Circulation in the Arctic Ocean: Results from a high-resolution coupled ice-sea nested Global-FVCOM and Arctic-FVCOM system

Abstract

47 48 A high-resolution unstructured-grid global-regional nested ice-current coupled 49 FVCOM system was configured for the Arctic Ocean and used to examine the impact of 50 model resolution and geometrical fitting on the basin-coastal scale circulation and 51 transport in the pan-Arctic. With resolving steep bottom slope and irregular coastal 52 geometry, the model was capable of simulating the multi-scale circulation and its spatial 53 variability in the Arctic Basin and flow through the Bering Strait, Fram Strait and 54 Canadian Archipelago. The model-simulated annual-mean velocities were in good 55 agreement with observations within the measurement uncertainty and variability due to 56 insufficient sampling. The errors in the flow direction varied with the flow speed, larger 57 in the weak velocity zone and smaller as the velocity increased. In the upper 50-m layer, 58 the annual-mean circulation pattern was dominated by the wind-and ice-drifting-induced 59 anticyclonic circulation in the Arctic Basin and a relatively strong cyclonic slope current 60 along the edge of the continental shelf. In the deep 200-600-m layer, a relatively 61 permanent cyclonic circulation occurred along the steep bottom slope. These annual-62 mean circulations accounted for ~85% of the total kinetic energy variance. De-trending 63 the mean flow, an empirical orthogonal function (EOF) analysis showed that the semi-64 annual and seasonal variability of the sub-tidal flow was dominated by the first and 65 second modes that accounted for ~46% and ~30% of the total variance in the upper 50-m 66 layer and ~58% and 20% in the deep 200-600-m layer. Consistent with observations, the 67 AO-FVCOM-simulated cyclonic slope flow was characterized by a large positive 68 topostrophy. Sensitivity experiment results with various grid configurations suggested 69 that the currents over slopes, narrow straits and water passages featured topographic and 70 3 baroclinic frontal dynamical scales associated with bathymetric slope and internal Rossby 71 deformation radius. Over the Arctic slope, since these two scales are in the same order, 72 the along-slope current could be captured, as the cross-isobath model resolution was 73 refined to resolve the steep bottom topography. Under this condition, there is no need to 74 add Neptune forcing into the momentum equations. The accuracy of the estimation of the 75 transport through the strait and narrow water passage was affected by the model 76 resolution. In Fram Strait where the flow is characterized by strong lateral current shear 77 resulting from the Atlantic inflow and Arctic outflow, the transport estimation could have 78 a significant uncertainty due to both horizontal and vertical sampling resolutions.

Seasonal and interannual variability of the Arctic sea ice: A comparison between AO-FVCOM and observations

“…The AO‐FVCOM is an integrated Arctic‐global ice‐sea coupled ocean model system configured with the version 3.1 source code of FVCOM [ Chen et al ., ]. The FVCOM is a prognostic, unstructured‐grid, Finite‐Volume, free‐surface, 3‐D primitive equation Community Ocean Model [ Chen et al ., ].…”

“…The AO‐FVCOM was configured with a nonoverlapped triangular grid with a horizontal resolution varying from 2 to 40 km (Figure ). The grid was designed to better resolve topographic and baroclinic dynamical scales over the Arctic slope and narrow straits in the CAA [ Chen et al ., ], and the triangles were created by following the criteria to make sure the areas of adjacent meshes satisfied a ratio that could avoid the discontinuity and stability in numerical performance.…”

“…Examples are shown in Figure for the comparison of the spatial distribution of seasonally averaged model‐simulated and observed sea ice drift velocities. We first interpolated the model‐simulated sea ice drift velocity onto the locations where the observational data were available, and then rescaled both model‐simulated and observed data using the root‐square vector scale [ Chen et al ., ] and resampled them with 80 km resolution to make vectors in the figure visible. In the spring, the observed sea ice featured an anticyclonic drift, with a center in the Beaufort Sea and seasonally averaged maximum velocity of ∼5 cm/s.…”

“…Chen et al . [] made a comparison between the AO‐FVCOM‐simulated and observed water velocities in the Arctic Ocean, and also found that the direction error was larger in the weak velocity zone, which was consistent with measurement uncertainties: a small error in the weak flow regime could lead to a large error in the flow direction. It was believed that their finding was also applied to the sea ice drift velocity.…”

“…Availability of the long‐term sea ice data in the past decades provides us such an opportunity to conduct a more comprehensive model‐data comparison experiment with inclusion of all types of sea ice properties such as sea ice extent, concentration, thickness, and drift velocity. A high‐resolution, fully coupled sea ice‐ocean, Arctic‐global nested model was developed based on the three‐dimensional unstructured‐grid, Finite‐Volume Community Ocean Model (hereafter referred to as AO‐FVCOM) [ Chen et al ., ; Gao et al ., ]. The sea ice model implemented in the AO‐FVCOM is UG‐CICE‐the Unstructured Grid version of the Los Alamos Community Ice Code (CICE) developed by Gao et al .…”