A More Than Two Decade, in-Situ Record of Air-Sea Coupling in the Eastern South Pacific: Observations and Comparison to Models

The eastern South Pacific, with its coastal upwelling and frequent low stratus cloud cover, has been challenging for atmospheric and coupled models, which have shown warm biases in predicted sea surface temperatures. Historically data-sparse, a lack of knowledge of air-sea coupling in the region slowed efforts to understand the role of air-sea coupling in maintaining the stratus clouds and governing the temperature and depth of the ocean surface mixed layer. We report on analysis of a new, high-quality in-situ record of air-sea coupling from the region. Since 2000, high quality in-situ measurements of surface meteorology have been recorded at the Stratus Ocean Reference Station (ORS) surface mooring in the eastern South Pacific Ocean, 1500 km west of northern Chile. This record as been withheld from assimilation to provide an independent benchmark for the validation of computer simulations. Time series of the air-sea fluxes of heat, freshwater, and momentum are derived from the surface meteorological record. The time series are used to characterize the temporal variability of the surface meteorology, sea surface temperature and salinity, and air-sea fluxes over periods from hours to decades. One focus for the analysis is on low-frequency variability and trends, including strengthening of the surface stress and resultant changes in heat flux when the South Pacific High shifts to the northeast. Co-located time series from atmospheric model reanalyses (ERA5, NCEP2, MERRA2) have also been analyzed and the model surface meteorology and air-sea fluxes compared to the Stratus ORS record. Interestingly, reanalyses consistently under-estimate the net input into the ocean. This cooling bias is also seen at other sites in the Trade Wind region, however, it is at odds with the warm SST bias at the Stratus site.

The eastern South Pacific, with its coastal upwelling and frequent low stratus cloud cover, has been challenging for atmospheric and coupled models, which have shown warm biases in predicted sea surface temperatures. Historically data-sparse, a lack of knowledge of air-sea coupling in the region slowed efforts to understand the role of air-sea coupling in maintaining the stratus clouds and governing the temperature and depth of the ocean surface mixed layer. We report on analysis of a new, high-quality in-situ record of air-sea coupling from the region. Since 2000, high quality in-situ measurements of surface meteorology have been recorded at the Stratus Ocean Reference Station (ORS) surface mooring in the eastern South Pacific Ocean, 1500 km west of northern Chile. This record as been withheld from assimilation to provide an independent benchmark for the validation of computer simulations. Time series of the air-sea fluxes of heat, freshwater, and momentum are derived from the surface meteorological record. The time series are used to characterize the temporal variability of the surface meteorology, sea surface temperature and salinity, and air-sea fluxes over periods from hours to decades. One focus for the analysis is on low-frequency variability and trends, including strengthening of the surface stress and resultant changes in heat flux when the South Pacific High shifts to the northeast. Co-located time series from atmospheric model reanalyses (ERA5, NCEP2, MERRA2) have also been analyzed and the model surface meteorology and air-sea fluxes compared to the Stratus ORS record. Interestingly, reanalyses consistently under-estimate the net input into the ocean. This cooling bias is also seen at other sites in the Trade Wind region, however, it is at odds with the warm SST bias at the Stratus site.