Emergence of deep convection in the Arctic Ocean under a warming climate

Abstract

The appearance of winter deep mixed layers in the Arctic Ocean under a warming climate is investigated with the HiGEM coupled global climate model. In response to a four times increase of atmospheric CO2 levels with respect to present day conditions, the Arctic Basin becomes seasonally ice-free. Its surface becomes consequently warmer and, on average, slightly fresher. Locally, changes in surface salinity can be far larger (up to 4 psu) than the basin-scale average, and of a different sign. The Canadian Basin undergoes a strong freshening, while the Eurasian Basin undergoes strong salinification. These changes are driven by the spin up of the surface circulation, likely resulting from the increased transfer of momentum to the ocean as sea ice cover is reduced. Changes in the surface salinity field also result in a change in stratification, which is strongly enhanced in the Canadian Basin and reduced in the Eurasian Basin. Reduction, or even suppression, of the stratification in the Eurasian Basin produces an environment that is favourable for, and promotes the appearance of, deep convection near the sea ice edge, leading to a significant deepening of winter mixed layers in this region (down to 1000 m). As the Arctic Ocean is transitioning toward a summer ice-free regime, new dynamical ocean processes will appear in the region, with potentially important consequences for the Arctic Ocean itself and for climate, both locally and on larger scales. 2015), indicating a decrease in the intensity of deep convection, and a decline in Arctic sea ice 46 cover (Stroeve et al., 2012), along with a northward migration of the sea ice edge into the Arctic 47 Ocean and the Barents Sea. Given that the location of deep convection sites is inherently tied to 48 the location of the sea ice edge, one might expect that new regions of deep convection could appear 49 at higher latitudes (Rainville et al., 2011). We expect that changes in deep convection location and 50 MLD will alter water mass properties and ultimately the deep branch of the AMOC (Rahmstorf, 51 2002; Heuzé, 2017) and ocean heat transport (Exarchou et al., 2015). 52 53 Today, the deepest layers of the Canadian and Eurasian Basins are mostly filled with dense waters 54 formed by brine rejection during sea ice formation on the continental shelves, or transformed in the 55 Barents Sea, or advected from the Greenland Sea through Fram Strait. These dense water masses 56 are eventually advected to the Nordic Seas through Fram Strait (Jones et al., 1995; Lique et al., 57 2010). As the sea ice edge migrates northward, the expectation is that the relative contribution 58 of deep waters formed locally in the Arctic Basin by open ocean convection will increase relative 59 to those waters formed on continental shelves or in the Barents Sea. In the deep Greenland Sea, 60 observations suggest that the amount of dense water originating from the deep Arctic has recently 61 increased compared to the dense water formed locally in the Greenland Sea during deep convection 62 events (Langeh...

Recent Sea Ice Decline Did Not Significantly Increase the Total Liquid Freshwater Content of the Arctic Ocean

“…Although its contribution to the changes of the liquid FWC is relatively small compared to the larger changes of the Barents Sea branch AW and the Pacific Water, the Fram Strait inflow can also impact the deep AW layer circulation. Previous studies have indicated that weaker sea ice and changes in ocean surface circulation can influence the AW layer circulation located at depth (Karcher et al 2012;Itkin et al 2014;Lique et al 2015). In this paper we focus on the Arctic surface freshwater, while the model results indicate that dedicated future work is needed to better understand the processes that can influence the fate of the AW in the Arctic Ocean in a changing climate, in particular, in terms of possible changes in the AW layer circulation.…”

“…By using a coupled climate model Lique et al (2018) showed that the Amerasian basin will become fresher while the Eurasian basin will become more saline due to sea ice decline in future warmer climate (in a 4 3 CO 2 scenario). Our study indicates that the sea ice decline in the past decade has already started this changing tendency.…”

“…For example, Arctic warming can change the surface properties in the Barents Sea, and thus the Barents Sea Water formed there; this signal can propagate to the Arctic basin within the AW layer. Recent climate model simulations imply that AW circulation at Fram Strait and in the Eurasian basin might also change in a warming climate (Lique et al 2018).…”

Arctic Ocean Freshwater Content and Its Decadal Memory of Sea‐Level Pressure

“…HiGEM is a fully coupled climate model based on the Hadley Centre Global Environmental Model version 1 (HadGEM1; Johns et al, 2006) with a relatively high resolution ocean and a reasonable representation of the Arctic (Lique et al, 2015(Lique et al, , 2017. HiGEM is a fully coupled climate model based on the Hadley Centre Global Environmental Model version 1 (HadGEM1; Johns et al, 2006) with a relatively high resolution ocean and a reasonable representation of the Arctic (Lique et al, 2015(Lique et al, , 2017.…”

“…We analyze monthly mean output from a 150-year control integration performed with the High-Resolution Global Environmental Model (HiGEM). HiGEM is a fully coupled climate model based on the Hadley Centre Global Environmental Model version 1 (HadGEM1; Johns et al, 2006) with a relatively high resolution ocean and a reasonable representation of the Arctic (Lique et al, 2015(Lique et al, , 2017. The model is described and evaluated in Shaffrey et al (2009).…”