Studying the Impact of Oceanic Barrier Layer on Hurricane Activity over the Amazon-Orinoco Plume Region Using the Community Regional Earth System Model

Hurricanes are one of the important climate extremes in the tropical Americas. The role of oceanic sea surface temperature (SST) on hurricane activity is widely known, but the role of oceanic salinity is less well-known. An important factor affecting oceanic salinity in this region is the Amazon river discharge, which spreads towards Northwest Tropical Atlantic and the Caribbean, combining with Orinoco discharge to form the Amazon–Orinoco River plume in one of the most hurricane active regions. The lower density of the freshwater from the river discharge stabilizes the vertical density gradient of the ocean, often leading to the formation a fresher and shallower upper ocean layer known as the “barrier layer” which weakens turbulent vertical mixing at the mixed layer base. From 1960 to 2000, 68% of category-5 hurricanes in the Caribbean passed over this plume region, thus potentially interacting with the barrier layer.

Within the Amazon-Orinoco plume region, the sea surface temperature (SST) can be up to 3°C warmer than surroundings. This pre-existing high SST provides more energy for hurricanes to intensify as they pass over the region. Besides the warmer SST effect, the inhibition of vertical mixing can also play an important role. Normally, the primary impact of the passage of a hurricane over the ocean is a dramatic increase in the vertical mixing triggered by the strong surface winds. This entrains the cooler waters below the ocean surface mixed layer into the warmer waters above, and leading to a significant cooling of the SST, which can inhibit further hurricane intensification. The stable barrier layer in the plume area could effectively reduce or prevent surface cooling through weakening the vertical mixing, causing the warm SST conditions to persist and thus intensifying hurricanes passing over the plume. Our study will not only address role of high pre-existing SST before the arrival of the hurricane, but also consider the attenuation of hurricane-induced cooling on the ocean and its impact on hurricane intensification. By separating and comparing these two effects, we can have clearer understanding of how barrier layers can affect the evolution of hurricanes.

The Community Regional Earth System Model (CRESM) developed jointly by TAMU, NCAR and OUC will be used for this study. The CRESM has three components: atmospheric model (Advanced Research Weather Research and Forecasting, WRF-ARW, model), ocean model (Regional Ocean Modeling System, ROMS), and land component (Community Land Model, CLM). We select ten hurricanes, the tracks of which pass over Amazon-Orinoco Plume area. We conduct CRESM control experiments for each hurricane, to simulate the hurricane track and intensity. Then, we carry out a set of perturbation experiments where we alter the plume properties and assess its impact on hurricane properties. In order to distinguish the effects of the inhibition of vertical mixing from that of pre-existing high SST, we alter the ocean vertical structure by eliminating the effect of the Barrier Layer, while keeping the SST the same. By comparing the hurricane intensity, SST, and mixed layer depth in the control and perturbation experiments, we assess the thermodynamic and dynamic impacts of the barrier layer on hurricane activity in the Caribbean region.