The influence of salinity on tropical Atlantic instability waves

Abstract

Sea surface salinity (SSS) data derived from the Aquarius/SAC-D satellite mission are analyzed along with other satellite and in situ data to assess Aquarius' capability to detect tropical instability waves (TIWs) and eddies in the tropical Atlantic Ocean and to investigate the influence that SSS has on the variability. Aquarius data show that the magnitude of SSS anomalies associated with the Atlantic TIWs is 60.25 practical salinity unit, which is weaker than those in the Pacific by 50%. In the central equatorial Atlantic, SSS contribution to the mean meridional density gradient is similar to sea surface temperature (SST) contribution. Consequently, SSS is important to TIW-related surface density anomalies and perturbation potential energy (PPE). In this region, SSS influences surface PPE significantly through the direct effect and the indirect effect associated with SSS-SST covariability. Ignoring SSS effects would underestimate TIW-related PPE by approximately three times in the surface layer. SSS also regulates the seasonality of the TIWs. The borealspring peak of the PPE due to SSS leads that due to SST by about one month. Therefore, SSS not only affects the spatial structure, but the seasonal variability of the TIWs in the equatorial Atlantic. In the northeast Atlantic near the Amazon outflow and the North Brazil Current retroflection region and in the southeast Atlantic near the Congo River outflow, SSS accounts for 80-90% of the contribution to mean meridional density gradient. Not accounting for SSS effect would underestimate surface PPE in these regions by a factor of 10 and 4, respectively.

Assessment of Aquarius Sea Surface Salinity

“…Figure 6b shows the global averages of regional temporal STD values of Aquarius SSS with respect to Argo-SIO SSS for various spatial scales. The difference between Figure 6a [4,21], Madden-Julian Oscillation [22,23], El Niño-Southern Oscillation [24], and Indian Ocean Dipole [25]. It is noteworthy that the global average STD value for non-seasonal SSS anomalies is less than 0.1 psu for 1 • × 1 • scale and 0.04 psu for 10 • × 10 • scale.…”

“…Figure 6c,d shows the global average of STD values for composite seasonal anomalies and nonseasonal anomalies (referenced to the composite seasonal cycle) of the SSS differences between Aquarius and Argo-SIO. The non-seasonal anomalies are associated with the differences between Aquarius and Argo-SIO in representing non-seasonal signals associated with phenomena such as tropical instability waves [4,21], Madden-Julian Oscillation [22,23], El Niño-Southern Oscillation [24], and Indian Ocean Dipole [25]. It is noteworthy that the global average STD value for non-seasonal SSS anomalies is less than 0.1 psu for 1° × 1° scale and 0.04 psu for 10° × 10° scale.…”

Spatial and temporal scales of sea surface salinity variability in the Atlantic Ocean

“…The spatial pattern of the remaining variance closely resembles the total SSS variability: values up to 0.5 g/kg at the sea ice edge in the Nordic seas and Labrador Sea; also comparably large values north of the Gulf Stream and somewhat larger values (up to 0.7 g/kg) at the mouth of the Amazon River and at the exit of the Baltic Sea. Along the equator, values of up to 0.4 g/kg (confirmed by the independent results at the mooring stations) partly reflect the influence of the tropical instability waves with periods of 20–40 days [ von Schuckmann et al ., ; Lee et al ., ]. These are regions where large‐amplitude fast processes make observing SSS challenging.…”

Importance of the Equatorial Undercurrent on the sea surface salinity in the eastern equatorial Atlantic in boreal spring

“…Satellite measurements offer the opportunity to observe the SSS with an unprecedented resolution about 50–150 km, 3–7 days [ Lee et al ., ; Hernandez et al ., ; Reul et al ., ; Lu et al ., ]. In the tropical Atlantic, satellite observations allow to monitor the seasonal SSS variability [ Tzortzi et al ., ] and to detect tropical instability waves (TIWs) [ Lee et al ., ]. Lee et al .…”

“…Lee et al . [] showed that SSS horizontal gradient significantly contributes to the dynamical balance of TIWs by enhancing the meridional density gradient in the tropical Atlantic, especially during boreal spring.…”