Pacific Subtropical High: An Overview

Pacific Subtropical High: An Overview

Jin-Yi YuDepartment of Earth System Science

University of California, Irvine

The Two Types of ENSO

Central-Pacific El Niño

Eastern-Pacific El Niño

(Yu and Kao 2007; Kao and Yu 2009)

Regression-EOF Method for EP/CP-ENSO

Central-Pacific (CP) ENSOEastern-Pacific (EP) ENSO

(Kao and Yu 2009)

CP-ENSO SST Variations

-14

-4

-10

-6

-12

-8

+20-2

(Yu, Kao, and Lee 2010)

North Pacific Oscillation (NPO) and Associated SST Anomalies

EO

F2

SSTNPO (SLP EOF mode) Correlated SST

Possible Forcing Mechanisms for CP ENSO

Monsoon forcing

Subtropical forcing

CP ENSO

(Yu et al. 2009)

(Yu et al. 2010)

OUTLINES

Seasonal Cycle: Maintenance Mechanisms; Summer vs. Winter

Interannual Variability: WPSH; El Nino vs. Monsoon

Decadal Variability: Before and After 1990; Two Types of El Nino

July

January

Sea Level Pressure (SLP)

Zonally Symmetric Circulation View

Equator (warmer)

Pole(colder)

30° 60°

Hadley Cell Polar CellFerrel Cell

(warm) (cold)

LL H H

thermally direct circulationthermally indirect circulation

(driven by eddies)

JS JP

Off-Equatorial Heating

“ .. We find that moving peak heating even 2 degree off the equator leads to profound asymmetries in the Hadley circulation, with the winter cell amplifying greatly and the summer cell becoming negligible.”

--- Lindzen and Hou (1988; JAS)

Equator

winterhemisphere

Vertical Velocity ω500mb(June-August 1994)

Eq

Southern (winter) subtropical descent

Northern (summer) subtropical descent

Diabatic cooling is larger in the winter hemisphere, not summer

(Hoskins 1996)

Subtropical Highs

Localized Highs(summer)

A Belt of Highs(winter)

July (northern summer)

Winter subtropical highs can be explained by the Hadley circulation Summer subtropical highs has to be explained in the contect of planetary

waves

Maintenance Mechanism for the Summertime Subtropical Highs

It is still not fully understood how the subtropical highs in the NH summer are forced and maintained dynamically and thermodynamically.

In the past, Hadley circulation is used to explain the formation and maintenance of the subtropical highs.

However, a zonally symmetric Hadley circulation is supposed to produce a much weaker subtropical subsidence in the summer hemisphere than in the winter hemisphere (Lindzen and Hou 1988).

It has been suggested that dynamics of the highs may be better understood in the context of planetary waves rather than in a framework of zonally symmetric circulation.

(Miyasaka and Nakamura, 2005; JCLI)

Summer Subtropical Highs

July

Center around 35˚N Reside over the eastern sectors of ocean basins A “cell” not a “belt” of high pressure Isobars almost parallel to the west coasts of the continents H cells extend westward reaching western boundary of the basin

H35˚N

Asia America

Pacific Ocean Basin

Possible Mechanisms - Summer

The underlying mechanisms are still disputed:(1) Monsoon-desert mechanism (Rodwell and Hoskins 1996, 2001)(2) Local land-sea thermal contrast (Miyasaka and Nakamura 2005)(3) Diabatic amplification of cloud-reduced radiative cooling(4) Air-sea interaction

Monsoon-Desert Mechanism(Rodwell and Hoskins 1996)

Desert/descendingAsian monsoon

10N 25N

Sinking Branches and Deserts

(from Weather & Climate)

Global Distribution of Deserts

(from Global Physical Climatology)

Monsoon-Desert Mechanism for North Pacific

AsianMonsoon

North AmericanMonsoon

North Pacific

?

Pacific Subtropical High and North American monsoonω674mb ѱ887mb

20% of the obs

43% of the obs

80% of the obs

southward extension

PE Model Expt.

Mountains only

Mt + N. A. monsoon

Mt + N. A. monsoon +local cooling fromNorth Pacific

Mt + N. A. monsoon +local cooling fromNorth Pacific + local Hadley circulation

(Rodwell and Hoskins 2001)

It is demonstrated that the descent over the eastern North Pacific is a Rossby wave response to the North American summer monsoon heating, which is further enhanced by local North Pacific SSTs.

ѱ887mb

(Rodwell and Hoskins 2001)

Kelvin Wave

Pacific Subtropical High and Asian monsoon

Subtropical high extends all the way from Pacific to Atlantic

In summer, the North Pacific subtropical anticyclonic easterlies are primarily a Kelvin wave response to the east of the Asian monsoon heating.

Asian Monsoon

Monsoon-Desert Mechanism

Monsoon Heatingdescending

R Subtropicalhigh

K

AsianMonsoon

North AmericanMonsoon

subtropical high descent

North Pacific

Local Sea-Land Contrast Mechanism

Subtropical High and Eastern-Boundary Current

(Figure from Oceanography by Tom Garrison)

Global Surface Currents

(from Climate System Modeling)

ESS220Prof. Jin-Yi Yu

Step 4: Boundary Currents



Costal Upwelling/Downwelling

A result of Ekman transport and mass continuity.



Eastern Boundary Current

Cold water from higher latitude ocean.

Costal upwelling associated with subtropical high pressure system.

Atmospheric subsidence produce persistent stratiform clouds, which further cool down SSTs by blocking solar radiation.

(from Global Physical Climatology)



Local Sea-Land Contrast Mechanism(Deep vs. Shallow Heating)

Warm North AmericaCool NE Pacific

deep monsoon convection

shallow sea-land contract convection

“The authors demonstrate through numerical experiments that those (i.e. subtropical) highs can be reproduced in response to a local shallow cooling–heating couplet associated with this thermal contrast, ........... Since each of the subtropical highs can be reproduced reasonably well, even for the premonsoon season (i.e., May), in response to a local shallow land–sea heating contrast, it is suggested that the monsoonal convective heating may not necessarily be a significant direct forcing factor for the formation of the summertime subtropical highs.” (Miyasaka and Nakamura 2005)

Pacific Subtropical High and Local Land-Sea Contrast

SLP

PE Model Expt.

(Miyasaka and Nakamura 2005)

Global Heating

20˚-50˚N Heating (no tropical heating)

Local Heating (no Asian monsoon

heating)

Lower Tropospheric Heating

70% of the obs

coolingheating


(Miyasaka and Nakamura, 2005)

SUMMER

North Pacific Subtropical High (NPSH)

WPSH has profound impacts on EASM and typhoon.

monsoonal flowDrier, cooler flow

Seasonal Evolution of NPSH

June

August

The northward shift of WPSH affects the onset and retreat of the EASM.(from Lu and Dong 2001)

WPSH vs. Monsoon & Typhoon

(from Wang et al. 2013)

(extremely strong WPSH years) (extremely weak WPSH years)

An enhanced WPSH signifies reduced TS days in the subtropical WNP and decreased numbers of TSs that impact East Asian (Japan, Korea, and East China) coastal areas.

Possible Causes for the Interannual WPSH Variability

WPSH

ENSO Indian Ocean Wrming Others

EOF Modes of Interannual WPSH Variability

EOF 1 EOF 2

IO warming Pacific cooling Developing CP La Nina

(from Wang et al. 2013)

WPSH and W. Pacific Warm Pool(Lu and Dong 2001) Vertical Structure of WPSH

+-

+

+

-

-

suppressed convection

westward extension

SST<0monsoon ENSO

Interannual Variability of WPSH

(Lu and Dong 2001)

(Sui et al. 2007)

NPSH shows a remarkable zonal extension/contraction over the western Pacific on interannual timescales.

Western Pacific Subtropical High (WPSH)

Two Bands of WPSH

Western Pacific Subtropical High (WPSH)

(Sui et al. 2007)

3-5yr Walker circulation ENSO

2.5yr Hadley circulation TBO

rising

sinking

risingsinking

Tropospheric Biennial Oscillation (TBO)

(from Meehl and Arblaster 2002)

Decadal Changes in the Two Bands of WPSH

(Sui et al. 2007)

1990

3-5yr (ENSO-related)

2.5yr (monsoon-dominated)

Decadal Change in EASM-WNPSM Relation

(Kwon et al. 2005) more negatively correlated after 1993

Precipitationanomalies

ENSOWNPSM

--

Two Mode of WPSH Variability

WNPSM

-

ENSO- risingsinking

risingsinking

(Kwon et al. 2005) (Sui et al. 2007) (Wang et al. 2013)

Decadal Change in EA-WNP Summer Monsoon and El Nino Relation

(Yim et al. 2008)

Monsoon-RelatedMode

Eastern-PacificEl Nino

Before 1993

After 1993

ENSO-RelatedMode

Central-PacificEl Nino

ENSO

WNPSM

El Niño shifted from EP to CP(Yu, Lu, and Kim 2012)

Walker Circulationweakened

before1990 after

1990

Wal

ker C

ircul

ation

Str

engt

h (×

10-1

Pa/s

ec)

Hadley Circulation

stren

gthe

ned

before1990

after1990

Hadl

ey C

ircul

ation

Str

engt

h (m

/sec

)

EP

before 1990

after 1990

NPO

CP

The increased extratropical forcing to the tropics after 1990 is a likely cause for the recent emergence of the Central-Pacific El Niño.

NPO and Tropical Pacific SST Variations

CP EP

NPO

NPO Index and Niño Index1990

1990

Central Pacific SSTAis closely related toExtratropical atmosphere (i.e. NPO), but less relatedto eastern tropical Pacific.

After 1990

Central T. Pacific SSTA is less related to extratropical atmosphere, but more related toeastern tropical Pacific.

Before 1990

NPO

Niño4

Niño3

(5-year running means; using CFS Reanalysis)

EP/CP-ENSO Correlates with SLP

EP ENSO

CP ENSO

Hadley Circulation

Walker Circulation(Kao and Yu 2009)

Central-Pacific SST Variability

EPbefore 1990

after 1990

NPO

CP

The increased extratropical forcing to the tropics after 1990 is a likely cause for the recent emergence of the Central-Pacific El Niño.

WPSH and the Two Types of El Nino

Monsoon forcing

Subtropical forcing

CP ENSO

(Yu et al. 2009)

(Yu et al. 2010)

WPSH

Interdecadal Variability

WPSH has extended westward since the last 1970s shifted rain bands in China more rainfalls in the south and less rainfalls in the North

Causes unknown, but may be related to the forcing from Indo-Pacific Ocean.

(Zou et al. 2009; JCLI)

Asian Monsoon

Strength of Walker/Hadley Circulation

HC : [v200mb]-[v850mb] averaged over Pacific 120E-80W along 10NWC : 500mb vertical velocity difference b/w (180W-120W) and (100E-150E) along equator

Walker Circulation

weakenedbefore1990 after

1990

Wal

ker C

ircul

ation

Str

engt

h (×

10-1

Pa/s

ec) Hadley Circulation

stren

gthe

ned

before1990

after1990

Hadl

ey C

ircul

ation

Str

engt

h (m

/sec

)

Diabatic Heating (June-August 1994)

Eq

Southern (winter) subtropical cooling

Northern (summer) subtropical cooling

Diabatic cooling is larger in the winter hemisphere, not summer

(Hoskins 1996)

How Many Monsoons Worldwide?North America Monsoon

Africa MonsoonSouth America Monsoon

Asian Monsoon

Australian Monsoon

(figure from Weather & Climate)

Seasonal Cycle of Rainfall

AustralianMonsoon

IndianMonsoon

(from IRI)


Gill’s Response to Symmetric Heating

• This response consists of a eastward-propagating Kelvin wave to the east of the symmetric heating and a westward-propagating Rossby wave of n=1 to the west.

• The Kelvin wave low-level easterlies to the east of the heating, while the Rossby wave produces low-level westerlies to the west.

• The easterlies are trapped to the equator due to the property of the Kelvin wave.

• The n=1 Rossby wave consists of two cyclones symmetric and straddling the equator.

• The meridional scale of this response is controlled by the equatorial Rossby radius, which is related to the β-effect and the stability and is typically of the order of 1000km.

(from Gill 1980)

Climate Roles of WPSH

Linking Asian summer monsoon to tropical forcing (i.e., El Nino)

Influencing the transport of water vapor into East Asia

Documents

Pacific Subtropical High: An Overview