Warm Deep Water Variability During the Last Millennium in the CESM–LME: Pre-Industrial Scenario versus Late 20th Century Changes

Abstract

Water transformation around Antarctica is recognized to significantly impact the climate. It is where the linkage between the upper and lower limbs of the Meridional Overturning Circulation (MOC) takes place by means of dense water formation, which may be affected by rapid climate change. Simulation results from the Community Earth System Model Last Millennium Ensemble (CESM–LME) are used to investigate the Weddell Sea Warm Deep Water (WDW) evolution during the Last Millennium (LM). The WDW is the primary heat source for the Weddell Sea (WS) and accounts for 71% of the Weddell Sea Bottom Water (WSBW), which is the regional variety of the Antarctic Bottom Water (AABW)—one of the densest water masses in the ocean bearing directly on the cold deep limb of the MOC. Earth System Models (ESMs) are known to misrepresent the deep layers of the ocean (below 2000 m), hence we aim at the upper component of the deep meridional overturning cell, i.e., the WDW. Salinity and temperature results from the CESM–LME from a transect crossing the WS are evaluated with the Optimum Multiparameter Analysis (OMP) water masses decomposition scheme. It is shown that, after a long–term cooling over the LM, a warming trend takes place at the surface waters in the WS during the 20th century, which is coherent with a global expression. The subsurface layers and. mainly. the WDW domain are subject to the same long–term cooling trend, which is decelerated after 1850 (instead of becoming warmer like the surface waters), probably due interactions with sea ice–insulated ambient waters. The evolution of this anomalous temperature pattern for the WS is clear throughout the three major LM climatic episodes: the Medieval Climate Anomaly (MCA), Little Ice Age (LIA) and late 20th century warming. Along with the continuous decline of WDW core temperatures, heat content in the water mass also decreases by 18.86%. OMP results indicate shoaling and shrinking of the WDW during the LM, with a ~6% decrease in its cross–sectional area. Although the AABW cannot be directly assessed from CESM–LME results, changes in the WDW structure and WS dynamics have the potential to influence the deep/bottom water formation processes and the global MOC.

Past to Future South Atlantic Overturning Circulation: upper-ocean pathways and low-frequency variability

“…Previous studies have shown a shoaling and warming trend of WDW since 1990's, although it has experienced some periods of cooling (Robertson et al, 2002;Fahrbach et al, 2004Fahrbach et al, , 2011. This pattern in the WDW has also been reproduced by climate models (Tonelli et al, 2019). In our study, climatologies (Figure 4 c and d) show that the intermediate layer temperature is on average within the range of observations for the WDW (Whitworth et al, 1998;Orsi et al, 1993;Robertson et al, 2002).…”

Climate Projections for the Southern Ocean Reveal Impacts in the Marine Microbial Communities Following Increases in Sea Surface Temperature

“…Significant increases in SST are not expected to happen in the northwestern Antarctic Peninsula (NWAP) coast before 2080 nor until the end of the century in the northwestern Weddell Sea (NWWS; Figure 2A). This is particularly relevant to reassure the Weddell Sea (WS) ability to cushion the offshore warming signal advected by the Weddell Gyre (Ryan et al, 2016) due to the local dynamics and cryosphere-related processes (Fahrbach et al, 2011;Tonelli et al, 2019).…”

“…These human-induced changes have already spread to about half (20-55%) of the world oceans (Silvy et al, 2020), and the largest heat gain has occurred in the Southern Ocean (Roemmich et al, 2015;Trenberth et al, 2016). However, the ice-insulated regions of the SO protect its sea surface waters from climate forcing (Gille, 2008(Gille, , 2002Fahrbach et al, 2011;Tonelli et al, 2019). As a result, heat and carbon uptake happens mostly through the formation and subduction of water masses (Heuzé et al, 2013;Sallée et al, 2013), and the earliest detection of the anthropogenic warming fingerprint in the SO occurs in the ocean interior rather than in surface waters (Silvy et al, 2020).…”

“…Despite the ability of the ice-insulated regions of the SO to delay the warming trends throughout the global ocean due to the local circulation and the interaction with sea ice and ice shelves (Fahrbach et al, 2011;Tonelli et al, 2019), warmer surface waters lead to increased sea ice melting, loss of ice mass, and increased surface water flux (Rignot et al, 2008), ultimately leading to a slowdown of bottom water formation and the stalling of the CO 2 sink in the SO (Lovenduski et al, 2008;Lenton et al, 2009;Purkey and Johnson, 2012). In particular, the surroundings of the Antarctic Peninsula and the western Weddell Sea have been exposed to intense warming and sea ice melting (Fan et al, 2014).…”