The El Nino Southern Oscillation (ENSO) and the Seasonal Outlook

Sea surface temperature anomalies showing the three ENSO cycles: El Nino, La Nina, and the neutral state

Some oceanic and atmopheric phenomena ossilate (change back and forth from one state to another) only every season, every few years, or every few decades, occurring in seasonal to multiyear to multidecadal cycles. The state they are in during a given hurricane season may contribute to shaping the amicability of the environment in which developing hurricanes might find themselves.

El Nino (low panel) is an oceanic phenomenon in the equatorial Pacific in which warmer ocean water - normally blown westward by the trade winds, leaving only colder surface water near the South American continent - instead stays atop the sea surface close to the continent. La Nina (top) occurs when the warmer water is blown even farther westward than in the neutral state (middle), allowing the the colder water to stretch all the way out into the central Pacific.

The illustration shows not absolute sea surface temperature but temperature anomaly, such that El Nino (warm phase) appears as a "finger" of anomalous warm sea surface temperature readings protruding out from the West Coast of South America into the Pacific Ocean.

The El Nino Southern Oscillation (ENSO), of which El Nino is only one of the three possible states, is believed to affect many of the wind and sea surface conditions that shape the tropical Atlantic during hurricane season. El Nino/La Nina events change pressure, wind, and temperature relationships all over the world, and every region is affected differently. ENSO changes its state slowly, gradually, between the three poles: El Nino, La Nina, and the neutral state. This occurs on a short multiyear cycle. The state it is in during hurricane season is one factor that is believed to affect the favorability of the general environment with respect to hurricane development and intensification.

El Nino appears to influence the Atlantic hurricane environment in a way that is hostile to storm development and intensification, with La Nina, the cold state of ENSO, seeming to produce the opposite effect (this situation is approximately reversed with respect to hurricanes in the Eastern Pacific). El Nino events tend to be associated with increased vertical wind shear conditions in the tropical Atlantic, such that the variation in wind speed and direction with height in the atmosphere is greater than normal. This produces a hostile environment for hurricanes, as too much vertical wind shear decapitates the developing storm. El Nino years are therefore associated with less hurricane activity in the tropical Atlantic.

La Nina events tend to be associated with less vertical wind shear in the tropical Atlantic, a condition very favorable to hurricane development and hence, associated with more active seasons. Although the ENSO cycle is believed to be one factor affecting hurricane activity in a given season, it is far from the only one. Research to identify and analyze other atmospheric and oceanic contributors to hurricane seasonal activity is ongoing.

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