Neil Malan: How leaky is a marine heatwave?
Are marine heatwaves (MHWs) coherent bodies of warm water, or regions that water simply passes through? MHWs are generally defined either from temperature at fixed locations, or as a semi-Lagrangian moving ‘object’. In both cases they appear as long, intense episodes. But organisms experience extreme temperatures differently depending on how they move: sessile species such as kelp and corals are fixed in space, passive drifters such as plankton and larvae feel temperature along their paths, and swimmers such as fish sit in between. In this work we analyse existing output from ACCESS-OM2-01 to fully characterise the horizontal and vertical structure of the eponymous 2011 Western Australian marine heatwave. We then seed passive particles into the MHW at its peak severity and advect them forward and backward, comparing fixed-point (Eulerian) and along-trajectory (Lagrangian) severity.
We find that the MHW leaks and is not coherent. Water starting in the core of the MHW has an ~8-day 50% survival time in both frames of reference, but the two persistence curves diverge at longer lags - after 60 days ~7% of fixed points remain over the MHW threshold, versus ~49% of advected particles - so the MHW footprint and its original water do not persist together. In a case study of the Ningaloo shelf, cross-shelf exchange dynamics controlled the difference between Lagrangian and Eulerian exposure, with broader shelf areas showing stronger communication between the coastal and open ocean. Moving forward, the comparison of Lagrangian and Eulerian metrics may further illuminate the complex ecological impacts of MHWs.