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On the Impact of the Indian Summer Monsoon On the ENSO (B.N.Goswami and V. Jayavelu, GPL, 2001)

On the Impact of the Indian Summer Monsoon On the ENSO (B.N.Goswami and V. Jayavelu, GPL, 2001) 2005. 3. 16. 박혜숙 , 이윤복 , 김옥희 E W NE SW Indian monsoon-La Nina & El Nino Indian Summer Monsoon (ISM) Indian Winter Monsoon (IWM) (Wu and kirtman,2003) Indian monsoon-ENSO connection

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On the Impact of the Indian Summer Monsoon On the ENSO (B.N.Goswami and V. Jayavelu, GPL, 2001)

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  1. On the Impact of the Indian Summer Monsoon On the ENSO (B.N.Goswami and V. Jayavelu, GPL, 2001) 2005. 3. 16. 박혜숙,이윤복, 김옥희

  2. E W NE SW Indian monsoon-La Nina & El Nino Indian Summer Monsoon (ISM) Indian Winter Monsoon (IWM)

  3. (Wu and kirtman,2003) Indian monsoon-ENSO connection SST lead SST lag • Correlation of monthly NINO-3.4 SST with JJAS Indian Monsoon Rainfall (IMR) •  negative correlation is seen along the equatorial CP and EP •  Positive correlation in western North and South Pacific •  Cold ENSO is correlated with a wet strong monsoon

  4. Introduction • Many ‘weak’ and ‘strong’ phase of the ISM and ‘warm’ and ‘cold’ phase of the ENSO tend to occur simultaneously in nature. • However, large SST anomalies and surface wind anomalies in the Pacific are characteristically associated with the ENSO even without ISM.  Therefore, the efficacy of the ISM in influencing the ENSO depends on its ability to induce significant surface wind anomalies in the active regions of central and eastern equatorial Pacific in the absence of enhanced ENSO related SST forcing.

  5. (Case III) (Case I) (Case II) Criterion relaxed to strong (weak) > 0.5 s.d (< -0.5 s.d) Data and Methods • NCEP/NCAR reanalysis data(1949.Jan – 1998.Dec) • Rain gauge station data over continental India • IMR : seasonal mean (June-Sept.) all India monsoon rainfall • Strong (weak) : normalized anomalies (inter-annual S.D) > +1 (<-1) • Global analysis of SST + OISST - SST anomalies for Nino3(150-90W, 5S-5N)

  6. W E W E W E C W C W W C WP EP IO CP (Case I) Lead-lag composites • Significant easterly anomalies in CP and EP during the winter following a ‘strong’ ISM • Upper level winds are nearly opposite to those at lower level by walker circulation • Positive SST anomalies in IO prior to strong ISM, move eastward to WP • Eastward movement of the enhanced precipitation zone to the WP • Wind anomalies associated with ISM and SST induced precipitation forcing related with the ENSO events

  7. ENSO Monsoon E W C W (Case II) “Pure” ISM effect • Surface zonal wind following a ‘pure’ strong ISM remains westerly till Dec/Jan. • Maximum zonal wind anomalies at sfc wind in the CP and EP goes up to only 0.8 m/s (cf. Fig.1 goes up to 2.3 m/s)  Surface wind and SST anomalies throughout the equatorial Pacific associated with a ‘pure’ ISMtend to be very weak

  8. (Case III) ENSO effect except ISM ENSO Monsoon • Large surface wind anomalies in the CP seen the monsoon composite arise due to simultaneous ENSO related heating in the Pacific E E W C

  9. (Case IV) Long simulation of the GFDL AGCM forced by climatological SST • a) Simulated IMR : seasonal mean anomalies for 20 years • Standard deviation(0.53 mm/day) of the model internal IMR is comparable to the observed one(0.72 mm/day) • b) Regression of simulated zonal stress on the model IMR index • Strong monsoon associated with westerly stress between equator and 15°N • Throughout the EP, the easterly stress anomalies associated with the Indian monsoon are quite weak • The largest easterly anomalies in the EP occurs not around the equator but north of 10°N • Maximum zonal sfc stress anomalies are of the order of 0.01 dynes/cm2 [dyne/cm2]

  10. Conclusion • Indian monsoon, by itself, does not produce significant surface wind anomalies in the equatorial Pacific either during or following the monsoon season • Large surface wind anomalies in the central and eastern Pacific associated with strong ISM are essentially due to the SST anomalies associated with El Nino and La Nina • Weak equatorial zonal stress anomalies associated with the Indian monsoon is related to the fact that the rainfall anomalies associated with a ‘pure’ Indian monsoon do not tend to extend eastward of 160°E  Indian monsoonis unlikely tointroduce significant wind response in the central and eastern Pacific and hence unlikely toinfluence the ENSO related air-sea interactions in a significant way

  11. References • B.N. Goswami and V. jayavelu, On Possible Impact of The Indian Summer Monsoon on the ENSO, GRL, vol.28,No.4, 571-574,2001 • K.K. Kumar, B.Rajagopalan, M.A. Cane, On the Weakening Relationship Between Indian Monsoon and ENSO, Science, vol284, 2156-2159,1999 • R. Wu and B.P. Kirtman, Impacts of the Indian Ocean on the Indian Summer Monsoon-ENSO relationship • B.Wang, R. Wu, and K.-M. Lau, Interannual Variability of the Asian Summer Monsoon : contrasts between the Indian and the Western North Pacific-East Asian Monsoon, J.Climate, vol14,4073-4090, 2001

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