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Monthly Archive

Notes from an Applied Climatologist: How does El Niño affect the monsoon in the Southwest?

Sunday, October 18, 2015

Research on the interactions between El Niño events and the North American Monsoon System suggests that during past El Niño events, there was a slowing of the onset of monsoon precipitation across the Southwest U.S.  

In summer 2014 the monsoon moved in right on time, ushered in by surges of moisture that moved up the Gulf of California into the desert Southwest, several of which were triggered by tropical storms and hurricanes in the eastern Pacific that flourished in highly favorable conditions, including light upper level winds and very warm ocean water.  These conditions were not necessarily related to El Niño, as conditions that favor an El Niño event were largely absent in the atmosphere, with only glimmers of favorable patterns of sea surface temperatures across the eastern Pacific Ocean, and the fact that the current El Niño had been struggling to form (in 2014) may be one reason we had a relatively "normal" start to the monsoon (in 2014)

In summer 2015, monsoon moisture moved in ahead of schedule in late June with dewpoints in southern Arizona skyrocketing from the low 30’s (F) in mid-June to above 60F by the end of the month. Abundant tropical moisture was drawn north up into the Southwest as the subtropical ridge expanded and strengthened over the western U.S. Previous research on connections between monsoon onset and El Nino suggest that during strong El Nino events, the subtropical ridge struggles to form early on and delays the onset of the season. This was not the case this year as a moderate to strong El Nino was fully engaged with the atmospheric circulation pattern across the globe. It is possible that strong Madden-Julian Oscillation activity through June helped offset some of the typical ENSO effects on the North American Monsoon early on in the season.


For more on this year's monsoon, see the monsoon recap in the Oct 2015 CLIMAS SW Climate Outlook

2015 Monsoon Recap - Oct 2015

Friday, October 16, 2015

Originally published in the October 2015 CLIMAS Southwest Climate Outlook

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). Following a break in the monsoon circulation, rainfall in August and September was average in Arizona and below average for New Mexico (Figs. 1c-1d).  These statewide rankings do little to capture the spatial and temporal variability of the monsoon (see below), but they do give a sense as to the general character of the monsoon.  As noted in the October SW Climate Podcast, the monsoon started strong, but tended to fizzle for most of August and September. However we did see a late season push from tropical storm activity, which helped push some of the monsoon seasonal precipitation totals just above average values.  

Image Source - NOAA - National Centers for Environmental Information

Shifts in the persistence and intensity of the monsoon 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 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. 

A particularly notable characteristic of the monsoon this year was the persistently above average dewpoint temperatures we saw over the season (Fig. 2).  This was likely linked to eastern Pacific tropical storm activity driving moisture into the region. In some cases, this provides a supplement to monsoon precipitation, as we saw this year with Linda, 16-E (and last year with Norbert and Odile). These storms can have intense and spatially variable effects, including increased precipitation and serious flooding, as well as driving up the humidity while adding little additional precipitation.

Image Source - National Weather Service

Monsoon Summary (June 15 - Sept 30)

Looking at cumulative totals 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.

Image Source - Climate Science Applications Program

El Niño Tracker - Oct 2015

Thursday, October 15, 2015

Originally published in the Oct 2015 CLIMAS Southwest Climate Outlook

We spent the better part of 2014 (and the first part of 2015) waiting in anticipation for an El Niño event that was initially forecast to be one of the stronger events on record. By early 2015, the event in question had not yet materialized, and some questioned whether El Niño would ever arrive. Eventually it did, and has been going strong for months, with most forecasts indicating that it will remain a strong event through the winter. As this event unfolds, there are numerous impacts we might expect to see across the Southwest over the course of our cool season (approximately Oct - Mar). In the coming months, CLIMAS will aggregate news, information, and commentary about the possible and expected impacts of El Niño, from the perspective of what is most relevant and applicable to the Southwest. This will include what we have learned from past events, and what forecasting and models can tell us about planning for this event.

For more information, please visit our repository for ENSO related materials, which we will update with timely and relevant information about El Niño throughout the winter.


2015 El Niño Tracker

El Niño conditions continued for an eighth straight month, and models continue to forecast a strong El Niño event that likely will last through spring 2016 and remain 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, enhanced convective activity in the central and eastern Pacific, and El Niño-related ocean-atmosphere coupling.

Image Source - Australian Bureau of Meteorology

Image Source - NOAA/NWS - Climate Prediction Center

On October 8, the NOAA-Climate Prediction Center (CPC) extended its El Niño advisory, predicting a 95 percent chance that the current strong El Niño conditions 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 October 9, the Japan Meteorological Agency identified ongoing El Niño conditions in the equatorial Pacific, noting “remarkably above-normal” SST anomalies and convective activity, and forecast the current El Niño conditions would continue into spring. On October 13, the Australian Bureau of Meteorology maintained its tracker at official El Niño status, noting the strongest event seen since 1997, and forecast the event will persist into early 2016. On October 15, the International Research Institute for Climate and Society (IRI) and CPC forecasts indicated a 100 percent chance of El Niño through early 2016, and saw continued evidence in both the ocean and atmosphere that this event would be one of the strongest events on record.

Image Source - International Research Institute for Climate & Society

The North American multi-model ensemble currently shows a strong event extending into 2016 with gradual weakening heading into spring (Fig. 4). We continue to grapple with this El Niño event and the potential impacts it might bring to the Southwest over the winter and into spring, especially given current forecasts that have it pegged as one of the strongest events on record. Seasonal forecasts (see Fig. 5 on Maps Page) and past events suggest we might expect well above-average precipitation totals across our cool season. This does not mean we should expect a winter of daily rains, however. Variability across these months means we may see periods of below-average precipitation as well. 

Image Source - NOAA/NWS - Climate Prediction Center

Looking at the 1997–1998 event—the strongest El Niño event on record—most of Arizona and New Mexico received above-average precipitation in December but below-normal precipitation for all of January before returning to normal or above-normal precipitation in February and March (Figs. 5–8). At this point, we are fairly certain that El Niño will remain on the current trajectory, and we will see one of the top three strongest events on record since 1950. This is likely to bring above-average winter precipitation to the Southwest, particularly later in the season, but it is far from a guarantee. Additionally, what happens in late spring and early summer may determine the longer-term impacts. If we bounce back into La Niña conditions, as happened after the 1997–1998 event, we may see a return to more below-average precipitation forecasts heading into 2016–2017. 

Image Source - Western Regional Climate Center - WestWide Drought Tracker

Image Source - Western Regional Climate Center - WestWide Drought Tracker

Image Source - Western Regional Climate Center - WestWide Drought Tracker

Image Source - Western Regional Climate Center - WestWide Drought Tracker