This site uses a method to determine what the heat content is of the oceans. You can read the opening page above and also the
methodology page for information about the method.
Then take a look at the imagery available, like these areas for the Atlantic:
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Atlantic
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Caribbean
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Gulf of Mexico
They are a bit technical, but basically you want to know to look for the highest regions of TCHP. You do that by looking for warmest colors on the image. They have the highest heat content. The higher the number, the more energy available to the cyclone. Another important measurement is the depth of the 26 degree isotherm. If the depth of the 26 degree isotherm is 100m, that storm has 100 meters of water than is 26 degrees or higher. If the depth is less, like 25m, then the warm water is much more shallow, which means the cyclone has warm water under it that doesn't go as deep. Upwelling of cooler waters could then have a greater impact on the storm's intensity.
Previously, if you wanted to compare one year's image to another year's image, you simply changed the date:
2019:
https://www.aoml.noaa.gov/phod/dataphod1/work/HHP/NEW/2019152go.jpg
2005:
https://www.aoml.noaa.gov/phod/dataphod1/work/HHP/NEW/2005152go.jpg
When testing this in July 2019, older data was not accessible, even through the full list of dates page
here. When available, you couldn't go back farther than 2005. The 152 means that the image is for the 152nd day of the year. (To get the day of the year you can either guess or use a calendar like the one
here)