Pacific Subtropical High: An
Overview
Jin-Yi Yu
Department of Earth System Science
University of California, Irvine
The Two Types of ENSO
(Yu and Kao 2007; Kao and Yu 2009)
Central-Pacific El Niño
Eastern-Pacific El Niño
Regression-EOF Method for EP/CP-ENSO
(Kao and Yu 2009)
Eastern-Pacific (EP) ENSO
Central-Pacific (CP) ENSO
CP-ENSO SST Variations
(Yu, Kao, and Lee 2010)
-14
-12
-10
-8
-6
-4
-2
0
+2
North Pacific Oscillation (NPO) and Associated SST Anomalies
NPO (SLP EOF mode)
Correlated SST
SST
EOF2
Possible Forcing Mechanisms for CP ENSO
(Yu et al. 2010)
Subtropical
forcing
Monsoon
forcing
(Yu et al. 2009)
CP ENSO
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
Sea Level Pressure (SLP)
July
January
Zonally Symmetric Circulation View
thermally indirect circulation
thermally direct circulation
JP
JS
Hadley Cell
Ferrel Cell
Polar Cell
(driven by eddies)
L
H
Equator
(warmer)
30°
(warm)
L
60°
(cold)
H
Pole
(colder)
Equator
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)
winter
hemisphere
Vertical Velocity ω500mb(June-August 1994)
Northern (summer) subtropical descent
Diabatic cooling is larger in the winter hemisphere, not summer
Eq
(Hoskins 1996)
Southern (winter) subtropical descent
Subtropical Highs
July (northern summer)
Localized Highs
(summer)
A Belt of Highs
(winter)
 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
Asia
35˚N
July





America
H
Pacific Ocean Basin
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
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)
Asian monsoon
Desert/descending
10N
25N
Sinking Branches and Deserts
(from Weather & Climate)
Global Distribution of Deserts
(from Global Physical Climatology)
Monsoon-Desert Mechanism for North Pacific
Asian
Monsoon
?
North Pacific
North
American
Monsoon
Pacific Subtropical High and North American monsoon
(Rodwell and Hoskins 2001)
ω
674mb
ѱ
887mb
PE Model Expt.
Mountains only
20% of the obs
It is demonstrated that the descent over the eastern North Pacific is a
Mt +
Rossby wave response to the North American summer
monsoon
N. A. monsoon
43%heating,
of the obs
which is further enhanced by local North Pacific SSTs.
Mt + N. A. monsoon +
local cooling from
North Pacific
80% of the obs
southward extension
Mt + N. A. monsoon +
local cooling from
North Pacific +
local Hadley circulation
Pacific Subtropical High and Asian monsoon
Kelvin Wave
In summer, the North Pacific subtropical anticyclonic easterlies are
primarily a Kelvin wave response to the east of the Asian monsoon
heating.
ѱ
887mb
Subtropical high extends all the way from Pacific to Atlantic
(Rodwell and Hoskins 2001)
Asian
Monsoon
Monsoon-Desert Mechanism
descending
Asian
Monsoon
R
K
Monsoon
Heating
subtropical
high
descent
North Pacific
Subtropical
high
North
American
Monsoon
Local Sea-Land Contrast Mechanism
Subtropical High and Eastern-Boundary Current
(Figure from Oceanography by Tom Garrison)
Global Surface Currents
(from Climate System Modeling)
Step 4: Boundary Currents
(Figure from Oceanography by Tom Garrison)
ESS220
Prof. Jin-Yi Yu
Costal Upwelling/Downwelling
 A result of Ekman
transport and mass
continuity.
(Figure from Oceanography by Tom Garrison)
ESS220
Prof. Jin-Yi Yu
Eastern Boundary Current
 Cold water from higher
latitude ocean.
 Costal upwelling
associated with subtropical
high pressure system.
(from Global Physical Climatology)
 Atmospheric subsidence
produce persistent stratiform
clouds, which further cool
down SSTs by blocking
solar radiation.
ESS220
Prof. Jin-Yi Yu
Local Sea-Land Contrast Mechanism
ESS220
Prof. Jin-Yi Yu
Local Sea-Land Contrast Mechanism
(Deep vs. Shallow Heating)
deep monsoon 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
shallow sea-land contract convection
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)
Cool NE Pacific
Warm North America
Pacific Subtropical High and Local Land-Sea Contrast
SLP
(Miyasaka and Nakamura 2005)
PE Model Expt.
Global Heating
Lower Tropospheric Heating
20˚-50˚N Heating
(no tropical heating)
cooling
heating
Local Heating
(no Asian monsoon
heating)
70% of the obs
Local Sea-Land Contrast Mechanism
SUMMER
(Miyasaka and Nakamura, 2005)
North Pacific Subtropical High (NPSH)
WPSH has profound impacts on EASM and typhoon.
Seasonal Evolution of NPSH
August
June
(from Lu and Dong 2001)
The northward shift of WPSH affects the onset and
retreat of the EASM.
WPSH vs. Monsoon & Typhoon
An enhanced WPSH signifies reduced TS days in the
subtropical
andyears)
decreased
numbers
of years)
TSs that
(extremely WNP
strong WPSH
(extremely
weak WPSH
impact East Asian (Japan, Korea, and East China)
coastal areas.
(from Wang et al. 2013)
Possible Causes for the Interannual WPSH Variability
WPSH
ENSO
Indian Ocean
Wrming
Others
EOF Modes of Interannual WPSH Variability
(from Wang et al. 2013)
EOF 1
IO warming
Pacific cooling
EOF 2
Developing CP La Nina
WPSH and W. Pacific Warm Pool
(Lu and Dong 2001)
Vertical Structure of WPSH
+
-
westward extension
+
+
suppressed convection
monsoon
SST<0
ENSO
Interannual Variability of WPSH
Western Pacific Subtropical High
(WPSH)
(Sui et al. 2007)
(Lu and Dong 2001)
NPSH shows a remarkable
zonal extension/contraction
over the western Pacific on
interannual timescales.
Two Bands of WPSH
(Sui et al. 2007)
sinking
rising
Western Pacific Subtropical High
(WPSH)
3-5yr  Walker circulation  ENSO
2.5yr  Hadley circulation  TBO
sinking
rising
Tropospheric Biennial Oscillation (TBO)
(from Meehl and Arblaster 2002)
Decadal Changes in the Two Bands of WPSH
2.5yr (monsoon-dominated)
3-5yr (ENSO-related)
(Sui et al. 2007)
1990
Decadal Change in EASM-WNPSM Relation
(Kwon et al. 2005)
more negatively correlated after 1993
Precipitation
anomalies
-
ENSO
WNPSM
-
Two Mode of WPSH Variability
-
ENSO
(Kwon et al. 2005)
WNPSM
-
sinking
rising
(Sui et al. 2007)
sinking
rising
(Wang et al. 2013)
Decadal Change in EA-WNP Summer Monsoon
and El Nino Relation
(Yim et al. 2008)
ENSO-Related
Mode
ENSO
Eastern-Pacific
El Nino
Before 1993
After 1993
Monsoon-Related
Mode
WNPSM
Central-Pacific
El Nino
(Yu, Lu, and Kim 2012)
Walker Circulation
Hadley Circulation
before
1990
Hadley Circulation Strength
(m/sec)
Walker Circulation Strength (×10-1
Pa/sec)
El Niño shifted from EP to CP
after
1990
after
1990
before
1990
NPO
The increased extratropical
forcing to the tropics
CP
EP the recent
after 1990 is a likely cause
for
emergence of the Central-Pacific El Niño.
after 1990
before 1990
NPO and Tropical Pacific SST Variations
NPO
CP
EP
NPO(5-year
Index
and
Niño
Index
running means; using CFS Reanalysis)
1990
NPO
After 1990
Niño4
Before 1990
Central T. Pacific
SSTA is less related
to extratropical
atmosphere, but
more related to
eastern tropical
Pacific.
Niño3
1990
Central Pacific SSTA
is closely related to
Extratropical
atmosphere (i.e.
NPO), but less related
to eastern tropical
Pacific.
EP/CP-ENSO Correlates with SLP
(Kao and Yu 2009)
Walker Circulation
EP ENSO
CP ENSO
Hadley Circulation
Central-Pacific SST Variability
NPO
after 1990
CP
EP
before 1990
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
(Yu et al. 2010)
WPSH
Monsoon
forcing
(Yu et al. 2009)
Subtropical
forcing
CP ENSO
Interdecadal Variability
(Zou et al. 2009; JCLI)
 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.
Asian
Monsoon
Hadley Circulation
Walker Circulation
before
1990
after
1990
Hadley Circulation Strength (m/sec)
Walker Circulation Strength (×10-1 Pa/sec)
Strength of Walker/Hadley Circulation
after
1990
before
1990
HC : [v200mb]-[v850mb] averaged over Pacific 120E-80W along 10N
WC : 500mb vertical velocity difference b/w (180W-120W) and (100E-150E) along equator
Diabatic Heating (June-August 1994)
Northern (summer) subtropical cooling
Diabatic cooling is larger in the winter hemisphere, not summer
Eq
(Hoskins 1996)
Southern (winter) subtropical cooling
How Many Monsoons Worldwide?
North America Monsoon
Asian Monsoon
Australian
Monsoon
South America Monsoon
(figure from Weather & Climate)
Africa Monsoon
Seasonal Cycle of Rainfall
(from IRI)
Indian
Monsoon
Australian
Monsoon
Gill’s Response to Symmetric Heating
(from Gill 1980)
• 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. ESS228
Prof. Jin-Yi Yu
Climate Roles of WPSH
 Linking Asian summer monsoon to tropical forcing (i.e., El Nino)
 Influencing the transport of water vapor into East Asia