Barrier Layers of the Atlantic Warm Pool: Formation Mechanism and Influence on Weather and Climate

The aim of this research is to study the formation mechanism of Barrier Layers (BL) in the western tropical Atlantic and their influence on the tropical Atlantic climate at both short and long timescales. Many Coupled General Circulation Models (CGCMs) tend to overestimate the salinity in the Atlantic warm pool or the Northwestern Tropical Atlantic (NWTA) and underestimate the surface salinity in the subtropical salinity maxima region. Most of these models also suffer from a seasurface temperature (SST) bias in the NWTA region, leading to suggestions that the upper ocean salinity stratification may need to be improved in order to improve the BL simulations and thus the SST through BL-SST-Intertropical Convergence Zone (ITCZ) feedbacks. We used a CGCM to perform a set of idealized numerical experiments to understand the sensitivity of the BL and consequently SST in the NWTA region to freshwater flux and hence the upper ocean salinity strati cation. We find that the BL of the western tropical Atlantic is quite sensitive to upper ocean salinity changes in the Amazon River discharge region and the subtropical salinity maxima region. The BL phenomenon is further manifested by the formation of winter temperature inversions in our model simulations. However, in the region of improved BL simulation, the SST response is not statistically significant. SST response to Tropical Cyclones (TCs) is studied for the Atlantic region using a high-resolution coupled regional climate model (CRCM) and observational data sets. The presence of a BL, defined as the layer below the mixed layer that separates the base of the isothermal layer from the base of the isohaline layer, is found to modulate the SST response. The amplitude of TC-induced surface cooling is reduced by more than 35 percent in the presence of a BL, as a consequence of the weak thermal stratification. Furthermore, in locations when the BL exhibits a temperature inversion, TC-induced mixing can result in weak surface warming. BLs considerably reduce the rightward bias for tropical storms, but the effect is less conspicuous for TCs. The enthalpy flux into the atmosphere at the air-sea interface is enhanced by 16 percent and the increase in upper ocean potential energy due to TC-induced mixing is reduced by 25 percent in the presence of BLs. The results from the coupled model are supported by an observational analysis performed using re-analysis data sets, as well as data from Argo floats and TRMM satellite. As previous modeling and observational studies have indicated that the surface cooling caused by TC-induced mixing acts as a negative feedback for its intensity, results from our study suggest that BLs may have potential implications for TC intensity prediction.