El Niño may influence hurricanes

A developing El Niño in the tropical Pacific Ocean could hinder development of tropical cyclones in the Atlantic basin this year.

Rain and waves strike Manzanillo, Mexico

Rain and strong waves hit the beach as hurricane Andres arrives in the Pacific resort city of Manzanillo, Mexico Tuesday. Andres strengthened into the Pa-cific season’s first hurricane as it flooded homes, toppled trees and killed at least one person on its way up Mex-ico’s southwestern coast. Andres weakened from a tropical storm to a depres-sion Wednesday as it headed out to sea. Late Sunday, Andres became the first named storm of the eastern Pacific hurricane season, which began May 15 and ends Nov. 30. It was the latest arrival of a named storm in 40 years. Photo: AP

Earlier this month, the Climate Prediction Center of the US-based National Oceanic and Atmospheric Administration issued an El Niño watch as part of its monthly El Niño/Southern Oscillation diagnostic discussion. That synopsis concluded that conditions were favorable for a transition from ENSO-neutral to El Niño conditions during June to August 2009.

Last Monday, in its weekly ENSO forecast update, the Climate Prediction Center noted that warmer-than-average sea surface temperatures continue to increase across much of the equatorial Pacific Ocean.

‘Current observations and dynamical model forecasts indicate conditions are favourable for a transition from ENSO-neutral conditions to El Niño conditions during June-August 2009,’ the CPC reaffirmed.

El Niño is a cyclical weather pattern involving an anomalous warming of sea surface temperatures in the equatorial Pacific. Based on historical observations, El Niño years tend to lead to fewer hurricanes in the Atlantic basin because it creates wind shear in the upper atmosphere that inhibits tropical cyclone formation.

However, Cayman’s Head of Meteorological Services Fred Sambula said residents here should not let their guard down even if the expected El Niño materialises.

‘El Niño does tend to reduce tropical cyclone activity in the Atlantic,’ he said. ‘But while the activity may be diminished, it does not mean there will be no storms to form.’

Mr. Sambula repeated his mantra that it only takes one storm to impact the Cayman Islands for it to be considered an active hurricane season here. He said the case of Hurricane Andrew in 1992 was a classic example of how an El Niño year could still spawn powerful Atlantic hurricanes.

The 1992 hurricane season only produced seven tropical storms and four hurricanes, both numbers below average, but Hurricane Andrew became the second strongest hurricane to make landfall in the United States in August of that year, causing 65 deaths and US$26.5 billion in damage.

In addition to hindering tropical cyclone development in the Atlantic basin, El Niño also affects rainfall patterns in different places, Mr. Sambula said.

‘Parts of Australia become dry and certain parts of the US, based on experience, get more rain,’ he said.

The Precipitation Outlook issued by the Caribbean Institute for Meteorology and Hydrology in Barbados calls for below-normal rainfall in the Caribbean, especially in the northern and western part of the region. The institute cites ENSO conditions as one of the reasons for the anticipated decrease in rainfall.

Saharan dust

According to a NASA study released in 2007, huge dust storms coming off the western coast of Africa can also disrupt tropical cyclone formation.

These dust storms created in the Sahara Desert are sometimes so vast they block sunlight from reaching the ocean surface in the tropical Atlantic, causing the sea surface temperatures to decrease. The warmer sea surface temperatures are, the more conducive it is for tropical cyclone formation and intensification. The dust also hinders updraft in thunderstorms that give energy to hurricanes.

An experimental seasonal prediction of African dust over the Atlantic produced by University of Wisconsin researcher Amato Evan suggests that summertime dust storm activity in 2009 will be below the 1982-2008 average.

Less Saharan dust would normally support more hurricane activity. However, Mr. Sambula said dust was just one more variable for the development of hurricanes.

‘There are a number of variables for tropical cyclones to form,’ he said. ‘Some of the variables are necessary conditions, but not sufficient on their own; other variables come into play.’

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