Pacific Subtropical High: An Overview

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

2. The Two Types of ENSO (Yu and Kao 2007; Kao and Yu 2009) Central-Pacific El Niño Eastern-Pacific El Niño

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

4. CP-ENSO SST Variations (Yu, Kao, and Lee 2010) -14 -12 -10 -6 -4 -8 0 +2 -2

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

6. Possible Forcing Mechanisms for CP ENSO (Yu et al. 2010) Subtropical forcing 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 JP JS Hadley Cell Ferrel Cell Polar Cell (driven by eddies) L H L H 60° Pole (colder) Equator (warmer) 30° (warm) (cold)

10. 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

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 Asia America H 35˚N July 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 25N 10N

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

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

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

20. Pacific Subtropical High and North American monsoon ѱ887mb ω674mb (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 summermonsoon 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 K Subtropical high R descending North American Monsoon Asian 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 premonsoonseason (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 WPSH 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 Vertical Structure of WPSH (Lu and Dong 2001) - + - - + + westward extension suppressed convection SST<0 ENSO monsoon

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

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 sinking rising (Kwon et al. 2005) (Sui et al. 2007) (Wang et al. 2013) sinking WNPSM - - rising ENSO

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 Hadley Circulation weakened after 1990 before 1990 after 1990 Walker Circulation Strength (×10-1 Pa/sec) strengthened Hadley Circulation Strength (m/sec) 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. NPO after 1990 CP EP before 1990

48. NPO CP EP NPO and Tropical Pacific SST Variations

49. (5-year running means; using CFS Reanalysis) 1990 NPO After 1990 Central Pacific SSTA is closely related to Extratropical atmosphere (i.e. NPO), but less related to eastern tropical Pacific. Niño4 Before 1990 Central T. Pacific SSTA is less related to extratropical atmosphere, but more related to eastern tropical Pacific. Niño3 NPO Index and Niño Index 1990

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