Sea Science: by Jordanna Sheermohamed
As we near the end of the Atlantic Hurricane Season, which is June 1-Nov. 30, many wonder how we can become better at predicting cyclones by understanding the signs from the skies and seas. By the way, the term “cyclone” is synonymous with “hurricane” and “typhoon,” the only difference being where they are geographically located. Tropical cyclones are called hurricanes in the Atlantic and Eastern Pacific oceans, typhoons in the Central and Western Pacific, and simply cyclones in the Indian and South Pacific ocean basins.
Cyclogenesis, the birth of a cyclone, requires that a specific set of conditions be in place in order for development/intensification. Let’s dissect these six major ingredients to fully understand the complexity of one of nature’s most powerful forces.
The first is sea surface temperatures, or SST. Ocean waters must be above 26C (79F) to provide the heat to “fuel” the system. A Bureau of Meteorology study demonstrated that 98.3% of global cyclone formation occurs when SST values exceed 25.5 C (77.9 F)
The second is an unstable atmosphere, which provides lots of vertical motion. Warm air continues to rise until it finds itself surrounded by air of an identical temperature. Provided there is adequate moisture present in the atmosphere, this rising warm air and moisture work in tandem to develop clouds. If the rising motion continues unchecked, this will allow the clouds to continue building vertically, which brings the potential for thunderstorms.
Third is relative humidity, or RH, which is the amount of moisture available in the atmosphere, compared with how much it could fully hold (100% humidity). High values of RH need to be present from the lower to middle portions of the atmosphere. Low values of RH cannot support cloud/thunderstorm development, and the 50% threshold of RH is borderline at best. RH values of 70% and above are considered prime.
The preexisting condition of a simple thunderstorm blossoms into a potential area for development, if it is able to develop over or migrate into a region with the above mentioned factors. So the fourth ingredient is wind shear, or how wind speed and direction change with height. These changes must be at the ideal level: enough to sustain a counterclockwise flow – low pressure spins counterclockwise in the Northern Hemisphere – but not strong enough to move the heat and moisture away from the center of the system and essentially destroy the vertical integrity of the cloud column.
Fifth is the Coriolis force. As a result of the Earth’s rotation, this force induces motion to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. A prime example of where the Coriolis force is regularly observed is in ballistic trajectories, which are often aimed slightly off from a target, never directly at it, to compensate for the Earth’s rotation. The amount of Coriolis force increases as the distance from the equator increases. The sweet spot for cyclone development is about 500 km (310 miles) from the equator, although formation outside of that zone is entirely possible. It is physically difficult for formation to occur within 5 degrees of the equator, because the amount of Coriolis force is simply too weak. Consequently, once a system rises above 20 degrees latitude, the other above-mentioned conditions become harder to achieve or maintain, so the ideal, or “Goldilocks Zone,” for cyclogenesis remains between 5 degrees and 20 degrees.
The sixth ingredient is the time frame. The window for formation and intensification of hurricanes in the Atlantic Basin is generally from June 1 to Nov. 30. Of all the Atlantic storms on record, 97% have formed within that time frame. What about the other 3%? The earliest known system has been re-analyzed to have occurred in January 1938, and the latest known system occurred in December 1954, toward the end of the month. So it’s important to remember that, while unlikely, it’s both historically and statistically conceivable for conditions to be ripe outside of the general “hurricane season” time frame.
When ocean basins reach the required temperatures to support a cyclone breeding ground and all the other conditions are met, then a hurricane/typhoon/cyclone is a-comin’.
Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a weather-forecasting firm (WeatherForecastSolutions.com). Comments are welcome below.