 40-50 day oscillation during the
northern summer as inferred from outgoing longwave radiation. Mon. Wea. Rev.,
114, 1354-1367.
30
Li, T., and S. G. H. Philander, 1996: On the annual cycle of the eastern equatorial Pacific.
J. Climate, 9, 2986-2997.
Li, X., Y. Chao, J. C. McWilliams, and L.-L. Fu, 2001: A comparison of two vertical-
mixing schemes in a Pacific Ocean general circulation model. J. Climate, 14,
1377-1398.
Lu, P., J. P. McCreary, and B. A. Klinger, 1998: Meridional circulation cells and the
source waters of the Pacific equatorial undercurrent. J. Phys. Oceanogr., 28, 62-
84.
Luo, J.-J., S. Masson, E. Roeckner, G. Madec, and T. Yamagata, 2005: Reducing
climatology bias in an ocean-atmosphere CGCM with improved coupling physics.
J. Climate, in press.
Madden, R. A., and P. R. Julian, 1971: Detection of a 40-50 oscillation in the zonal wind
in the tropical Pacific. J. Atmos. Sci., 28, 702-708.
Maloney, E. D., and D. L. Hartmann, 2000: Modulation of hurricane activity in Gulf of
Mexico by the Madden-Julian Oscillation. Science, 287, 2002-2004.
Maloney, E. D., and S. K. Esbensen, 2003: The amplification of east Pacific Madden-
Julian Oscillation convection and wind anomalies during June-November. J.
Climate, 16, 3482-3497.
Mapes, B. E., and P. Zuidema, 1996: Radiative-dynamical consequences of dry tongues
in the tropical troposphere. J. Atmos. Sci., 53, 620-638.
McCaa, J. R., and C. S. Bretherton, 2004: A new parameterization for shallow cumulus
convection and its application to marine subtropical cloud-topped boundary
31
layers. Part II: Regional simulation of marine boundary layer clouds. Mon. Wea.
Rev., 132, 883-896.
McCreary, J. P., and Z. J. Yu, 1992: Equatorial dynamics in a 2.5-layer model. Progress
in Oceanography, Vol. 29, Pergamon, 61-132.
McCreary, J. P., P. K. Kundu, and R. L. Molinari, 1993: A numerical investigation of
dynamics, thermodynamics and mixed-layer processes in the Indian Ocean.
Progress in Oceanography, Vol. 31, Pergamon, 181-244.
McPhaden, M. J., and Co-authors, 1998: The tropical ocean-global atmosphere observing
system: A decade of program. J. Geophys. Res., 103, C7, 14169-14240.
McPhaden, M. J., and X. Yu, 1999: Equatorial waves and the 1997-1998 El Nino.
Geophys. Res. Let., 26, 2961-2964.
Mechoso, C., and Co-authors, 1995: The seasonal cycle over the tropical Pacific in
general circulation models. Mon. Wea. Rev., 123, 2825-2838.
Mechoso, C., J. Y. Yu, and A. Arakawa, 2000: A coupled GCM pilgrimage: from climate
catastrophe to ENSO simulations. In General Circulation Model Development:
Past, Present and Future, D. A. Randall, Ed., Academic Press, pp 539-575.
Meehl, G. A., G. J. Boer, C. Covey, M. Latif, and R. J. Stouffer, 2000: The Coupled
Model Intercomparison Project (CMIP). Bull. Am. Meteorol. Soc., 81, 313-318.
Meehl, G. A., P. R. Gent, J. M. Arblaster, B. L. Otto-Bliesner, E. C. Brady, and A. Craig,
2001: Factors that affect the amplitude of El Nino in global coupled models. Clim.
Dyn., 17, 515-527.
32
Miller, M. J., T. N. Palmer, and R. Swinbank, 1989: Parameterization and influence of
sub-grid scale orography in general circulation and numerical weather prediction
models. Meteor. Atmos. Phys., 40, 84-109.
Mo, K. C., 2000: Intraseasonal modulation of summer precipitation over North America.
Mon. Wea. Rev., 128, 1490-1505.
Murakami, T., B. Wang, and S. W. Lyons, 1992: Summer monsoons over the Bay of
Bengal and the eastern North Pacific. J. Meteor. Soc. Japan, 70, 191-210.
Neelin, J. D., and Co-authors, 1992: Tropical air-sea interaction in general circulation
models. Clim. Dyn., 7, 73-104.
Neelin, J. D., D. S. Battisti, A. C. Hirst, F.-F. Jin, Y. Wakata, T. Yamagata, and Zebiak,
S. E., 1998: ENSO theory. J. Geophys. Res., 103, C7, 14261-14290.
Nordeng, T. E., 1994: Extended versions of the convective parameterization scheme at
ECMWF and their impact on the mean and transient activity of the model in the tropics.
ECMWF Research Dept. Tech. Memo., 206, European Center for Medium-Range
Weather Forecasts, Reading, United Kingdom, 41 pp.
Perigaud, C., and B. Dewitte, 1996: El Nino-La Nina events simulated with Cane and
Zebiak’s model and observed with satellite or in situ data. Part I: Model data
coimparison. J. Climate, 9, 66-84.
Philander, S. G. H., D. Gu, D. Halpern, D. Lambert, N.-C. Lau, T. Li, and R. C.
Pacanowski, 1996: Why the ITCZ is mostly north of the equator. J. Climate, 9,
2958-2972.
33
Rajendran, K., A. Kotoh, S. Yukimoto, 2004: South and East Asian summer monsoon
climate and variation in the MRI coupled model (MRI_CGCM2). J. Climate, 17,
763-782.
Rayner, M. A., E. B. Horton, D. E. Parker, and C. K. Folland, 1998: Versions 2.3b and
3.0 of the global sea-ice and sea surface temperature data set. Hadley Centre
Internal Note 85.
Roeckner, E., and Co-authors, 1996: The atmospheric general circulation model
ECHAM-4: Model description and simulation of present-day climate. Max-
Planck-Institute for Meteorology Report 218, 90pp.
Schneider, E. K., Z. Zhu, B. S. Giese, B. Huang, B. P. Kirtman, J. Shukla, and J. A.
Carton, 1997: Annual cycle and ENSO in a coupled ocean-atmosphere model.
Mon Wea. Rev., 125, 680-706.
Seager, R., S. E. Zebiak, and M. A. Cane, 1988: A model of the tropical Pacific sea
surface temperature climatology. J. Geophys. Res., 93, 1265-1280.
Slingo, J. M., and Co-authors, 1996: Intraseasonal oscillations in 15 atmospheric general
circulation models: Results from am AMIP diagnostic subproject. Clim. Dyn., 12,
325-357.
Sperber, K. R., J. M. Slingo, P. M. Inness, and W.K.-M. Lau, 1997: On the maintenance
and initiation of the intraseasonal oscillation in the NCEP/NCAR reanalysis and
in the GLA and UKMO AMIP simulations. Clim. Dyn., 13, 769-795.
Sperber, K. R., J. M. Slingo, P. M. Inness, S. Gualdi, W. Li, P. J. Gleckler, C. Doutriaux,
and the AMIP and CMIP Modeling Groups, 2003: The Madden-Julian Oscillation
34
in general circulation models. ECMWF/CLIVAR Workshop on Simulation and
Prediction of Intraseasonal Variability with Emphasis on the MJO (UCRL-PROC-
200601). Reading, United Kingdom.
Stendel, M., and E. Roeckner, 1998: Impacts of horizontal resolution on simulated
climate statistics in ECHAM4. Max-Planck-Institute for Meteorology Rep. 253,
Hamburg, 120 pp.
Taylor, K. E., D. Williamson, and F. Zwiers, 2000: The sea surface temperature and sea-
ice concentration boundary condition for AMIP II simulations, PCMDI Rep. 60,
Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore
National Laboratory, Livermore, CA, 25 pp. [Available online at http://www-
pcmdi.llnl.gov/amip/AMIP2EXPDSN/BCS/amip2bcs.html.]
Teng, H. Y., and B. Wang, 2003: Interannual variations of the boreal summer
intraseasonal oscillation in the Asian-Pacific region. J. Climate, 16, 3572-3584.
Tiedtke, M., 1989: A comprehensive mass flux scheme for cumulus parameterization in
large-scale models. Mon. Wea. Rev., 117, 1779-1800.
Tompkins, A. M., 2001: Organization of tropical convection in low vertical wind shears:
The role of water vapor. J. Atmos. Sci., 58, 529-545.
Waliser, D. E., K. M. Lau, and J. H. Kim, 1999: The influence of coupled sea surface
temperatures on the Madden-Julian oscillation: A model perturbation experiment.
J. Atmos. Sci., 56, 333-358.
Waliser, D. E. and Co-authors, 2003: AGCM simulations of intraseasonal variability
associated with the Asian summer monsoon. Climate Dyn., 21, 423-446.
35
Wang, B., and H. Rui, 1990: Synoptic climatology of transient tropical intraseasonal
convection anomalies: 1975-1985. Meteor. Atmos. Phys., 44, 43-61.
Wang, B., T. Li, and P. Chang, 1995: An intermediate model of the tropical Pacific
ocean. J. Phys. Oceanogr., 25, 1599-1616.
Wang, B., and X. Xie, 1996: Low-frequency equatorial waves in sheared zonal flow. Part
I: Stable waves. J. Atmos. Sci., 53, 449-467.
Wang, B., and X. Fu, 2001: Physical processes determining the rapid reestablishment of
the equatorial Pacific cold tongue/ITCZ complex from March to May. J. Climate,
14, 2250-2265.
Wang, W. Q., S. Saha, H. L. Pan, S. Nadiga, and G. White, 2004: Simulation of ENSO in
the new NCEP coupled forecast system model (CFS03). Submitted to Bull. Amer.
Meteor. Soc.
Wang, Y., H. Xu, and S. P. Xie, 2004: Regional model simulation of marine boundary
layer clouds over the southeast Pacific off South America. Part II: Sensitivity
experiments. Mon. Wea. Rev., In press
Xie, P., and P. A. Arkin, 1997: Global precipitation: A 17-year monthly analysis based on
gauge observations, satellite estimates, and numerical model outputs. Bull. Amer.
Meteor. Soc., 78, 2539-2558.
Xie, S. P., 1994: On the genesis of the equatorial annual cycle. J. Climate, 7, 2008-2013.
Yasunari, T., 1979: Cloudiness fluctuations associated with the Northern Hemisphere
summer monsoon. J. Meteor. Soc. Japan, 57, 227-242.
36
Yasunari, T., 1980: A quasi-stationary appearance of 30 to 40 day period in the
cloudiness fluctuations during the summer monsoon over India. J. Meteor. Soc.
Japan, 58, 225-229.
Yeh, S. W., J. G. Jhun, I.-S. Kang, B. P. Kirtman, 2004: The decadal ENSO variability in
a hybrid coupled model. J. Climate, 17, 1225-1238.
Yoneyama, K., and D. B. Parsons, 1999: A proposed mechanism for the intrusion of dry
air into the tropical western Pacific region. J. Atmos. Sci., 56, 1524-1546.
Zebiak, S. E., and M. A. Cane, 1987: A model El Nino Southern Oscillation. Mon. Wea.
Rev., 115, 2262-2278.
Zhang, R.-H., and S. E. Zebiak, 2003: Embedding a SST anomaly model into a z-
coordinate oceanic GCM for producing El Nino oscillation in the tropical Pacific
climate system. Geo. Res. Let., 30(4), 1176, doi:10.1029/2002GL015428.
37
Figure Captions
Figure 1. Annual-mean SST from the observations (100-year mean from GISST) (a),
from the hybrid coupled model (40-year mean) (b), and model bias ((b)-(a)). Contour
interval is 1oC.
Figure 2. Seasonal-mean zonal wind vertical shear (850 hPa-200 hPa) and rainfall in
boreal summer (JJAS) from the observations (a), from the model (b); and in boreal winter
(DJFM) from the observations (c) and the model (d). Shadings are for rainfall (mm day-1)
and contours are for vertical shear (m s-1).
Figure 3. 1000-hPa wind vector and wind speed (contours) in boreal summer (JJAS) from
the observations (a) and the model (b); and in boreal winter (DJFM) from the
observations (c) and the model (d). Contour interval is 2 m s-1 (larger than 6 m s-1 are
shaded).
Figure 4. 200-hPa zonal wind speed in boreal summer (JJAS) from the observations (a)
and the model (b); and in boreal winter (DJFM) from the observations (c) and the model
(d). Contour interval is 5 m s-1 (larger than 40 m s-1 are shaded).
Figure 5. Annual means of (a) SSTs (oC) and (b) surface zonal winds (m s-1) from the
observations and the model along the equatorial Indo-Pacific Oceans.
Figure 6. Annual cycles of SSTs along the equatorial Indo-Pacific Oceans from the
observations (a) and the model (b). Contour interval is 0.5oC (positive values are shaded).
38
Figure 7. Annual cycles of surface zonal winds along the equatorial Indo-Pacific Oceans
from the observations (a) and the model (b). Contour interval is 0.5 m s-1 (positive values
are shaded).
Figure 8. Rainfall standard deviations associated with tropical intraseasonal oscillations
(with periods between 20-90 days) in boreal winter (NDJFMA) from the observations (a)
and the model (b); and in boreal summer (MJJASO) from the observations (c) and the
model (d). Contour interval is 1 mm day-1 (larger than 2 are shaded).
Figure 9. Wavenumber-frequency spectra of rainfall associated with westward-eastward
propagating disturbances in boreal winter (NDJFMA) averaged between 10oS and 5oN
from the observations (a) and the model (b). Contour interval is 3 (mm day-1)2.
Figure 10. Wavenumber-frequency spectra of rainfall associated with southward-
northward propagating disturbances in boreal summer (MJJASO) averaged between
120oE and 150oE from the observations (a) and the model (b). Contour interval is 3 (mm
day-1)2.
Figure 11. Standard deviation of SST anomalies in Indo-Pacific sector from the
observations (a) and the model (b). Contour interval is 0.1oC (larger than 0.2 are shaded).
Figure 12. Time series of Nino-3.4 SST anomalies (oC) from the observations (a) and the
model (b).
Figure 13. Seasonal cycles of SST internannual standard deviations (oC) at Nino-3.4
region from the observations (a) and the model (b).
39
Figure 14. Maps of correlation of the Nino-3.4 SST anomaly time series with global sea-
level pressure anomaly from the NCEP reanalysis (a) and the model (b). Correlation
coefficients larger than 0.6 or smaller than -0.6 are shaded.
40
Figure 1. Annual-mean SST from the observations (100-year mean from GISST) (a),
from the hybrid coupled model (40-year mean) (b), and model bias ((b)-(a)). Contour
interval is 1oC.
41
Figure 2. Seasonal-mean zonal wind vertical shear (200 hPa-850 hPa) and rainfall in
boreal summer (JJAS) from the observations (a), from the model (b); and in boreal winter
(DJFM) from the observations (c) and the model (d). Shadings are for rainfall (mm day-1)
and contours are for vertical shear (m s-1).
42
Figure 3. 1000-hPa wind vector and wind speed (contours) in boreal summer (JJAS) from
the observations (a) and the model (b); and in boreal winter (DJFM) from the
observations (c) and the model (d). Contour interval is 2 m s-1 (larger than 6 m s-1 are
shaded).
43
Figure 4. 200-hPa zonal wind speed in boreal summer (JJAS) from the observations (a)
and the model (b); and in boreal winter (DJFM) from the observations (c) and the model
(d). Contour interval is 5 m s-1 (larger than 40 m s-1 are shaded).
44
Figure 5. Annual means of (a) SSTs (oC) and (b) surface zonal winds (m s-1) from the
observations and the model along the equatorial Indo-Pacific Oceans.
45
Figure 6. Annual cycles of SSTs along the equatorial Indo-Pacific Oceans from the
observations (a) and the model (b). Contour interval is 0.5oC (positive values are shaded).
46
Figure 7. Annual cycles of surface zonal winds along the equatorial Indo-Pacific Oceans
from the observations (a) and the model (b). Contour interval is 0.5 m s-1 (positive values
are shaded).
47
Figure 8. Rainfall standard deviations associated with tropical intraseasonal oscillations
(with periods between 20-90 days) in boreal winter (NDJFMA) from the observations (a)
and the model (b); and in boreal summer (MJJASO) from the observations (c) and the
model (d). Contour interval is 1 mm day-1 (larger than 2 are shaded).
48
Figure 9. Wavenumber-frequency spectra of rainfall associated with westward-eastward
propagating disturbances in boreal winter (NDJFMA) averaged between 10oS and 5oN
from the observations (a) and the model (b). Contour interval is 3 (mm day-1)2.
49
Figure 10. Wavenumber-frequency spectra of rainfall associated with southward-
northward propagating disturbances in boreal summer (MJJASO) averaged between
120oE and 150oE from the observations (a) and the model (b). Contour interval is 3 (mm
day-1)2.
50
Figure 11. Standard deviation of SST anomalies in Indo-Pacific sector from the
observations (a) and the model (b). Contour interval is 0.1oC (larger than 0.2 are shaded).
51
Figure 12. Time series of Nino-3.4 SST anomalies (oC) from the observations (a) and the
model (b).
52
Figure 13. Seasonal cycles of SST internannual standard deviations (oC) at Nino-3.4
region from the observations (a) and the model (b).
53
Figure 14. Maps of correlation of the Nino-3.4 SST anomaly time series with global sea-
level pressure anomaly from the NCEP reanalysis (a) and the model (b). Correlation
coefficients larger than 0.6 or smaller than -0.6 are shaded.
54