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Ocean Surface Topography from Space


Yves Quilfen - (LOS - IFREMER)

  Bertrand Chapron
Nicolas Reul
A. Mouche
S. Jullien
Jean François Piolle
Fabrice Ardhuin
C. Combot - PhD

Jason-2 SSH anomalies every ten days after the altimeter track intersected the wake of hurricane Igor.
Jason-2 SSH anomalies every ten days after the altimeter track intersected the wake of hurricane Igor. Areas filled in black indicates lower sea level by comparison with the first analyzed Jason-2 pass on the left. The dotted black line gives the location of the Amazon/Orinoco plume as determined from the SMOS salinity measurements.
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 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. 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.

Satellite altimeters are unique to provide robust estimates of the sea states generated by tropical cyclones (Young, 2003; Quilfen et al., 2006, 2010) and 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.

The first objective of the project is to advance our knowledge of sea state conditions using the current altimeter datasets conjointly with numerical and analytical models, and to investigate how these waves participate to the ocean mixing and cold wake formation. With the actual satellite constellation and Argo floats deployment, coverage is far from complete but it is better than ever and will keep increasing.

The second objective is to investigate the re-stratification phase. What is of primary interest is what processes control the recovery of the wake toward pre-cyclone conditions, and how the balance and rates of these processes change depending on the parameters of the wake: buoyancy gradient, depth and width. The purpose here is to evaluate the processes of re-stratification of the cold wake, to characterize how dependent they are on pre-cyclonic ocean conditions and finally to quantify the ocean heat uptake by TCs. These objectives can only be achieved by using all the observations available conjointly with the understanding of processes that modeling experiments can offer.

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