Warming of tropical oceans increase hurricanes

In response to the public interest and media press on the relationship of extreme weather patterns and climate change, the global community of tropical cyclone researchers put together a statement on new research at the 6th International Workshop on Tropical Cyclones of the World Meteorological Organization (IPCC) in November 2006.

They stated "The surfaces of most tropical oceans have warmed by 0.25-0.5 degree Celsius during the past several decades. The Intergovernmental Panel on Climate Change (IPCC) considers that the likely primary cause of the rise in global mean surface temperature in the past 50 years is the increase in greenhouse gas concentrations.... Some recent scientific articles have reported a large increase in tropical cyclone energy, numbers, and wind-speeds in some regions during the last few decades in association with warmer sea surface temperatures. Other studies report that changes in observational techniques and instrumentation are responsible for these increases."

Hurricanes need warm ocean waters to strengthen and sustain them. Hurricanes do not form unless water temperatures are at least 80 degrees Fahrenheit — hot enough to create atmospheric convection that casts moisture 10 miles up into the atmosphere. Ocean waters were generally two to three degrees Fahrenheit warmer than average during the 2005 season, which favored stronger. Hurricanes can only form in areas of low wind shear, regardless of the ocean temperatures. During 2005, wind shear was very low from the central tropical Atlantic to the Gulf of Mexico.

It is now the 11th year of a new era of heightened Atlantic hurricane activity. This era has been unfolding in the Atlantic since 1995, and is expected to continue for the next decade or perhaps longer. This is attributed to the increase in activity to natural occurring cycles in tropical climate patterns near the equator. These cycles, called “the tropical multi-decadal signal,” typically last several decades (20 to 30 years or even longer). As a result, the North Atlantic experiences alternating decades long (20 to 30 year periods or even longer) of above normal or below normal hurricane seasons. NOAA research shows that the tropical multi-decadal signal is causing the increased Atlantic hurricane activity since 1995, and is not related to greenhouse warming.

The tropical climate patterns now favoring very active hurricane seasons are similar to those seen in the late 1920s to the late 1960s. The current active hurricane era began in 1995, meaning the nation is now 11 years into an active era that could easily last several decades (20-30 years or even longer). We can expect ongoing high levels of hurricane activity — and very importantly high levels of hurricane landfalls — as long as the active era continues.

Most people associate twisters with tornadoes, but in fact tropical twisters come from hurricanes. Hurricanes are what scientists call "strong Tropical Cyclones". They are formed when large areas of the ocean become heated, and the air pressure over that area drops. This causes thunderstorms and strong surface winds. Cyclones develop over tropical or sub-tropical waters (for example, in the Atlantic off the coast of Africa, or in the Pacific). As they travel long distances gathering energy from the ocean, they are likely to be classified as strong Tropical Cyclones. When the winds of a tropical storm reach 74 mph, then the storm is classified as a hurricane.

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