The barrier layer and ocean colour in the Amazon and Orinoco plume: Competing for the oceanic control on tropical cyclone intensity

Abstract

The Amazon and the Orinoco river plumes modulate ocean stratification and colour in the tropical North Atlantic. This changes air-sea interactions and may thus be important for tropical cyclones (TCs). Using a regional ocean model, the potential impact of the rivers on ocean temperatures, stability, and TC intensity is investigated. The influence of riverine freshwater on the ocean is twofold: Firstly the freshwater plume stabilizes the water column, and secondly ocean colour in the plume modifies solar transmission. Within the Amazon and Orinoco plume, the two mechanisms have opposing and effective cancelling effects on TCs. On the one hand the freshwater plume thickens the barrier layer by up to 15 m locally, leading to increased stability and temperature inversions. The cooling inhibition index (CI) is +2.2 (J/m2) 1/3 larger when the river freshwater is present, potentially reducing SST feedbacks. Surface temperatures and upper ocean heat content are only slightly modified by the rivers. Ocean colour in the freshwater plume on the other hand, blocks the deeper ocean from sunlight, leading to moderate surface warming (+0.1o C) and substantial sub- surface cooling (-0.3o C 100m mean temperature). As a consequence cold water is more readily available to passing storms and the CI decreases by -2.1 (J/m2) 1/3 , hence increasing negative SST feedbacks. The net effect of the coloured plume on SST cooling is thus negligible. Simple, idealized relationships between expected surface cooling and TC intensity, suggest that the river-induced barrier layer enhances strong TCs by up to -5 to -12 hPa, while ocean colour may reduce intensity of strong storms by +8 hPa to +16 hPa. The net impact of the coloured plume is negligible for weak storms and a slight intensity reduction for stronger cyclones. Within the Amazon and Orinoco plume, the river freshwater effect may thus be substantially reduced or even offset by light absorbing particles.