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The earliest rainy season of the Asian Summer Monsoon; the Bay of Bengal Onset

The earliest rainy season of the Asian Summer Monsoon; the Bay of Bengal Onset. UAW5, Japan Speaker : Yuwei Lin 01Jul 2008. 1.The Asian summer monsoon evolution. LinHo and Wang, 2002.

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The earliest rainy season of the Asian Summer Monsoon; the Bay of Bengal Onset

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  1. The earliest rainy season of the Asian Summer Monsoon; the Bay of Bengal Onset UAW5, Japan Speaker : Yuwei Lin 01Jul 2008

  2. 1.The Asian summer monsoon evolution LinHo and Wang, 2002 BOB monsoon onset is important for the formation of a large scale circulation pattern that favors the SCS monsoon onset. (Ding and Liu,2001 ;Wu et al.,2005) Indian onset SCS onset BOB onset

  3. 2.What triggers the BOB onset? A wet phase ISO triggers the BOB onset From climatology, a northward-propagating ISO in early May is found associated with the BOB onset. • The first and strongest northward-propagating ISO in summer. • The tightest phase locks with calendar as seen from CISO (Climatological ISO) . The warmest SST of the BOB & northward-propagating rain in early May Northward ISO Time-lat. cross sections of 80E-100E

  4. 3.BOB Onset Composite • Two crucial events to stage BoB onset (need high-resolution data) • Find the date of the regional minimum OLRKZ( 20-80days filter)=> Onset date ! Composite period in 1998-2007 • Check by 200mb H • H >12470 • Continue more than 2 PTD. =>South Asian High development OLRkz & SSTkz in BOB 80E~105E,15N~20N 80E~100E,EQ~10N SSTkz warming occurs before the northward ISO.

  5. 4. BOB onset is Classified into 3 stages Charge stage | | | Trigger stage | | | Discharge stage Day-20 ~Day -10 Day-8 ~Day -3 Day-2 ~ Day 2 Day 3 ~Day 8 Day 10~Day 20 Contours show OLRkz anomalies at 5 w/m2 and arrows show anomalies 850hPa winds. Shading denote areas with statistically significant wet (dry) singularity at 95% confidence level.

  6. 5.The three stages (I) OLRkz(shaded), wnd85kz 500mb H(contour) • The dry phase ISO Inhibits convection over the BOB. • The wet phase ISO moves eastwards along the equator. Charge stage | | | Trigger stage | | | Discharge stage vort85kz, wnd85(stream) The strongest convection induces a Gill response. Then an explosion of Rossby dipole leads to BoB onset. • A north-eastward propagating ISO brings strong low level south-westward flow. • India-Burma trough is formed.

  7. 6. Pre-onset condition 28Apr~03May • Small warm pool in northern Indian Ocean serves as heat battery • For duration of ISO time scale • Effect of Hot Peninsula in spring • the effect of Tibetan Plateau The reversed meridional SST gradient. 08Apr~23Apr • . Qnet: Net heat flux C: the specific heat of sea water ρ: density of sea water h: mixed layer depth (m) Low level trough

  8. Low-level moisture convergence (shaded), moisture fluxkz SSTkz (shaded), Vaporkz 8.The three stages (II) In BOB • SST warms up and the water vapor increases. • BOB provides moisture flux to eastern equatorial Indian ocean. • The low-level trough flow is decease in BOB by equatorial wet ISO. Charge stage | | | Trigger stage | | | Discharge stage • Max moisture convergence is in south BOB. • The meridional SST gradient reverses. • Northward-propagating ISO discharges the warm SST and low-level moist potential energy. • The moist SW flow supports the convection in BOB.

  9. 7. Air-sea interaction of the northward ISO Composite ISO anomaly data, time-latitude cross sections of 85E-97.5E EQ OLRkz & SSTkz SSTkz & LHkz EQ OLRkz & Qconvkz SSTkz & SWkz Kemball-cook and Wang,2000 Day 0

  10. 8.The large scale circulation change The release of latent heat of condensation is the primary heat source driving the Asian summer monsoon. Krishnamurti et al. 1976

  11. 9. Summary 1.The northward-propagating ISO in early May induces the BOB onset. 2.The evolution of BOB onset can be classified into three stages: • Charge stage: • The moisture convergence induced by the eastward-propagating ISO neutralized pre-onset wind field in BoB as part of a low-level hot trough of Peninsula, leading to weak (total) wind stress in BoB lasting around 20 days. • SST increases rapidly around 13N。 • Trigger stage: • Eastward-propagating wet phase ISO reaches Sumatra and excites an n=1 Rossby wave. • Only the northward component thrives under the favorable basic state, which starts the BOB Onset. • Discharge stage: • The Rossby wave moves northwards by consuming warm SST on its northern flank. • India-Burma trough is formed, further enhances moisture convergence in North rim of BoB. • South Asian High develops rapidly.

  12. Thanks for your attention!

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