Pacific Subtropical High: An Overview Jin-Yi Yu Department 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) Central-Pacific El Niño Eastern-Pacific El Niño 2

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 +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 THIRD THING we like to suggest is that atmospheric forcing for CP ENSO MAY BE RELATED to THE SUBTROPICAL FORCING and THE MONSOON FORCING, LET ME BEGIN TO TELL YOU WHY WE MAKE THESE SUGGESTIONS Monsoon forcing CP ENSO (Yu et al. 2009)

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 JS JP Hadley Cell Ferrel Cell Polar Cell (driven by eddies) L H L H Equator (warmer) 30° 60° Pole (colder) (warm) (cold)

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 July Asia America H 35˚N 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: Monsoon-desert mechanism (Rodwell and Hoskins 1996, 2001) Local land-sea thermal contrast (Miyasaka and Nakamura 2005) Diabatic amplification of cloud-reduced radiative cooling 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 American Monsoon North Pacific

Pacific Subtropical High and North American monsoon ω674mb ѱ887mb (Rodwell and Hoskins 2001) PE Model Expt. Mountains only 20% of the obs 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. Mt + N. A. monsoon 43% of the obs Mt + N. A. monsoon + local cooling from North Pacific 80% of the obs Mt + N. A. monsoon + local cooling from North Pacific + local Hadley circulation southward extension

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. ѱ887mb Subtropical high extends all the way from Pacific to Atlantic (Rodwell and Hoskins 2001)

Asian Monsoon

Monsoon-Desert Mechanism Monsoon Heating descending R K Subtropical high Asian Monsoon North American Monsoon 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)

Step 4: Boundary Currents (Figure from Oceanography by Tom Garrison)

Costal Upwelling/Downwelling A result of Ekman transport and mass continuity. (Figure from Oceanography by Tom Garrison)

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

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 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) shallow sea-land contract convection Warm North America Cool NE Pacific

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) Drier, cooler flow monsoonal flow WPSH has profound impacts on EASM and typhoon.

Seasonal Evolution of NPSH August June The northward shift of WPSH affects the onset and retreat of the EASM. (from Lu and Dong 2001)

WPSH vs. Monsoon & Typhoon 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. (extremely strong WPSH years) (extremely weak WPSH years) (from Wang et al. 2013)

Possible Causes for the Interannual WPSH Variability ENSO Indian Ocean Wrming Others

EOF Modes of Interannual WPSH Variability (from Wang et al. 2013) EOF 1 EOF 2 IO warming Pacific cooling Developing CP La Nina

WPSH and W. Pacific Warm Pool (Lu and Dong 2001) Vertical Structure of WPSH - + - - + + westward extension suppressed convection SST<0 monsoon ENSO

Interannual Variability of WPSH Western Pacific Subtropical High (WPSH) (Sui et al. 2007) NPSH shows a remarkable zonal extension/contraction over the western Pacific on interannual timescales. (Lu and Dong 2001)

Two Bands of WPSH 3-5yr  Walker circulation  ENSO (Sui et al. 2007) rising sinking Western Pacific Subtropical High (WPSH) 3-5yr  Walker circulation  ENSO 2.5yr  Hadley circulation  TBO rising sinking

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 WNPSM - - ENSO

Two Mode of WPSH Variability ENSO - rising sinking (Kwon et al. 2005) (Sui et al. 2007) (Wang et al. 2013) WNPSM - rising sinking

Decadal Change in EA-WNP Summer Monsoon and El Nino Relation (Yim et al. 2008) ENSO-Related Mode Eastern-Pacific El Nino ENSO Before 1993 After 1993 Monsoon-Related Mode Central-Pacific El Nino WNPSM

El Niño shifted from EP to CP (Yu, Lu, and Kim 2012) Walker Circulation weakened before 1990 after Walker Circulation Strength (×10-1 Pa/sec) Hadley Circulation strengthened Hadley Circulation Strength (m/sec) The increased extratropical forcing to the tropics after 1990 is a likely cause for the recent emergence of the Central-Pacific El Niño. NPO after 1990 CP EP before 1990

NPO and Tropical Pacific SST Variations CP EP

NPO Index and Niño Index (5-year running means; using CFS Reanalysis) 1990 NPO Central Pacific SSTA is closely related to Extratropical atmosphere (i.e. NPO), but less related to eastern tropical Pacific. After 1990 Central T. Pacific SSTA is less related to extratropical atmosphere, but more related to eastern tropical Pacific. Before 1990 Niño4 Niño3

EP/CP-ENSO Correlates with SLP (Kao and Yu 2009) Walker Circulation EP ENSO CP ENSO Hadley Circulation

Central-Pacific SST Variability after 1990 NPO 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) Subtropical forcing WPSH THIRD THING we like to suggest is that atmospheric forcing for CP ENSO MAY BE RELATED to THE SUBTROPICAL FORCING and THE MONSOON FORCING, LET ME BEGIN TO TELL YOU WHY WE MAKE THESE SUGGESTIONS Monsoon forcing CP ENSO (Yu et al. 2009)

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

Strength of Walker/Hadley Circulation Walker Circulation weakened before 1990 after Walker Circulation Strength (×10-1 Pa/sec) Hadley Circulation strengthened before 1990 after Hadley Circulation Strength (m/sec) 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 Africa Monsoon South America Monsoon (figure from Weather & Climate)

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.

Climate Roles of WPSH Linking Asian summer monsoon to tropical forcing (i.e., El Nino) Influencing the transport of water vapor into East Asia