Effects of Model Resolution and Ocean Mixing on Forced Ice‐Ocean Physical and Biogeochemical Simulations Using Global and Regional System Models

Abstract

The current coarse‐resolution global Community Earth System Model (CESM) can reproduce major and large‐scale patterns but is still missing some key biogeochemical features in the Arctic Ocean, e.g., low surface nutrients in the Canada Basin. We incorporated the CESM Version 1 ocean biogeochemical code into the Regional Arctic System Model (RASM) and coupled it with a sea‐ice algal module to investigate model limitations. Four ice‐ocean hindcast cases are compared with various observations: two in a global 1° (40∼60 km in the Arctic) grid: G1deg and G1deg‐OLD with/without new sea‐ice processes incorporated; two on RASM's 1/12° (∼9 km) grid R9km and R9km‐NB with/without a subgrid scale brine rejection parameterization which improves ocean vertical mixing under sea ice. Higher‐resolution and new sea‐ice processes contributed to lower model errors in sea‐ice extent, ice thickness, and ice algae. In the Bering Sea shelf, only higher resolution contributed to lower model errors in salinity, nitrate (NO3), and chlorophyll‐a (Chl‐a). In the Arctic Basin, model errors in mixed layer depth (MLD) were reduced 36% by brine rejection parameterization, 20% by new sea‐ice processes, and 6% by higher resolution. The NO3 concentration biases were caused by both MLD bias and coarse resolution, because of excessive horizontal mixing of high NO3 from the Chukchi Sea into the Canada Basin in coarse resolution models. R9km showed improvements over G1deg on NO3, but not on Chl‐a, likely due to light limitation under snow and ice cover in the Arctic Basin.

Multi‐Model Intercomparison of the Pan‐Arctic Ice‐Algal Productivity on Seasonal, Interannual, and Decadal Timescales

“…It is well known that anti‐cyclonic wind associated with the Beaufort High accumulates oligotrophic fresher water inside the central Canada Basin (McLaughlin & Carmack, ; Proshutinsky et al, ). However, as described in section and Jin et al (), the Arctic grid size of 18–62 km in the UAF‐G model would be insufficient to keep steep isopyncal front along the shelf‐basin boundary, in contrast to the UAF‐R model. According to the WOA13 annual climatology, nitrate concentrations higher than 5 mmol N m −3 are seen in the Chukchi Sea and the Nordic seas (Figure c).…”

“…The UAF simulations are based on a common marine biogeochemical module (Jin, Deal, et al, ; Moore et al, ), which is incorporated into the global Community Earth System Model (CESM) (Moore et al, ) and the Regional Arctic System Model (RASM) (http://www.oc.nps.edu/NAME/RASM_PhaseIII.html), respectively. The detailed model configuration and experimental design of both the global and regional frameworks were introduced in Jin et al (). The sea‐ice ecosystem consists of ice algae and three nutrients (nitrate, ammonium, and silicate) (Jin et al, ).…”

“…The nitrate value widely varies among the five models and among the GLODAPv2 station data, respectively. The higher‐resolution models tend to retain the bowl‐shaped structure of the nutricline in the Beaufort Gyre region (Jin et al, ). A distinct inter‐model spread appears in the simulated snow depth even when the common atmospheric forcing data set is used (Table : CFSR for the JAMSTEC and UW experiments, CORE II for the UAF‐G and UAF‐R experiments).…”

CSIB v1: a sea-ice biogeochemical model for the NEMO community ocean modelling framework

“…In contrast, the nutrient control on ice algal production is less pronounced; although high ice algal GPP usually concides with high surface seawater nitrate, it is also present in a few areas where the nitrate levels are relatively low (Baffin Bay and Chukchi Sea; Figure 8b). Overall, ice algal production is mostly confined to shelf regions (water depth <100 m; Figure 3), consistent with previous model studies (Deal et al, 2011;Dupont, 2012;Jin et al, 2012Jin et al, , 2018 There are a few noteworthy similarities and differences in the spatial variability in modelled ice algal annual production between the present study and previous model studies. All studies show a moderate-to-high level of ice algal production in Baffin Bay.…”

“…A possible explanation for the lower ice algal production in this region in the latter studies is due to an insufficient nutrient supply from the Pacific boundary as discussed in Dupont (2012). Lastly, the recent study by Jin et al (2018) finds the Sea of Okhotsk to be a region of elevated 25 ice algal annual production, which we are unable to assess in the present study due to the limited model domain.…”