Mesoscale Circulation Variability from Five years of Quasi-continuous Glider Observations and Numerical Simulation at a Key Sub-basin 'Choke' Point.

Quasi-continuous glider observations over 5 years have uniquely characterised a high frequency variability in the circulation through the Ibiza Channel, an important ‘choke’ point in the Western Mediterranean Sea. This ‘choke’ point governs the basin/sub-basin scale circulation and the north/south exchanges of different water masses. The resulting multi-scale variability impacts the regional shelf and open ocean ecosystems, including the spawning grounds of Atlantic bluefin tuna. Through the unique glider record we show the relevance of the weekly/mesoscale variability, which is of same order as the previously established seasonal and inter-annual variability. To understand the drivers of this variability we combine the glider data with numerical simulations (WMOP) and altimetry. Two key drivers are identified; extreme winter events, which cause the formation of a cold winter mode water (Winter Intermediate Water) in the shelf areas to the north of the Ibiza Channel, and mesoscale activity, which to the north produce channel ‘blocking’ eddies and to the south intermittent and vigorous flows of fresher ‘Atlantic’ waters through the Ibiza Channel. Results from the 2 km resolution WMOP are compared with the high-resolution (2 – 3 km.) glider data, giving insight into model validation across different scales, for both circulation and water masses. There is an emerging consensus that gliders can uniquely access critical time and length scales and in this study gliders complement existing satellite measurements and models, while opening up new capabilities for multidisciplinary, autonomous and high-resolution ocean observation.