Seasonal cycle of near‐surface freshwater budget in the western tropical Atlantic

Abstract

We investigate differences of the ocean response in the Amazon domain to the seasonal variability of the river discharge that are either introduced via assimilating climatological temperature and salinity or by specifying seasonally varying river runoff. The role of the seasonal cycle of the Amazon freshwater discharge for the evolution of the barrier layer (BL) in the western tropical Atlantic and on the freshwater budget is estimated. During the experiments, three different runoff fields are being applied, including a time‐mean runoff, a seasonally varying runoff, and one that results from the GECCO assimilation approach. The simulation forced with a seasonal Amazon discharge appears to be closer to the constrained solution and moves away from the run with a constant runoff, demonstrating that the seasonal variability of the Amazon is an essential contributor in the freshwater forcing of the western tropical Atlantic. The modeled time‐mean BL thickness seems to be overestimated by the model relative to the data. On the seasonal timescale, the simulated spatial mean BL is found to vary between 13 and 30 m, with a maximum occurring in July, following the Amazon high discharge period in May. Analyzing the freshwater content balance, we find integrated near‐surface freshwater import from the western tropical Atlantic interior of around 0.20 Sv in October–November at 38°W and cumulative freshwater export out of the domain with a maximum of around 0.4 Sv in June as an effect of the Amazon flood in May.