Structure of Arctic Cyclones During MOSAiC and Their Surface Impacts

Abstract:

Recent studies of primarily summertime Arctic cyclones (ACs) have revealed a unique structure, where long-lived tropopause polar vortices (TPVs) phase with surface baroclinic zones and rapidly become quasi-barotropic and nearly axisymmetric in nature. Cold lower and warm upper thermal anomalies result in a lowered tropopause, and ACs impact sea ice through both thermodynamic and kinematic processes. Though most studies utilize reanalysis data, the recently completed, year-long MOSAiC field program allow observational examination of the structure and surface impacts of High Arctic ACs during the entire annual cycle (Oct 2019-Oct 2020).

Over 20 cyclones impacted the MOSAiC observational domain. Atmospheric structure was sampled with high-frequency rawinsondes, a cloud radar, a wind profiler radar, and on-ice radiation, turbulence, and basic meteorological measurements. Observations of surface impacts include surface energy budget measurements, precipitation gauges, ice radar, SAR imagery, and an array of GPS buoys. Synoptic-scale spatial structure is provided by validated reanalysis data and satellite imagery. This preliminary work examines several cases. One mid-November case showed many classical structural features, but also features not previously associated with Arctic cyclones. These include a suggestion of topographical vortex-stretching influences from Greenland, and a low-level jet structure to the quasi-axisymmetric near-surface winds. Changes in direction of the strong surface winds produced ice fracturing, which later led to significant shearing with the passage of a subsequent weaker cyclone. Thermodynamic impacts on the sea ice resulted from dramatic changes in turbulent heat fluxes and longwave radiation, resulting in a brief reversal of the typical heat loss from the sea ice to the atmosphere. During the winter and following spring, numerous additional cyclones, including ones with much lower central pressure than the November cyclone, passed near the MOSAiC site. Many showed the LLJ structure, some produced thermodynamic impacts on the sea ice, and others kinematic impacts. This presentation will examine the structure of a number of these cyclones and their surface impacts. If time permits, cyclone structure from a coupled model will also be compared to the observations.