Seasonal variation in the long-term warming trend in water temperature off the Western Australian coast

Previous studies have demonstrated that one area of greatest increase in surface sea temperatures (SST) (0.02 degrees C per year) in the Indian Ocean over the last 50 years occurs off the lower west coast of Australia, an area dominated by the Leeuwin Current. The present paper examines water temperature trends at several coastal sites since the early 1970s: two rock lobster puerulus monitoring sites in shallow water (<5 m); four sites from a monitoring program onboard rock lobster vessels that provide bottom water temperature (<36 m); and an environmental monitoring site at Rottnest (0-50 m depth). Two global SST datasets are also examined. These data show that there was a strong seasonal variation in the historic increases in temperature off the lower west coast of Australia, with most of the increases (0.02-0.035 degrees C per year) only focussed on 4-6 months over the austral autumn-winter with little or no increase (<0.01 degrees C per year) apparent in the austral spring-summer period. These increases are also apparent after taking into account the interannual variation in the strength of the Leeuwin Current. The warming trend results in a change to the seasonal temperature cycle over the decades, with a delay in the peak in the temperature cycle during autumn between the 1950s and 2000s of similar to 10-20 days. A delay in the timing of the minimum temperature is also apparent at Rottnest from August-September to October. This seasonal variation in water temperature increases and its effect on the annual temperature cycle should be examined in climate models because it provides the potential to better understand the specific processes through which climate change and global warming are affecting this region of the Indian Ocean. It also provides an opportunity to further test the climate models to see whether this aspect is predicted in the future projections of how increases will be manifest. Any seasonal variation in water temperature increase has important implications for fisheries and the marine ecosystem because it may affect many aspects of the annual life cycle such as timing of growth, moulting, mating, spawning and recruitment, which have to be taken into account in the stock assessment and management of fisheries.