Delayed and Quasi‐Synchronous Response of Tropical Atlantic Surface Salinity to Rainfall

Abstract

Zonally elongated patterns of tropical Atlantic rainfall variability associated with the Atlantic meridional (dipole) and the El Niño‐Southern Oscillation (ENSO) modes extend over the ocean and adjacent land and lead to a complex sea surface salinity (SSS) response. SSS response to local ocean rainfall is quasi‐instantaneous, while SSS response to land rainfall is delayed through river hydrology. Amazon rainfall associated with the Atlantic meridional mode concentrates over coastal north‐east Brazil and results in a fast Amazon response. In contrast, ENSO rainfall occupies vast inland areas and results in 3‐ to 7‐month delayed Amazon discharge response. Although ocean profile analyses represent well interannual SSS forced by open ocean rainfall, they miss interannual SSS in the plume. Including thermosalinograph transect data improves the representation of plume SSS, but its dynamic is better captured by ocean reanalyses. In Simple Ocean Data Assimilation, the plume SSS anomaly persists several months following the peak of land rainfall and is mixed out by seasonally accelerating winds in boreal winter. Its discharge‐correlated magnitude is only a modest few tenths of PSU. Perhaps, such weak projection of continental discharge variations on simulated SSS is not surprising due to other factors contributing in this dynamically active western boundary area. Significant transient variability in local currents not associated with rainfall/discharge variability is a factor explaining why ocean analyses based on occasional profile observations fail in resolving the rain‐induced SSS in the Amazon plume.

Tropical Instability Waves in the Atlantic Ocean: Investigating the Relative Role of Sea Surface Salinity and Temperature From 2010 to 2018

“…SSS in the tropical Atlantic is controlled by several seasonally varying processes among which high precipitation under the ITCZ and river discharge (Awo et al., 2018; Grodsky et al., 2020). Both processes are strongly influenced by precipitations, and the leading mode of interannual rainfall variability in the tropical Atlantic is the AMM (e.g., Grodsky & Carton, 2018). It peaks in spring and is related to a cross‐equatorial gradient in SST (Ruiz‐Barradas et al., 2000; Chiang & Vimont, 2004) that governs anomalous meridional atmospheric pressure differences and related shifts of the ITCZ which in turn affects rainfall.…”

“…To understand why the relation holds better some years, it is also interesting to look at the AMM trend. Indeed, a decaying phase of the AMM leads to a manuscript submitted to Journal of Geophysical Research: Oceans salinification north of the equator decreasing the salinity gradient at 1°N, whereas this gradient is enhanced during the build-up phase (Grodsky & Carton, 2018). It will therefore enhance the signal resulting from a positive or negative AMM index.…”

Satellite Salinity Observing System: Recent Discoveries and the Way Forward

“…In general, salinity data from satellite and Argo profiling floats are highly complementary: gridded satellite data have spatial resolutions as fine as a few tens of km on approximately weekly intervals, while the Argo array has a nominal sampling of one float per 3 • ×3 • at 10-day intervals. Thus, combining the two datasets improves detection and characterization of mesoscale features, such as fronts and eddies that are not well captured by Argo alone (e.g., Grodsky et al, 2012;Reul et al, 2014a;Grodsky and Carton, 2018;Kao and Lagerloef, 2018) while mitigating the large-scale biases of satellite SSS. These synergistic products show particular improvement of salinity variability in regions where Argo floats are sparse (Toyoda et al, 2015;Lu et al, 2016) or regions with high variability such as that caused by ocean currents (Chakraborty et al, 2015).…”

Interannual Variability and Multiyear Trends of Sea Surface Salinity in the Amazon‐Orinoco Plume Region From Satellite Observations and an Ocean Reanalysis

“…This suggests a minor influence of river discharge on interannual variability of the Amazon‐Orinoco plume. However, other studies (e.g., Grodsky & Carton, 2018) have pointed out the importance of southern tributaries in conveying NAO effects to the interannual variability of the Amazon system discharge. Since it is still challenging for ocean reanalyzes including SODA to accurately account for the effects of continental runoff due to limited observations, the role of river discharge on the Amazon‐Orinoco plume interannual variability may have been underestimated.…”