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Unraveling Southern Subtropical Anticyclones in Winter

Explore the driving forces of southern subtropical anticyclones in winter through AGCM and simple model experiments. Hypotheses involve monsoon heating differences and NH influences. Discover implications for interhemispheric connections and ocean gyre circulations.

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Unraveling 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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