Modelling Phytoplankton Blooms and the Resulting Biophysical Feedbacks within an Antarctic Coastal Polynya.

Recent studies have offered contrasting conclusions as to the primary physical driver of high net primary productivity (NPP) in coastal polynyas around Antarctica. Arrigo et al. (2015) show a correlation with ice shelf melt rate and thus supply of iron. Meanwhile Park et al. (2017), in comparing two adjacent polynyas, attribute the difference in their productivity to cloud cover and the resulting differences in surface irradiance. Determining the controls on NPP in these polynyas is important as collectively they feedback significantly onto global climate via air-sea exchanges of carbon. However the presence of large quantities of chlorophyll at and near the ocean surface during the spring bloom also has the potential to have more localised, shorter time-scale effects on the cryosphere. In this work the Biology Light Iron Nutrients and Gases model (BLING) (Galbraith et al. 2010) and MITgcm are used in an idealised domain to model the physics and biology of the Amundsen Sea Polynya, West Antarctica. A slab ice shelf with melting forced by ocean temperature is used to drive an overturning circulation (meltwater pump) and to supply the polynya with iron; light limitation is controlled primarily by the seasonal cycle in surface irradiance and by the modelled sea ice distribution. BLING is modified by adding an online chlorophyll-dependence into the attenuation coefficients in each grid cell (Manizza et al. 2005), with a decrease in annual NPP observed due to self-shading of photosynthetically available radiation (PAR). Then a new subroutine in MITgcm is implemented to allow this dynamic light field to alter the distribution of heating in the upper ocean. Thus for the first time biogeochemical processes in BLING feed back onto the physics of MITgcm, with the potential to impact both sea ice extent and the supply of freshwater from the ice shelf to the polynya.