Sources and Impacts of Variability of the Meridional Property Transports in the Atlantic Ocean
- (University of Washington, Applied Physics Laboratory)
Recent in situ observations are quantifying the variability of meridional heat and volume transport in the Atlantic Ocean at specific locations, but processes controlling the variability and meridional coherence of property transports remain elusive. Our goal is to explain observed decadal anomalies of meridional heat and freshwater transport in terms of changes in ocean circulation, surface fluxes and interbasin exchanges, as well as the potential for coupling to the atmosphere. Our focus will be on property transport anomalies and their linkages to the AMOC (Atlantic Meridional Overturning Circulation) anomalies; we will investigate the effect of wind-forced changes in SO (Southern Ocean) circulation and connections to the Artic, Indian Ocean (Agulhas eddies) and Pacific Ocean property inflows. The impact of MHT (meridional heat transport) anomalies on air-sea heat fluxes in the Atlantic western boundary currents will be examined, as well as their potential for influencing large-scale SST patterns. We propose a linked series of observational and modeling studies to characterize anomalies, examine specific forcing mechanisms and to suggest linkages to other climate relevant processes.
Analyses of altimetric sea surface height (SSH), GRACE (Gravity Recovery and Climate Experiment) equivalent water thickness, hydrographic estimates of thermosteric and halosteric sea level, and heat and freshwater fluxes will be central to our study. Our analysis period will begin in 1992 to characterize interannual to decadal anomalies, with the expectation that Argo and GRACE data will improve the accuracy for more recent time periods. We will extend a Kalman filter box model of mass and heat budgets to include salinity and the Atlantic sector of the SO. Initial results show MHT coherence from 31oS to 36oN, with the model accurately capturing the recent drop and reversal in the directly observed MHT (Kelly et al., submitted to J. Climate, hereafter KTL12). The box model also shows that positive anomalies of MHT are associated with increased heat loss in the Gulf Stream (GS) region, indicating feedback to atmospheric heat transport. Coherent MHT anomalies are correlated with the AAO (Antarctic Oscillation), suggesting a SO origin for the anomalies. We will examine SSH anomalies to identify fingerprints of these anomalies: for example, changes in flow through the Drake Passage or the Agulhas Current eddies or changes in eddy heat fluxes in the SO.
In addition to data analysis, we will use two modeling approaches to determine the relationships between property transports, ocean circulation and surface fluxes. We will compare the role of wind forcing in the SO with that of North Atlantic surface fluxes in forcing meridional transport anomalies throughout the Atlantic, and we will examine the source of the wind-stress anomalies. First, we will use the global ocean mass conserving isopycnal model GOLD (Global Ocean Layer Dynamics from the Geophysical Fluid Dynamics Laboratory) and a fully coupled counterpart to investigate the specific processes responsible for the MHT anomalies and the role that wind-forced changes play in those anomalies. Because we expect that eddies may play a crucial role in the adjustment of the circulation to changes in winds in the SO or in heat inflow from the Agulhas, we will also analyze the output of a high-resolution fully coupled simulation (based on the National Center for Atmospheric Research coupled climate model). In all of the runs, we will focus on relationships between changes in property transports, circulation and surface fluxes. SSH comparisons will be used as a check on the fidelity of the models and to determine to what extent SSH (alone or combined with other observations) can be used as an indicator of the property transport anomalies.