High-Latitude Ocean Convection and Gyre Dynamics

Abstract

High-latitude ocean deep convection substantially contributes to vertical mixing, vertical heat transport, deep-water formation, and sea-ice budget in the World Ocean. However, the extent of this contribution remains poorly constrained. The concept of ocean convective available potential energy (OCAPE) have been developed to improve the understanding and the prediction for these deep convection events. The kinetic energy (KE) budget of deep convection is explored analytically and numerically based on the observations in the Weddell Sea. OCAPE, which is derived from thermobaricity, is identified as a critical KE source to power ocean deep convection. Other significant contributions to the energetics of convection, including diabatic processes related to cabbeling and stratification, are also carefully quantified. An associated theory is developed to predict the maximum depth of convection. This work may provide a useful basis for improving the convection parameterization in ocean models. As an application of the theory above, basin-scale OCAPE is found to be significantly built up in the North Atlantic at the end of Heinrich Stadial 1 (~17,000 years ago). This OCAPE is ultimately released to power strong ocean deep convection in North Atlantic as simulated by numerical models. This causes a ~2 °C sea surface warming for the whole basin (~700 km) within a month and exposes a huge heat reservoir to the atmosphere. This may invigorate the Atlantic meridional overturning circulation and provide an important mechanism to explain the abrupt Bolling-Allerod warming. Mesoscale turbulence is another crucial process for high-latitude ocean dynamics. From the physical nature of baroclinic instability, the framework of eddy-size- constrained Available Potential Energy (APE) density is developed, which is capable of well-detecting individual eddies and local eddy kinetic energy (EKE) in the World Ocean. This new framework is likely useful in parameterizing mesoscale eddies in ocean GCMs. Mesoscale turbulence are found to be coupled to the wind-driven Ekman pumping in determining the temperature and salinity budgets in subpolar gyres such as the Weddell Gyre. A conceptual model of the evolution of isopycnals has been developed in which the isopycnal responds to a seasonal oscillation in the surface wind stress. The model accurately predicts the observed phases of the temperature and salinity variability in relationship to the surface wind stress. The model, despite its heavy idealization, also accounts for more than 50% of the observed oscillation amplitude, which depends on the strength of the seasonal wind variability and the parameterized eddy diffusivity. These results highlight the importance of mesoscale eddies in modulating the export of AABW in narrow boundary layers around the Antarctic margins.