Surface signature of Mediterranean water eddies in the Northeastern Atlantic: effect of the upper ocean stratification

[摘要] Meddies, intra-thermocline eddies of Mediterranean water, can often bedetected at the sea surface as positive sea-level anomalies. Here we studythe surface signature of several meddies tracked with RAFOS floats and AVISOaltimetry.

While pushing its way through the water column, a meddy raises isopycnalsabove. As a consequence of potential vorticity conservation, negativerelative vorticity is generated in the upper layer. During the initialperiod of meddy acceleration after meddy formation or after a stagnationstage, a cyclonic signal is also generated at the sea-surface, but mostlythe anticyclonic surface signal follows the meddy.

Based on geostrophy and potential vorticity balance, we present theoreticalestimates of the intensity of the surface signature. It appears to beproportional to the meddy core radius and to the Coriolis parameter, andinversely proportional to the core depth and buoyancy frequency. Thisindicates that surface signature of a meddy may be strongly reduced by theupper ocean stratification. Using climatic distribution of the stratificationintensity, we claim that the southernmost limit for detection in altimetry ofsmall meddies (with radii on the order of 10–15 km) should lie in thesubtropics (35–45° N), while large meddies (with radii of25–30 km) could be detected as far south as the northern tropics(25–35° N). Those results agree with observations.

[学科分类] 海洋学与技术