Vertical temperature gradients are computed from the temperatures and depths reported in BT observations. The vertical temperature gradients show the change in temperature over a given vertical distance. The gradients may be positive or negative. Positive gradients show an increase in temperature with depth, while negative gradients show a decrease in temperature with depth. The greater the change in temperature(+) or (), the stronger the gradient.

Positive gradients refract sound waves upward toward the sea surface. Negative gradients refract sound waves down toward the ocean bottom. Various combinations of positive and negative gradients exist within the mixed and/or sonic layer, while the main and conditional thermoclines have strong negative temperature gradients. Vertical temperature gradients are computed to assess their effect on sound waves. For exam-ple, go back to the temperature-versus-depth profile provided in figure 1-4-5. The temperature difference between the surface and 15 meters is zero. Because there is no change in temperature within the 15 meters, the gradient is 0.0C, and the sound wave would be refracted upward slightly. Between 15 meters and 55 meters, the temperature decreases 2.4C per 40 meters, the 40 meters being the difference between the 15 and 55 meter depths. For standardization purposes, all gradients are computed per 31 meters.

Therefore, the gradient of -2.4C per 40 meters must be changed to reflect the change in temperature per 31 meters. This is done using a ratio formula. In this case, 2.4C is to 40 meters as X (the unknown) is to 31.

Because the temperature is decreasing with depth, the gradient is classified as a negative (1.9C per 31 meters). Sound waves in this layer would be refracted down toward the bottom. For train-ing purposes, plot a few BT observations on graph paper, and determine the MLD, SLD, and vertical temperature gradients. Have your chief or immediate supervisor check your work.

Fundamentals of Oceanography, NWS-AG-J- 095, NOCF Bay St. Louis, Miss., February 1982.

Ocean Thermal Structure Forecasting, SP-105, U.S. Naval Oceanographic Office, 1966. 

Idyll, C. P. Exploring the Ocean World, Thomas Y. Crowell Co., New York, 1972.

Practical Methods of Observing and Forecasting Ocean Waves by Means of Wave Spectra and Statistics, H.O. Hydrographic Office, 1955.

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