Abstract

The seasonal cycle of surface chlorophyll in the Leeuwin Current off the southwest Western Australian coast is characterized by low austral summer concentrations that increase during the austral autumn and winter. Results from a one-dimensional numerical model show that a combination of solar heating and nutrient poor surface waters during the summer can restrict plankton growth to the base of the euphotic zone resulting in low surface chlorophyll concentrations that are consistent with observations. The model also suggests that increased vertical mixing caused by surface cooling can explain some of the observed increases in surface chlorophyll that occur during the winter. However, the magnitude of the autumn chlorophyll signal varies between years, and sometimes the bloom precedes the onset of surface cooling at the end of the summer. Two different mechanisms are proposed to explain this result. During years when the Leeuwin Current is weaker, southward transport of heat is reduced, enhancing the influence of surface cooling during autumn, which results in convective overturning and a moderate increase in surface chlorophyll. By contrast, during years when the Leeuwin Current is stronger, surface cooling is less effective and convective mixing is delayed by up to two months during the autumn. Instead, nonlocal forcing associated with increased Leeuwin Current flow is proposed to be responsible for relatively large autumn increases in these years. The autumn surface chlorophyll signal increases with the strength of the Leeuwin Current suggesting a link between climate variability and primary production in this region.

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