U.S. ENSO Precipitation During Winter Pre/Post 1997-98 El Nino

ENSO:  A look at Winter-time precipitation across the United States prior to and following the epic 1997-1998 El Niño episode

Olivia Kellner, PhD. Climate Impact Company Lead Research Scientist

Introduction

The El Niño-Southern Oscillation (ENSO) is one of the most influential global climate teleconnections on the United States winter-time precipitation and temperature regimes.  Reviewing the intensity of historic El Niño events using NOAA’s Oceanic Niño Index (ONI – 3 month running mean of ERSST.v5 SST anomalies) in the Niño 3.4 region (5^oN-5^oS, 120^o-170^oW)) the following events are of the most intense (each event reaching an average 3-month mean sea surface temperature departure of ≥ 2.0°C) on record: 1965-1966, 1972-1973, 1982-1983, 1997-1998 and 2015-2016.

Until the 2015-2016 event, the 1997-1998 El Niño episode was the strongest on record.  It had numerous, costly impacts on the United States economy in transportation, retail, housing, and agricultural sectors due to the shift and intensity of temperature and precipitation regimes.

Additional modifiers to the intensity of ENSO events include increasing global temperature. Global air and ocean temperatures have continually increased over the last several decades, inducing dynamic shifts in the global atmospheric weather patterns and ocean circulations such as ENSO.  While the exact influence of a warmer global climate regime on ENSO is not known, the general consensus is that ENSO events will become more influential (i.e. more intense) on climate regimes.

When reviewing global temperature anomalies since 1880, a distinct warm anomaly in global temperature began in the 1980s (Figure 1).  When reviewed in context of ENSO event intensity, the 1997-1998 event stands out as a marked, stronger event than previous events begging the question, has the influence of ENSO on climate regimes in the US changed when comparing ENSO events prior to and after the 1997-1998 El Niño event?  This analysis seeks to answer that question through a review of cold-season precipitation anomalies across the United States at State Climate Division level.

Figure 1: Starting in 1980, the marked increase in global temperatures can be seen. Source: https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature.

Figure 2: Annual global temperature anomalies as reviewed in context of ENSO events.  A marked increase in the intensity of El Niño events is seen after 1995-1996 El Niño event when the 1997-1998 event marks a new level of intensity (boxed in dark blue for clarity). Source: https://www.ncdc.noaa.gov/sotc/global/201113.

Data

This analysis reviews the following El Niño events, which were selected because the sea surface temperature anomaly (SSTA) intensity of the event reached or exceeded 1.0°C for at least (one) 3-month period during the event.  They are grouped as pre-1998 and post-1998 events.

Pre-1998 events: 1957-1958, 1963-1964, 1965-1966, 1968-1969, 1969-1970, 1972-1973, 1982-1983, 1986-1987 and 1987-1988.

Post-98 events: 2002-2003, 2009-2010, 2014-2015, and 2015-2016.Each El Nino episode is compared to the 1951-2010 base period of precipitation records to provide a realistic comparison across the time-frame during which ONI data is archived.

Precipitation anomalies during the cold-months (October through March) beginning with quarter 4 (OCT/NOV/DEC; Figure 3-4), quarter 1 (JAN/FEB/MAR; Figure 5-6) and meteorological winter (DEC/JAN/FEB; Figure 7-8) are compared for El Nino prior to and post the 1997-98 episode. Figure 9-10) summarize the cold season precipitation anomalies.

Analysis

Figure 3-4: Post-1998 events (right) show a marked increase in fall to early winter (OCT/NOV/DEC) precipitation across the south-central and eastern Great Plains, and much of the Midwest U.S.

Figure 5-6: Post-1998 events (right) show a marked shift in dry areas across the Midwest and in the West and Southeast U.S. during winter to early spring (JAN/FEB/MAR) precipitation.

Figure 7-8: Pre- (left) versus post-1998 (right) El Nino events show a slight shift in dry areas across the Midwest States during meteorological winter (DEC/JAN/FEB).  Post-events show a shift of drier areas across the Interior West U.S. while The Southeast is wetter.  Also note that post-events are drier in California.

Figure 9-10: Pre- (left) versus post-1998 (right) El Nino events indicate Washington is drier while the eastern Ohio Valley is also drier during the entire cool season (OCT-MAR). Post-events are wetter Texas to the Carolinas and drier in California.

Conclusion

The mid-to-late 1990’s are considered as the onset of long-term cycle change of the El Nino southern oscillation (ENSO), Pacific decadal oscillation (PDO) and Atlantic multi-decadal oscillation (AMO). Additionally, this period is recognized as the onset of climate change as related to CO2 increase into the atmosphere. The historic 1997-98 El Nino occurred during these macroscale climate regimes. So to identify whether El Nino wintertime precipitation has changed (or not) when comparing to El Nino’s prior to the 1997-98 event or afterward is evaluated.

Findings include a much broader wet regime post-1998 across the East, Southeast and to the Midwest U.S. during quarter 4 of an El Nino year. Usually El Nino onset occurs during this time. This regime seems to be occurring in 2018 with the current developing Ell Nino episode.

During quarter 1 of an El Nino year the post-1998 El Nino’s are markedly drier especially in the Southeast and California.

Meteorological winter is more broadly wetter in the Southeast States for post-1998 events and not as dry in the Ohio Valley and drier in the West.

During the 6-month period the post-1998 El Nino’s are wetter East and drier West.

The analysis reveals the most dramatic differences on the front and back end of the 6-month cold season and the tendency is drier West and wetter South/East.

Figure 11: Typical winter temperature and precipitation patterns during an El Niño event.  Source: weather.com

With global climate models (GCM) forecasting continued warming of land masses and oceans, the wet conditions observed in the Southeast U.S. westward to Texas and northward to the Mid-Atlantic States will likely continue to become more extreme during El Niño events while a drier trend occurs in the West. El Nino used to be a reliable wet weather producer for the western states but in this new El Nino climate depended upon rainfall and mountain snow fails. The historically understood upper air pattern associated with El Nino (Figure 11) is likely changing based on the post-1998 El Nino character with the Western Canada upper ridge more expansive to cause drying into the Southwest U.S. while the polar jet stream lifts northward allowing the southern storm track to vault increasing precipitation (also) farther north.