What are Geostrophic Currents?Geostrophic currents are the major oceanic flows created by a combination of gravity, winds, and the “Coriolis effect”. The Coriolis effect is the force being applied to the water due to the rotation of the earth. It’s more understandable when we relate it to the Altimetry data. On the Altimetry charts, we see different levels of the sea surface height, both highs and lows. Water naturally wants to flow downhill, from highs to lows. The rotation of the earth then applies the Coriolis force to the movement of the water, which causes the water to move around the highs (to the right, in the Northern Hemisphere). The currents are calculated using this surface height data from the satellite altimeters.

Clear as mud so far, I know. We’re not trying (nor are we qualified!) to teach Oceanography here, and I’m sure you mainly want to know how this can help find the fish. In some areas it’s pretty clear, the currents can help show the edges of the Gulf Stream or Loop Current, as well as identify and track eddies. SST and Chlorophyll data will also show these features, but aren’t much help during cloudy periods. Looking at both types of data should give you more information to work with during those times. In other areas the currents may not be as distinct, but the same theory should apply – we are generally looking for the transition zone, or the “edge”.

We display the same currents data in two different formats each day. The first option shows the currents overlaid on a color-coded field based on the Dynamic Surface Height of the water. This is the height of the ocean surface as determined by the satellite altimeter. In areas where the currents are not especially strong, this may be the best option for seeing the edges. The second option shows the currents overlaid on a color-coded field representing the velocity of the current. This is a good option for finding particularly strong currents, and indicating the edges of these areas. We suggest you look at both, and decide which is more applicable to your region at the time.

Interpreting the arrows used on the currents charts The current flow and direction is shown by the arrows on the chart. Areas with very little movement will show as just an arrow head, with no tail. The longer the tail, the faster the current is moving in that area. In regions where the currents move fast, the Currents/Velocity charts are the best indicator of specific speeds. On these charts the velocity of the current is represented not only by the arrows, but also by the color coding on the chart. This shows the velocity in meters per second. An easy approximate conversion to knots is to multiply by 2. (The actual conversion is 1 meter/second = 1.94 knots).

If you would like to do some further reading about currents, here are some links listed below.