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

Marcos Tonelli,Bruno Ferrero,Fernanda Marcello,Ilana Wainer

doi:10.3390/geosciences9080346

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.

Highlights

Unique exchange processes and water transformation over the Antarctic continental shelf and shelf slope are contributors in the formation of the Antarctic Bottom Water (AABW), one of the main components of the Meridional Overturning Circulation (MOC) lower limb [1,2,3].Of all the specific sites around the Antarctic continent, the Weddell Sea (WS) is the primary source of AABW [4,5]—through remote and local water mass interactions influenced by Weddell Gyre (WG) dynamics
As a first approach to assess the water column structure of the WS throughout the LME simulation, we examine the potential temperature anomaly along the WS transect during the Medieval Climate Anomaly (MCA) (950–1250) and the Little Ice Age (LIA) (1400–1700), as well as the late 20th century (1970–2000) with regards to the pre-industrial Last Millennium (LM)
During the MCA, the entire water column is warmer than the base period, with more intense anomalies located above the 2000 m depth

Summary

Introduction

Unique exchange processes and water transformation over the Antarctic continental shelf and shelf slope are contributors in the formation of the Antarctic Bottom Water (AABW), one of the main components of the Meridional Overturning Circulation (MOC) lower limb [1,2,3].Of all the specific sites around the Antarctic continent, the Weddell Sea (WS) is the primary source of AABW [4,5]—through remote and local water mass interactions influenced by Weddell Gyre (WG) dynamics. Deep Water (CDW) which gradually cool and freshen as they entrain the WS ambient waters. By this point they are referred to as the Warm Deep Water (WDW) [6,7,8,9]. As WDW interacts with dense shelf waters near the continental WS shelf break, Weddell Sea. Bottom Water (WSBW) is formed, the densest AABW regional variety (Figure 1). After further mixing of dense shelf waters with WDW, Weddell Sea Deep Water (WSDW) is produced in a similar manner, the less dense AABW regional variety [10,11]

Results

Discussion

Conclusion