Freshwater Input to the Upper Ocean by Tropical Cyclones
[12-Feb-2018] Reul, N.
Presented at the 2018 Ocean Sciences Meeting
A number of weather systems can produce extreme rainfall over the ocean, including squall lines, mesoscale convective complexes, and tropical cyclones (TCs). Intense TCs generate extreme rainfall events with rain rates that can reach local values significantly greater than 45 mm/h, generally found in the vicinity of the storm inner core, but also at the storm's periphery within the spiraling rain bands (Houze, 2010). These events can last sometimes more than 72 hours with such high rain fall rates (Shepherd et al., 2007). Due to those typically very heavy rains associated with TC, the upper ocean salinity can be substantially freshened by those very large and intermittent fresh water flux into the ocean. Intense vertical mixing at the base of the oceanic mixed layer that is generated concomitantly in the wake of these storms by the TC-induced inertial current shear generally upwells colder and saltier subsurface waters from below the mixed-layer (Price, 1981). The latter rapidly mixes with the fresh water that falls from the atmosphere into the upper ocean: the post-storm upper ocean salinity found in the wake of these extreme is therefore the resultant of these two counter-acting effects. Post-storm salinity in the deepened and homogenized mixed layer can thus exhibit in the storm's wake alternating spatial patterns of positive and negative salinity anomalies, depending on the pre-storm hydrological conditions and the local balance during the TC passage between the downward freshwater fluxes and the salty water upwelling/mixing strength.
Whether the imprints of these impacts on the sea surface salinity can be detected using current observation systems such as satellites and in situ data is one of the questions we attempt to answer here. The variability of both surface and mixed-layer salinity changes in all active TC areas are determined over the period 2000-2016 from the data of the SMOS, Aquarius and SMAP satellite missions and from the ARGO float upper level data. TC induced rainfall is characterized using the Tropical Rainfall Measuring Mission (TRMM) satellite and TC track databases. Averaged SSS changes in the wake of TCs are characterized as function of TC intensity, translation speed, vertical wind shear, rain fall rate, pre-storm ocean stratification and distance to TC track.