Analysis of the Interannual Variability of the Amazon-Orinoco River Plume

Abstract

TheAmazon-Orinoco river plume is a buoyant freshwater lens of 1.2 × 106km2, which has been traced over 2000 km from the Amazon river mouthinto the Caribbean Sea and along the Lesser Antilles. The river plume iswarmer than the surrounding open-ocean waters, with temperaturedifferences up to 1.5 ∘C caused by a stratification-induced barrier layerinhibiting vertical mixing and coloured matter increasing solar energyabsorption. Due to its magnitude, the river plume affects thehydrodynamics and the oceanic conditions in the Western Tropical NorthAtlantic (WTNA) substantially, but its variations on interannual time scales andthe corresponding relation to local sea-surface temperature (SST) are notwell understood. The Caribbean Sea is a region of high ecological value as itis home to extensive coral reefs, which are especially sensitive topersistent high SST. Therefore, this study investigates the interannualvariability of the Amazon-Orinoco river plume and its relationship to SSTsin the Caribbean Sea and the WTNA. It is hypothesised that fresh anomaliesof the river plume salinity pattern are indicative of a more extensivetransportation of the heat contained in the river plume. As a result, itis expected that interannual variations of dominant river plume pathwaysaffect the magnitude and location of anomalous SSTs. To test thishypothesis, model reanalysis fields of oceanic conditions from 1993 to 2017are used to conduct statistical analyses. In this context, the river plumevariability is determined using specific regions of freshwater influenceestablished using Empirical Orthogonal Function (EOF) analysis ofanomalous sea-surface salinity (SSS). Cross-correlations analysis relatingthese EOF modes of with atmospheric processes show that the interannualvariability of the river plume is dominated by wind-inducedadvective transport and -mixing. Strong winds along the Brazilian shelfare related locally increased SSS, while a weak southward component makesfor extensive spreading of the low-salinity plume waters. Additionally, weshow that high river discharge affects SSS east of the Lesser Antillesafter a lag of three months. Through its modulation of these atmosphericprocesses, there is a strong indication that the El Niño-SouthernOscillation affects SSS variability in the main along-shelf northwestplume pathway, with low SSS 1–9 months after a La Niña event. DecreasedSSS are found in phases 2 and 3 of the Madden-Julian Oscillation, whileincreased SSS was observed in phases 6 and 7. However, the evidence forthis relation is weak and should be investigated in further research.
The results show that, opposed to the hypothesis,a more extensive river plume is not associated with higher SSTs in theCaribbean Sea. However, strong correlations are found between river plumesurface area and SSTs at a lag of 1 year. Based upon results of previousstudies, we argue that the river plume has the ability to pre-heat themixed layer in the WTNA leading to extreme temperatures in the followingyear. It is wise to conduct a Lagrangian parcel back-tracking experimentto verify this mechanism.