Satellite altimetry’s contribution to ocean circulation studies.

Ocean circulation at subtropical latitudes is made up of large cells known as anticyclonic gyres; the famous Gulf Stream basically corresponds to the western boundary of the North Atlantic subtropical gyre.

Data used

Merged Maps of Sea Level Anomalies (MSLAs) and Maps of Absolute Dynamic Topography (MADTs) are required. (See description on the Aviso website).
– both are already gridded and easy to use, moreover no specific data reprocessing is required here,
– both provide their own view of the sea surface current, furthermore, their aspects are complementary,
– both have been merged, to provide the most precise dataset available,
– up-to-date (‘Upd’) data offer better quality for a given date, whereas reference (‘Ref’) data are more suited to long temporal studies (see the second part: ‘Seasonal variations’).

You can get access to these products by subscribing and by filling the form on the Aviso website. Then a user account will be sent and you will be able to download the relevant data.

 

madt_gulf_sm msla_gulf_sm
fig 1 & 2: Maps of Absolute Dynamic Topography (left) and Sea Level Anomaly (right), North Atlantic
madt_uv_gulf_sm msla_uv_gulf_sm
fig 3: Map of absolute geostrophic velocity fig 4: Map of geostrophic velocity anomalies

 

Methodology

Temporal extraction

From the authenticated service, download up-to-date (“Upd”) MSLA and MADT files for November 2, 2005 , in global/dt/upd/msla/merged/h/ or in global/dt/upd/madt/merged/h/
Then download reference (“Ref”) MSLA and MADT files from 2001 to 2005 (See description) in global/dt/ref/msla/merged/h/ or in in global/dt/ref/madt/merged/h/.

Geographic extraction

Our area of interest is defined by the following coordinates: 0°N-50°N,10°W-80°W.

Four different points of view

Select a date, for example here 2 November 2005 and then plot:

  • the Absolute Dynamic Topography map corresponding to your chosen file,
  • the Sea Level Anomaly map corresponding to your chosen file,
  • the absolute geostrophic velocity map provided with your MADT file,
  • the geostrophic velocity anomalies map provided with your MSLA file.

On the MADT map, an area with significant contrasts is visible, where the topography varies by approximately 80 centimetres within quite a short distance, all along what seems to be a front.
On the MSLA map, no such frontal area appears, but there are some separated spots in the same place.
On the absolute geostrophic velocity map, vector fields plot a continuous structure: vectors join together to form a significant surface current.
On the velocity anomalies map, vectors still plot a major current in the North Atlantic Ocean, but with more eddies.
These maps correspond with a significant current that flows along the coasts of North America towards European shelves. Eddies in its wake show that a lot of energy seems to be carried by this current.

Previous maps provided just a ‘snapshot’ of the Gulf Stream system. With a larger time scale, it becomes possible to see that this current does not always flow in the same location, nor with the same intensity: it changes with the seasons, and for seasonal or interannual studies, such maps are obviously not enough to quantify its variability.

 

The Gulf Stream, season by season

Here we are considering two seasons: autumn (October-November) and Spring (April-May). For each one we:

  • compute the temporal average of the absolute dynamic topography all along a cross-section in longitude (70°W),
  • plot diagrams showing how these averaged dynamic heights vary with latitude.

Seen in this way, the Gulf Stream appears at approximately 37°N, and seems further north in October (black curves) than in April (red curves), as well as appearing more intense. These differences could mirror a seasonal or inter-annual signal.

 

fig 5: Gulf Stream Seasonal variation Autumn (black) vs Spring (April) MDT along the Gulf Stream at 70º W.