Supervisers - Grant Bigg (University of Sheffield) and Ian Renfrew
The deep waters of the global ocean are formed through increasing the density of upper
ocean waters by either extreme cooling of a salty ocean or brine rejection during sea-ice formation in
(typically) fresher waters. Both mechanisms are thought to occur intermittently in polar and sub-polar
regions but have rarely been observed. Ocean and coupled atmosphere-ocean models tend to be poor
in reproducing the properties of deep water. One of the reasons for this is that they are generally
forced by climatological fields, thus missing small-scale or short-lived atmospheric weather systems.
In polar latitudes small but intense storms, known as polar mesocyclones or polar lows, are
ubiquitous and undoubtedly play a role both in pre-conditioning the ocean and initiating deep water
formation episodes. The number and distribution of these storms is strongly modulated by the large-
scale atmospheric circulation, particularly in the northern Atlantic.
In this project, the impact of polar mesocyclones on the ocean will be studied through ocean
modelling. To do so, it will first be necessary to examine to what extent mesocyclones appear in high
temporal resolution meteorological data sets, such as the analyses produced by the European Centre
for Medium Range Weather Forecasts. Existing satellite-based climatologies of mesocyclones for
both the NE Atlantic and Weddell Sea will be used for this purpose. Cyclone-tracking software could
be employed to develop a climatology of mesocyclones within the analyses fields, with scatterometer
data employed to check the surface wind fields for certain cases. This would then lead to ocean
general circulation modelling experiments with, and without, mesocyclones embedded in the
atmospheric forcing fields. These experiments would be performed for both particularly data-rich
case studies, and using the ECMWF climatology as a base forcing field. The modelling studies would
focus on the Nordic Seas and the Labrador Sea, for which a high spatial resolution ocean model
already exists. However, it is envisaged that similar experiments could be done in the Weddell Sea
near the sea-ice edge to contrast the two hemispheres. Southern Ocean mesocyclones tend to be
weaker than those of the North Atlantic.