The effect of surface flooding on the physical–biogeochemical dynamics of a warm-core eddy off southeast Australia

Warm-core eddies (WCEs) formed from the East Australian Current (EAC) play an important role in the heat, mass and biogeochemical budgets of the western Tasman Sea. The development and separation of an EAC WCE during July–December 2008 was observed using remotely sensed temperature, ocean colour and sea-level elevation, three Argo floats, a shipboard CTD, a shelf mooring array and a 15-day deployment of a Slocum glider. The eddy formed from an EAC meander during the first half of 2008 and in late August had a ∼275m deep surface mixed layer. In the two months before separation in early December, fresher and warmer EAC water flooded the top of the eddy, submerging the winter mixed layer. The rate of vertical transport due to submergence was estimated to be between 1 and 6 Sv, at the time accounting for a significant fraction of the mean southward flow of the EAC. The core of the eddy had a surface chlorophyll a   concentration of <0.4mgm−3 throughout the observations. A 20–40 m thick pycnocline formed at the interface of the flooding surface waters and the submerged layer. Chlorophyll a concentration in the pycnocline ranged from 0.5 to 2 mg m−3, with depth-integrated concentration ranging between 25 and 75 mg m−2. The development of a sub-surface maximum suggests that flooding increased light levels in the pycnocline. Elevated levels of coloured dissolved organic matter in the submerged layer correspond to oxygen depletion, suggesting respiration of organic matter. A comparison is made with observations from WCEs in 1978 and 1997 in which, unusually, surface flooding did not occur, but solar heating stratified the top 50 m. In the two eddies with surface capping, surface chlorophyll a concentrations were an order of magnitude higher than the 2008 flooded eddy, but depth-integrated chlorophyll a was similar. These findings suggest that EAC WCEs with relatively shallow surface flooding contain more phytoplankton biomass than surface images would suggest, with the vertical position of the chlorophyll a maximum depending on whether, and to what depth, the winter surface mixed layer is submerged.