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During boreal winter, there is a prominent maximum of intraseasonal sea-surface temperature (SST) variability associated with the Madden-Julian Oscillation (MJO) along a Thermocline Ridge located in the southwestern Indian Ocean (5 o S−10°S, 60°E-90°E; TRIO region). There is an ongoing debate about the relative importance of air-sea heat fluxes and oceanic processes in driving this intraseasonal SST variability. Furthermore, various studies have suggested that interannual variability of the oceanic structure in the TRIO region could modulate the amplitude of the MJOdriven SST response. In this study, we use observations and ocean general circulation model (OGCM) experiments to quantify these two effects over the 1997−2006 period. Observational analysis indicates that Ekman pumping does not contribute significantly (on average) to intraseasonal SST variability. It is, however, difficult to quantify the relative contribution of net heat fluxes and entrainment to SST intraseasonal variability from observations alone. We therefore use a suite of OGCM experiments to isolate the impacts of each process. During 1997−2006, wind stress contributed only 20% of the intraseasonal SST variability (averaged over the TRIO region), while heat fluxes contributed to 70%, with forcing by shortwave radiation (75%) dominating the other flux components (25%). This estimate is consistent with an independent air-sea flux product, which indicates that shortwave radiation contributes 68% of intraseasonal heat flux variability. The time scale of the heat-flux perturbation, in addition to its amplitude, is also important in controlling the intraseasonal SST signature, with longer periods favouring a larger response. There are also strong year-to-year variations in the respective role of heat fluxes and wind stress. Intraseasonal-wind stress dominates the SST signature for one (in 2001) and contributes significantly to another (in 2000) of the 5 strong cooling events identified in both observations and the model (2 in 1999, 1 in 2000, 2001 and 2002). Interannual variations of the subsurface thermal structure associated with the Indian Ocean Dipole or El Niño/La Niña events modulate the MJOdriven SST signature only moderately (by up to 30%), mainly by changing the temperature of water entrained into the mixed layer. Our results therefore suggest that the primary factor that controls year-to-year changes in the amplitude of TRIO, intraseasonal SST anomalies is the amplitude and timescale of the intraseasonal heat-flux perturbations.

During summer, the northern Indian Ocean exhibits significant atmospheric intraseasonal variability associated with active and break phases of the monsoon in the 30-90 days band. In this paper, we investigate mechanisms of the Sea Surface Temperature (SST) signature of this atmospheric variability, using a combination of observational datasets and Ocean General Circulation Model sensitivity experiments. In addition to the previously-reported intraseasonal SST signature in the Bay of Bengal, observations show clear SST signals in the Arabian Sea related to the active/break cycle of the monsoon. As the atmospheric intraseasonal oscillation moves northward, SST variations appear first at the southern tip of India (day 0), then in the Somali upwelling region (day 10), northern Bay of Bengal (day 19) and finally in the Oman upwelling region (day 23). The Bay of Bengal and Oman signals are most clearly associated with the monsoon active/break index, whereas the relationship with signals near Somali upwelling and the southern tip of India is weaker. In agreement with previous studies, we find that heat flux variations drive most of the intraseasonal SST variability in the Bay of Bengal, both in our model (regression coefficient, 0.9, against *0.25 for wind stress) and in observations (0.8 regression coefficient); *60% of the heat flux variation is due do shortwave radiation and *40% due to latent heat flux. On the other hand, both observations and model results indicate a prominent role of dynamical oceanic processes in the Arabian Sea. Wind-stress variations force about 70-100% of SST intraseasonal variations in the Arabian Sea, through modulation of oceanic processes (entrainment, mixing, Ekman pumping, lateral advection). Our *100 km resolution model suggests that internal oceanic variability (i.e. eddies) contributes substantially to intraseasonal variability at small-scale in the Somali upwelling region, but does not contribute to largescale intraseasonal SST variability due to its small spatial scale and random phase relation to the active-break monsoon cycle. The effect of oceanic eddies; however, remains to be explored at a higher spatial resolution.

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Abstract:Basin-wide wintertime surface warming is observed in the Indian Ocean during El Niño years. The basin-wide warming is found to be stronger when El Niño and Indian Ocean Dipole (IOD) co-occur. The mechanisms responsible for the basin-wide warming are different for the years with El Niño only (El Niño without IOD) and for the co-occurrence (both El Niño and IOD) years. Strong westward propagation of downwelling Rossby waves is observed in the southern Indian Ocean during the IOD years. Such strong propagation is not seen in the case of the El Niño-only years. This indicates that the ocean dynamics play an important role in winter warming of the western Indian Ocean during the IOD years. The weak easterly wind anomalies in the El Niño-only years show no measurable impact on the Wyrtki Jets, but weakening or reversal of these jets is seen in the IOD years. This strongly suggests that the variability related to surface circulation is due to the local IOD forcing rather than El Niño induced wind anomaly. For the El Niño-only composites, surface heat fluxes (mainly latent heat flux and short wave radiation) play an important role in maintaining the basin-wide surface warming in the Indian Ocean. In the IOD-only composites (when there is no El Niño in the Pacific), such basin-wide warming is not seen because of the absence of ENSO (El Niño and Southern Oscillation) induced subsidence over the eastern Indian Ocean. For the years in which both El Niño in the Pacific and dipole in the Indian Ocean co-occur, warming in the western Indian Ocean is due to the ocean dynamics and that in the eastern Indian Ocean is due to the anomalous latent heat flux and solar radiation.

This study discusses the impact of the Pacific–Japan (PJ) pattern on Indian summer monsoon (ISM) rainfall and its possible physical linkages through coupled and uncoupled pathways. Empirical orthogonal function analysis of 850-hPa relative vorticity over the western North Pacific (WNP) is used to extract the PJ pattern as the leading mode of circulation variability. The partial correlation analysis of the leading principal component reveals that the positive PJ pattern, which features anticyclonic and cyclonic low-level circulation anomalies over the tropical WNP and around Japan respectively, enhances the rainfall over the southern and northern parts of India. The northwestward propagating Rossby waves, in response to intensified convection over the Maritime Continent reinforced by low-level convergence in the southern flank of westward extended tropical WNP anticyclone, increase rainfall over southern peninsular India. Meanwhile, the anomalous moisture transport from the warm Bay of Bengal due to anomalous southerlies at the western edge of the low-level anticyclone extending from the tropical WNP helps to enhance the rainfall over northern India. The atmospheric general circulation model forced with climatological sea surface temperature confirms this atmospheric pathway through the westward propagating Rossby waves. Furthermore, the north Indian Ocean (NIO) warming induced by easterly wind anomalies along the southern periphery of the tropical WNP–NIO anticyclone enhances local convection, which in turn feeds back to the WNP convection anomalies. This coupled nature via interbasin feedback between the PJ pattern and NIO is confirmed using coupled model sensitivity experiments. These results are important in identifying new sources of ISM variability/predictability on the interannual time scale.

A suite of satellite and in‐situ observations are used to study ocean‐atmospheric conditions over the tropical Southwest Indian Ocean (SWIO) during 2006–2007 when El Nino and an Indian Ocean dipole took place simultaneously. Argo profiles reveal a pronounced up‐westward propagation of subsurface warming in the southern tropical Indian Ocean associated with Rossby waves traveling on the sloping thermocline. With the thermocline deepening by 60 m, a thick barrier layer forms and propagates with the Rossby wave, potentially contributing to the mixed layer warming.

[1] Interannual variability of the Wyrtki jets is studied in the context of Indian Ocean Dipole (IOD) and El Niño and Southern Oscillation (ENSO) wind-forcing using a three dimensional numerical ocean model and observations. The boreal fall (October-November) Wyrtki jet is more significantly affected than the boreal spring (April-May) Wyrtki jet since both the IOD and ENSO tend to peak toward the end of the calendar year. Various statistical methods are used in an attempt to separate the impacts of the IOD and ENSO on these jets, with emphasis on the fall jet. The first two modes of an Empirical Orthogonal Function (EOF) decomposition account for about 90% and 85% of variability in zonal currents and wind stress respectively along the equator in the Indian Ocean, but EOF analysis does not cleanly separate out IOD and ENSO forcing and response. Partial correlation analysis reveals that IOD wind-forcing and zonal equatorial current response are stronger on average than for ENSO and extend further west across the basin. Composite analysis of IOD only, ENSO only, and combined IOD and ENSO years provides a complementary definition of the relative contributions of these two phenomena on Wyrtki jet variability and in general is consistent with the results of the partial correlation analysis.