Coupled Simulations of a Wintertime Arctic Cyclone with Significant Surface Impacts Observed During MOSAiC

Between Jan 30 and Feb 2, 2020, a surface cyclone developed over Svalbard and moved into the Central Arctic, crossing over the ~50 km X 50 km observational domain of the Multidisciplinary Observatory for the Study of the Arctic Climate (MOSAiC), centered on the German icebreaker R/V Polarstern and located near (87.5N, 96.4E). The minimum measured MSLP was 974.6 hPa, which was the second lowest pressure measured during the year-long MOSAiC campaign. This extensive set of air, ice, and ocean observations sampled this cyclone very well as it passed over. The upper-air structure showed that most of the disturbance and clouds were below 500 hPa. There were clear signatures of a warm front and a warm sector to the system, followed by a cold front aloft and then, a few hours later, a surface cold front. A low-level jet (LLJ) at 250 m height (15 m/s from the SW) was present in the warm sector, which was followed by a second LLJ at 350 m (21 m/s from the N) behind the surface cold front about 12 h later. The sea-ice moved primarily by the atmospheric stress (wind), with the ice motion about 35 degrees to the right of the wind. However, changes in the wind speed and direction, especially between the LLJs and near the frontal passages, appeared to be the cause of rapid changes in ice motion and ice deformation (opening of leads) during these wind transition periods. Furthermore, the ice drag on the upper ocean produced a current signature in the upper-ocean Ekman layer resulting from the passage of this atmospheric cyclone. Finally, the cyclone passage appeared to set off “inertial ringing” in both the sea-ice and the upper ocean, which continued for several days afterwards.

While the observations of this case are interesting by themselves, this presentation will focus on simulations using the Coupled Atmospheric Forecast System (CAFS) developed at NOAA/PSL. CAFS fully couples the air, ice, and ocean with a 9-10 km horizontal resolution and utilizes the WRF atmospheric model, the CICE sea-ice model, and the POP ocean model. The model is initialized with NOAA Global Forecast System atmospheric analyses and satellite-derived sea ice concentrations and sea surface temperatures. The model has been run quasi-operationally since 2015 and is used to support the National Weather Service and observational campaigns. This Feb 1 case provides an excellent opportunity to test the viability of realistically representing air-ice-ocean interaction processes with a coupled model. A 96-h simulation of this case (00Z Jan 30 – 00Z Feb 3) has shown a reasonable ability to represent the key air, ice, and ocean features and interactions, though there are clearly aspects which are not well represented. Aspects of this case that will be examined in more detail include 1) the evolutionary processes leading to this deep surface low but only occupying the lower half of the troposphere; 2) the development of a LLJ (possibly axisymmetric) and its role in modulating sea ice motion and deformation; 3) other possible atmospheric dynamical impacts on the sea-ice; and 4) an attempt at evaluating model coupling aspects in the ice momentum equation, including estimates of the internal ice stress.