On deep convection events and Antarctic Bottom Water formation in ocean reanalysis products

Abstract

Abstract. Open ocean deep convection is a common source of error in the representation of Antarctic Bottom Water (AABW) formation in ocean general circulation models. Although those events are well described in non-assimilatory ocean simulations, the recent appearance of a massive open ocean polynya in the Estimating the Circulation and Climate of the Ocean Phase II reanalysis product (ECCO2) raises questions on which mechanisms are responsible for those spurious events and whether they are also present in other state-of-the-art assimilatory reanalysis products. To investigate this issue, we evaluate how three recently released high-resolution ocean reanalysis products form AABW in their simulations. We found that two of the products create AABW by open ocean deep convection events in the Weddell Sea that are triggered by the interaction of sea ice with the Warm Deep Water, which shows that the assimilation of sea ice is not enough to avoid the appearance of open ocean polynyas. The third reanalysis, My Ocean University Reading UR025.4, creates AABW using a rather dynamically accurate mechanism. The UR025.4 product depicts both continental shelf convection and the export of Dense Shelf Water to the open ocean. Although the accuracy of the AABW formation in this reanalysis product represents an advancement in the representation of the Southern Ocean dynamics, the differences between the real and simulated processes suggest that substantial improvements in the ocean reanalysis products are still needed to accurately represent AABW formation.

A novel hydrographic gridded data set for the northern Antarctic Peninsula

“…High concentrations of phytoplankton have been documented in the Bransfield and Gerlache straits, with implications for local carbon dioxide uptake (Costa et al, 2020;Monteiro et al, 2020a, b). The high primary production also supports an abundant krill stock (Atkinson et al, 2020), which is the base of the food chain up to top predators (Seyboth et al, 2018). If the changes reported for the region continue (e.g.…”

“…sea ice cover and seasonality declining, land ice melting, atmospheric and oceanic temperature warming), an imbalance or even a collapse in the regional food chain is expected (Ferreira et al, 2020). A change in the composition of phytoplankton, from diatoms to cryptophytes, has already been observed in the NAP (Mendes et al, 2013) as well as a reduction in the recruitment of juvenile krill and a general decline of krill stocks in areas of the Atlantic sector of the Southern Ocean, including the NAP (Atkinson et al, 2020).…”

Southern Ocean polynyas in CMIP6 models

“…Polynya detection in observational data (e.g. Markus and Burns, 1995;Kern et al, 2007;Ohshima et al, 2016) and ocean reanalysis products (Aguiar et al, 2017) and the role of OWPs in spurious deep water formation in CMIP5 (Heuzé, 2015) have been discussed before. The formation of OWPs in models has been shown to be very sensitive to vertical mixing parameters (e.g.…”

Impacts of Ice-Shelf Melting on Water-Mass Transformation in the Southern Ocean from E3SM Simulations

“…Meanwhile, there is no AABW formation in either the Ctrl or ISM simulations and transformation rates of LCDW in the two simulations are minimal. Formation of AABW is notably difficult to represent in low resolution of general circulation models (Aguiar et al 2017). In addition, most CMIP5 models have temperature and salinity biases over the entire water column in Southern Ocean, which is a factor influencing the density of seawater (Sallée et al 2013).…”