The efficiency of baroclinic instability in the global ocean

This work will provide a better understanding of the pathways of energy in the global ocean and the exchanges with the atmosphere. These processes occur on different time and space scales and regulate Earth's climate. The ocean generally gains energy (heat) in the tropics from the sun, which is then transported poleward by major ocean currents along the western boundaries of ocean basins, such as the Gulf Stream, and eventually lost back to the atmosphere in the midlatitudes. Recent research shows that oceanic turbulence in these midlatitude regions associated with strong current systems leads to a loss of energy to the atmosphere, not accounted for in traditional ocean circulation models. This sink of energy to the atmosphere at these turbulent time and space scales modulates the ocean circulation in unforeseen ways that need further investigation. This project will enhance our understanding of these processes and will develop parameterizations for their inclusion in general ocean circulation models leading to improving projections of Earth's climate system. The study will combine remotely sensed data of sea surface height, surface temperature, and net heat flux collected over several decades with numerical simulations of coupled and uncoupled high-resolution Earth system models. The analysis will lead to better quantifying the rates and mechanisms of how energy is transferred from the ocean to the atmosphere and how this is associated with mesoscale eddies in the ocean. This proposal will support and help build the laboratory of an early career PI and the training of a PhD student. Public outreach lectures will be used to communicate the importance of the Gulf Stream in a changing climate.

This work will aid in a better understanding of the pathways of energy in the global ocean. High-resolution climate models persistently have high sea surface temperature variance, but surface EKE comparable to observations. This proposal will help shed light on why this discrepancy occurs. Work in the Kuroshio Extension using a regional coupled model shows that this sink accounts for more than 70% of the EPE that would be available for conversion to eddy kinetic energy (EKE). However, it is still an open question how the OME-A sink connects to a modulation of EKE globally. This study will use multiple decades of available remote-sensed surface observations and a suite of global high-resolution simulations in coupled and uncoupled configurations of the Community Earth System Model (CESM-H) to access energy conversion rates through a local and global analysis. A local analysis requires a decomposition of the eddy heat flux field into divergent and rotational fluxes. The principal investigator will use Helmholtz decomposition to solve the Poisson equation globally in order to extract the dynamically important divergent fluxes. The main objectives are to 1) determine how the OME-A feedback modulates the efficiency of baroclinic instability to convert potential to kinetic energy in CESM-H; and 2) examine the spatial and temporal scale dependence of the EPE sources and sinks in state-of-the-art observations and CESM-H. There will be particular focus on the northern hemisphere Western Boundary Currents and their correspondence with decadal climate modes such as the North Atlantic Oscillation and Pacific Decadal Oscillation. A new parameter will be defined that does not depend on absolute model energetics and will allow model evaluations in current and future High-Resolution Model Intercomparison Projects such as the international Laboratory for High-Resolution Earth System Prediction (iHESP). The proposed team will collaborate with participants in the climate process team (CPT) on ocean transport and eddy energy and will ultimately contribute to a framework for parameterization development of the OME-A EPE sink in coarse-resolution coupled climate models.

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.