Source: NASA/Goddard Space Flight Center
Warm water fuels the tropical storms that ultimately form hurricanes. In this video segment adapted from NASA's Goddard Space Flight Center, learn how El Niño events — climatic anomalies that occur periodically in the Pacific Ocean — alter the course of atmospheric circulation and lessen hurricane formation in the Atlantic Ocean.
Hurricanes can originate over warm tropical oceans when one of three scenarios exist: (1) ripples indicating instability develop in the easterly trade winds; (2) mid-latitude weather systems intrude into the tropics; or (3) low pressure areas form where the trade winds of the northern and southern hemispheres converge, or meet. For the Atlantic Ocean basin, the most common precursor to a hurricane is ripples in the easterly trade winds, called easterly waves, which come off the African continent.
In order for any of these weather scenarios to generate a hurricane, ocean waters must be a least 26°C (79°F), and winds must be blowing at about the same speed and in the same direction at all atmospheric levels, from the ocean surface to the atmosphere's upper levels. Since warm ocean waters fuel the thunderstorms associated with hurricanes, most Atlantic Ocean hurricanes occur in the late summer or early autumn, when Caribbean and Atlantic temperatures are typically at their warmest.
During El Niño years, the Pacific basin experiences a slackening and reversal of easterly trade winds, which disrupts the normal flow of ocean currents. As a result, hurricane activity generally increases in the Pacific basin, especially in the South Pacific, the central North Pacific near Hawaiʻi, and the western North Pacific between 160° East longitude and 180° East longitude (the International Dateline).
In contrast, the Atlantic basin generally experiences an increase in upper-level wind speeds during El Niño years, resulting in decreased hurricane activity, particularly in the Caribbean Sea. These faster upper-level winds disrupt the relatively uniform wind speed and direction needed for hurricane formation, and thunderstorm complexes rip apart, or shear, before they can develop into hurricanes.
In 2004 and 2005, the Atlantic hurricane season was very active, but 1997 — an El Niño year — experienced relatively few hurricanes compared with the long-term average, and those that did form were weak and short-lived. However, El Niño years do not always mean a quiet hurricane season in the Atlantic. In fact, the El Niño years of 1965 and 1972 were some of the costliest in terms of hurricane damage. Hurricanes can strike in any year and cause large losses — both financially and to human life.
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