The importance of the Indian Ocean sea surface temperature (SST) anomalies in modulating regional climate has recently received increasing recognition. Surface variability has been implicated in widespread changes to precipitation around Indian Ocean rim countries, including East Africa, India, Indonesia, and Australia. Non-uniform warming of the Indian Ocean has recently been described and characteristics of the dominant mode of tropical Indian Ocean variability are suggested to be changing. These factors highlight the need for a better understanding of the mechanisms by which Indian Ocean variability modulates regional rainfall. This has major implications for seasonal rainfall forecasting and will ultimately aid in improvements to water and agricultural management.
In a series of atmospheric general circulation model experiments, the potential impact of Indian Ocean SST anomalies in modulating low- to mid-latitude precipitation around the Indian Ocean-rim countries is examined (Ummenhofer et al. 2008, 2009a,b). SST anomalies closely resembling a leading mode of Indian Ocean variability with features of both tropical and subtropical Indian Ocean dipoles induce a basinwide re-organization of the atmospheric circulation. This induces significant changes to precipitation distributions over the surrounding landmasses (Fig. 1).
Variations in eastern Indian Ocean upper-ocean thermal properties are assessed for the period 1970–2004, with a particular focus on asymmetric features related to opposite phases of Indian Ocean dipole events, using high-resolution ocean model hindcasts. Sensitivity experiments, where interannual atmospheric forcing variability is restricted to the Indian or Pacific Ocean only, support the interpretation of forcing mechanisms for large-scale asymmetric behavior in eastern Indian Ocean variability. Years are classified according to eastern Indian Ocean subsurface heat content (HC) as proxy of thermocline variations. Years characterized by an anomalous low HC feature a zonal gradient in upper-ocean properties near the equator, while high events have a meridional gradient from the tropics into the subtropics. The spatial and temporal characteristics of the seasonal evolution of HC anomalies for the two cases is distinct, as is the relative contribution from Indian Ocean atmospheric forcing versus remote influences from Pacific wind forcing: low events develop rapidly during austral winter/spring in response to Indian Ocean wind forcing associated with an enhanced southeasterly monsoon driving coastal upwelling and a shoaling thermocline in the east; in contrast, formation of an anomalous high eastern Indian Ocean HC is more gradual, with anomalies earlier in the year expanding from the Indonesian Throughflow (ITF) region, initiated by remote Pacific wind forcing, and transmitted through the ITF via coastal wave dynamics. Implications for seasonal predictions arise with high HC events offering extended lead times for predicting thermocline variations and upper-ocean properties across the eastern Indian Ocean.
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Ummenhofer CC, Sen Gupta A, Taschetto AS, and England MH. (2009). Modulation of Australian precipitation by meridional gradients in East Indian Ocean sea surface temperature. Journal of Climate, 22, 5597-5610. Reprint
Ummenhofer CC, Schwarzkopf FU, Meyers GA, Behrens E, Biastoch A, and Böning CW. (2013). Pacific Ocean contribution to the asymmetry in eastern Indian Ocean variability. Journal of Climate, 26, 1152-1171. Reprint