Circumpolar Deep Water Circulation and Variability in a Coupled Climate Model

Abstract

The natural variability of Circumpolar Deep Water (CDW) is analyzed using a long-term integration of a coupled climate model. The variability is decomposed using a standard EOF analysis into three separate modes accounting for 68% and 82% of the total variance in the upper and lower CDW layers, respectively. The first mode exhibits an interbasin-scale variability on multicentennial time scales, originating in the North Atlantic and flowing southward into the Southern Ocean via North Atlantic Deep Water (NADW). Salinity dipole anomalies appear to propagate around the Atlantic meridional overturning circulation on these time scales with the strengthening and weakening of NADW formation. The anomaly propagates northward from the midlatitude subsurface of the South Atlantic and sinks in the North Atlantic before flowing southward along the CDW isopycnal layers. This suggests an interhemispheric connection in the generation of the first CDW variability mode. The second mode shows a localized ϪS variability in the Brazil-Malvinas confluence zone on multidecadal to centennial time scales. Heat and salt budget analyses reveal that this variability is controlled by meridional advection driven by fluctuations in the strength of the Deep Western Boundary and the Malvinas Currents. The third mode suggests an Antarctic Intermediate Water source in the South Pacific contributing to variability in upper CDW. It is further found that NADW formation is mainly buoyancy driven on the time scales resolved, with only a weak connection with Southern Hemisphere winds. On the other hand, Southern Hemisphere winds have a more direct influence on the rate of NADW outflow into the Southern Ocean. The model's spatial pattern of ϪS variability is consistent with the limited observational record in the Southern Hemisphere. However, some observations of decadal CDW ϪS changes are beyond that seen in the model in its unperturbed state.

Projected Changes to the Southern Hemisphere Ocean and Sea Ice in the IPCC AR4 Climate Models

“…This warm, salty signal connects into the upper CDW. This is consistent with a modeling study by Santoso et al (2006), which showed that a significant portion of upper CDW T-S variability originates within the Brazil-Malvinas confluence via meridional advection. The anomalous signal then propagates into the Southern Ocean CDW via the ACC.…”

“…The projected salination and warming signal seen in most models south of 408S reflect changes in CDW. While changes in CDW could occur through property changes in NADW or via changes in the rate of diapycnal mixing with neighboring AAIW/AABW (Santoso et al 2006), such mechanisms would not result in changes consistent with those projected. Instead, the dominant mechanism appears to be an increased southward and upward transport of freshwater and heat, within the CDW, associated with the spinup of the Deacon circulation.…”

Southern Ocean Sector Centennial Climate Variability and Recent Decadal Trends

“…The variations of the vertically integrated heat content amount to about 640 3 10 21 J in the Atlantic Ocean basin-Indian Ocean Basin, defined here as the region 408-908S and 708W-808E. The heat is provided by the lower branch of the AMOC, a mechanism previously described from uncoupled ocean model simulations (Mikolajewicz and Maier-Reimer 1990;Yin and Sarachik 1995;Pierce et al 1995) and coupled climate model simulations (Santoso et al 2006). During phases of deep convection, large amounts of heat are released to the atmosphere in the KCM, similar to what was observed during the Weddell Polynya (Moore et al 2002).…”

Using Eulerian and Lagrangian Approaches to Investigate Wind-Driven Changes in the Southern Ocean Abyssal Circulation

“…Observational studies estimate that the Southern Ocean transports distinct varieties of AABW equatorward into the subtropical Atlantic, Pacific, and Indian basins at a net rate of roughly 20 Sverdrups (Sv; 1 Sv [ 10 6 m 3 s 21 ), primarily via a network of deep western boundary currents (DWBCs) (Lumpkin and Speer 2007;Purkey and Johnson 2012). The inflow of LCDW is also thought to occur primarily via DWBCs and is composed of a mixture of North Atlantic Deep Water (NADW) from the Atlantic basin and recirculated deep waters from the Indian and Pacific Oceans (Jacobs 2004;Santoso et al 2006). Together, these meridional flows into and out of the Southern Ocean play an important role in the storage and transport of carbon, heat, and other geochemical tracers (e.g., Levitus et al 2005) and consequently may play a significant role in global climate change (e.g., Meehl et al 2006).…”