Thursday, March 28, 2013

Having fun with the map viewer!

(March 28) Tomorrow, our SailBuoy will have been deployed for two weeks and it continues to collect data as planned and expected. All indicators we receive on an hourly basis are green, indicating that all sensors and components are functioning. The battery levels are still very high. 

However, we haven’t made much ground moving in the direction of the De Soto Canyon. As the image below shows, the buoy has moved to the southeast instead of sailing to the northwest following the isobaths (lines of same depths) in the Canyon like we had planned.  This image was created using the map viewer function of the Deep-C Gulf of Mexico Atlas.


This can be explained by wind conditions that have prevailed this past week. From Monday through Thursday, the winds were very strong and originating from the northwest (the direction in which the buoy was programmed to go). And just like a full-size sailboat, there is an angle (compared to the wind direction) that the buoy cannot sail towards (this is referred to as the “no-go area” on the image at right). If she tries, the buoy will find herself “in irons” and will drift with the currents instead of sailing. 

When the buoy starts to drift, her speed is reduced significantly compared to when she sails. Consequently, two consecutive blue dots on the map above (recorded one hour apart) will be closer to each other when the buoy drifts and farther apart when the buoy sails. By the way, I am sure you had already figured this out... but the red dot is the actual location of the buoy.

The Deep-C Gulf of Mexico Atlas is loaded with options and functionalities that makes it fun to play with. For instance, we can superimpose the SailBuoy trajectory on the currents at the surface of the Gulf of Mexico, as produced by our high resolution HYCOM model. The image below shows the surface currents for this last Tuesday (March 25). They are oriented from the northwest to the southeast and help illustrate and explain the buoy drift as described above. Of course, the currents seen here were forced and generated by the local winds, in the preceding hours. 

On the map viewer we can also plot or superimpose the temperature or salinity fields, at different depths or at the surface. We can even make a vertical section from the surface to the bottom!  Personally, I like to compare the values seen by the buoy to the ones predicted by the HYCOM model. You will see that they are very close to each other! Something else that I have noticed on the recent SailBuoy data is that the surface of the Gulf has been getting slightly colder these past days. Once gain, this is probably a consequence of the recent strong winds, which have provoked vertical mixing (with the colder waters underneath) and evaporation.  Wind is the main force for evaporation, more than the sun radiations, and evaporation has a tendency to cool the ocean’s surface. 

Today, the winds have shifted and are now originating from the southeast, as reported in real time by the meteorological SGOF1 tower, conveniently located nearby (click here to view the SGOF1 tower station information - historical data are also archived). These conditions, called “astern” are still not optimal for the SailBuoy's navigation. This is why today we see a drift to the northwest. But as soon as the wind shifts a little bit compared to the direction in which the buoy is programmed to go (northwest) then we expect to observe the buoy back in her full “sailing mode” at higher velocities (up to 1.5 m/s which is about 3 knots!) 

Check out the Deep-C Gulf of Mexico Atlas for yourself.  To map the SailBuoy's, select CMR Sailbuoy on the Base Layer drop-down menu (located in the upper left corner).  You can map its locations (points on the map) or its path.

Posted by:

Dr. Nico Wienders
Florida State University

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