Effect of local SST on inland rainfall

E. Linacre and B. Geerts

3/’02

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A lot of recent research has focused on how remote sea surface temperature (SST) anomalies affect weather patterns (teleconnections, Section 12.7). For instance, El Niño warming in the central/eastern equatorial Pacific tend to cause droughts in Indonesia and Australia, and floods in California. And above-normal SSTs off Angola, Africa, tend to produce wetter conditions in Brazil's Nordeste (Fig 16.11 in book).

Perhaps it is too obvious to note that local SST anomalies too affect coastal rainfall patterns. For instance, El Niño warming off Peru causes above-normal rainfall in coastal Peru. It is intuitive that warmer waters are associated with wetter conditions, but the details of how sensitive precipitation is to SST, and how far inland the effect can be felt, depend on coastal topography as well as on prevailing wind, tropospheric stability, and rain-producing weather patterns.

Generally, rainfall totals are very sensitive to the offshore SST in areas where most of the rain occurs when onshore winds prevail. For instance, in New South Wales (Australia) places closer to the coast appear to be affected more by coastal SST anomalies than places slightly further inland (Fig 1) (1). Further inland in New South Wales, especially west of the Dividing Range, most rain results from offshore moving weather systems (towards the east), hence east coast SST anomalies have virtually no effect on rainfall anomalies there. The association between coastal SST and rainfall may not be a cause-and-effect relation. Some feature of winds or ocean currents may govern both. For instance, a change in wind speed/direction from strong/along-shore (southerly) to weak/onshore in coastal Peru may be responsible for both a SST and a rainfall increase. Some ingenious logic indicates that the rainfall in coastal New South Wales is enhanced by two factors independently - the nearby SST and the wind direction (1).

Fig 1. Relation between monthly-mean coastal sea-surface temperature SST) anomaly and monthly total rainfall anomaly at two places near Sydney, Australia. Narrabeen is 0.3 km from the ocean and Richmond is about 50 km inland (from (1)).

Reference

(1) Hirst, A. and E.T. Linacre 1978. Associations between coastal sea-surface temperatures, onshore winds and rainfalls in the Sydney area. Search, 9, 325-7.