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Meddies in Atlantic Ocean

Lively Data, June 18, 2009

Top: Map of merged SLA on a Hofmoeller diagram (xt) averaged on a latitude band between 35.5°N and 37.7°N. The black square represents the same spot on the bottom map. To plot such a diagram, you have to select the variable (here Delayed-time Maps of Sea Level Anomalies Merged), then the area (21°W-32°W and a constant latitude 36°N), and afterwards click in the left menu on "Define variable". Choose "average", and the axis "y" . Then define the limits in latitude (35.5°N-37.7°N) on the interactive map, without touching anything on the longitude. Then click on next, to go back to the previous screen, define the time period (Oct.2004-Jan.2005).
Bottom: Maps of Geostrophic Velocity Anomalies represented with vectors (U,V) on 2005/08/17. Red lines matrialized the Azores current.

'Meddies' (Mediterranean eddies) are defined as warm and salty eddies of Mediterranean water sinking in the northeast Atlantic through the Gibraltar Strait. They are generated by the interaction of the Mediterranean Waters with the bottom topography. Even they propagate at mid-depth northward or westward, they have a significant signature on the sea level profile that the satellites altimeter can detect. They are generally anti-cyclonic (they rotate in a clockwise direction) and correspond to a positive sea level anomaly with about several cm of amplitude.

Using the LAS, Maps of Sea Level Anomalies showing evolution wrt to longitude and time or maps with geostrophic currents can help to show these meddies or detail their characteristics (number, size, main trajectory, temporal evolution, ...).  One example is given to the South of Azores (near 30° W, 36°N) in the Atlantic Ocean with a meddy visible on a Hovmoeller diagram including MSLA data (top). It moved over ten degrees of longitudes (22°W - 32°W)  during more of one year (October 2004 - November 2005). A map with geostrophic current anomalies (bottom) corresponding to one date and one spot of this diagram (represented with little black squares) shows an instantaneous picture of this meddy just to the north to the Azores current (represented  with red lines).

 

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