CoAuthors

Bertrand Chapron (IFREMER/LOPS, France); Clément Combot (IFREMER/LOPS, France); Swen Jullien (IFREMER/LOPS, France); Nicolas Reul (IFREMER/LOPS, France)

Combined satellite-based observations offer means to better question the role of extremes on the upper ocean heat budget at local and global scales. Energy inputs in the region of intense storm tracks are indeed thought to represent the main kinetic energy sources necessary to maintain the deep ocean stratified and to strengthen ocean stirring processes. Strong winds associated with tropical cyclones (TCs) generate extreme sea states and vigorous vertical mixing in the upper ocean (about 10 times the usual mixing), stirring warm surface waters with colder waters below. The strong wind and associated wind vector rotation induce an energetic inertial current response. It is generally characterized by a very strong asymmetrical upwelling of cold deep water under the cyclone track with weaker and more widespread downwelling on the sides. The upwelling strongly participates in the surface and subsurface thermal response by uplifting the thermocline.  After the TC passage, the sea surface cold anomaly can then quickly dissipate by means of positive net air–sea heat fluxes, whereas a subsurface warm anomaly is believed to persist over a longer period.
Satellite altimeters are unique to provide robust estimates of the sea states generated by tropical cyclones and to help quantify changes in sea surface height in storm-affected regions during the recovery period (months) following tropical cyclone passages. Changes in sea surface height are closely linked to changes in ocean heat content, which enable direct estimates of the vertically integrated changes in ocean temperatures caused by tropical cyclones. Altimeter observations can further be completed with other satellite observations: winds from scatterometers and passive radiometers (including new-class of L-band radiometer measurements SMOS, SMAP, and CYGNSS bi-static ones), swell waves from multiple SAR missions (Sentinel 1 A and B), sea surface temperature (combined passive and IR measurements), sea surface reflectance (optical measurements). Furthermore, Argo floats deployment, while far from offering a complete coverage, is better than ever and will keep increasing to provide interior profiles to augment the ensemble of satellite observations.
The project includes the proper modeling of the sea state and surface winds during the extreme phases. Results of this step are presented that combine altimeter, scatterometer, and SAR observations with analytical and more complex numerical models outputs. The main objective shall then be directed to more precisely depict the ocean mixing and restratification processes and the ocean heat uptake by TCs. As foreseen, this can only be achieved using all available observations, conjointly with the understanding of processes that simplified models and hindcast experiments can offer.