From the Mountains to the Penguins in the Deep Blue Sea: Importance of Atmospheric Forcing Resolution to the Simulation of the Ocean for a Biological Hotspot off the Antarctic Peninsula

Many biological hotspots, which are critical to the marine ecosystem structure and function along the Western Antarctic Peninsula (WAP), are located near submarine canyons. A project (SWARM) is currently underway to study how the circulation dynamics associated with the Palmer Deep canyon near Anvers Island contribute to a persistent hotspot above the canyon, as shown by known penguin foraging locations. An extensive observing campaign was carried out during summer 2019/2020 to observe the ocean physical and biological characteristics of the area. As part of this, an existing high resolution (1.5 km) ocean circulation model was modified to run simulations covering the field season.

In order to force the model, two atmospheric products were selected that covered the spatial extent of the model domain: the ERA5 reanalysis (~ 30 km horizontal resolution) and archived forecasts from the Antarctic Mesoscale Prediction System (AMPS: Grid “2” at 8 km horizontal resolution). Wind observations were available from two AWS stations situated on islands ~21 km apart that bracket the northern end of the canyon. Because of the orography of Anvers Island to the northeast (which is the direction the prevailing winds come from), there is often a significant difference in the winds at the two AWS stations.

When compared to the AWS stations over summer 2019/2020, both wind products represent the daily averaged variability quite well. However, the ERA5 winds, as would be expected because of the resolution differences, were significantly weaker over the canyon than the AMPS winds. The AMPS winds also were able to represent some of the temporal variability in the difference in wind speed between the two AWS stations.

Ocean model simulations in this area often suffer from having too shallow surface mixed layers during the summer. Properly representing the mixed layer depth can be critical to simulating ocean primary productivity in the area, which will be important to the ecosystem. Here, simulations were run with both wind products, as well as changing the temporal frequency of the ERA5 winds used to force the ocean, and modifying the vertical mixing parameterization. Changing the wind frequency or the vertical mixing parameterization had little effect on the mean simulated summer mixed layer depth over Palmer Deep (mean increase of ~ 1 m). However, switching the winds from ERA5 to AMPS deepened the summer mixed layer by ~ 11 m. We plan to use this ocean model to explore several biological hot spots along the WAP, and as many of these are located in near-coastal environments, the higher resolution AMPS winds should be quite helpful.