Everyone knows something about tropical cyclones, the broad class of tropical low-pressure systems that includes hurricanes and typhoons (Fig. 1). Many of us know when and where they are more likely to form and how they are named. If you were asked about the most favorable conditions under which these tropical systems develop, among the first things you would probably say would be sea surface temperature. Some of you may also know that a sea surface water temperature above about 80oF is a very favorable factor. No wonder that hurricanes form over low-latitude ocean areas.
A hurricane or its antecedent weather system could be thought as an engine, drawing heat energy from the water body underneath it. The system does this in two ways. First, air touching warm water is warmed by contact (conduction), and air heated from below wants to rise (convection)*. Second, water evaporates from the warm ocean surface (with latent heat trapped in the water vapor) and is then carried upward by rising air currents (updrafts). The latent heat is released during the phase change from water vapor to water droplets (condensation and cloud formation) and that heat is then distributed vertically within the updraft (Fig. 2).
Now, think about a weather situation modeled in your kitchen. Imagine that you have a bowl with hot, steaming soup. Would you rather cool it down by blowing on the whole bowl or a small portion of the soup in a spoon? Almost everybody would choose the second option. Well, now think about the force of the winds in a hurricane. Don’t you think they are strong enough to lower the water temperature? And if you take also into account the large amounts of chilled precipitation that falls on the ocean surface, then you would realize that we need something else to keep providing the necessary energy for hurricanes.
The depth of the isothermal layer (a layer in the ocean that has an almost constant temperature) plays an important role in sustaining and intensifying hurricanes and tropical cyclones. Considering its depth and its temperature, we could then estimate the available potential energy for the hurricane’s heat engine. It is important to note that the hurricane circulation is strong enough and its pressure low enough to actually lift the surface of the ocean, causing deeper layers with a lower temperature to rise, thereby squeezing this isothermal layer somewhat. Depending on the strength of the storm’s circulation, how low the atmospheric pressure is in the center of the storm, and the storm’s speed of motion, the effect can be quite strong. It could possibly even generate a cool surface water “footprint.” Obviously, such cool “footprints” can weaken a hurricane.
Conversely, we can say that there must be certain areas where a tropical cyclone could intensify. Tropical meteorologists call these “warm pools.” Forecasters can monitor these and ascertain how they affect a hurricane’s growth and/or intensification.
*Sensible heat is almost negligible, since it depends of the temperature difference between the water surface and the air, which in the tropics is usually less than 1oC ( Emanuel, K: The theory of hurricanes, 1991).
© 2017 Mayguen Ojeda
Originally posted 6/17/17