3.6.7.2 Indian Ocean Dipole

Large interannual variability of SST in the Indian Ocean has been associated with the Indian Ocean Dipole (IOD), also referred to as the Indian Ocean Zonal Mode (IOZM; Saji et al., 1999; Webster et al., 1999). This pattern manifests through a zonal gradient of tropical SST, which in one extreme phase in boreal autumn shows cooling off Sumatra and warming off Somalia in the west, combined with anomalous easterlies along the equator. The magnitude of the secondary rainfall maximum from October to December in East Africa is strongly correlated with positive IOD events (Xie et al., 2002). Several recent IOD events have occurred simultaneously with ENSO events and there is a significant debate on whether the IOD is an Indian Ocean pattern or whether it is triggered by ENSO in the Pacific Ocean (Allan et al., 2001). The strongest IOD episode ever observed occurred in 1997 to 1998 and was associated with catastrophic flooding in East Africa. Trenberth et al. (2002b) showed that Indian Ocean SSTs tend to rise about five months after the peak of ENSO in the Pacific. Monsoon variability and the SAM (Lau and Nath, 2004) are also likely to play a role in triggering or intensifying IOD events. One argument for an independent IOD was the large episode in 1961 when no ENSO event occurred (Saji et al., 1999). Saji and Yamagata (2003), analysing observations from 1958 to 1997, concluded that 11 out of the 19 episodes identified as moderate to strong IOD events occurred independently of ENSO. However, this was disputed by Allan et al. (2001), who found that accounting for varying lag correlations removes the apparent independence from ENSO. Decadal variability in correlations between SST-based indices of the IOD and ENSO has been documented (Clark et al., 2003). At inter-decadal time scales, the SST patterns associated with the inter-decadal variability of ENSO indices are very similar to the SST patterns associated with the Indian monsoon rainfall (Krishnamurthy and Goswami, 2000) and with the North Pacific inter-decadal variability (Deser et al., 2004), raising the issue of coupled mechanisms modulating both ENSO-monsoon system and IOD variability (e.g., Terray et al., 2005).

3.6.8 Summary

Decadal variations in teleconnections considerably complicate the interpretation of climate change. Since the TAR, it has become clear that a small number of teleconnection patterns account for much of the seasonal to interannual variability in the extratropics. On monthly time scales, the SAM, NAM and NAO are dominant in the extratropics. The NAM and NAO are closely related, and are mostly independent from the SAM, except perhaps on decadal time scales. Many other patterns can be explained through combinations of the NAM and PNA in the NH, and the SAM and PSA in the SH, plus ENSO-related global patterns. Both the NAM/NAO and the SAM have exhibited trends towards their positive phase (strengthened mid-latitude westerlies) over the last three to four decades, although both have returned to near their long-term mean state in the last five years. In the NH, this trend has been associated with the observed winter change in storm tracks, precipitation and temperature patterns. In the SH, SAM changes are related to contrasting trends of strong warming in the Antarctic Peninsula and a cooling over most of interior Antarctica. The increasing positive phase of the SAM has been linked to stratospheric ozone depletion and to greenhouse gas increases. Multi-decadal variability is also evident in the Atlantic, and appears to be related to the THC. Other teleconnection patterns discussed (PNA, PSA) exhibit decadal variations, but have not been shown to have systematic long-term changes.

ENSO has exhibited considerable inter-decadal variability in the past century, in association with the PDO (or IPO). Systematic changes in ENSO behaviour have also been observed, in particular the different evolution of ENSO events and enhanced El Niño activity since the 1976–1977 climate shift. Over North America, ENSO- and PNA-related changes appear to have led to contrasting changes across the continent, as the west has warmed more than the east, while the latter has become cloudier and wetter. Over the Indian Ocean, ENSO, monsoon and SAM variability are related to a zonal gradient of tropical SST associated with anomalous easterlies along the equator, and opposite precipitation and thermal anomalies in East Africa and over the Maritime Continent. The tropical Pacific variability is influenced by interactions with the tropical Atlantic and Indian Oceans, and by the extratropical North and South Pacific. Responses of the extratropical ocean become more important as the time scale is extended, and processes such as subduction, gyre changes and the THC come into play.