Relationships between autumn precipitation anomalies in southeastern South America and El Niño event classification

PABLO L. ANTICO

INTERNATIONAL JOURNAL OF CLIMATOLOGY

Wiley

Lugar: Chichester; Año: 2009 vol. 29 p. 719 - 727

0899-8418

The classification of El Niño events was performed based on the time evolution of sea surface warming in the tropical Pacific Ocean during the period 1950-2000. Two sets of events were constructed: one in which the warming core migrates eastward along the tropical Pacific until April-June of the following year, and another one in which it evolves westward until November-January. The first type has associated positive precipitation anomalies over southeastern South America during April-June. It results from a favorable combination of cyclonic vorticity advection and humidity convergence. At high-level, cyclonic vorticity advection is explained in terms of an eastward extension of the subtropical jet. Enhanced humidity advection takes place by an increased low-level northwesterly flow to the east of the Andes. It provides enough moisture availability that, in combination with the upper-level cyclonic vorticity advection, supports heavy precipitation during April-June. The second type of events exhibits slight negative or near normal precipitation anomalies over the same region. Both low and high-level circulation anomalies are also weaker in this case. The 1997-98 El Niño is analyzed separately because it cannot be classified into any of the previously described event types.The observed distribution of both types of events along the analyzed period changes after the 1970s. Comparison with other authors- results suggests the influence of low-frequency processes such as the Pacific Decadal Oscillation.