Depth of 26° C Isotherm (Year 2005)

Description:

Scientists often look at the depth of the 26°C isotherm to determine how deep the warm surface layer of the ocean extends. An isotherm is a line of equal temperature and 26°C is equal to 78.8°F. The depth of the 26°C isotherm is estimated from satellite altimeter measurements of the sea surface. The density of sea water decreases with increasing temperature (above 4°C), making the warm water more buoyant than cold water. This means that the warmest water is always at the surface and the coldest water is always at the ocean floor. Where the water is warm through a significant depth, it bulges up (the surface is centimeters higher) compared to the surrounding cooler water. Satellites can measure this bulge, which is statistically correlated, through many ocean temperature measurements, with the depth of the 26°C isotherm.

The 26°C isotherm has been of interest to scientists recently because the heat content of the ocean is related to hurricane intensification. As a hurricane travels over the ocean, it churns the ocean up causing deeper, typically cooler water to come to the surface. When the 26°C isotherm is very deep, the deep water the hurricane brings to the surface through mixing is still relatively warm allowing the hurricane to grow even more. In the case of Hurricane Katrina, even the deep water was warm, allowing Katrina to grow to the category 5 hurricane that it became in the Gulf of Mexico. During the formation of Katrina, the 26°C isotherm extended well over 100 feet below the surface providing plenty of warm water for rapid growth. The pocket of the deepest warm water in the middle of the Gulf of Mexico is called a loop current. When well organized hurricanes pass over the loop current, major hurricanes can form. The strength of Katrina can be correlated to the depth of the warm water. Katrina was at its strongest when it was over the loop current, and then weakened as it approached the shore and moved away from the pocket of warm water in the loop current.

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Notable Features:

• Loop Current right in the middle of the Gulf of Mexico during August
• The depth that the 26°C water extended

Data Category

Major: Ocean

Keywords:

Ocean, Hurricane, Katrina, Temperature, 26°C Isotherm

Data Set Name	Depth of 26 degree isotherm 2005
Data Set Directory Name	26_isotherm
Data Set Source	NOAA/AOML
Data Set Developer	NOAA/AOML
Visualization Developer	Steve Albers, NOAA/GSD
Audio	No
Download	FTP