Purdue University researchers have found evidence that tropical cyclones and hurricanes play an important role in the ocean circulation patterns that transport heat and maintain the climate of North America and Europe.
According to Matthew Huber, Purdue University professor of earth and atmospheric sciences who led the research group, the findings support a 2001 theory by MIT professor Kerry Emanuel that suggests there is an additional factor to be included in climate models that may change predictions of future climate scenarios.
"It was thought that hurricanes occurred over too short of a time period and over too small of an area to affect the global system. This research provides evidence that hurricanes play an important role and may be one of the missing pieces in the climate modelling puzzle," said Prof. Huber.
Prof. Huber said their research also showed that hurricanes cool the tropics, forming in response to higher temperatures and acting as a thermostat for the area.
"Warm water fuels hurricanes, which have been shown to leave cold water in their wake. I like to say the good news is that hurricanes function like a thermostat for the tropics, and the bad news is that hurricanes function like a thermostat for the tropics," said Prof. Huber.
"The logical conclusion of this finding, taking into account past research into the impact of rising temperatures on cyclone and hurricane intensity, is that as the world and the tropics warm, there will be an increase in the integrated intensity of hurricanes," he said.
Ryan Sriver, the paper's lead author and a Purdue graduate student said, that while normally, the upper part of the atmosphere works like a conveyer belt travelling from the south to the north, passing through the Pacific Ocean and Indian oceans and past warmer latitudes warming the water brought to North America and Europe, in the tropics, this pattern must be reversed.
"Warm, buoyant water must be mixed downward, and cold, dense water must be mixed upward. This process, called vertical mixing, plays an important role in the conveyer belt's circulation. It was known that this mixing occurred, but the cause was not well-understood," Sriver said.
"Climate models today use what is called 'background mixing' to solve this problem. They represent the mixing as an average of the total amount that is needed and apply it over these regions consistently. However, we believe this mixing is not consistent; it is not everywhere all of the time. It is sporadic and happens over a small area for a limited amount of time.
"In some areas of the world, such as the equator, there are no cyclones, and no mixing occurs," he said.
"If cyclones were added to models in place of the background mixing, there would be zero mixing at the equator. This is very important because it is well known that to get El Niño right in a climate model, the background mixing at the equator must be greatly reduced. Our data has a beautiful no-mixing zone right where there should be no mixing," added Prof. Huber.
According to Prof. Huber, this study also explains the mystery of the observed temperature from the distant past during a greenhouse climate.
"The poles were much warmer than today, about 82 degrees Fahrenheit, but the tropics were not much warmer than the present," said Prof. Huber.
"Using the best, most comprehensive models in existence, we could not obtain results that matched this past climate that we know existed. We knew a basic, fundamental process that cooled the tropics was missing from the models," he said.
"Our results suggest that this is the missing mixing and it is a vital part of ocean circulation," he added.
The findings appear in the May 31 issue of the journal Nature.