Interannual sea surface salinity variations observed in the tropical North Pacific Ocean

Abstract

Analysis of observational data sets reveals pronounced interannual variations of sea surface salinity (SSS) in the tropical North Pacific (7°N–15°N) during 2000–2012. SSS anomalies with maximum magnitudes >0.2 occur in the central Pacific and translate westward at a speed of 15–20 cm s–1. The signals are weakened during their westward movement but reinforced in the Philippine Sea. Budget analysis for the mixed layer salinity suggests that in the central Pacific, El Niño–Southern Oscillation‐related atmospheric freshwater forcing and ocean advection changes are both important in generating and maintaining these large SSS anomalies. In the advection term, the most contributing component is the meridional Ekman advection induced by trade winds. These SSS anomalies are subsequently carried westward by the North Equatorial Current, which is the primary cause of SSS variations in the Philippine Sea. Freshwater forcing is also at work in the Philippine Sea, but its role is generally secondary.

Causes of the Northern Gulf of Guinea Cold Event in 2012

“…To examine the causes of the anomalously cold SST in 2012, we used the mixed‐layer heat budget which can be written as follows (see Vialard et al., 2001, or Jouanno et al., 2011, 2017): with here, T is the model potential temperature, the surface reference density (set to 1,021 kg.m −3 as in Li et al., 2013), the heat capacity (set to 3984 J kg −1 °C −1 as in Wade et al., 2011), h the MLD, Q ns the nonsolar surface heat flux (sum of the net longwave radiation, the sensible and latent heat fluxes), Q s the net shortwave radiation, f z =− h the fra...…”

Mixed-layer salinity budget in the tropical Indian Ocean: seasonal cycle based only on observations

“…Þwhere τ x is the zonal wind stress, τ y is the meridional wind stress, f is the Coriolis parameter, and σ o (set to 1027 kg m −3 , Li et al 2013) is the reference density of seawater. The geostrophic velocity is deduced from the difference between total horizontal velocity and Ekman velocity.…”

Glider observations of the North Equatorial Current in the western tropical Pacific

“…The annual minimum (maximum) in surface salinity in January (March) follows anomalously high (low) precipitation over the western Pacific. The annual salinity minimum lags local rainfall by 4 months, suggesting that advection contributes to the salinity minimum, as previous researchers have found [ Delcroix and Hénin , ; Li et al ., ]. The exceptionally high standard deviation in surface salinity, from the surface to 100 m and between 8°N and 13°N, is caused by the presence and absence of NPTSW due to interannual precipitation associated with ENSO.…”

“…NPTSW is a fresh, shallow layer covering the southern part of the NEC and the northern part of the NECC [Wyrtki and Kilonsky, 1984]; the origin is either from local rainfall or meridional advection [Delcroix and H enin, 1991;Li et al, 2013]. In the glider sections, NPTSW is between 88N and 138N, with potential density less than 22 kg m 23 , salinity less than 34.1 psu, and a potential temperature of 298C (Figures 9 and 10a).…”