Indian Ocean Sea Surface Temperature and El Niño-Southern Oscillation: A New Perspective

International audienceHere we show that the 1976-1977 climate regime shift was accompanied by aremarkable change in the lead-lag relationships between Indian Ocean Sea SurfaceTemperature (SST) and El Niño evolution. After the 1976-1977 regime shift, acorrelation analysis suggests that southern Indian Ocean SSTs observed during lateboreal winter are a key precursor in predicting El Niño evolution as the traditionaloceanic heat content anomalies in the equatorial Pacific or zonal wind anomalies overthe equatorial western Pacific. The possible physical mechanisms underlying this highlysignificant statistical relationship are discussed. After the 1976-1977 regime shift,southern Indian Ocean SST anomalies produced by Mascarene High pulses duringboreal winter trigger coupled air-sea processes in the tropical eastern Indian Oceanduring the following seasons. This produces a persistent remote forcing on the Pacificclimate system, promoting wind anomalies over the western equatorial Pacific andmodulating the regional Hadley cell in the southwest Pacific. These modulations, inturn, excite Rossby waves, which produce quasi-stationary circulation anomalies in theextratropical South Pacific, responsible for the development of the southern branch ofthe “horseshoe” El Niño pattern.The change of the background SST state that occured in the late 1970s over the IndianOcean may also explain why ENSO evolution is different before and after the 1976-1977 regime shift. These results shed some light on the possible influence of globalwarming or decadal fluctuations on El Niño evolution through changes inteleconnection patterns between the Indian and Pacific Oceans

Improvement of ENSO prediction using a linear regression model with a Southern Indian Ocean Sea Surface Temperature Predictor

International audienceThis study presents a detailed comparison between three ENSO precursors which canpredict across the spring persistence barrier: the anomalous equatorial Pacific upper oceanheat content, the zonal equatorial wind stress anomaly in the far-western Pacific and SSTanomalies in the South-East Indian Ocean (SEIO) during the late boreal winter. A newcorrelation analysis confirms that El Niño (La Niña) onsets are preceded by significant cold(warm) SST anomalies in the SEIO during the late boreal winter after the 1976-77 climateregime shift. Thus, the objective is to examine the respective potential of these three ENSOprecursors to predict ENSO events across the boreal spring barrier during recent decades.Surprisingly, in this focus, cross-validated hindcasts of the linear regression models based onthe lagged relationship between Niño3.4 SST and the predictors suggest that SEIO SSTanomalies during the late boreal winter is the more robust ENSO predictor

Role of the southern Indian Ocean in the transitions of the monsoon-ENSO system during recent decades

International audienceThe focus of this study is to document the possible role of the southern subtropical IndianOcean in the transitions of the monsoon-ENSO system during recent decades.Composite analyses of Sea Surface Temperature (SST) fields prior to El Niño-SouthernOscillation (ENSO), Indian Summer Monsoon (ISM), AUstralian Summer Monsoon(AUSM), Tropical Indian Ocean Dipole (TIOD) and Maritime Continent Rainfall (MCR)indices reveal the South East Indian Ocean (SEIO) SSTs during late boreal winter as theunique common SST precursor of these various phenomena after the 1976-1977 regime shift.Weak (strong) ISMs and AUSMs, El Niños (La Niñas) and positive (negative) TIOD eventsare preceded by significant negative (positive) SST anomalies in the SEIO, off Australiaduring boreal winter. These SST anomalies are mainly linked to subtropical Indian Oceandipole events, recently studied by Behera and Yamagata (2001). A wavelet analysis of aFebruary-March SEIO SST time series shows significant spectral peaks at 2 and 4-8 yearstime scales as for ENSO, ISM or AUSM indices. A composite analysis with respect toFebruary-March SEIO SSTs shows that cold (warm) SEIO SST anomalies are highlypersistent and affect the westward translation of the Mascarene high from austral to borealsummer, inducing a weakening (strengthening) of the whole ISM circulation through amodulation of the local Hadley cell during late boreal summer. At the same time, thesesubtropical SST anomalies and the associated SEIO anomalous anticyclone may be a triggerfor both the wind-evaporation-SST and wind-thermocline-SST positive feedbacks betweenAustralia and Sumatra during boreal spring and early summer. These positive feedbacksexplain the extraordinary persistence of the SEIO anomalous anticyclone from boreal springto fall. Meanwhile, the SEIO anomalous anticyclone favors persistent southeasterly windanomalies along the west coast of Sumatra and westerly wind anomalies over the westernPacific, which are well-known key-factors for the evolution of positive TIOD and El Niñoevents, respectively. A correlation analysis supports these results and shows that SEIO SSTsin February-March has higher predictive skill than other well-established ENSO predictors forforecasting Niño3.4 SST at the end of the year. This suggests again that SEIO SST anomaliesexert a fundamental influence on the transitions of the whole monsoon-ENSO system duringrecent decades

Interannual relationships between Indian Summer Monsoon and Indo-Pacific coupled modes of variability during recent decades

International audienceVarious SST indices in the Indo-Pacific region have been proposed in the literature in light of a long-range seasonal forecasting of the Indian Summer Monsoon (ISM). However, the dynamics associated with these different indices have never been compared in detail. To this end, the present work re-examines the variabilities of ISM rainfall, onset and withdrawal dates at interannual timescales and explores their relationships with El Niño-Southern Oscillation (ENSO) and various modes of coupled variability in the Indian Ocean. Based on recent findings in the literature, five SST indices are considered here: Niño3.4 SST index in December-January both preceding [Nino(-1)] and following the ISM [Nino(0)], South East Indian Ocean (SEIO) SST in February-March, the Indian Ocean Basin (IOB) mode in April-May and, finally, the Indian Ocean Dipole (IOD) averaged from September to November, also, both preceding [IOD(-1)] and following the ISM [IOD(0)]. The respective merits and associated dynamics of the selected indices are compared through various correlation and regression analyses. Our first result is a deceptive one: the statistical relationships with the ISM rainfall at the continental and seasonal scales are modest and only barely significant, particularly for the IOD, IOB and Nino(-1) indices. However, a detailed analysis shows that statistical relationships with the ISM rainfall time series are statistically biased as the ISM rainfall seems to be shaped by much intraseasonal variability, linked in particular to the timing of the onset and withdrawal of the ISM. Surprisingly, analysis within the ISM season shows that Nino(-1), IOB and SEIO indices give rise to prospects of comparatively higher ISM previsibility for both the ISM onset and the amount of rainfall during the second half of the ISM season. The IOD seems to play only a secondary role. Moreover, our work shows that these indices are associated with distinct processes occurring within the Indian Ocean from late boreal winter or early spring onwards. The regression analyses also illustrate that these (local) mechanisms are dynamically and remotely linked to different phases of ENSO in the equatorial Pacific, a result which may have useful implications in terms of forecasting strategies since the choice of the better indices then hinges on the concurrent phasing of the ENSO cycle