Vertical Structure of Migratory Anticyclones Affecting the Mediterranean

A comprehensive climatology of migratory anticyclones affecting the Mediterranean was generated by the University of Melbourne finding and tracking algorithm (MS algorithm), applied to 34 years (1979–2012) of ERA-Interim mean sea level pressures on a 1.5°x1.5° horizontal resolution. The algorithm was employed for the first time for anticyclones in this region, thus, its robustness and reliability in efficiently capturing the individual characteristics of the anticyclonic tracks in a closed basin with complex topography, such as the Mediterranean, were checked and verified. Then, the tracks and the statistical properties of the migratory systems were calculated and analysed.

As large variations are taking place in the thermal characteristics of anticyclones and in order to get a perspective on the dynamic and thermodynamic processes in their formation, the vertical thermal extent of the anticyclonic systems is studied. This can provide information on the structural features of anticyclones, such as their cold or warm core structure. Cold anticyclones are linked to low-level cooling, developing over continental interiors, while warm anticyclones result from convergence in the upper troposphere and subsidence beneath. This produces warmer than normal temperatures in the middle and lower troposphere. Moreover, in the middle latitudes, warm anticyclones may be associated with blocking action.

For this purpose, an algorithm developed as an extension module of the MS algorithm is applied using ERA-Interim temperature datasets on several isobaric levels in the troposphere from 1000hPa to 100hPa on an 1.5°x1.5° horizontal resolution. The thermal signal is examined at several vertical levels so that the morphological features and characteristic properties of anticyclones can be revealed.

The module's basic idea is that the average temperature difference between the anticyclone core and an environmental ring set at a distance corresponding to the anticyclone size is calculated throughout the troposphere during the lifecycle of an anticyclone as a metric of the core temperature anomaly, thus, we can explore how the thermal signature of an anticyclone may evolve in time. The environmental temperature is considered as the average temperature over the circle of radius R and centered on the cyclone centre, while the radius R is taken to be the anticyclone radius as calculated by the MS algorithm.

Additionally to the vertical thermal structure of the anticyclonic systems, the study of mean fields of potential vorticity, temperature advection, vorticity advection at various isobaric levels is performed in order to elucidate the role of upper and low levels during anticyclogenesis and system evolvement and to further understand the dynamic mechanisms which are responsible for the development and movement of anticyclones over the Mediterranean region.

Acknowledgement: This research project is implemented within the framework of the Action «Supporting Postdoctoral Researchers» of the Operational Program "Education and Lifelong Learning" (Action's Beneficiary: General Secretariat for Research and Technology), and is co-financed by the European Social Fund (ESF) and the Greek State. Some funding from the Australian Research Council is also acknowledged.