How to differentiate between coastal cooling and upwelling events on SST images?

Abstract

We aim to find on MODIS (Aqua, Terra) SST-pictures of the South-Eastern Baltic those characteristic features, which allow to distinguish the differential coastal cooling from the coastal upwelling events, even though their manifestations look much alike. The basis is data of MODIS (Aqua, Terra) spectroradiometers for 2000–2013. We analyze horizontal SST-profiles above coastal slopes under conditions of autumnal cooling and during well known upwelling events. The SST profiles above slopes are formed as a result of joint contribution of heat exchange with the atmosphere and horizontal transport of heat from the sea. Satellite images of October-November, corresponding to the periods of well pronounced decrease in air temperature (with time rate of change as large as 0.86–2.54 C/day) were analyzed. Typical shape of the profiles, demonstrating the presence of differential coastal cooling over the distance of 10–20 km from the shore, allows for allocation of the region where thermocline meets the slope, what gives (for the given bathymetry) an information on current thickness of the upper mixed layer (UML). Shape of the SST profile is quite conservative, not sensitive to both the steepness of the bottom slope, bathymetry peculiarities, and intensity of cooling and even wind conditions, indicating that sea-shelf heat exchange is in general close to the steady state. The SST drop from open to coastal area is about 2–3 °C and does not depend on the thickness of the UML or the steepness of the slope. On the other hand, about 90 events of coastal upwelling in May-November 2000–2013 in South-Eastern Baltic were selected and the features of the horizontal SST-profiles above the same coastal slopes were analyzed. It appeared that the shape of the SST-profiles demonstrates the presence of upwelling quite definitely, what allows for effective differentiation from the coastal cooling effects.