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What drives the southern subtropical anticyclones in winter?

What drives the southern subtropical anticyclones in winter?. Sang-Ki Lee 2,1 Carlos Mechoso 3 , Chunzai Wang 1 and David Neelin 3 1 NOAA-AOML, 2 Univ. of MIAMI-CIMAS, 3 UCLA. Outline Background and Hypothesis AGCM Experiments Simple Model Experiments.

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What drives the southern subtropical anticyclones in winter?

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  1. What drives the southern subtropical anticyclones in winter? Sang-Ki Lee2,1 Carlos Mechoso3, Chunzai Wang1 and David Neelin3 1NOAA-AOML, 2Univ. of MIAMI-CIMAS, 3UCLA Outline • Background and Hypothesis • AGCM Experiments • Simple Model Experiments

  2. Background: Subtropical Anticyclones • Subtropical anticyclones tied to trades and westerlies • Northern subtropical anticyclones stronger & better defined in boreal summer • Summer: “monsoon heating” paradigm (Rodwell and Hoskins. 2001; RH01) • Winter: topography effect on zonal mean winds (RH01)

  3. Background: “Monsoon Heating” Paradigm – RH01 Pressure velocity Meridional velocity NPACHeating NATL NPACHeatingNATL • Heating over North America  poleward low-level wind (Sverdup balance: ·v ~ f·/p) • Adiabatic subsidence to the west (Rossby wave)  equatorward low-level wind

  4. Southern Subtropical Anticyclones • The southern subtropical anticyclones remain strong in austral winter  inconsistent with aa“monsoon heating” aaparadigm of RH01 • What drives the southern subtropical anticyclones in austral winter?

  5. Hypotheses • 1st Hypothesis: monsoon heating weaker in SH  summer: subtropical aaanticyclones stronger aain NH  winter: blocking effect aaon zonal winds weaker aaover Africa & Australia • 2nd Hypothesis: Inter-hemispheric influence of NH monsoon  Wang, Lee & Mechoso aa(2010)  Richter, Mechoso & aaRobertson (2008)

  6. AGCM Experiments • NCAR Community Atmospheric Model version 4 (CAM4)  global atmosphere-land model with FV dynamic core;  2.5°(zonal)  1.9° (meridional) resolution; 26 hybrid sigma-pressure levels • CTRL: Control Experiment  climatological SSTs & sea-ice prescribed;  20-year long simulations • SYNC: Inter-hemispheric Synchronization Experiment  minimize inter-hemispheric connections;  TOA solar insolation shifted by 6 months only in NH;  climatological SSTs & sea-ice shifted by 6 months in NH

  7. AGCM Experiments • CTRL – SYNC: net inter-hemispheric influences of NH on SH

  8. AGCM Experiments

  9. CAM4: Zonally Averaged SLPs for Each Basin • SYNC: S-PAC subtropical high maximized in austral summer • SYNC: S-ATL & S-IND subtropical highs weakened greatly in austral winter • Southern subtropical highs strengthened by NH monsoon in austral winter

  10. CAM4: Interhemispheric Hadley Cell • CTRL - SYNC: inter-hemispheric Hadley cell  rising motion at 5N-30N  sinking motion at 5N-15S • Sinking motion at 5N-15S  warm & moist surface air aareplaced by cold & dry air above  increased SLP  slowly sinking air heated due to aaadiabatic compression  increased lower tropospheric aastability  limited vertical development of aaconvection

  11. CAM4: VPOT & DIV Winds at 200hPa • CTRL:  rising motion - WHWP  rising motion - Asian aasummer monsoon  sinking motion - aaSE-PAC and ATL • SYNC:  nearly symmetrical wrt aaequator  rising motion - aaEQ-IND; WPWP; aawestern EQ-ATL  much weakened aasinking motion - aaSE-PAC and ATL

  12. CAM4: VPOT & DIV Winds at 200hPa • Three regions of rising motions  Indian Summer aaMonsoon;  Summer expansion aaof WPWP; WHWP • Three regions of sinking motions  EQ-IND;  western EQ-ATL;  SC-PAC • But, sinking motions are limited to tropics

  13. CAM4: Poleward Propagation of Stationary Waves • CTRL – SYNC:  SLP response mostly aasouth of 20S  SLP response is due to aabarotropic motion • Hypothesis:  SH SLP response to NH aamonsoon linked to aapoleward propagation of aastationary Rossby aawaves forced from the aatropics

  14. CAM4: Tropical Forcing of Stationary Rossby Waves

  15. Simple Model Experiments • Simple two-level model of Lee, Wang & Mapes (2009)  minimum complexity model of both local and remote aaresponses to tropical heating anomalies  Gill-model and barotropic vorticity equation combined  not designed to reproduce AGCM simulations • Stationary waves are forced in six tropical regions

  16. Simple Model Experiments • SC-PAC cooling  S-PAC subtropical high aastrengthened • Western EQ-ATL cooling  S-ATL subtropical high aastrengthened • EQ-IND cooling  S-IND subtropical high aastrengthened • Direct impact of NH heating much smaller • But, S-PAC and S-ATL subtropical highs moderately strengthened by direct effect of ISM (not shown)

  17. Summary • NH summer monsoon  equatorial & tropical SH convection suppressed  poleward propagation of stationary waves  SH subtropical highs strengthened

  18. Discussion • Is the interhemispheric connection stationary?  ex: AGW, PDO and AMO • Does it also work at interannual and longer time scales?  ex: WHWP, ISM, PMM, AMM and AMO • Do atmosphere-ocean interactions play any role?  CAM4-SOM experiments • Implication for SH ocean gyre circulations  Southern subtropical ocean gyres are likely to be aaenhanced by NH summer monsoon

  19. Publications • Lee, S.-K., C. R. Mechoso, C. Wang and J. D. Neelin, 2013: Interhemispheric influence of the northern summer monsoons on the southern subtropical anticyclone. J. Climate, 26, 10193-10204. • Wang, C., S.-K. Lee and C. R. Mechoso, 2010: Inter-hemispheric influence of the Atlantic warm pool on the southeastern Pacific. J. Climate, 23, 404-418. • Ji, X., J. D. Neelin, S.-K. Lee and C. R. Mechoso, 2014: Interhemispheric teleconnections from tropical heat sources in intermediate and simple models. J. Climate, 27, 684-697. • Wang, C., L. Zhang, S.-K. Lee, L. Wu, and C. R. Mechoso, 2014: A global perspective on CMIP5 climate model biases. Nature Clim. Change. In-press.

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