Influence of Nonseasonal River Discharge on Sea Surface Salinity and Height. (1st February 2022) - Chandanpurkar, Hrishikesh A.
Abstract: River discharge influences ocean dynamics and biogeochemistry. Due to the lack of a systematic, up‐to‐date global measurement network for river discharge, global ocean models typically use seasonal discharge climatology as forcing. This compromises the simulated nonseasonal variation (the deviation from seasonal climatology) of the ocean near river plumes and undermines their usefulness for interdisciplinary research. Recently, a reanalysis‐based daily varying global discharge data set was developed, providing the first opportunity to quantify nonseasonal discharge effects on global ocean models. Here we use this data set to force a global ocean model for the 1992–2017 period. We contrast this experiment with another experiment (with identical atmospheric forcings) forced by seasonal climatology from the same discharge data set to isolate nonseasonal discharge effects, focusing on sea surface salinity (SSS) and sea surface height (SSH). Near major river mouths, nonseasonal discharge causes standard deviations in SSS (SSH) of 1.3–3 practical salinity unit (1–2.7 cm). The inclusion of nonseasonal discharge results in notable improvement of model SSS against satellite SSS near most of the tropical‐to‐midlatitude river mouths and minor improvement of model SSH against satellite or in‐situ SSH near some of the river mouths. SSH changes associated with nonseasonal discharge can be explained by salinity effects on halosteric height and estimated accurately through the associated SSS changes. A recent theory predicting river discharge impact on SSH is found to perform reasonably well overall but underestimates the impact on SSH around the global ocean and has limited skill when applied to rivers near the equator and in the Arctic Ocean. Plain Language Summary: River discharge is important to the physics and biogeochemistry of the ocean. While the effects of seasonal discharge on the ocean have been studied extensively, there is inadequate understanding about the impacts of nonseasonal discharge (the deviation from seasonal climatology) on the ocean. Here we use a daily varying global discharge data set to study the latter effects, focusing on sea surface salinity (SSS) and sea surface height (SSH). We used model sensitivity experiments to isolate the effects of nonseasonal discharge. It is found that nonseasonal discharge has measurable impacts on SSS and SSH near major river mouths. The inclusion of nonseasonal discharge leads to some improvements of the model ocean near major river mouths. We also assessed the skill of a recently proposed theory that predicts the impact of river discharge on SSH using the output from the model sensitivity experiments. The theory is found to predict the discharge‐induced SSH variation relatively well in terms of temporal correlation. However, we identified several limitations in the theoretical prediction of the discharge‐induced SSH variations, including the amplitude of the SSH variation and the applicability for rivers near the equator and in the Arctic Ocean. Key Points: Nonseasonal discharge (deviation from seasonal climatology) has measurable impacts on model salinity and sea level near major rivers Inclusion of nonseasonal discharge leads to notable improvement of model‐observation comparison near major river mouths A recent theory underpredicts the impact of discharge on sea level and has limited skill when applied to equatorial and Arctic rivers