2. Reports of current polar low research

2.9 The impact of polar mesocyclones on deep water formation

Ian Renfrew, Physical Sciences Division, British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET,

Grant Bigg, University of East Anglia

The project is being supervised by Dr Grant Bigg (University of East Anglia) and Dr Ian Renfrew (British Antarctic Survey). The hard work will be carried out by Scott Jermy, who has recently graduated with a M. Physics degree from Kings College, University of London. The proposed work is outlined below, and we would hope to have results to present in a couple of years time at the next Polar Lows Workshop.

Project Outline
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.