Patterns of sea surface temperature variability on seasonal to sub-annual scales at and offshore the Río de la Plata estuary

Abstract

SST variability on seasonal to sub-annual scales in the coastal region of South America between 30 and 39° S, largely influenced by the Rio de la Plata estuary’s plume, and its relation with wind variability is explored. Data are six years of daily ensembles of gridded satellite SST and sea surface winds with spatial resolutions of about 11 and 25 kilometers, respectively. Observations from oceanographic cruises are used to validate results. It is found that the seasonal cycle can be explained in terms of two modes. The first one, characterizing fall-early winter/spring-early summer, is related to the radiative cycle. The second one, corresponding to late summer and winter, displays warm/cold anomalies along the Uruguayan coast forced by the prevailing winds during those seasons. In the upper estuary and the northern part of the area of influence of the fresh water plume, variability in sub-annual scales is significant. A large portion of this variance is related to zonal wind anomalies that force warm/cold SSTs along that coast. Cold anomalies of up to -5°C occur under anomalously intense easterly winds, indicating upwelling. These events are very frequent and show large persistence, occurring up to one and a half months. They also display a marked seasonal cycle -being more frequent in late spring and summer-, large inter-annual variability and seem to be modulated by the continental runoff. When discharge is low, the fresh water plume retracts to the west, reducing the inner-shelf stratification and increasing the likelihood of a full upwelling to the surface. In winter, short time-scale SST variability is mostly due to variability in the atmospheric cold fronts crossing the region. Weaker or less frequent (stronger or more frequent) fronts produce a generalized warming (cooling) over the region. As the estuary heats (colds) faster than the shelf, a warm (cold) anomaly develops in the upper Río de la Plata. On inter-annual time scales, probably because ENSO activity was weak during the studied period, SST variability was not important.