Remote Sensing Project
back to top page
It is possible to compute currents from the changes over time of the sea surface temperature patterns. This amounts to finding the location of a feature in the second image that was identified in the first image. To eliminate human intervention this is done with cross correlations which give the resulting current that would move the first image pattern into the second image pattern. Unfortunately this method requires that the area be cloud free in both of the images. Thus it is not possible to compute currents for every part of a satellite image. We retrieve images for those areas where the cloud cover is absent in both images.
Because of the high frequency of cloud cover we are forced to combine vector current maps over time to get enough current vectors for our area of interest. We have found that about a ten day compositing period works well with the vectors. For our analysis of the area between Sydney and Hobart we have used this 10 day compositing period for our surface current maps. In these composites we vector average at locations where more than one vector occurs. At locations where only one vector is present it is taken as the representative 10-day vector.
To compensate for the cloud effect on the spatial coverage we have used an optimum interpolation to interpolate the open areas due to persistent cloud cover. This interpolation smoothes the features but does not remove them or totally distort them. The resulting current maps appear quite complex due to the interpolation filling in cloud cover generated gaps in the current field. It is most important to view the vectors as a total field that changes in time. The first 10-day map covers the period from Nov. 15 to Nov. 24, the second from Nov. 25 to Dec. 4, the third from Dec. 5 to Dec. 14 and the fourth from Dec. 15 to Dec. 21.
We believe that these vectors and their changes in time represent well what has happened over the past two months in the area between Sydney and Hobart. We believe that while the directions of these currents is quite accurate, the velocity magnitudes are an underestimate of the real surface current. The persistence in these features is seen from the lack of dramatic month-to-month changes in the current field expressed by this time series of 10-day images. We believe that it is reasonable to interpolate in time to estimate what can be expected for the surface currents. It should be noted that these currents combine all types of currents that reach the surface and it is not possible to separate wind-driven from density driven ocean currents.
