Weather’s impact on ocean circulation – Project outline

Atmosphere-ocean interactions, which play a fundamental role in driving the circulations of both the oceans and atmosphere in our climate system, are observed to vary on a range of spatial and temporal scales. A significant part of these air-sea heat and momentum fluxes is caused by mesoscale weather systems that are too small and too short-lived to be properly resolved by the current generation of global climate models. Recent modelling studies^1,2 suggest that the ocean circulation can be very sensitive to air-sea fluxes associated with unresolved weather systems such as polar lows and tip jets. However, a systematic study of the impact of these weather systems on global ocean circulation is still lacking. Furthermore, the underlying physical mechanisms remain unclear.

The overall aim of this project is to improve our understanding of mesoscale weather system effects on regional and global ocean circulations. This is an exciting new research area with many important climatic implications including, for example, future mesoscale weather parameterizations in global climate models. Such parameterizations can be deterministic¹ or stochastic³; both approaches have been tried, but neither approach properly explored.

You will join a productive research team of physical oceanographers and meteorologists. The tools to tackle this project will include a combination of simple theory and numerical models of different complexity. You will start with a simple 1D upper ocean model to build up an understanding of the physical processes involved in ocean response to deterministic and then stochastic heating and wind forcing. The state-of-the-art MIT ocean circulation model at both coarse and fine resolutions will then be used to investigate the impact of mesoscale weather systems on the ocean general circulation and associated heat transport. The student will be encouraged to pursue his or her own particular interests under the general aim of the project. This project will provide you with a thorough training in ocean dynamics, air-sea interactions, numerical modelling and data analysis. Researchers in the School regularly lead and take part in field campaigns and there will very likely be an opportunity for field work if you wish.

Requirement: We seek an enthusiastic candidate with strong scientific interests and self-motivation. He or she will have at least a 2.1 honors degree in mathematics, physics, oceanography, meteorology, or climate science with good numerical skills.

Further details:

The mechanism and impact of unresolved mesoscale weather systems on the ocean remain poorly understood. A recent study³ on climatic impacts of stochastic air-sea fluxes associated with these weather systems proposed the following mechanism: surface cooling destabilizes the water column and deepens the mixed layer, whereas surface heating simply stabilizes the water column and cannot shoal the mixed layer. The net effect of stochastic air-sea heat fluxes is therefore a deeper mixed layer and reduced sea surface temperature (SST). This newly-conjectured mechanism, intriguing as it is, seems to be at odds with previous studies on the diurnal cycle of the ocean surface mixed layer, where diurnal heating/cooling is found to increase the time-mean SST because daytime heating causes the mixed layer to shoal and reduces its heat capacity. Further study is clearly needed in order to sort out these seemingly conflicting statements about climate impacts of stochastic air-sea fluxes. This project will carry out dedicated process studies of the upper ocean response to deterministic as well as stochastic air-sea heat fluxes using a simple yet robust 1D upper ocean model.

In addition to contributing to air-sea heat fluxes, mesoscale weather systems also play an important role in supplying momentum, vorticity and mechanical energy to the ocean general circulation, particularly at mid and high latitudes, owing to the quadratic stress law⁴. However, the response of the ocean to the mechanical forcing associated with mesoscale weather systems is yet to be fully explored. We propose to use the state-of-the-art MIT ocean circulation model at both coarse and fine resolutions to investigate how the ocean general circulation and associated heat transport respond to thermal and mechanical forcing associated with poorly-resolved mesoscale weather systems such as polar lows or orographic jets.

References:

¹Condron, A., and I. A. Renfrew, 2013: The impact of polar mesoscale storms on northeast Atlantic Ocean circulation, Nature Geoscience, 6, 34-37.

²Pickart, R. S., M. A. Spall, M. H. Ribergaard, G. W. K. Moore, and R. F. Milliff 2003: Deep convection in the Irminger Sea forced by the Greenland tip jet, Nature, 424, 152–156.

³Williams, P. D., 2012: Climatic impacts of stochastic fluctuations in air-sea fluxes, Geophys. Res. Lett., 39, L10705, doi:10.1029/2012GL051813.

⁴Zhai, X., H. L. Johnson, D. P. Marshall, and C. Wunsch, 2012: On the wind power input to the ocean general circulation, J. Phys. Oceanogr., 42, 1357-1365.