Loading of coloured dissolved organic matter in the Arctic Mediterranean Sea and its effects on the ocean heat budget

Abstract

Currently, the most rapid increase in near-surface air temperature takes place in the Arctic, accompanied by a decline in sea ice cover. Consequently, the underwater shortwave radiation, and thus, the type and amount of phytoplankton are changing. In this context, the thawing permafrost, accompanied by increased precipitation and freshwater discharge, is expected to result in higher loads of coloured dissolved organic matter (CDOM) and total suspended matter (TSM) entering the Arctic Ocean. The amount of these optically active water constituents determines how much light is absorbed in the surface waters and how much can reach greater depths, affecting the vertical distribution of heat.

In this thesis, I first examine the potential of CDOM and TSM in enhancing the radiative heating and sea ice melting in the shelf waters of the Laptev Sea, an area heavily influenced by one of the largest river systems in the Arctic region. By using in situ observations, I simulate the in-water radiative heating utilizing coupled atmosphere-ocean radiative transfer modelling (RTM). The results indicate that CDOM and TSM highly affect the energy budget of the Laptev Sea shelf waters, absorbing most of the solar energy in the first 2 meters of the water column. The increased absorbed energy leads to higher sea ice melt rates and changes in the heat exchange with the atmosphere. By using satellite remote sensing and RTM, I quantify the spatial distribution of radiative heating in the Laptev Sea for a typical summer day. The spatial patterns of radiative heating closely follow the distribution of the optically active water constituents, with the highest energy absorption occurring over river-influenced waters.

As a next step, I upscale the previous one-dimensional and regional study by means of general circulation modelling for the entire Arctic Mediterranean Sea. By operating an ocean biogeochemical model coupled to a general circulation model with sea ice (Darwin-MITgcm), the effect of phytoplankton and CDOM is incorporated into the in-water shortwave radiation penetration scheme. Accounting for their radiative effect increases the sea surface temperature (SST) in summer, decreases the sea ice concentration, and induces more heat loss to the atmosphere, primarily through sensible and latent heat flux. In some parts of the Eastern Arctic, the sea ice season is reduced by up to one month. CDOM drives 48% of the summertime changes in SST, suggesting that an increase in its concentration will amplify the observed Arctic surface warming. Additionally, the CDOM effect alters the vertical diffusion, advection, and non-local vertical mixing of heat. The shortwave heating and vertical diffusion terms account for a large part of the Arctic-wide changes in the heat budget throughout the year. On the contrary, in the Atlantic sector, differences in the sub-surface heating can be largely determined by advective and non-local mixing processes in spring and winter. In the Norwegian Sea, the sub-surface wintertime indirect dynamical effect is 2.7 times larger than the effect of shortwave heating. These results underline the potential of indirect changes in advective and mixing processes in intensifying or dumping the direct effect of CDOM at the sub-surface.

The changes induced by CDOM feed back on phytoplankton and CDOM itself, leading to higher annual mean surface concentrations for both of them. On the contrary, phytoplankton reduces at the sub-surface resulting in a 16.6% overall biomass decrease in the upper 100 m. The areas where light limits phytoplankton growth, expand at the expense of nutrient limitation. In spring, reduced light availability causes a phytoplankton bloom delay and an increase in nutrient concentrations. However, in summer the excess of nutrients together with the light limitation confine phytoplankton growth in a few tens of meters from the ocean surface leading to an intensification and delay of the end of the bloom, especially at the Barents Sea. These findings indicate that a future increase of CDOM will ignite a secondary positive feedback mechanism on the Arctic’s surface warming, through increased phytoplankton and CDOM light absorption close to the surface.

Annotation: The Arctic Mediterranean Sea comprises the Arctic Ocean and the Greenland, Iceland and Norwegian (the Nordic) Seas.