CoAuthors

Bertrand Chapron (Ifremer, France); Nicolas Reul (Ifremer, France); Swen Jullien (Ifremer, France)

Satellite-based observations offer means to better question the role of extreme conditions for the state of ocean at local and global scales, and effects on ocean circulation and ocean heat transport. 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 (Emanuel, 2001; Sriver and Huber, 2007). Strong winds associated with tropical cyclones (TCs) generate vigorous vertical mixing in the upper ocean (about 10 times the usual mixing), stirring warm surface waters with colder waters below. Cyclonic rotating winds also induce an Ekman pumping that is particularly strong for slow or static storms. It is characterized by a very strong 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 (Price, 1981; Shay et al., 2000; Jullien et al., 2012). After the TC passage, the sea surface cold anomaly quickly dissipates by the mean of positive net air–sea heat fluxes, whereas the subsurface warm anomaly is believed to persist over a much longer period. It then corresponds to a net ocean heat uptake whose fate is the subject of active debate. Such a flux of heat into the low-latitude ocean has been proposed to be an important modulator of local and remote climate.
Techniques using satellite altimetry can help to quantify changes in sea surface height in storm-affected regions during the months following tropical cyclones (Jansen et al., 2010; Haney et al., 2012; Mei et al., 2013; Sriver, 2013). 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. However, limitations in the observational systems and in methodological approaches are likely to hamper severely significance of the results, because TC-induced SSH steric anomalies are small (~1cm) with respect to large scale SSH variations (~1m) and background variability. While SSH measurements from the current altimeter constellation shall be used for this study, it can be anticipated that the much larger coverage by the 2D SSH imaging capability of the SWOT satellite will enable more accurate estimate of these anomalies, and more direct and better tracking of the cold wake mesoscale processes. Indeed, as the surface fluxes quickly restore the pre-cyclonic SSTs (10-day timescale) and then erase the SST gradients, a 2D SSH imaging will enable to track at the monthly/seasonal timescale, for a given event, the sub-surface fronts associated with the sub-surface bolus of warm water anomaly, to better evaluate the spatial scales of the processes. Improved knowledge of the wakes restratification spatial and temporal scales is crucial to define the best methodology for heat content uptake estimation. First results will be presented to discuss the feasibility and representativeness of an approach combining all the observations available (Argo floats, satellite SSH, SSS, SST) conjointly with the understanding of processes that modeling experiments can offer.