Atmospheric Response to an Open-Ocean Polynya in a High-Resolution Climate Model

Although large open-ocean polynyas in the Southern Ocean have been rare since the mid-70s, they are commonly seen in coupled climate models. Hence, understanding their impact on the ocean and the atmosphere is important to understand model biases. Here we explore the atmospheric response to an open-ocean polynya in the Southern Ocean in a synoptic-scale resolving Community Earth System Model (CESM) simulation. While coarser-resolution versions of CESM generally do not produce open-ocean polynyas in the Southern Ocean, they do emerge and disappear on interannual timescales in the synoptic-scale simulation. This provides an ideal opportunity to study the polynya's impact on the overlying and surrounding atmosphere. This has been pursued here by investigating the seasonal cycle of differences of surface and air-column variables between polynya and non-polynya years. Our results indicate significant local impacts on turbulent heat fluxes, precipitation, cloud characteristics, and radiative fluxes. In particular, we find significant differences in the character of the cloud deck, with much thinner and lower clouds (fog) during non-polynya years. Reduced longwave absorption and emission by these clouds largely counteract a reduction in longwave radiation emitted from the colder sea ice surface. Impacts are found to be sensitive to the synoptic wind direction. Strongest regional impacts are found when northeasterly winds cross the polynya and interact with katabatic winds. Surface air pressure anomalies are only found to be significant when cold, dry air masses strike over the polynya, i.e. in case of southerly winds.

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