A strong pulse of warm water traversed the equatorial Pacific Ocean from west to east during the last several months, setting in motion the emergence of a possible El Niño event. Consequently, the NOAA Climate Prediction Center (NOAA-CPC) issued an El Niño Watch in March. El Niño events are characterized by unusually warm sea surface water from the middle Pacific Ocean (near the International Date Line) to South America. Very warm water has emerged this spring along the coast of Peru and into the Pacific Ocean along the equator (Figure 1). The timing and pattern of these warm waters resembles conditions in 1997, a year in which El Niño became one of the strongest on record. NASA recently released satellite images that showed similarities in sea surface height anomalies between this May and those of May 1997; height anomalies are related to sea surface temperatures (http://1.usa.gov/QMZwfE). While it is too early to estimate the strength of this year’s nascent El Niño, wind conditions suggest it will continue to strengthen. Near the surface around the equator, winds typically blow westward, pushing warm water towards Indonesia. Recent observations indicate these winds have weakened and at times even reversed direction. These changes can help reinforce the pooling of warm water in the eastern Pacific by enabling the warm water piled in the west to move eastward. The slackened winds give rise to the belief that it’s not a question of if, but when an official El Niño will be declared.
For an El Niño event to be designated official, SSTs in the mid-Pacific Ocean along the equator need to remain above average for several consecutive months. According to the ENSO forecast issued by the NOAA-CPC and International Research Institute for Climate and Society (IRI) in mid-May, chances for an El Niño event occurring in coming months rises sharply, from 48 percent in the May–July period to 69 percent in the September–November period (Figure 2). The chance of a La Niña event returning during this time is very small, and neutral conditions also look unlikely. Even though El Niño seems a near lock, climate model simulations in May struggle to determine the short-term evolution of El Niño. Nonetheless, confidence is growing that at least a weak to moderate event is very likely to persist through the 2014–2015 winter. Moreover, there are hints that this event could become strong, similar to 1997–1998.
The speed and eventual strength of the burgeoning El Niño event will influence the impact on the Southwest in coming months. If the event quickly gains strength and persists through next winter, changes in weather patterns across Arizona and New Mexico may include the weakening of the monsoon ridge and a delay in monsoon precipitation; enhanced late summer and early fall tropical storm activity in the eastern Pacific Ocean, which increases the risk of storms striking land and drenching the Southwest; and increased winter storm activity starting in December and persisting through February or March.