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
SST
NPO (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 Heating
descendingR Subtropical
high
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
(Figure from Oceanography by Tom Garrison)
ESS220Prof. Jin-Yi Yu
Costal Upwelling/Downwelling
A result of Ekman transport and mass continuity.
(Figure from Oceanography by Tom Garrison)
ESS220Prof. Jin-Yi Yu
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)
ESS220Prof. Jin-Yi Yu
Local Sea-Land Contrast Mechanism
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
Local Sea-Land Contrast Mechanism
(Miyasaka and Nakamura, 2005)
SUMMER
North Pacific Subtropical High (NPSH)
WPSH has profound impacts on EASM and typhoon.
monsoonal flowDrier, cooler flo
w
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
ENSOENSO Indian Ocean Wrming
Indian Ocean Wrming OthersOthers
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
rising
sinking
(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
Had
ley
Circ
ulati
on S
tren
gth
(m/s
ec)
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
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.
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/
sec) Hadley Circulation
stren
gthe
ned
before1990
after1990
Had
ley
Circ
ulati
on S
tren
gth
(m/s
ec)
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)
ESS228Prof. Jin-Yi Yu
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
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