Surface Thermal Structure and Variability of Upwelling Shadows in the Gulf of Arauco, Chile

The upwelling shadow in the Gulf of Arauco (GA) is studied using 15 years of daily satellite images of Sea Surface Temperature (SST) from Geostationary Operational Environmental Satellites (GOES), as well as heat flux and wind data from the ERA5 reanalysis product. An Upwelling Shadow Index (USI) is developed based on the SST differences between the GA and the region off Punta Lavapie (PL) farther offshore, characterized by active upwelling. USI values greater than 0.91°C and SST differences less than 1.1°C, correspond to an upwelling shadow event. These cases occurred 10.29% of the time and were more frequent in spring/summer during events of upwelling‐favorable winds with strong cyclonic wind stress curl, interspersed with wind relaxations. Multiple correlations between USI and wind stress curl and solar radiation showed an r2 = 40%–80% for some summer months. Most events persisted for only 1 day (53%), whereas 34% of upwelling shadow events lasted between 2 and 4 days and 9% of events were longer than 4 days. Water residence times as long as 15 days were observed in the GA during upwelling shadow events. During southerly wind relaxation (less than 2 days), cold surface waters flowing north from PL curved onshore and entered the GA from the north, weakening the thermal gradient between PL and GA and the upwelling shadow. Long periods of wind relaxation (at least 1 week) resulted in the dissipation of the thermal gradient due to the warming of offshore waters. Plain Language Summary Coastal upwelling has been defined as the rise of subsurface cold waters in response to wind forcing. When this phenomenon occurs close to embayments facing the equator, the inner sea can remain sheltered to the alongshore winds, which induces a local surface warming. Thus, an upwelling shadow has been defined as “a localized region of an active coastal upwelling system within which upwelling is reduced, and a coherent pattern of anomalously high surface temperature is observed.” Here, the upwelling shadow in the Gulf of Arauco (GA) (Chile) was studied using 15 years of daily satellite images of sea surface temperature, solar radiation, and wind data. As a result, an Upwelling Shadow Index was developed based on the temperature differences between the GA and the offshore waters off Punta Lavapie, characterized by active upwelling. This index included a daily factor from 0 to 1, based on the regional SST average, where 0 is the coldest day and 1 is the warmest day in the 15 yea