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

Abstract

This study investigates sea surface salinity (SSS) and sea surface temperature (SST) variations in the tropical Atlantic east of the Lesser Antilles, a region where freshwater advection from the Amazon and Orinoco Rivers, may potentially impact air‐sea interaction. Observations are used to document later‐summer variability and evaluate offshore riverine transport from 2010 to 2014. During this period, the largest difference in plume‐affected areas, defined as the extent covered by SSS lower than 35.5 pss, is found between 2011 and 2014. Plume waters covered 92% of the study region in 2011 and 60% in 2014, with the average SSS in the study region being 2 pss lower in 2011. Lagrangian particle tracking based on satellite‐derived ocean currents is used to diagnose the impact of the river plumes on SSS and SST from 2010 to 2014. Northward freshwater flux in summer 2014 was significantly weaker than fluxes in 2010–2013. This difference is not due to interannual discharge variability, but to significant changes in eddy‐driven transport and cross‐shore winds. In particular, the stronger cross‐shore wind in May 2014 restricted offshore freshwater flow and lead to a smaller plume‐affected area. Persistent SST gradients are often found near the plume edge, which may have implications for ocean‐atmosphere coupling associated with atmospheric convection. SST in the study region was 1°C higher in 2010 compared to other years, and is related to basin‐scale ocean‐atmosphere processes. Interannual variation in Amazon advective pathways and the associated SSS changes are also influenced by changes in the ITCZ position between 2011 and 2014.

Intraseasonal Variability of Surface Salinity in the Eastern Tropical Pacific Associated With Mesoscale Eddies

“…SSS is a key parameter influencing the ocean circulation through its effect on density but is also a tracer of air‐sea freshwater fluxes and ocean dynamics. Satellite missions have revealed that SSS anomalies associated with eddies can be monitored for months near river outflows (Fournier, Vandemark, et al, ; Fournier, Vialard, et al, ) and large anomalies can be seen in the tropical Pacific Ocean following El Niño and La Niña events (Hasson et al, , ). Studies have also shown that remotely sensed SSS in the tropical Pacific Ocean be used to trace mesoscale features such as tropical instability waves (Lee et al, ; Melnichenko et al, ; Yin et al, ) and heavy rainfall associated with large convective cells in the ITCZ (Supply et al, ).…”

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

“…A salinification tendency northeast of the Lesser Antilles (Figure a) may be attributed to wind‐driven Amazon plume contraction in response to strengthening northeasterly trade winds (opposite to the wind anomalies in Figure a). Based on Molleri et al () hypothesis, Fournier et al () have demonstrated that strengthening northeasterly trades (coincident with anomalous cooling of SST in the north tropical Atlantic and southward shift of the ITCZ) significantly contract the spatial dispersion of the Amazon/Orinoco plume by suppressing the export pathway that delivers fresh water into the north subtropical Atlantic northeast of the Lesser Antilles. Missing Amazon fresh water in this area results in apparent contraction of the plume and corresponding up to 0.8 psu (mission long) salinification in the 60 W–50 W, 20 N–30 N sector (Figure a).…”

“…Noticeably, the meridional wind pattern is responsible for stronger/weaker onshore component of the northeasterly trade winds along the coast of northeastern South America during cold/warm north tropical Atlantic SST events, respectively. As shown by Molleri et al (2010) and Fournier et al (2017), the strength of onshore wind component is a factor impacting the spatial dispersion and areal extent of the Amazon/Orinoco plume.…”

“…Although ENSO-induced variations explain about 50% of observed SSS variability in the Amazon plume (Zeng et al, 2008), other factors contribute as well. They include cross-shore winds that modify offshore freshwater dispersal (Molleri et al, 2010) and appear to be related to the tropical Atlantic meridional SST mode (Fournier et al, 2017). It is also probable that interannual variability in regional surface currents not associated with local winds (Grodsky, Reverdin, et al, 2014) may account for the remaining portion of interannual SSS variability.…”

PIRATA: A Sustained Observing System for Tropical Atlantic Climate Research and Forecasting

“…On interannual timescales, there is some evidence that changes in ocean circulation dominate in the western tropical Atlantic (Coles et al, 2013;Foltz et al, 2015;Fournier et al, 2017). However, there are also strong interannual variations in Amazon outflow that can contribute (Grodsky et al, 2014).…”