Pago Pago, AMERICAN SAMOA — Climatologists from NOAA’s Climate Prediction Center forecast are predicting an 82% chance an El Niño pattern will develop between May and July, with the probability of El Niño conditions persisting through early 2027 rising above 96%.

El Niño conditions occur when abnormally warm waters build in tropical region of the central and eastern Pacific Ocean and are usually associated with a weakening of the easterly trade winds, sometimes even reversing to westerlies. Consequently, tropical rains that usually fall over Indonesia move eastward; sea level decreases in the western Pacific; and the vertical, thermal structure of the ocean and coastal and upwelling currents are changed.

EL NIÑO AND ITS IMPACTS ON AMERICAN SAMOA

Rainfall during the year of a strong El Niño starts out below normal in August and dryness continues through June of the year after El Niño. These long -term dry conditions may be punctuated with intermittent wet conditions. The level of these dry conditions depends on the intensity of the El Niño event. For strong El Niño’s, rainfall begins to return to normal by August of the year following that strong El Niño event. With weak and moderate El Niño’s rainfall can be above normal, however this response is not consistent.

During strong El Niño’s, the tropical cyclone season exhibits an enhanced risk of a damaging tropical cyclone from November through January. The risk falls away to near or below average from February through April as TC activity pushes farther to the east. These features of the tropical cyclone season are less distinct during weak El Niño’s.

Sea level in American Samoa usually starts falling during October or November of the El Niño year. The fall is 3-6 months later, when compared to North Pacific islands. Sea levels are lower than average for the entire year after El Niño. Taimasa is the local term given to these conditions.

During El Niño, ocean waters tend to warm substantially in the central and eastern equatorial Pacific at both the surface and subsurface. In American Samoa, however, near or even slightly cooler than normal ocean waters are commonly observed during strong El Nino’s.

EVERY EL NIÑO IS A LITTLE BIT DIFFERENT

El Niño conditions can start to develop as early as May or June and typically reaches maximum strength during December; the conditions then subside towards normal conditions by June of the following year. However, the evolution and duration, strength and impacts of individual El Niño events can vary, in some cases greatly. This makes constant monitoring and awareness extremely important for decision makers across multiple sectors.

BACKGROUND

The El Niño – Southern Oscillation is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean and the patterns of sea level pressure, lower- and upper-level winds, and tropical rainfall across the Pacific basin. On periods ranging from about two to seven years, the surface waters across a large swath of the tropical Pacific Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal. This irregular oscillation between warm and cool patterns, referred to as the ENSO cycle, directly affects rainfall distribution in the tropics and can have a strong influence on weather across the Pacific basin.

(Sources: NOAA, Agricultural Weather Forum)