No abstract

Phytoplankton blooms on Australia's southern shelves are revisited using satellite‐derived monthly data of chlorophyll a concentrations for the period 2003–2015. It is known that the region hosts a seasonal coastal upwelling system that develops in austral summer (January–March) with chlorophyll a concentrations of >2 mg/m3. While this summer upwelling is spatially limited to a few hot spots, here we show that widespread phytoplankton blooms of moderate (~1 mg/m3) chlorophyll a concentrations develop during autumn and early winter on most of Australia's extensive southern shelves—from the vast shelves of the Great Australian Bight (GAB) in the west to Bass Strait in the east. This surprising finding disproves the widespread belief that shelf waters of the GAB are generally oligotrophic and may explain the relatively high abundance of both forage fish (sardines) and upper trophic‐level predators (e.g., tuna and whales) in the region.

This study focuses on the regional wind variability that controls the intensity of cold-water upwelling off Sumatra -a key feature of the Indian Ocean Dipole (IOD). Our analysis of daily atmospheric data reveals the existence of convectively triggered synoptic-scale atmospheric cyclones in the South-East Tropical Indian Ocean (SETIO). The northern branch of the cyclones corresponds to westerly equatorial wind events, whereas the eastern branch involves northwesterly winds that operate to suppress cold-water upwelling off Sumatra's west coast. Data for the period 1988-2022 show that 5-9 SETIO cyclones normally form each year during the boreal summer-autumn season, effectively suppressing upwelling in the region. In contrast, there are only few (1-2) cyclone events in years identified as positive phases of the IOD, when the absence of cyclones concurs with the development of strong coastal upwelling off Sumatra. Our findings suggest that the absence or presence of SETIO cyclones contributes to IOD variability.