Chances are high that an El Niño event will take hold in coming months, according to the June ENSO forecast issued by the NOAA Climate Prediction Center (CPC) and International Research Institute for Climate and Society (IRI; Figure 1). Most models suggest a moderate event will materialize, which is a slight downgrade in intensity from forecasts made last month. One reason for this change is that subsurface temperatures in the eastern Pacific Ocean, while still well above average (Figure 2), have cooled slightly in the past month. The initial subsurface warming was triggered by a wave that traversed the equatorial Pacific in the spring and has been slowly delivering warm water to the surface over the last two months. The surfacing of this warm water, however, has also led to subsurface cooling that may, in turn, lower sea surface temperatures (SSTs) in upcoming months.
Even though SSTs likely will cross the El Niño threshold, reaching moderate or stronger levels will require the atmosphere to act in concert, which it has yet to do. The strength of easterly winds along the equator and the Southern Oscillation Index are near average, both indicating that the atmosphere is paying little attention to the warm SST pattern. For the nascent El Niño to gain steam, convection normally in the western Pacific Ocean will need to move east towards the emerging warm SSTs in the central Pacific. Convection in the central Pacific Ocean will then weaken the easterly winds along the equator, helping to stall the upwelling of cool water in the far eastern Pacific Ocean and further reinforcing unusually warm SSTs across the eastern and central parts of the Pacific basin. This coupling between atmospheric winds and SSTs is the hallmark of a mature El Niño event.
For those rooting for El Niño, a jumpstart to the atmosphere may be forthcoming if a forecasted Madden Julian Oscillation (MJO) appears. An MJO is a flare-up of convection in the Indian Ocean that moves headstrong into the easterly winds in the tropics and therefore reduces their strength. The fate of ENSO likely will become clearer next month.
The take-home message is an El Niño is expected but it is uncertain whether the event will blossom into a strong El Niño similar to 1997–98, limp along as a weak event, or be in-between. The best guess (for now) sides with moderate strength. For winter precipitation in the Southwest, the intensity appears to matter. Arizona and New Mexico, particularly southern parts of both states, often receive more precipitation during moderate and strong events than weak ones; the statistical relationship, however, suffers from a small sample size. Nonetheless, El Niño events tend to bring wetter conditions to many parts of the Southwest (Figure 3), a welcome sign after four consecutive winters of below-average precipitation and widespread drought.