El Nino's Impact on Winter 2015-2016 for North & South Carolina

El Niño is expected to continue through this upcoming winter and into the spring of 2016.  Although El Niño is a warming of the tropical eastern Pacific Ocean and the overlying atmosphere, it has global weather impacts with changes in temperature and precipitation patterns across the planet.  El Niño typically reduces the severity of our hurricane season due to increased wind shear across the tropical Atlantic and Caribbean.  However its largest local impacts are noted during the winter with heavier precipitation typically occurring across the southern United States including the Carolinas.

El Niño is actually just one phase of the El Niño/Southern Oscillation, often called by the acronym ENSO.  ENSO has two phases:  a warm phase we call El Niño and a cool phase called La Niña.  These alternate at irregular intervals of 1 to 4 years.

This particular El Niño is expected to become strong this winter, perhaps rivaling some of the strongest events in recent history.  Looking back over the past 60 years there have been six strong El Niño events we can use to base a winter outlook on.  These strong events occurred during the winters of 1957-1958, 1965-1966, 1972-1973, 1982-1983, 1991-1992, and 1997-1998.

Here are satellite loops from two low pressure systems that developed during the strong El Niño winter of 1997-1998.  Both of these systems developed within an active subtropical jet stream and brought substantial rainfall to the southeastern United States.  The first (January 6-8, 1998) brought 1-2 inches of rain to eastern North and South Carolina.  The second storm (January 23, 1998) dumped another 1 to 3 inches of rain across the eastern Carolinas.  Similar storm systems should be a common occurrence during this upcoming winter.

El Niño should bring lots of rain to the Carolinas...

In almost all cases a strong El Niño brings above-normal rainfall to the Carolinas during the winter months of December through February.  During the winters of 1981-1982 and 1997-1998 extremely large amounts of rain fell causing long periods of river flooding across the eastern Carolinas.  Some of the highest river crests ever observed occurred on the Cape Fear River at William O. Huske Lock and Dam, on the Little Pee Dee River at Gallivant's Ferry, and on the Black River at Kingstree, SC during El Niño winters.

This map shows the observed winter precipitation anomalies averaged across the last six strong El Niño events.  Over the three winter months (December through February) rainfall was consistently above normal across all of the Carolinas.  Precipitation anomalies were largest near the coast; the southern coastal plain of North Carolina averaged 3.04" above normal while the northern coastal plain of South Carolina averaged 3.37" above normal.  Above-normal precipitation typically extends across much of the Southeastern United States and also along the west coast during El Niño winters.

What produces this heavy rain?  Huge thunderstorm clusters over the unusually-warm tropical eastern Pacific Ocean lift tremendous amounts of moisture into the atmosphere.  Powerful subtropical jet streams (frequently observed during El Nino winters) efficiently transport this moisture across the southern United States where it falls as rain -- or occasionally snow!

No clear tendency for cold vs. warm winter temperatures...

During strong El Niños there is a tendency for winter temperatures to run below normal for the southernmost United States from southern California eastward across the Gulf Coast states.  This tendency for cool winter temperatures becomes much weaker across the Carolinas.  Averaging the previous six strong El Niños, we find only a small deviation from normal in either winter mean temperatures or coldest winter temperatures for North or South Carolina. 

Interestingly the strong El Niños of 1957-1958 and 1965-1966 had very cold winter temperatures relative to normal for the Carolinas.  This temperature pattern was reversed during the El Niños of 1991-1992 and 1997-1998.

As we have seen over the past few winters (particularly since 2010) another atmospheric pattern called the North Atlantic Oscillation (NAO) can overwhelm the weather pattern across eastern North America, creating periods of extremely cold temperatures regardless of the state of ENSO or other patterns. 

A positive NAO features a strong west-to-east jet stream across eastern North America and the North Atlantic Ocean, preventing arctic air from plunging south.  A negative NAO is associated with a weaker east-to-west jet stream flow with much greater tendency for cold air to move into the eastern United States.  A negative NAO pattern during the middle of February 2015 helped bring the coldest air in 19 years into Wilmington with 13 degrees measured on the morning of February 20th. 

El Niño brings infrequent, but sometimes large snowstorms...

In contrast to rainfall, snowfall anomalies are not nearly as easy to predict during strong El Niño winters.  During the El Niño winter of 1972-1973 the single largest snowfall ever seen in Florence and Columbia, SC occurred February 9-11, 1973.  This was also the second-largest snowstorm on record in Wilmington, NC and Myrtle Beach, SC.  Strong El Niños in 1991-1992 and 1997-1998 brought very little (if any) snowfall to the Carolinas. Nearly all stations reported below-normal or zero totals for those winters.

Even in a normal (non El Niño) winter snowfall statistics for the coastal Southeastern U.S. are -- strange.  For example Wilmington's annual average snowfall is 1.6 inches.  However snowfall statistics also show we average less than one measurable snow event per year.  The standard deviation computed for Wilmington's historic snowfall events is much larger than the annual average!

Computing a simple average snowfall across the six previous strong El Niño winters shows above-normal totals all across the Carolinas.  However this statistic is dominated by a small number of very large snowstorms, particularly during the winters of 1972-1973 and 1982-1983, which overwhelms the totals.  El Niño does enhance the frequency of wintertime low pressure systems with heavy precipitation, a few of which may encounter cold enough air to produce snow across the Carolinas. 

Winter Outlooks from the NWS Climate Prediction Center...

These outlook graphics from the Climate Prediction Center show the probability of above- or below-normal precipitation (left) and temperatures (right) for the upcoming winter months of December 2015 through February 2016.  Darker green colors on the left indicate progressively higher probabilities of above-normal precipitation.  Southeastern North Carolina and eastern South Carolina collectively have between a 60-70 percent chance of above-normal precipitation this winter. 

The white-shaded area marked "EC" on the temperature map (right) is a forecast for "Equal Chances" of above- or below-normal temperatures for the upcoming winter season.  A forecast discussion to accompany these graphics is available at this link.

Online resources for the 2015-2016 El Niño

• NOAA/Climate Prediction Center:  El Niño/Southern Oscillation (ENSO)
• NOAA/Climate Prediction Center:  ENSO Diagnostic Discussion
• NOAA/Climate Prediction Center:  ENSO: Recent Evolution, Current Status and Predictions (pdf)
• NOAA/Climate Prediction Center:  Oceanic Niño Index since 1950  (values 0.5 or greater for 5+ overlapping seasons constitute an El Niño)
• NOAA/Climate Prediction Center:  Seasonal Temperature/Precipitation Outlooks || Experimental Two-Class Outlooks
• NOAA/Climate Prediction Center:  Prognostic Discussion for Long-lead Seasonal Outlooks
• NOAA/Earth System Research Laboratory:  El Niño/Southern Oscillation (ENSO)
• NASA/JPL:  Ocean Surface Topography from Space (uses satellites to measure sea level changes resulting from El Niño)

Research and Page Author: Tim Armstrong
Page Created: August 23, 2015
Last Updated: August 24, 2015