Update Frequency - Please Read This! - Altimetry charts are updated daily. The Altimetry data is about 18 hours delayed when we receive it, so the images will always be 1 day behind. As with all government satellite data, at times the data is not available to us due to issues within the processing pipeline. When that happens we will get the images caught up as soon as the data becomes available.
What is Altimetry?- Altimetry satellites calculate the distance from the satellite to the surface (ocean or land) by measuring the round trip time of a radar pulse. Based on this, ths surface height of the water can be determined.
Interpreting the information - The charts that we are producing at this time show the Sea Suface Height (SSH) Anomaly (the difference from sea level), measured in centimeters. The general concept is that the oceans currents and eddies will push the water to different levels. From a fishing perspective, the usual interpretation is that this shows areas of upwelling and downwelling. Upwelling water would be bringing nutrients to the surface, while in downwelling areas the nutrients will be moving away from the surface. Note that the way this shows on the charts is actually opposite of what you would assume - high levels on the chart indicate areas of downwelling, while low levels on the chart indicate areas of upwelling. The more productive areas for fishing would likely be the boundaries between the highs and lows.
Another theory in some parts of the country is that warm eddies will be higher than the basic level, while cold eddies will be lower than the surrounding water. However, this is a generalization and it doesn't seem to apply in all areas. At times we will see highs that are associated with areas of cooler water, so you can't just assume that the highs are warm water. The areas between these highs and lows would have stronger currents and are likely to have more activity.
It's important to realize that this data is looking at the ocean on a very large scale. You are not going to see the subtle changes and edges that we look for in the much smaller scale SST and Chlorophyll charts. But, the altimetry data can give a good overview of the bigger picture and the general movement of the waters. When we have a long stretch of cloud cover that prevents us from getting and useful SST data, this can at least be a good reference to what is happening out there, since this data is not affected by clouds. It can be helpful to use the Loop Tool to get a feel for how the waters are moving.
If you would like to do some further reading about satellite altimetry, try the link below.
Ocean Surface Topography Overview