Abstract
The Atlantic Ocean is known to have higher sea surface salinity than the Pacific Ocean at all latitudes. This is thought to be associated with the Atlantic Meridional Overturning Circulation and deep water formation in the high latitude North Atlantic – a phenomenon not present anywhere in the Pacific. This asymmetry may be a result of salt transport in the ocean or an asymmetry in the surface water flux (evaporation minus precipitation; ) with greater over the Atlantic than the Pacific. In this paper, we focus on the surface water flux. Seven estimates of the net freshwater flux ( including run-off, R), calculated with different methods and a range of data sources (atmospheric and oceanic reanalyses, surface flux data-sets, hydrographic sections), are compared. It is shown that over the Atlantic is consistently greater than over the Pacific by about 0.4 Sv (1 Sv m s). The Atlantic/Pacific asymmetry is found at all latitudes between 30S and 60N. Further analysis with ERA-Interim combined with a run-off data-set demonstrates that the basin asymmetry is dominated by an evaporation asymmetry in the northern high-latitudes, but by a precipitation asymmetry everywhere south of 30N. At the basin scale, the excess of precipitation over the Pacific compared to the Atlantic (S N) dominates the asymmetry. Also it is shown that the asymmetry is present throughout the year and quite steady from year to year. Investigation of the interannual variability and trends suggest that the precipitation trends are not robust between data-sets and are indistinguishable from variability. However, a positive trend in evaporation (comparable to other published estimates) is seen in ERA-Interim, consistent with sea surface temperature increases.
Highlights
The Atlantic Ocean is known to have higher sea surface salinity (SSS) than the Pacific Ocean at all latitudes
Salinity patterns are linked to the hydrological cycle (Schmitt, 2008) with regions of high SSS corresponding to regions of positive E − P and regions of low SSS corresponding to regions of negative E − P (Fig. 1b)
Using ERAInterim, which compares favourably with other estimates, we proceed on exploring the Atlantic/Pacific asymmetry on spatial (10◦ latitudinal bands) and temporal scales not accessible with some other data-sets as well as investigating the role of precipitation, evaporation and run-off separately on the E − P − R asymmetry
Summary
The Atlantic Ocean is known to have higher sea surface salinity (SSS) than the Pacific Ocean at all latitudes. The higher subpolar North Atlantic mean evaporation rate noted by Warren (1983), Emile-Geay et al (2003) and Wills and Schneider (2015) was attributed to higher Atlantic sea surface temperatures (SSTs). Many previous studies have focused on the E − P asymmetry between the far northern regions of both oceans, (Rahmstorf, 1996) focused on the positive Atlantic E − P north of 30◦S It is unclear where E − P is the critical quantity since the SSS asymmetry between the basins exists at all latitudes. For completeness, results for the Indian Ocean are shown but that our discussion largely focuses on the Atlantic and Pacific basins
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