TropicalM. D. Eastin Intraseasonal and Interannual Variability

Tropical M. D. Eastin

Intraseasonal and Interannual Variability


Outline

Tropical phenomena

• Madden Julian Oscillation (MJO)

• Quasi-Biennial Oscillation (QBO)

Mid-Latitude phenomena with links to the Tropics

• North Atlantic Oscillation (NAO)

• Pacific Decadal Oscillation (PDO)

Comments on Global Warming and Data Records


Madden-Julian Oscillation (MJO)Definition and Background

• The MJO is an intraseasonal oscillation between periods of enhanced and suppressed tropical rainfall with a period of 30-60 days• Discovered by R. Madden and P. Julian in 1971

Evolution

• Each regime or phase (wet and dry) tends to develop in the Indian Ocean and propagate eastward at ~5 m/s into the central Pacific• More pronounced in the eastern Hemisphere• Upon crossing in the dateline, the convective intensity decreases and the regimes begin to propagate eastward at >10 m/s• Often hard to “track” in the western Hemisphere• Intensity is influenced by ENSO

Impacts

• Affects the intensity and breaks periods of the Asian (Indian) and Australian monsoon • Wet phase tends to enhance TC activity

30-60 Day Filtered OLR Anomalies(Blue = Convection Red = dry)


Madden-Julian Oscillation (MJO)Structure

• Wet phase is characterized by an “envelope” of deep convection composed of numerous higher frequency “events” (equatorial waves, TCs, and tropical MCS) that propagate both east and west through the envelope

Causes (or Forcing Mechanisms)

Unknown• Currently believed to result from both external forcing and internal instabilities

30-60 Day Filtered OLR, Streamfunction, and Wind Anomalies

shading → convectioncontours → streamfunction

vectors → winds

Adapted from Kiladis et al. (2005) and Zhang (2005)

850 mb

200 mb

Wave motion

Zonal Cross-Section

Cool SST Warm SST Cool SST

Warm

Warm

Cool

Cool Cool

Moist

MoistDry Dry


Madden-Julian Oscillation (MJO)Current Status:

Let’s go to: http://www.cpc.noaa.gov/products/precip/CWlink/MJO/mjo.shtml


Quasi-Biennial Oscillation (QBO)Definition and Background

• The QBO is a quasi-periodic interannual oscillation of the tropical stratospheric zonal winds between easterlies and westerlies with a mean period of 28-29 months

• First recognized by R. J. Reed and R. A. Ebdon in 1960• First referred to as the QBO by Jim Angell in 1964

• Each wind regime or phase (east and west) develops at the top of the stratosphere and propagates downward at ~1 km per month until they dissipate at the tropical tropopause• The west phase (westerly winds) descends faster than the east phase• The east phase amplitude (20-30 m/s) is larger than the west phase amplitude (10-20 m/s) • The phases are coherent through the entire equatorial belt at any given time• Peak amplitudes are over the Equator at an altitude of 24 km (~30 mb)• Amplitudes decrease away from the equator (i.e. “equatorially trapped” → Why?)

Time-Height (Pressure) diagram of Equatorial Zonal Wind

East

WestWest WestWest

EastEastEastEast


Quasi-Biennial Oscillation (QBO)Causes (or Forcing Mechanisms)

QBO is equatorially trapped → Forcing is from equatorial waves (also trapped) that vertically propagate into the stratosphere → QBO physics described by Holton and Lindzen (1972), Plumb (1977), Plumb (1984), Baldwin et al. (2001)

Vertical Propagation of Equatorial Waves (the Basics)

• Waves can propagate vertically until they reach a “critical layer” (which acts like a sponge)• Critical layers are determined by the mean flow characteristics and the wave type• When a wave encounters a critical layer, the wave is first damped and then dissipates• Damped waves deposit their propagation momentum into the mean flow, accelerating the mean flow in the direction of wave propagation “Critical Layer” Wave Flow the waves Flow the waves Propagation CAN vertically can NOT vertically Momentum

Wave type Direction propagate through propagate through Acceleration

Inertio-Gravity eastward westward strong eastward eastward Kelvin

Inertio-Gravity Rossby-Gravity westward eastward strong westward westward Rossby


Quasi-Biennial Oscillation (QBO)QBO Physics: Switching from the East phase to the West phase in the lower stratosphere

WestwardPropagating

Waves

EastwardPropagating

Waves

passunimpeded

dissipate atupper levels

start newwest phase

aloft

damped andacceleratemean flow

Time 1

EastwardFlow

Time 0

eastwardflow maxdescends

westwardflow maxdescends



WestwardPropagating

Waves

EastwardPropagating

Waves

passunimpeded


Time 2

EastwardFlow

Time 1




WestwardFlow



WestwardPropagating

Waves

EastwardPropagating

Waves

passunimpeded


Time 3Time 2

thin eastward maxdestroyed by

viscous diffusion



WestwardFlow


Quasi-Biennial Oscillation (QBO)QBO Physics: Switching from the West phase to the East phase in the lower stratosphere

WestwardPropagating

Waves

EastwardPropagating

Waves

passunimpeded

Time 4Time 3



WestwardFlow


start neweast phase

aloft




WestwardPropagating

Waves

EastwardPropagating

Waves

passunimpeded

Time 5Time 4



EastwardFlow


WestwardFlow




WestwardPropagating

Waves

EastwardPropagating

Waves

passunimpeded

Time 6Time 5


EastwardFlow



thin westward maxdestroyed by

viscous diffusion



WestwardPropagating

Waves

EastwardPropagating

Waves

passunimpeded


start newwest phase

aloft


Time 1 or Time 7

EastwardFlow

Time 0 or Time 6




Quasi-Biennial Oscillation (QBO)Why is the QBO Important?

• The phase of the QBO affects Atlantic TC activity

West phase → Increased TC activityEast phase → Decreased TC activity

• West Pacific TC frequency increases during the West phase

• Southwest Indian Ocean TC frequency increased during the East phase

• Believed to impact the onset and intensity of monsoon circulations

• Used by some statistical predictions of ENSO to forecast strength and timing of events

• Believed to impact west African rainfall

• Decay of aerosol loading from volcanic eruptions depends on QBO phase


North Atlantic Oscillation (NAO)Definition and Background

• The NAO is an interannual meridional oscillation of mass within the North Atlantic defined by the normalized surface pressure difference between the Iberian Peninsula and (minus) Iceland

• First recognized by the Danish in the 1770s• First researched and defined by Sir Gilbert Walker in the 1920s

• Primarily influences the strength of mid-latitude westerly winds and storm tracks• Influence is most prominent in the winter months (Nov-Apr)• Impacts the seasonal climate from eastern North America to central Asia and from tropical to polar latitudes

Source: http://www.cgd.ucar.edu/cas/jhurrell/indices.html

Winter normalized NAO index: 1900-2012


North Atlantic Oscillation (NAO)Positive NAO Index

• Low pressure anomaly over Iceland (a stronger Icelandic Low) and a high pressure anomaly over the subtropical Atlantic (a stronger Azore High)

• Enhanced mid-latitude westerlies• Enhanced tropical trade winds• More intense SAL outbreaks

• Warm over Europe and the eastern U.S.• Cold over the polar North Atlantic and the tropical North Atlantic

• Wet from Iceland to Scandinavia (contributes to advancing glaciers)• Dry over southern Europe

• More intense winter storms in northern Europe• Milder winters in eastern U.S.

Dominant phase from the early 1980s through the mid-2000s (will it continue?)


North Atlantic Oscillation (NAO)Negative NAO Index

• High pressure anomaly over Iceland (a weaker Icelandic Low) and a low pressure anomaly over the subtropical Atlantic (a weaker Azore High)

• Suppressed mid-latitude westerlies• Suppressed tropical trade winds• Less intense SAL outbreaks

• Cold over Europe and the eastern U.S.• Warm over the polar North Atlantic (enhanced ice sheet melting) and the tropical North Atlantic

• Dry over Iceland to Scandinavia• Wet over southern Europe and eastern North America

• More intense winter storms in the eastern U.S.

Dominant phase from the mid 1950s to the late 1970s


North Atlantic Oscillation (NAO)Causes (or Forcing Mechanisms)

• Thought to be driven by “natural internal atmospheric processes” (i.e. unknown)• High correlation with Atlantic SST and ice variability, but it’s unclear which causes which

Interesting “Relationship” to Global Warming

• The negative phase (cold North America and Europe) was dominant 1955-1979• The positive phase (warm North America and Europe) has dominated since (1980-2005)• The majority of “reliable” global surface observations are made over land (in particular over North America, Europe, and Asia) Could the recent “global warming” be an artifact of the NAO and our observation networks?

Global ClimateObserving Network

981 total stations


Pacific-Decadal Oscillation (PDO)Definition and Background

• The PDO is an multi-decadal (20-30 year) oscillation of SST within the North Pacific defined by normalized (and de-trended) SST anomalies north of 20ºN

• First recognized by Steven Hare (a fisheries scientist) in 1996• First related to climate by Yuan Zhang in 1997

• Influence is most prominent in the winter months (Oct-Mar)• Impacts the seasonal climate from eastern Asia and North America• Climate impacts that occur during each phase are similar to the ENSO impacts but are less intense and occur over a longer timescale• The cause of the PDO is unknown

Monthly values of the normalized PDO index: 1900-2012

Source: http://jisao.washington.edu/pdo/


Pacific-Decadal Oscillation (PDO)Positive PDO index (Warm phase)

• Colder SSTs in the central North Pacific• Warmer SSTs along the west coast of North America and in the equatorial central Pacific (similar to El Nino)• Low pressure anomaly over North Pacific

• Warm winters in the Pacific Northwest• Cold winters in the southeast U.S.• Wet winters in western U.S.• Dry winters in U.S. Midwest

Anomalous SST, Pressure, and WindsWarm Phase of PDO

Correlation of Winter Surface Temperature and Precipitation with the PDO Index

Temperature Precipitation


Pacific-Decadal Oscillation (PDO)Negative PDO index (Cold phase)

• Warmer SSTs in the central North Pacific• Colder SSTs along the west coast of North America and in the equatorial central Pacific (similar to La Nina)• High pressure anomaly over North Pacific

• Cold winters in the Pacific Northwest• Warm winters in the southeast U.S.• Dry winters in western U.S.• Wet winters in U.S. Midwest

Anomalous SST, Pressure, and WindsCold Phase of PDO

Correlation of Winter Surface Temperature and Precipitation with the PDO Index

Temperature Precipitation


Global Warming?



Reliable global observations:

• It is well accepted that reliable global weather observations began in the late 1940s•Hence → global warming “evidence”


Global Warming?



?

?

?

Reliable global observations:

• Notice how the NAO and PDO exhibit a negative trend from 1920-1960•Could the recent trends in ENSO NAO and PDO be an oscillation?


Summary:

• Madden-Julian Oscillation (MJO)• Definition and Evolution• Causes / Forcing• Impacts

• Quasi-Biennial Oscillation (QBO)• Definition and Basic Characteristics• Causes / Physics• Impacts

• North Atlantic Oscillation (NAO)• Definition• Impacts during each Phase• Causes / Forcing

• Pacific Decadal Oscillation• Definition• Impacts during each Phase• Causes / Forcing

Intraseasonal and Interannual Variability


ReferencesBaldwin, M. P., and Coauthors, 2001: The Quasi-Biennial Oscillation. Reviews of Geophysics, 39, 179-229.

Bond, N. A., and D. E. Harrison (2000): The Pacific Decadal Oscillation, air-sea interaction and central north Pacific winteratmospheric regimes. Geophys. Res. Lett., 27(5), 731-734.

Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage( http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl )

Hurrell, J. W., Y. Kushnir, M. Visbeck, and G. Ottersen, 2003: An Overview of the North Atlantic Oscillation. The NorthAtlantic Oscillation: Climate Significance and Environmental Impact, J.W. Hurrell, Y. Kushnir, G. Ottersen,and M. Visbeck, Eds. Geophysical Monograph Series, 134, 1-35.

Trenberth, K. E. and J. W. Hurrell, 1994: Decadal atmosphere-ocean variations in the Pacific. Clim. Dyn. 9, 303-319.

Zhang, C., 2005: Madden-Julian Oscillation, Reviews of Geophysics, 43, 1-36.

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TropicalM. D. Eastin Intraseasonal and Interannual Variability