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El Niño is here…what exactly does that mean for Arizona and New Mexico?

Thursday, September 24, 2015

“El Niño” has been all over the news lately, even garnering comparisons to a Godzilla – a prehistoric sea monster awakened and empowered by nuclear radiation (thank you Wikipedia). This characterization is in response to the near record strength of this El Niño event, which is exciting for climate enthusiasts, but leaves most people wondering; what does a strong El Niño event actually mean for Arizona and New Mexico? Are we talking floods? Droughts?  Plagues of locusts? Additionally, how soon can we expect this “El Niño” character to show up?  In other words, what does a realistic assessment look like?

First of all, El Niño is not a singular weather event that will storm ashore on a particular date.  Climate.gov has come up with an excellent analogy – tying the effects of El Niño to a big construction project - you will definitely feel the effects, but these vary with space and time and are dependent on a number of factors. There are a number of El Niño primers (e.g. the climate.gov ENSO blog) that explain how El Niño works in technical terms, but to put it simply, El Niño is part of the El Niño-Southern Oscillation (ENSO) which is a semi-regular shift on the order of several years (Figure 1, Oceanic Niño Index) in sea surface temperatures, atmospheric pressure, and rainfall patterns, along the equator in the Pacific Ocean. These shifts (between El Niño and La Niña event) reorganize global weather patterns that can drive drought and flood patterns on a regional basis.

Our understanding of the global impact of El Niño events has improved considerably over the last few decades. In particular, we can often detect the development of an El Niño event months in advance, long before it will ever have an impact on seasonal weather conditions.  For Arizona and New Mexico, an El Niño event means increased probability of receiving above-average winter precipitation, generally between October and March. However, there is still plenty of variability when comparing El Niño events, and there is no guarantee that any given El Niño event will lead to wetter than average conditions.

In order to look more closely at this relationship, it helps to take a look at plots describing the relationship between past El Niño and La Niña events, and October-March total precipitation for select regions in Arizona and New Mexico.[1] A couple key points:

  1. We will be looking at precipitation totals within climate divisions, which represent average values over large multi-county areas (Figure 2, AZ & NM Climate Divisions).  These totals are calculated using station observations within these regions. 
  2. We will be using a simple metric called the Oceanic Nino Index (ONI) to assess the phase and strength of ENSO. The ONI describes temperature anomalies (warm or cool) in the central and eastern Pacific Ocean along the equator, which in turn relates to shifts in tropical thunderstorm activity and resultant impacts to global atmospheric circulation

Figures 3a & 3b compare the ENSO-precipitation relationship for two regions in Arizona (climate division 3 and climate division 7).  Total cool-season precipitation is on the y axis, and the average ONI value for the same Oct-March period is on the x axis. The dots are color coded: red to reflect El Niño events (ONI values > 0.5), blue for La Niña events (ONI values < -0.5), and green for ENSO neutral years (ONI between -0.5 and 0.5).. At first glance, the blue dots (La Niña years) are clustered much lower on the y-axis, while the red dots (El Niño years) are much higher on this axis.  This characterizes the historical connection between La Niña events and dry winters, while El Niño years are associated with wet winters. The horizontal lines on the plots are the average cool-season precipitation for all years (black line), La Niña years (blue line), and El Niño years (red line), and help illustrate this relationship further. 

The more you stare at this graph (trust me, we’ve done our share of staring!), the more you see that this relationship is complicated, and even varies between the two regions in Arizona. In general there is a relationship between the ONI index value and the amount of cool-season precipitation (see correlation values on each figure), but this relationship is noisy at best, and demonstrates that the strength of the ENSO phase (in this case using ONI) isn’t a perfect linear fit with total cool season precipitation. Some of the biggest outliers are linked to weak or even moderate strength El Niño events, but are associated with below average winter precipitation (We're looking at you, 2007).  This is especially evident as you move north in the region (see climate division #3), where the relationship between El Niño strength and cool season precipitation is even weaker. What’s the point of all this?  El Niño is not a slam dunk forecast for unusually wet cool season conditions for the Southwest.

But…this year we are in rare territory with respect to the current and forecasted strength of this El Niño event. Current forecast models indicate that ONI values should reach approximately 2 deg C (above normal) over the upcoming winter season, making it one of the strongest El Niño events on record and on par with past big events like 1982-83 and 1997-98. When we look for these years on the plots in figure 2, we see that both of these years were associated with above-average precipitation.  In fact, the three “strong” events (1973, 1983, 1998) and the as well as the five “moderate” years (1958, 1966, 1987, 1992, 2010) all saw above average precipitation, which means that as long as this event stays strong (all indications are that it will) we will likely see a wetter than average winter.  A large caveat being the sample size of this relationship is VERY small.  

This pattern is even stronger for climate division 7 (southern Arizona) where the overall relationship between ONI and cool season precipitation is stronger. This is consistent with observations that the relationship between ENSO and winter precipitation is stronger over southern parts of Arizona (Figure 4, Percent of normal winter precipitation 1983 & 1998) and New Mexico. This can be attributed to a stronger than average and more persistent sub-tropical jet stream that favors southern Arizona and New Mexico with above-average winter storm activity. 

The current seasonal precipitation forecasts for the upcoming winter season issued by the NOAA Climate Prediction Center also reflect this expectation for additional moisture, especially in the SW. The best chances for observing above-average winter precipitation cover all of Arizona and New Mexico, but are slightly better for the southern half of each state. There is a pretty good chance that the next red dot for this upcoming year that shows up on these plots will be above-average for most climate divisions across the Southwest, but how much above-average will be very interesting to see.


[1] We are borrowing this idea from the Western Regional Climate Center.

Monsoon Summary Jun 15-Sep 17

Saturday, September 19, 2015

The monsoon started strong in late June and early July. This early start centered on Arizona, which recorded its second wettest June on record (Fig. 1a), with a return to relatively normal rainfall totals in July. New Mexico saw an increase in precipitation, recording its 10th wettest July on record (Fig. 1b). Rainfall in August and September was mostly below average, which is characteristic of the North American monsoon’s sporadic and spatially limited precipitation events.

These shifts in persistence and intensity are tied to the strength and location of the monsoon ridge, which, depending on its location, can facilitate the flow of organized storm activity from the south or east (during increased monsoon activity) or can shift the flow such that we see extended periods of decreased precipitation. The later season weakening of the monsoon ridge since early July is likely due at least in part to increasing El Niño convection. With this El Niño event set to be one of the strongest on record, it is not surprising that it may have had an expected disruptive effect on monsoon circulation. We also continue to watch eastern Pacific tropical storm activity as it helps drive moisture into the region, a pattern we’ve seen repeatedly in the past few weeks with a number of tropical storm systems, particularly Linda. The effects of these systems can be intense and spatially variable, with southern Arizona experiencing increased humidity and little additional precipitation, compared to serious flooding in northern Arizona and southern Utah. Regional dewpoint readings also illustrate the variability of monsoon activity and the influence of tropical storm activity (Fig. 2).

Looking at cumulative totals to date for the 2015 monsoon, precipitation as a percent of average demonstrates the spatial variability of monsoon precipitation (Figs. 3a–b), while raw precipitation totals show the wide range of normal precipitation totals we see across the Southwest (Figs. 4a–b). These totals can be skewed by a few strong events or even a single strong storm; the percent of days with rain (Figs. 5a–b) highlights the regularity of monsoon precipitation thus far, with much of Arizona and nearly all of New Mexico recording rain events (greater than 0.01 inch) on at least 25 percent of days since June 15. The daily intensity index (Figs. 6a–b) further illustrates the steady nature of most of this monsoon precipitation; higher values indicate much of the rain fell in a single event and lower values indicate more frequent and less intense events.

El Niño Tracker - September 2015

Friday, September 18, 2015

El Niño conditions continued for a seventh straight month, and forecasts and models indicate this event likely will last through spring 2016, remaining strong through the early part of the year. Forecasts focused on the persistence of sea-surface temperature (SST) anomalies (Figs.1–2) and weakened trade winds, ongoing convective activity in the central and eastern Pacific, and El Niño-related ocean-atmosphere coupling.

On September 10, the Japan Meteorological Agency identified persistent El Niño conditions in the equatorial Pacific, especially SST anomalies and convective activity, and forecast that the current El Niño conditions were likely to persist through winter. That same day, the NOAA-Climate Prediction Center (CPC) extended its El Niño advisory, predicting a 95 percent chance that El Niño will continue through winter 2015–2016, with gradual weakening into spring 2016 (Fig. 3). The center cited persistent positive SST anomalies in the central and eastern Pacific and ongoing ocean-atmospheric coupling and convection activity as indicators of an ongoing and strengthening event. On September 15, the Australian Bureau of Meteorology maintained its tracker at official El Niño status; a strong event with the potential to exceed the 1997–1998 El Niño in strength. On September 17, the International Research Institute for Climate and Society (IRI) and CPC forecasts corroborated the forecast of a strong El Niño with the potential to rival the strongest events on record.

The North American multi-model ensemble currently shows a strong event extending into 2016 (Fig. 4). Emergent questions have centered on how this event compares to other strong events such as those in 1982–83 and 1997–98. If El Niño remains on this trajectory, it will likely be one of the top three strongest events on record since 1950. Sensationalistic media coverage already has started but it will be important to temper expectations without minimizing possible impacts. Forecast consensus is for a strong El Niño that extends into winter 2015–2016 and would likely bring above-average winter precipitation in the Southwest, particularly later in the season. It is important to note that this relationship suggests that a strong El Niño event gives the Southwest a much better chance at increased precipitation totals by March or April, but it is far from a guarantee of increased precipitation. In the more immediate future, El Niño conditions could lead to a repeat of 2014’s above-average eastern Pacific tropical storm season, when conditions favorable to El Niño were thought to be driving increased tropical storm activity in the Southwest in September and October.

Southwest Climate Outlook September 2015

Thursday, September 17, 2015

Precipitation:  In the past 30 days, much of Arizona and New Mexico recorded below-average precipitation, although isolated areas received above-average precipitation (Fig. 1). This is consistent with the variable nature of the monsoon, especially during the seasonal transition. Water year precipitation to date (since Oct 1, 2014) offers hope in terms of drought relief, with much of Arizona and New Mexico recording above-average precipitation for the water year (Fig. 2).

Temperature: Temperature anomalies in the past 30 days were between 0 and 6 degrees F above average across most of Arizona and New Mexico (Fig. 3). Despite similarly near-record warm temperatures so far in 2015, temperatures have not felt as hot as they did last year, with above-average humidity suppressing daytime high temperatures and boosting nighttime lows. Arizona recorded near-record high statewide average temperatures from January to August 2015, while average temperatures in New Mexico for the same time period were among the state’s top 10 warmest (Fig. 4). Extremely hot days have been rare, with fewer than average very hot days across the region.

Monsoon: Variable spatial coverage and intensity along with intermittent frequency of precipitation events makes it difficult to characterize any monsoon as “normal.” That said, 2015 has been fairly typical for monsoon precipitation; the storms have been variable, most locations saw regular precipitation events that brought their total precipitation close to long-term averages, and, unlike last year, fewer locations recorded high-intensity precipitation events that dropped a full season or years’ worth of precipitation in a single storm event (see Monsoon Tracker on pgs. 6-7 for details).

Tropical Storm Activity: During late summer and into fall, tropical storms in the eastern Pacific Ocean have a better chance to recurve back into the Southwest, rather than heading west across the Pacific Ocean. Record sea surface temperatures intensify these storms, upping the chance that an organized system will bring moisture (humidity) and precipitation to the Southwest. Despite regular incursions of tropical moisture, we have not seen many heavy precipitation events associated with tropical storms, as we did with Norbert and Odile in 2014, save for recent extreme flooding in northern Arizona and Utah. However, we are just past the midway point of the tropical storm season, leaving time for tropical storm systems to bring additional (possibly heavy) rainfall to the region.

Drought & Water Supply: The U.S. Drought Monitor identifies persistent multi-year drought across the West. Arizona and New Mexico are grappling with years of accumulated drought and water deficits, but water year precipitation has helped scale back drought conditions, particularly in New Mexico (see Reservoir Volumes on pg. 8 for details).

Precipitation & Temperature Forecasts: The Sept. 17 NOAA-Climate Prediction Center seasonal outlook predicts above-average precipitation for most of the Southwest this fall (Fig. 5, top). Notable exceptions are northern California and most of the Northwest. Temperature forecasts are split, with elevated chances for above-average temperatures along the West Coast and into southwestern Arizona, and increased chances for below-average temperatures centered over Texas and extending across most of New Mexico (Fig. 5, bottom).