El-Niño 1997-1998 Modified from .

El-Niño 1997-1998

http://clima.casaccia.enea.it/staff/marullo/

Modified from http://www.eeb.ucla.edu/test/faculty/nezlin/ElNino.htm

The ocean water is stratified, i.e., warm upper layer of low density is separated from deep cold layer of low density.

The zones where the upper part of the water column has lower density (e.g., the upper mixed layer is warmer) is characterized by increased sea surface height and deepened pycnocline.

A combined influence of pressure gradient and the Earth rotation results in formation of eddies: cyclonic where the seawater is more dense and water level is lower (left), and anticyclonic where the seawater is less dense and water level is higher (right).

25 days

125 days

50 days

150 days

75 days

175 days

100 days

200 days

Il risultato dei trades ha prodotto un mixded layer più spesso sul lato occidentale

Questo produce un effetto sull’elevazione della superfice del mare

Different types of global climatological indices are used to characterize the actual El Nino level.

NINO3 index is an anomaly of sea surface temperature averaged over the rectangle 5 ｡ S-5 ｡ N; 150 ｡W-90 ｡W.

Southern Oscillation Index (SOI) is the difference between the Sea Level Atmospheric Pressure in Tahiti and Darwin.

High values of NINO3 and low values of SOI indicate El Nino events. ow NINO3 and high SOI are attributed to La Nina events

1. March 1997, the warm near-surface water layer was thicker in the western Pacific, near New Guinea, and thinner in the eastern Pacific, near Peru

2. In April 1997, the warm near-surface water layer was thicker near the International Date Line (longitude = 180°.

3. June 1997, the warm near-surface water layer was thickest in the central Pacific

4. October 1997, the warm near-surface water layer was thickest in the central Pacific, with water temperature as much as 12 ｼ C above normal.

5. In January 1998, the warm near-surface water layer was thickest in the central and eastern Pacific.