Using Observational In situ Argo Float data to analyze Amazon-Orinoco plume structure and its impact on Atlantic hurricane activity

The Amazon has the largest river discharge in the world, which reaches its maximum in May each year and then spreads northward towards Caribbean Sea through the whole hurricane season. The Orinoco is another major river discharge with a maximum in August each year. It combines with Amazon discharge to form the Amazon-Orinoco Plume, which is characterized by lighter freshwater in the upper ocean in the Northwestern Atlantic region, surrounded by more saline water. Previous research has indicated that the strong vertical density gradient in the plume will stabilize the upper ocean, and can lead to intensification of hurricanes passing over it. The lighter freshwater favors the formation a barrier layer in the upper ocean, which inhibits the cooler deep ocean water from being mixed upwards due to hurricane-induced vertical stirring.

However, our statistical analysis of hurricane tracks from 1960 to 2017 shows that major hurricane intensification happens along the climatological Amazon-Orinoco plume boundary, rather than occurring over the entire region of the plume as has been previously suggested. To better understand the plume vertical structure and the formation of barrier layers, we analyzed in situ Argo float profiles from 2000 to 2019. Our analysis suggests that in the summer, unlike in the winter, temperature inversion occurrences are concentrated along the plume boundary. Barrier layer thickness values derived from the Argo profiles also show higher values near the plume boundary, where there are strong horizontal and vertical salinity gradients. Temperature inversions and barrier layers can lead to intensification of hurricanes passing above them through ocean-atmosphere interaction. Therefore, we suggest that attention must be paid to the Amazon-Orinoco plume boundary to better understand and predict hurricane activity in this region.