The contrast between Atlantic and Pacific surface water fluxes

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; E − P) with greater E − P over the Atlantic than the Pacific. In this paper, we focus on the surface water flux. Seven estimates of the net freshwater flux (E − P − R 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 E − P − R over the Atlantic is consistently greater than E − P − R over the Pacific by about 0.4 Sv (1 Sv ≡ 10 6 m 3 s −1 ). The Atlantic/Pacific E − P − R asymmetry is found at all latitudes between 30 • S and 60 • N. Further analysis with ERA-Interim combined with a run-off data-set demonstrates that the basin E − P − R asymmetry is dominated by an evaporation asymmetry in the northern high-latitudes, but by a precipitation asymmetry everywhere south of 30 • N. At the basin scale, the excess of precipitation over the Pacific compared to the Atlantic (∼ 30 • S −60 • 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 datasets 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.

Water transport among the world ocean basins within the water cycle

“…These values are close to the 0.13-0.32 Sv estimated from ocean models, as needed to maintain the salinity balance in the Atlantic Ocean (Zaucker et al, 1994 1812,2021]) of water to the Atlantic and Indian oceans, where it is reincorporated into the water cycle via net E − P . As in previous studies (see Craig et al, 2017, for a synthesis), the freshwater lost in the Indian Ocean is similar to that in the Atlantic Ocean. In these studies, P − E + R is close to zero in the Pacific Ocean, producing a difference of 0.4 Sv between the Atlantic and Pacific oceans.…”

Monsoon‐Induced Zonal Asymmetries in Moisture Transport Cause Anomalous Pacific Precipitation Minus Evaporation

“…The annual mean moisture flux field (Figure 2b) over the Indian Ocean, Maritime Continent, and North‐West Pacific is dominated by JJA, as is Pacific since it is negative during the rest of the year, but strongly positive in JJA, enabling the Pacific basin to be neutral in the annual mean (Figure 3). Emile‐Geay et al (2003) first suggested a role for the Asian Monsoon in the asymmetry but only for the subpolar regions, while Craig et al (2017) showed that the asymmetry is dominated by stronger precipitation per unit area south of 30°N across the Pacific.…”

“…Previous studies have suggested that enhanced northward heat transport in the Atlantic by the Atlantic MOC (Trenberth & Caron, 2001) results in stronger Atlantic evaporation and that the excess water vapor is transported in the Trade Winds across Central America to the Pacific causing weak net evaporation or net precipitation across the Pacific (Broecker, 1991). However, the Atlantic evaporation is only notably stronger in the subpolar region (Craig et al, 2017; Czaja, 2009; Emile‐Geay et al, 2003; Warren, 1983; Wills & Schneider, 2015). There, sea surface temperatures are higher than in the Pacific (Manabe & Stouffer, 1988; Warren, 1983), and a greater fraction of the narrower basin is affected by advection of air off continents with low relative humidity (Schmitt et al, 1989) resulting in stronger area‐averaged evaporation.…”

“…There, sea surface temperatures are higher than in the Pacific (Manabe & Stouffer, 1988; Warren, 1983), and a greater fraction of the narrower basin is affected by advection of air off continents with low relative humidity (Schmitt et al, 1989) resulting in stronger area‐averaged evaporation. However, the P − E asymmetry exists at all latitudes (Craig et al, 2017). South of 30°N, the P − E asymmetry is dominated by greater precipitation per unit area over the Pacific than the Atlantic.…”

On freshwater fluxes and the Atlantic meridional overturning circulation

“…S1) to which a small global correction has been applied to ensure global freshwater flux balance, and freshwater fluxes are converted to salt fluxes using a mean ocean salinity of 35 psu. The total surface freshwater flux integrated over the Atlantic basin (between 35 S and 60 N) amounts to a net evaporation of about 0.75 Sv, which is at the upper end of a number of recent estimates (Craig et al 2017). The effect of a potential bias in the freshwater flux over the Atlantic will be discussed in Section 4.…”