Mesoscale dynamics of the Balearic Front, integrating glider, ship and satellite data

Abstract

An intensive observational programme of the Balearic Front, in the Western Mediterranean Sea, was carried out using conventional (CTDs from a ship) and new technologies (gliders). The strategy consisted of repeated sampling of a section almost perpendicular to the Balearic Current situated to the north of Mallorca Island. This section is coincident with track 773 of the ENVISAT satellite. In this work we present results of missions undertaken in July and September 2007. In July the sampling was designed to collect in-situ data from a ship and a coastal glider, almost simultaneously with the passage of the radar altimeter on board the ENVISAT satellite. In September the sampling was done only by the glider platform, and also simultaneously with the passage of the satellite. The Balearic Front was clearly detected by salinity (not temperature), with lower values (37.4 PSU) near the coast that are likely to be related to the input of recent Atlantic Water through the Mallorca Channel. The observed width of the front was about 35–40 km, with a vertical extension of 200 m. It had a significantly different pattern between the July and September samplings, which confirms the high mesoscale variability of this frontal area. Using a reference depth level of 180 m we have obtained maximum geostrophic velocities of 40–45 cm/s, flowing northeastwards along the north Mallorca coast. These velocity estimates are not sensitive to the test reference level (600 m), indicating that the layer between 200 and 600 m does not play a key role in the dynamics of the upper layer (200 m). A dynamic height rise from July to September (in 70 days) of about 3–7 cm reflects the seasonal cycle of sea level due mainly to thermosteric expansion of the water column. This gives a rise rate in the range of 1.3–3 cm/month, which is in agreement with previous estimates in the Mediterranean Sea. Absolute dynamic topography from altimetry data reveals good coherence with the dynamic height from the glider data in the area where there are common observations. In September, ENVISAT captured the sharp gradient observed with in-situ data, with mean velocities of about 24 cm/s, but with a smaller variance than the glider due to the larger wavelength of the filter applied to the altimeter data. From the technological point of view the observational experiment has confirmed the feasibility of using a coastal glider to monitor the spatial and low-frequency variability of the coastal ocean. The coastal glider was appropriate in this particular case since the vertical extension of the front only reaches up to 200 m. The autonomous platform was shown to be strongly robust, and able to monitor the area even under adverse meteorological conditions. This represents an important achievement, since under the same conditions the CTD sampling from a traditional research vessel would normally have been cancelled. Moreover, this type of platform allows autonomous collection of CTD casts and biogeochemical measurements at high spatial resolutions and at very low costs compared to conventional methods.