The Zonal Overturning Circulation

Vertical transports in the ocean are an important component of the coupled climate system because they are able to move heat, carbon, nutrients, and other tracers into and out of the surface mixed layer, where they can be exchanged with the atmosphere. Almost all research on the Ocean’s overturning circulation has focused on the two-dimensional Meridional Overturning Circulation, a two-dimensional latitude/depth projection.

In reality the overturning circulation is three-dimensional, with large zonal variations in vertical and diapycnal transports, often at different latitudes, that are connected by zonal flows. This project would examine this Zonal Overturning Circulation, the three-dimensional non-zonally averaged aspects of the two-dimensional Meridional Overturning.

The primary focus will be on the expected upwelling along midlatitude western boundaries, which is expected to mirror the downwelling along eastern boundaries that has been examined in the past, both associated with alongshore pressure gradients. This upwelling is found in several models at significant transport magnitudes, affecting the middle to upper water column and potentially influencing the exposure of those waters to the atmosphere, air-sea exchange, heat and salt storage and variability.

The project will relate the magnitude, horizontal, and vertical scales of the upwelling regions to their fundamental driving mechanisms, surface wind stress, and buoyancy forcing, using high resolution idealized numerical models, low and high resolution realistic state estimates, and boundary layer theory. The work will provide insight into the role of western boundary upwelling in the global climate system, support a graduate student, contribute to graduate and undergraduate course content, including the WHOI GFD summer school and classes at U.

Delaware, and involve undergraduates through the WHOI summer student fellow program and the U. Del. REU program. Public outreach is planned through webpages and the U. Del. annual Coast Day presentations.

The primary objectives of the proposed study are: to apply a hierarchy of idealized models and boundary layer theory to gain a fundamental understanding of what maintains the observed pressure gradient along mid-latitude western boundaries and how this connects to the three-dimensional ocean circulation and heat transport; to diagnose vertical transports and buoyancy budgets in realistic low- and high-resolution ocean state estimates and relate these findings to the idealized modeling and theory; and to use Lagrangian diagnostics to diagnose the source regions of waters upwelled near the western boundary and their fate after upwelling, including possible entrainment into the surface mixed layer and influence on air-sea interaction. Idealized modeling will be based on the HYCOM primitive equation model, which can be configured as purely isopycnal, z-coordinate, sigma-coordinate, or a combination of these coordinate systems.

The model will be applied in increasingly complex configurations in order to better isolate and understand the physics controlling the source waters for the zonal overturning circulation. Realistic modeling will include two ocean state estimates from the “Estimating the Circulation and Climate of the Ocean (ECCO)” consortium: the low-resolution, non-eddy resolving ECCO version 4 (ECCO v4); and the eddy-permitting ECCO, phase II (ECCO2).

In addition to examining the dynamics controlling upwelling, the models will also be used to explore the source and fate of upwelled water, using the introduction of both tracers and floats at a range of initial depths.

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.