Modulating role of the Interdecadal Pacific Oscillation on the relationship between interannual variation of the long rains over Tanzania and south‐central tropical Indian Ocean sea surface temperature

Abstract

AbstractSeveral studies have demonstrated the influence of sea surface temperature (SST) in the Indian Ocean on the interannual variability of the Tanzanian long rains (TLR). A new investigation into the south‐central tropical Indian Ocean (SCTIO) SST–TLR relationship has been undertaken, focusing on the modulating role played by the Interdecadal Pacific Oscillation (IPO). The relationship experienced interdecadal change during the early 1990s and was strong (weak) during the 1950–1990 (1991–2010) period. Further diagnosis revealed that the interdecadal change is closely associated with changes in atmospheric circulation anomalies induced by the SCTIO SST anomalies (SSTAs). Prior to the early 1990s, warm SCTIO SST induced anomalous upper‐level cyclone and easterlies over Tanzania, which led to evidence of abnormal subsidence over this region. These were accompanied by an early onset of the Southeast Asia monsoon (SAM) to promote dryness in Tanzania. Easterlies, cyclones and SAMs ceased to exist after the early 1990s. The SCTIO SSTA alone does not induce distinct atmospheric circulation anomalies linked to the TLR variation. It was found that IPO is a crucial factor modulating the nonstationary SCTIO SST–TLR relationship. Prior to the early 1990s, IPO‐related SSTA induced anomalous descending motion through the atmospheric bridge, resulting in northeasterly (easterly) over the north Indian (eastern Indian) Ocean. These anomalies also affect the mixed‐layer depth (latent heat flux), which in turn alters the SST, warming the whole Indian Ocean basin. In this case, the SCTIO SSTA exerts significant influence on the TLR, independent of the IPO. After the early 1990s, sea surface height anomalies from the Pacific propagate through the oceanic pathway to the SCTIO, which in turn modifies SSTs. During the 1991–2010 period, a weaker SCTIO SST–TLR association correlated with a stronger SCTIO SST–IPO relationship. We suggest that the IPO may act as a crucial modulator for the SCTIO SST–TLR relationship.