Interannual Rainfall Extremes over Southwest Western Australia Linked to Indian Ocean Climate Variability

Abstract

Interannual rainfall extremes over southwest Western Australia (SWWA) are examined using observations, reanalysis data, and a long-term natural integration of the global coupled climate system. The authors reveal a characteristic dipole pattern of Indian Ocean sea surface temperature (SST) anomalies during extreme rainfall years, remarkably consistent between the reanalysis fields and the coupled climate model but different from most previous definitions of SST dipoles in the region. In particular, the dipole exhibits peak amplitudes in the eastern Indian Ocean adjacent to the west coast of Australia. During dry years, anomalously cool waters appear in the tropical/subtropical eastern Indian Ocean, adjacent to a region of unusually warm water in the subtropics off SWWA. This dipole of anomalous SST seesaws in sign between dry and wet years and appears to occur in phase with a large-scale reorganization of winds over the tropical/subtropical Indian Ocean. The wind field alters SST via anomalous Ekman transport in the tropical Indian Ocean and via anomalous air-sea heat fluxes in the subtropics. The winds also change the large-scale advection of moisture onto the SWWA coast. At the basin scale, the anomalous wind field can be interpreted as an acceleration (deceleration) of the Indian Ocean climatological mean anticyclone during dry (wet) years. In addition, dry (wet) years see a strengthening (weakening) and coinciding southward (northward) shift of the subpolar westerlies, which results in a similar southward (northward) shift of the rainbearing fronts associated with the subpolar front. A link is also noted between extreme rainfall years and the Indian Ocean Dipole (IOD). Namely, in some years the IOD acts to reinforce the eastern tropical pole of SST described above, and to strengthen wind anomalies along the northern flank of the Indian Ocean anticyclone. In this manner, both tropical and extratropical processes in the Indian Ocean generate SST and wind anomalies off SWWA, which lead to moisture transport and rainfall extremes in the region. An analysis of the seasonal evolution of the climate extremes reveals a progressive amplification of anomalies in SST and atmospheric circulation toward a wintertime maximum, coinciding with the season of highest SWWA rainfall. The anomalies in SST can appear as early as the summertime months, however, which may have important implications for predictability of SWWA rainfall extremes.

The impact of the positive Indian Ocean dipole on Zimbabwe droughts

“…Thus conclusively separating their projection onto Zimbabwe rainfall is beyond the scope of this study. At the same time, while we did not establish a direct link between ENSO-related variability and Zimbabwe precipitation, there are numerous climate teleconnections from the Pacific to the IO (England et al, 2006) and so an indirect ENSO influence is quite likely. For example, it is possible that ENSO can play a role via sea level pressure response through the downward branch of the Walker circulation.…”

Mechanisms of Summertime Subtropical Southern Indian Ocean Sea Surface Temperature Variability: On the Importance of Humidity Anomalies and the Meridional Advection of Water Vapor*

“…Tripolar interannual SST anomalies, roughly similar in pattern to the ones described here, have been discussed recently by England et al (2006). In this case, the SST anomalies were found using the National Oceanic and Atmospheric Administration (NOAA) Ex- tended Reconstructed SST composites based on annual rainfall extremes in southwest Western Australia.…”

Impact of the Indian Ocean high pressure system on winter precipitation over western and southwestern Australia

“…1(b) of Ummenhoffer et al (2009) who carried out simulation experiments with the NCAR-CAM3 atmospheric GCM showing that, in the May to September season, this SST anomaly pattern leads to increased rainfall in Western Australia and the reverse SST pattern corresponds to dry conditions in the same region. This SST pattern has been referred to as the 'meridional SST gradient' by Ummenhoffer et al (2008) and England et al (2006. Figure 9 suggests that this SST pattern is related to longitudinal shifts of the Indian Ocean High.…”