Wind-driven Variability in Heat Storage and the Meridional Overturning Circulation

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Observations and climate models show that winds are responsible for much of the seasonal to interannual variability in ocean heat content and the meridional overturning circulation (MOC). This study is focused on gaining a better understanding of how the ocean adjusts to variable winds, with particular focus on climatically important quantities such as ocean heat content and the MOC strength and pathways. A theoretical framework will be developed to better understand how these quantities depend on the controlling parameters, including mean stratification; region, structure, and frequency of wind variability; bottom topography; and mean state (including wind-driven gyres and a mean overturning circulation). The idealized approach adopted will allow for a general understanding of the controlling physics, including how they will vary in different oceans around the world, and in a future climate where the mean state and forcing variability might be different than at present. The results of this study will be incorporated into graduate level classes and lectures at the Woods Hole Oceanographic Institute (WHOI) Geophysical Fluid Dynamics Summer School, and participation will be sought from undergraduate students through the WHOI Summer Student Fellow Program. This will provide training in numerical methods, applied mathematics, climate, and data analysis. Results from this work will be made available through the peer-reviewed literature, national and international meetings, and web page content. Despite its known importance for climate, there is currently no analytic theory for how transients in wind-forcing modify ocean heat storage and transport by the overturning circulation. This work will combine known features of the large-scale, low frequency ocean circulation, such as geostrophy, wave transients, and eddy fluxes, in a simple and compact way to represent climatically important integrated quantities such as ocean heat content and mass transports. The primary intellectual merit of the proposed work lies in developing a theoretical framework to connect regions of variable wind stress to remote ocean currents. This inevitably involves understanding how heat is stored and transported on both gyre- and global-scales in the ocean. The starting point is a linear theory based on isopycnal mass budgets in the interior subject to Sverdrup dynamics and Rossby and boundary wave transients. The predictions from the theory, and the importance of such nonlinear processes as outcropping, eddy fluxes, closed potential vorticity contours, and inertial western boundary currents, will be assessed with the use of idealized numerical models and published ocean state estimates from the ECCO (Estimating the Circulation and Climate of the Ocean) consortium. Diagnostics from these more complete models will also be used to guide revisions to the linear theory.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.