On the decoupling of the IODZM from southern Africa Summer rainfall variability

摘要

The decoupling of the Indian Ocean Dipole/zonal mode (IODZM) from southern Africa's dominant mode of rainfall variability since 1997 is investigated. An approach of using variability as the major descriptive parameter for the observational time series is adopted. This approach enables the identification of epochs differentiated by the examination of low-frequency signals in the climate data of the Indian Ocean basin. The period of analysis stretches from 1922 to 2008 and focuses on the October to December season so as to capture the most active component of the IODZM events that coincide with the early summer rainfall. The period around 1997/1998 is noted as an important climate regime transition in the Indian Ocean climate system whereas the well known 1976/1977 climate shift appears to be of lesser significance in the region. The temporal manifestation of statistical parameters of the IODZM matches significantly with those of basic atmospheric variables in the western subtropical South Indian Ocean and consistently identifies 1961 and 1997 as important turning points in the Indian Ocean climate system. However, although the relatively high variability of the atmospheric variables quickly decreases after 1997, a corresponding fall in the rainfall and IODZM time series variability is not found. Instead, an abrupt reduction in the correlation between the two time series to insignificant values occurs after 1997. Coincidentally, notable cooling over the western South Indian Ocean is observed during the pre-1997 positive IODZM events, followed by a synchronized anomalous warming and cooling over the respective western and eastern subtropical South Indian Ocean during the events after 1997. This change is associated with an eastward displacement of the South Indian Ocean anticyclone after 1997. Thus, 1997 is a significant year in the Indian Ocean climate system when a discontinuity acts to decouple the IODZM from the dominant mode of regional rainfall variability.