Originally published in the Mar 2016 CLIMAS Southwest Climate Outlook
El Niño conditions continued for a 13th straight month, but the peak of this event has passed. Monitoring and forecast discussions emphasize strong positive sea surface temperature (SST) anomalies (Figs. 1–2) and enhanced convective activity in the central and eastern Pacific. These positive temperature anomalies are waning, and trade wind activity is increasing, indications that this El Niño event is on the decline. Most forecasts emphasize this event will continue through spring or early summer before returning to ENSO-neutral status. There is also the possibility of swinging to La Niña conditions later in 2016, although there is considerable model and forecast uncertainty regarding the chances of La Niña vs. ENSO-neutral conditions.
On Mar. 10, the Japan Meteorological Agency identified ongoing El Niño conditions that had peaked and were actively decaying and expected to weaken to neutral conditions by summer. On the same day, the NOAA-Climate Prediction Center (CPC) extended its El Niño advisory, identifying current atmospheric and oceanic anomalies as reflecting a strong El Niño that will likely persist through most of the spring before transitioning to ENSO-neutral conditions in late spring or early summer. On Mar. 15, the Australian Bureau of Meteorology maintained its tracker at official El Niño status, but noted a slow and steady decline, with decreasing positive temperature anomalies and near-normal trade winds as two key indicators of this event’s ongoing deterioration. On Mar. 17, the International Research Institute for Climate and Society (IRI) and CPC forecasts described mixed signals regarding El Niño, with zonal winds and SSTs in decline, while convective activity and the Southern Oscillation Index (SOI) remained strong. The IRI/CPC forecast still identifies a return to ENSO-neutral conditions by summer, with a 50 percent chance of transition to La Niña conditions later in 2016 (Fig. 3), but also points out that the spring predictability barrier will affect our certainty regarding ENSO-neutral vs. La Niña outlooks. The North American multi-model ensemble currently shows a strong event extending into early spring with gradual weakening to neutral conditions by early summer (Fig. 4).
This El Niño is not over; atmospheric and oceanic conditions are still indicative of a strong El Niño event. The CPC/IRI forecast noted this fact, stating that “El Niño is not done yet,” and that at a global scale, strong signals are still associated with El Niño, particularly in Brazil and southeast Asia (Fig. 5). In the southwestern U.S., we are nearing our dry season, meaning limited time remains for additional El Niño-influenced precipitation events of significance.
The IRI/CPC forecast also made note of the lack of “typical teleconnections” in this El Niño event. In the Southwest, for example, winter precipitation has been sparse following the storms of early January. These storms were exactly the sort of events expected in an El NIño year, but they were followed by a persistent ridge of high pressure that set up and limited the influx of additional moisture into the Southwest. This diverted moisture resulted in well above-average precipitation in the coastal northwestern U.S. and northern California, even while the Southwest was drier than normal (Fig. 6), a pattern which more closely aligns with La Niña. This occurred at an especially inopportune time in terms of southwestern climate patterns, as it effectively limited the opportunity for El Niño-associated storms during much of the the winter season. Sub-seasonal variability limited El Niño’s potential, and with the Southwest already characterized by dry conditions in a normal year, conditions that limit opportunities for precipitation can cut into seasonal totals significantly.
Next month’s issue will include a seasonal recap of El Niño and comparisons to seasonal averages as well as other El Niño events, but this high pressure ridge is likely one of the major reasons why the Southwest (and Arizona in particular) have not seen as frequent or as intense precipitation events as were forecast in seasonal outlooks. These forecasts and projections were dependent on the influence of a strong El Niño signal at a climate timescale (i.e., how these events cluster over years or decades), without the benefit of foresight of how a persistent high pressure ridge operating at a weather timescale (i.e., days or weeks) would knock the precipitation signal out of alignment for weeks on end.