Celebrating the Monsoon Bangalore, India 7-24 2007 East Asian Monsoon In contrast to Indian monsoon Bin Wang Department of Meteorology and IPRC, SOEST University of Hawaii, Honolulu, HI 96822, USA
Understanding physical processes determining the differences between IM and EAM in • Annual cycle • Interannual variability • Interdecadal variability of ENSO-monsoon relation • Issues remain to be addressed
Annual Variation Why compare the annual variation? • Indian and East Asian (EA) monsoon subsystems are driven by different lower-boundary thermal forcing associated with land-ocean configuration and topography. • Examination of the different characteristics of the annual variability of the two subsystems may provide useful insight to understand how tectonic forcing and solar orbital forcing affect monsoon circulation.
Asian-Australian Monsoon System JA-JF 925 hPa winds and precipitation rate (mm/day) EA-WNP sector Indian sector Circulation systems differ between Indian and EA sectors Fig. 1
Seasonal Distribution of rainfall IM WNPM EAM An eastward shift of convection centers from Indian (in June-July) to the WNP (in August) during boreal summer . Peak and retreat dates differ. WNP is the largest heat source during NH summer. Fig. 2 Wang, Clemens and Liu 2003
(Climatology 1979-2001) Rainy Season 7/11 9/15 7/01 6/21 6/11 7/11 6/01 7/01 6/11 5/21 6/21 6/01 8/10 7/20 5/21 5/11 6/01 5/01 6/15 5/21 4/21 5/11 Wang and LinHo 2002
Indo-China 100-110E Seasonal March of ITCZ (SA Monsoontrough) and EA Monsoon front East Asia: 110-145E Indianmonsoon: 70-95E
How important is land-sea contrast and orography in Controlling monsoon AC? Chang et al. 2006 • Marked cross-equatorial flows in the South China Sea and Celebes. Annual cycle of the Australian monsoon has a firmer link to the EA monsoon than to the Indian monsoon. • Active convection and rainfall region shifts from Indian sector in boreal summer to the EA sector in austral summer
Equinoctial asymmetry In spring transition, EA sector has a well-defined extratropical precipitation band associated with the East Asian monsoon front. In fall transition, the Indian monsoon rain retreats to the south of the equator, whereas the rain in the EA sector remains in the Northern Hemisphere. April October
Differences in the annual cycle • Meridional extent and circulation systems: tropical system vs. coupled tropical and subtropical system (EA) • Seasonal march of major heat sources: BOB and WNP heat sources behave differently. • Rainy season onset and peak • Strong EA winter monsoon more closely coupled to Australian summer monsoon • Equinoctial asymmetry. • The differences in the annual cycle are attributed to the effects of differing land-ocean configuration on atmospheric response to the annual solar forcing, which resembles the effects of the external (tectonic and orbital) forcing on paleo-monsoon variability.
Interannual Variation Why compare the inter annual variation? • Are factors that determine annual cycle of monsoon also operate on interannual time scale? • Study of the different response and feedback of the Indian and EA monsoons to ENSO and warm pool conditions would shed light on the paleo-monsoon variability over the South China Sea and over the Arabian Sea.
Anomalous Monsoon Circulation and Teleconnection Observations have revealed that the year-to-year variations of the Indian and EA-WNP summer monsoons exhibit strikingly different spatial and temporal structure and teleconnection patterns (Wang et al. 2001a). ISM WNP- EA SM
What give rise to the differences between interannual variations of the EASM and ISM? • For the Indian monsoon, strongest anomalies occur during the fall of the El Niño developing year, while for the East Asia monsoon, the strongest anomalies occurs in the spring after the El Niño years. • What are the leading mode of IAV of the A-AM system?
S-EOF1 mode of A-AMS interannual variability The pattern has an SIO anticyclone (AC) in the developing year of El Nino and a WNP AC in decaying ElNino. El-Nino forcing alone cannot account for: 1) the amplification of the SIO AC;or 2) the maintenance of the WNP AC. These features result from local air-sea interactions and the impacts of the annual cycle. Leading Mode of S-EOF of 850 hPa winds and SST anomalies (1956–2004) Wang et al. 2003 J Climate
The evolution of SIO and WNP anticyclone are not in phase with El Nino “forcing”
Factors determining the IAV • Remote forcing from El Nino/La Nina • Monsoon-warm pool ocean Interaction --Equatorial Bjerkness positive feedback (IOD/IOZM) (Webster et al. 1999, Saji et al. 1999) --Off-equatorial Rossby Wave-SST feedback either positive or negative, depending on background annual cycle (Wang et al. 2000) --Negative feedback by monsoon-induced anomalies (Webster et al. 2002, Loschnigg et al. 2003, Lau and Nath 2000). --Memories of ocean mixed layer (Meehl 1994, 1997) • Regulation of the annual cycle (indirect role of continent) --Regulation of the monsoon-ocean interaction (Nicholls 1983) --Modify monsoon response to remote ENSO (Wang et al. 2003)
Monsoon-warm ocean interaction Monsoon- ocean interaction is characterized by Off-equatorial moist Rossby wave –“Dipole” SST feedback (Wang et al. 2000) The nature of this feedback depends on the basic state (monsoon annual cycle). (Wang et al 2003) (Wang et al. 2000)
Connections between ISM and EASM: Summer Circumglobal Teleconnection (CGT) Ding and Wang’05
Conclusions • The factors that control monsoon intensity may be classified as two groups: The forcing external to the coupled atmosphere-ocean-land system (tectonic forcing and solar orbital forcing) and the forcing internal to the coupled climate system, such as (remote) El Nino/La Nino, local monsoon-ocean interaction, land-atmosphere interaction and extratropical influences (ice or snow cover). • The mechanisms operating on the annual and interannual time scales are dominated, respectively, by the external and internal forcing. • The differences between the Indian and East Asian monsoon is essentially determine by the relative strengths of the external versus internal forcings.
Conclusion (Cont.) • The robust coupling between the East Asian and Australian monsoon on both the annual and interannual time scales is essentially established by tectonic forcing. Thus, the increase in solar procession could enhance both the Indian summer monsoon and the East Asian winter-Australian summer monsoons. • El Niño has little influence on the Arabian Sea summer monsoon, but considerable impacts on the South China Sea monsoon (about 10% on average and 40% in strong events), suggesting that drastic changes in the Pacific thermal conditions could remarkably alter the East Asian-Australian monsoon intensity.
Interdecadal variation of the ENSO-monsoon relationship • What are the differences between EASM and ISM? • What causes these differences?
ID Changes of Regional monsoon-ENSO relations: • ISM-ENSO relationship weakens in both developing and decaying ENSO • WNPSM-ENSO relation strengthened in both phases • Indonesian monsoon-ENSO relationship strengthened in all phases of ENSO. Dashed: post1979, Solid pre1979
Observed changes in the major modes • The overall coupling between the A-AM system and ENSO has become strengthened in post-1979 period. a) The ENSO induced FV (leading mode) increases from 24% to 31% for entire AAM system b) The second mode does not significantly related to ENSO in pre-1979 epoch but significantly leads ENSO after 1979, providing a precursor. c) While ENSO-ISM coupling weakens, the ENSO- WNPSM and ENSO-Indonesia MNS coupling strengthens.
Changes in ENSO behavior in late 1970s: Increased amplitude and periodicity Enhanced anomalous anti-Walker Cell Increased ENSO-induced monsoon-warm pool ocean Interaction Enhanced ENSO-Indonesian monsoon relation through all phases of ENSO Increase in ENSO- WNPSM/ EASM relation in both Dev. & Dec. ENSO Reduction in ENSO-ISM relation in both Dev. & Dec. ENSO Weaken biennial tendency of the A-AM 1st leading mode
Conclusion • Two major modes of IAV of AAM system (1956-2004). The first has prominent biennial tendency and concurs with ENSO turnabout. The second leads ENSO by one year. • The origin of the first mode is attributed to three factors: Remote El Niño forcing, the monsoon-warm pool ocean interaction, and the influence of the annual cycle. • The monsoon--ocean interaction is characterized by off-equatorial Convective coupled Rossby wave-ocean ML interaction. • IDV of the major modes Biennial tendency and eastward propagation Relation of the second mode and ENSO • Overall coupling between the A-AM system and ENSO has become strengthened since 1980. • The IDV is attributed to increased magnitude and periodicity of ENSO and the strengthened monsoon-ocean interaction.
Issues • How to define the domain of EASM? • How to measure the intensity of the EASM? Modern vs. Paleomonsoon • Interpretation of the intensity change of the EASM in orbital time scale (An 2000, Ding et al. 1995, Yancheva et al. 2007)
Questions • (1) What are the major patterns of interannual variability in the entire EA-WNP summer monsoon region (0-50N, 100-140E)? • (2) How do these patterns link to mid-latitude and tropical circulation anomalies? • (3) What processes give rise to these major patterns of variability?
EA-WNP Summer monsoon system ITCZ and subtropical monsoon front over the EA sector
Conclusion • The leading mode (38% of total variance) represents enhanced precipitation along the EA subtropical front, primarily associated with decaying phases of El Ninos (and after 1990 its reversed pattern links to developing phase of El Nino). • The response of the EASM to El Nino and La Nina forcing is nonlinear. • The second mode (11.3% of the total variance) is associated with developing phases of the El Nino and La Nina events and the third mode (7.4% of the total variance) links partially to the NINO4 warming. • Major modes are determined primarily by monsoon-warm pool ocean interaction, remote forcing from El Nino and NINO 4 SSTA. • The teleconnection patterns are dominated by a north-south tropical-polar teleconnection.
Conclusion • Meiyu/Changma/Baiu is anticorrelated with WNP ITCZ. Whether this anticorrelation exists on multi-decadal to orbital time scale deserves further study. • Recommendation: a strong EASM be defined by abundant Meiyu/Changma/Baiu.
Conclusion • The leading mode of the EA-WNP summer monsoon represents enhanced precipitation along the EA subtropical front, primarily associated with decaying phases of El Ninos (and after 1990 its reversed pattern links to developing phase of El Nino). • The response of the EASM to El Nino and La Nina forcing is nonlinear. • Major modes are determined primarily by monsoon-warm pool ocean interaction, remote forcing from El Nino and NINO 4 SSTA. • The teleconnection patterns are dominated by a north-south tropical-polar teleconnection. • Meiyu/Changma/Baiu is anticorrelated with WNP ITCZ. Whether this anticorrelation exists on multi-decadal to orbital time scale deserves further study. • Recommendation: a strong EASM be defined by abundant Meiyu/Changma/Baiu.
&. ISSUES How important are the East-Asian marginal seas in determining the mean monsoon structure and seasonal cycle? · Why do the most AGCMs have great difficulty in correct simulation of the summer rainfall in the WNP and the Western Pacific Subtropical High and the Meiyu/Baiu front regions? Why Sudden changes (singularities) at various geographic locations? · Mmechanisms for 10-20 day and 20-60 dayvariability in the WNP region? · What is the potential and practical predictability of the these oscillations? · How does the air-sea interaction influence these oscillation? · What is the coherent structure of the tropical biennial oscillation? What processes are responsible for the transition of the biennial tendency Roles of the land surface memories. How these land surface anomalies are generated and maintained? · What are the radiative impacts of clouds, especially cirrus, on monsoon evolution and intensity? · To what extent the mid-high latitude circulation anomalies prior to the summer monsoon can affect the EASM? How are they generated and maintained? ·
ISSUES (CONTINUE) H How are the teleconnection (the PJ and circumglobal teleconnection) modes associated with Asian summer monsoon excited and maintained? Are those modes intrinsic to the low frequency variability of the boreal summer mean states? · What is the predictability of the EA-WNP summer monsoon during the years when ENSO is in a near normal state? · How does the monsoon-warm ocean interaction affect the predictability and prediction of the seasonal mean rainfall? · What is the potential and practical predictability of the EA and WNP summer monsoon? · What are the impacts of the ISO on the seasonal mean climate forecast? Interdecadal variability · What is the dominant mode of the Interdecadal variation of the EA-WNP monsoons? What give rise to this variability? · Are the interdecadal variations in the EA-WNP region linked to that over the ISM? If not how different they are and why they are different? ) From Wang et al. 2005,
P52-55 P40-42 P34-36 P30-32 P40-42 P41 P34-35 P40-46 P44-46 P52-53 P52-56 P34-35 P30-33 P34-35 P30-33 Wang and LinHo 2002
Hydrological issues in RCM of monsoon: Uncertainty in moisture influxes of driving fields Vertically integrated water vapor convergence differ by 47% between: NCEP/DOE R2 (Blue) and EAR40 (Green)
Definition of EASMI EASMI = PC1*EV1[30N-50N,110E-145E] Regressed precipitation field Corr. (EOF1 of rainfall, WNPMI)= -0.70 Lee et al. (2005)
cold AC A Warm Sinking motion A Enhanced surface high Upper-level flow anomaly Enhanced 500hPa trough Reduced convection Warm SST anomalies Anomalous surface wind
Mechanism for the establishment of WNP AC • (i) El Nino enhances upper troposphere subtropical ridge and deepen the East Asian trough, encourage northward recurved tropical storms • (ii) The vigorous tropical-extratropical exchange of air mass and heat enhances the EA cold air outbreak into Philippine Sea • (iii) ISO and associated positive air-sea coupling further facilitating the abrupt establishment of the WNP AC • (iv) Cold SSTA in the WNP precondition the establishment of WNP AC • (v) Anticyclonic vorticity advection from the SA to Philippines.
Experimental design (Lau et al.) CTRL: Climatology SST outside DTEP MLM: Coupled GCM-Mixed layer Ocean (Alexander et. Al. 2000) GFDL R-30 L-14 Ensemble runs: MLM16; CTRL8
Time scales of monsoon variability Conceptual spectrum of monsoon variability on the annual to tectonic time scale. The periods of individual spectral peaks are labeled. Relative concentrations of variance at these periods are unknown. The two black peaks at the 41- and 23-ky periods indicate the Earth-orbital periods, which account for nearly all variability in incoming solar radiation. P.-X. Wang et al. 2005
U200(1) U200(2) RM2: U200(1)-U200(2) Summer monsoon index definition Summer monsoon indices 1) Indian summer monsoon ; All Indian Rainfall Index (AIRI, Parthasarathy et al., 1992) Webster and Yang Index (WYI, Webster and Yang, 1992) Monsoon Hadley Circulation Index (MHI, Goswami et al., 1999) 2) Western North Pacific monsoon ; Western North Pacific Monsoon Index (WNPMI, Wang et al., 2001) 3) East Asian monsoon Regional Monsoon Index (RM2, Lau et al., 2000) WNPMI (Wang and Fan 99) RM2 (Lau et al. 2000)