1. Pacific Subtropical High: An Overview
Jin-Yi Yu
Department of Earth System Science
University of California, Irvine

2. 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

3. Regression-EOF Method for EP/CP-ENSO
(Kao and Yu 2009)
Eastern-Pacific (EP) ENSO
Central-Pacific (CP) ENSO

4. CP-ENSO SST Variations
(Yu, Kao, and Lee 2010)
-14
-12
-10 -8 -6 -4 -2 +2

5. North Pacific Oscillation (NPO) and Associated SST Anomalies
NPO (SLP EOF mode)
Correlated SST
SST
EOF2

6. 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)

7. 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

8. Sea Level Pressure (SLP)
July
January

9. 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)

10. 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

11. 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

12. 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

13. 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)

14. 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

15. 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

16. Monsoon-Desert Mechanism (Rodwell and Hoskins 1996)
Asian monsoon
Desert/descending
10N
25N

17. Sinking Branches and Deserts
(from Weather & Climate)

18. Global Distribution of Deserts
(from Global Physical Climatology)

19. Monsoon-Desert Mechanism for North Pacific?
Asian
Monsoon
North American
Monsoon
North Pacific

20. 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

21. 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)

22. Asian Monsoon

23. Monsoon-Desert Mechanism
Monsoon Heating
descending R K
Subtropical
high
Asian
Monsoon
North American
Monsoon
subtropical
high
descent
North Pacific

24. Local Sea-Land Contrast Mechanism

25. Subtropical High and Eastern-Boundary Current
(Figure from Oceanography by Tom Garrison)

26. Global Surface Currents
(from Climate System Modeling)

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

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

29. 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)

30. Local Sea-Land Contrast Mechanism

31. 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

32. 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

33. Local Sea-Land Contrast Mechanism
SUMMER
(Miyasaka and Nakamura, 2005)

34. North Pacific Subtropical High (NPSH)
Drier, cooler flow
monsoonal flow
WPSH has profound impacts on EASM and typhoon.

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

36. 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)

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

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

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

40. 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)

41. 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

42. Tropospheric Biennial Oscillation (TBO)
(from Meehl and Arblaster 2002)

43. Decadal Changes in the Two Bands of WPSH
2.5yr (monsoon-dominated)
3-5yr (ENSO-related)
(Sui et al. 2007)
1990

44. Decadal Change in EASM-WNPSM Relation
(Kwon et al. 2005)
more negatively correlated after 1993
Precipitation
anomalies
WNPSM - -
ENSO

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

46. 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

47. 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 is a likely cause for the recent emergence of the Central-Pacific El Niño.
NPO
after 1990 CP EP
before 1990

48. NPO and Tropical Pacific SST VariationsCP EP

49. 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

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

51. Central-Pacific SST Variability
after 1990
NPO CP EP
before 1990
The increased extratropical forcing to the tropics after is a likely cause for the recent emergence of the Central-Pacific El Niño.

52. 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)

53. 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.

54. Asian Monsoon

55. 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

56. 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

57. How Many Monsoons Worldwide?
North America Monsoon
Asian Monsoon
Australian
Monsoon
Africa Monsoon
South America Monsoon
(figure from Weather & Climate)

58. Seasonal Cycle of Rainfall
(from IRI)
Indian
Monsoon
Australian
Monsoon

59. 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.

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