General: During La Niña, rainfall and thunderstorm activity diminishes over the central equatorial Pacific, and becomes confined to Indonesia and the western Pacific. The area experiencing a reduction in rainfall generally coincides quite well with the area of abnormally cold ocean surface temperatures. This overall pattern of rainfall departures spans nearly one-half the way around the globe, and is responsible for many of the global weather impacts caused by La Nina.

Detailed: In the left-hand panel you can see the seasonal rainfall totals over the Pacific Ocean, the United States, and South America during January-March 1989 when strong La Niña conditions were present. The heaviest rainfall is shown by the darker green and blue colors, and lowest rainfall is shown by the lighter green colors. The rainfall totals are shown in units of millimeters (mm). Since 25.4 mm is equal to 1 inch of rain, we see that the rainfall totals are more than 800 mm over the western tropical Pacific and Indonesia, which is more than 31 ½ inches of rain.

In the right-hand panel you can see the January-March 1989 seasonal rainfall departures from average for strong La Nina conditions. The areas where the rainfall is well above average are shown by darker green colors, and the areas where the rainfall is most below average are shown by the darker brown and yellow colors. These rainfall departures are shown in units of 100 millimeters. We see that rainfall totals were more than 200-400 mm above normal over the western tropical Pacific and Indonesia during the season, which is roughly 8-16 inches above normal! We also see well below-average rainfall across the central tropical Pacific, where totals in some areas were more than 400 mm (15 ¾ inches) below normal.