Bob Hallberg - Developing Parameterizations of Mesoscale Eddy Effects and Boundary Layer Mixing for Use in Global Ocean Models

Event type: 
Seminar
Date: 
18 October 2017
Time: 
2.00pm - 3.00pm
Location: 

Climate Change Research Centre, Seminar Room, Mathews Building 4th floor, UNSW, Sydney

Presenter: 
Bob Hallberg
NOAA/GFDL & Princeton University
Host: 
Climate Change Research Centre, UNSW

The oceans play a crucial role in regulating Earth’s climate. Predicting changes in the climate system requires the use of global ocean models capable of a skillful representation of the effects of both the large-scale processes in the ocean that can be explicitly resolved by model grids with horizontal scales of order tens of kilometers, and a vast range of processes that occur at smaller spatial scales. The effects of these smaller scale processes must be represented via parameterizations. Among the more important parameterizations in ocean models are the treatment of the turbulence of the ocean surface boundary layer (which mediates all of the interactions between the ocean and atmosphere), and the depiction of mesoscale eddies (which interact with the larger-scale balanced motions to regulate the overall dynamics of the ocean).

This talk describes some recent efforts to develop parameterizations for use in global climate system projections using the MOM6 ocean model. The discussion of GFDL’s new planetary boundary layer parameterization illustrates how an understanding of the straightforward energetic consideration governing boundary layer mixing can be combined with a careful numerical implementation to deliver robust  and accurate climate simulations. This is also a demonstration of an energetics framework for mixing that should facilitate the contribution of parameterizations of additional mixing processes.  The discussion of GFDL’s ongoing efforts to parameterize the effects of ocean mesoscale eddies in MOM6 illustrates that deciding where and whether to parameterize can be just as important as how to parameterize, while also illustrating how physical constraints and energy conservation apply in a rather different form from that used in boundary layer turbulence. Together, these two themes provide both an insight into the process of developing parameterizations for ocean and climate models, as well as giving a sense of some of the limitations and capabilities of today’s coupled climate models.

The intended purpose of this talk is threefold: to demystify the physics of the ocean’s surface boundary layer, to present a survey of efforts to develop physically based process parameterizations for use in global ocean models, and to extend an explicit invitation to participate in the community-based development of the MOM6 ocean model. This talk should be comprehensible without any particular prior knowledge.

Biography: Dr. Robert Hallberg is an Oceanographer and the head of the Ocean and Ice-sheet Processes and Climate group at NOAA’s Geophysical Fluid Dynamics Laboratory, a lecturer in the Princeton University Program in Atmospheric and Oceanic Sciences, and one of the lead developers of the MOM6 ocean and the SIS2 sea-ice models. During his 20 years at GFDL, he has worked extensively on the development of numerical ocean model algorithms and parameterizations, the configuration of coupled climate models, and on the application of ocean and climate models to study such problems as the role of ocean mesoscale eddies in the dynamics of the Southern Ocean and understanding how ocean mixing controls rates of sea-level rise.