Abstract
Abstract. In this study, we improve the representation of global river runoff in the Estimating the Circulation and Climate of the Ocean Version 4 (ECCOv4) framework, allowing for a more realistic treatment of coastal plume dynamics. We use a suite of experiments to explore the sensitivity of coastal plume regions to runoff forcing, model grid resolution, and grid type. The results show that simulated sea surface salinity (SSS) is reduced as the model grid resolution increases. Compared to Soil Moisture Active Passive (SMAP) observations, simulated SSS is closest to SMAP when using daily, point-source runoff (DPR) and the intermediate-resolution LLC270 grid. The Willmott skill score, which quantifies agreement between models and SMAP, yields up to 0.92 for large rivers such as the Amazon. There was no major difference in SSS for tropical and temperate coastal rivers when the model grid type was changed from the ECCO v4 latitude–longitude–polar-cap grid to the ECCO2 cube–sphere grid. We also found that using DPR forcing and increasing model resolution from the coarse-resolution LLC90 grid to the intermediate-resolution LLC270 grid elevated the river plume area, volume, stabilized the stratification and shoal the mixed layer depth (MLD). Additionally, we find that the impacts of increasing model resolution from the intermediate-resolution LLC270 grid to the high-resolution LLC540 grid are regionally dependent. The Mississippi River Plume is more sensitive than other regions, possibly because the wider and shallower Texas–Louisiana shelf drives a stronger baroclinic effect, as well as relatively weak sub-grid vertical mixing and adjustment in this region. Since rivers deliver large amounts of freshwater and anthropogenic materials to coastal regions, improving the representation of river runoff in global, high-resolution models will advance studies of coastal hypoxia, carbon cycling, and regional weather and climate and will ultimately help to predict land–ocean–atmospheric feedbacks seamlessly in the next generation of Earth system models.
Highlights
Coastal plume regions represent a small fraction of Earth’s surface and are an active component in global cycling of carbon and nutrients
We find that DPR greatly improves model representation of global rivers, with horizontal model resolution having a substantial control on sea surface salinity (SSS) in the vicinity of river mouths
A comparison with synchronized Soil Moisture Active Passive (SMAP) observations shows that the use of DPR forcing and intermediate grid resolution can increase the model performance in simulating SSS in the vicinity of river mouths
Summary
Coastal plume regions represent a small fraction of Earth’s surface and are an active component in global cycling of carbon and nutrients Recent satellite-based observations with quasi-global coverage have been greatly improved to monitor sea surface salinity (SSS), a key tracer for tracking the river plumes. The coastal plume regions have not been explicitly resolved in most global ocean general circulation models (OGCMs), Earth system models (ESMs), and Global Ocean Data Assimilation System (GODAS) products (Ward et al, 2020). The plume region produced by OGCMs, ESMs, and GODAS are not consistent with satellite observations. Riverine freshwater input to the ocean is forced in the top model layer over a pre-determined surface area in the vicinity of river mouths with climatological signal; the system disturbance by extreme weather events, e.g., floods and droughts, cannot been explicitly resolved (Griffies et al, 2005; Tseng et al, 2016). The above model configurations may limit the representation of coastal plume regions in global-scale models
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