Interannual Caribbean salinity in satellite data and model simulations

Abstract

Aquarius sea surface salinity (SSS) reveals the presence of interannual variations in the Caribbean with about 0.5 psu change between salty and fresh events, which propagate westward across that basin at an average speed of 11 cm/s and are preceded by corresponding SSS anomalies east of the Lesser Antilles. These upstream SSS anomalies are produced by interannual changes in the Amazon plume. Their presence is verified using in situ measurements from the northwest tropical Atlantic station. In contrast to SSS, which displays westward propagation, SST changes almost immediately across the Caribbean, suggesting large-scale atmospheric processes have a primary role in regulating interannual SST in contrast to SSS. A global 1/10 mesoscale ocean model is used to quantify possible origination mechanisms of the Caribbean salinity anomalies and their fate. Simulations confirm that they are produced by anomalous horizontal salt advection, which conveys these salinity anomalies from an area east of the Lesser Antilles across the Caribbean. Anomalous horizontal advection is dominated by mean currents acting on anomalous salinity. The model suggests that interannual Caribbean salinity anomalies eventually enter the Florida Current and reach the Gulf Stream 6-12 months after crossing the central Caribbean. Previous studies link the origin of salinity anomalies in the Amazon plume to variations in the annual freshwater discharge from the continent. In this model interannual discharge variations are absent while simulated SSS variability is in line with observations. This suggests that interannually forced ocean dynamics plays a key role in river plume variability and its spatial dispersion.

Interannual Variation in Offshore Advection of Amazon‐Orinoco Plume Waters: Observations, Forcing Mechanisms, and Impacts

“…Results do not indicate that August NPR salinity is measurably controlled by interannual spring‐summer Amazon discharge variations, a result that is consistent with previous studies (Grodsky et al, ; Masson & Delecluse, ). In contrast, Lagrangian back trajectory analyses (see Figures , and ) indicate that variable horizontal advection drives an order of magnitude difference between Amazon freshwater flux to the NPR in 2010–2011 versus 2013–2014.…”

“…Rugg et al (2016) present a 30 year study of AMM anomalies that highlights similarly strong upper-ocean contrasts between the positive phase years of 2010-2011 and the strong negative AMM seen in 2009 and 2014-2015. Results in the present study may indicate that these variable feedbacks also extend to interannual change in Amazon/Orinoco plume dispersion in the April-August timeframe, a result consistent with that surmised in the Caribbean basin salinity study of Grodsky et al (2015).…”

Freshwater exchanges and surface salinity in the Colombian basin, Caribbean Sea

“…Surface currents in the upper 50-to-100 m layer transport Caribbean Surface Water (CSW), which is formed by dilution of Atlantic Water by freshwater discharge mainly from the Amazon and Orinoco Rivers as it is transported into the Caribbean Sea by the Guiana Current [ 5 – 7 ]. Recently, Grodsky et al [ 8 ] investigated the propagation of interannual changes in the Amazon plume as salinity anomalies through the central Caribbean on into the Gulf Stream system. CSW is transported by the Caribbean Current, which accelerates toward the Yucatan Channel, and has an average speed greater than 25 cm s -1 [ 9 – 11 ].…”

Amazon Plume Salinity Response to Ocean Teleconnections

“…The Amazon River is by far the largest single source of terrestrial freshwater to the ocean, contributing ∼20-30% of the total river discharge to the Atlantic (Wisser et al, 2010;Salisbury et al, 2011;Korosov et al, 2015). SSS in the western tropical North Atlantic (WTNA) is strongly influenced by the Amazon River discharge, advection by surface currents, and rainfall (Grodsky et al, 2015;Ibánhez et al, 2017). The low salinity Amazon plume creates a nearsurface barrier layer that inhibits mixing, increases the sea surface temperature (SST), and enhances salinity stratification; thus, preventing vertical mixing between the upper warm mixed layer and the cold deep ocean (Ferry and Reverdin, 2004;Balaguru et al, 2012;Coles et al, 2013).…”