If El Niño develops over the summer or by the early fall – as NOAA is hinting – the Mid-Atlantic could be in for a cool summer, relative to normal. For each of the three big cities in the region – D.C., Philadelphia and Richmond – most of the summers in which El Niño developed since 1950 have featured temperatures cooler than the 30-year average.
More interestingly, temperatures averaged either near normal or below normal for the seven summers in which El Niño developed slowly. El Niño development – if set to happen this summer or early fall – could similarly occur at a slow pace.
The reason that El Niño could be slow to develop involves the current sea-surface temperature (SST) anomaly pattern in the Pacific Ocean. Anomalies indicate how much higher or lower a value deviates from some baseline value; in the case of SSTs, anomalies are represented in degrees Celsius, whereby a positive anomaly (say, 2C) tells you that the temperature of the sea surface is warmer than average by 2 degrees. A negative anomaly tells you that the water is colder than average.
To date in March, SST anomalies are very warm in the Gulf of Alaska, but, in the area monitored for El Niño (and La Niña) development along the eastern equatorial Pacific, they are cold. Further west toward the dateline (180 degrees longitude) and especially around the dateline, SSTs are above normal and trending warmer (refer to the graphic below). If and/or when this relatively warm water flows east of the dateline and fully or partially extends to the northwest coast of South America (toward Peru), then an El Niño event will have started.
Before the warmth, which is building more rapidly below the sea surface in the central and western equatorial Pacific, can expand and start to dominate in the eastern equatorial Pacific, it will have to overcome the stubbornly cold SSTs centered near 130 degrees west. El Niño events are associated with SSTs that are at least 0.5C above normal. Right now, in the cold region, SSTs are 0.4C below normal. Many of the models that attempt to project SST anomalies for the region favor substantial warming of these SSTs through the spring and into the fall.
The warming trend is strong enough on the models to lift anomalies to the 0.5C threshold by July. However, most of the same projections do not warm the SST anomalies higher than 0.7C or 0.8C by September. Meteorologists generally agree that SST anomalies between 0.5C and 1C define a “weak” El Niño event.
So, with the current pattern and projections in mind, I first looked at the average summer temperature for D.C., Philadelphia and Richmond during the seasons in which a weak El Niño event started. If the El Niño started before June or after August, its associated summer temperature data was not included in my analysis (with one exception: 1976, in which El Niño started in September, but SSTs were cold during the previous winter). This filtering produces seven summer-starting weak El Niño years: 1951, 1963, 1968, 1976, 1986, 2004 and 2009. Here are the results:
- D.C.’s average summer temperature over the seven years is 76.8F, or nearly 1 degree colder than average. The lowest summer temperature (75.7F in 1963) was 2 degrees colder than average, while the highest summer temperature (77.7F in 1968) was exactly normal.
- Philadelphia’s average over the seven years is 74.6F, or 1.3 degrees colder than average. The lowest summer temperature (72.5F in 1963) was nearly 3.5 degrees colder than average, while the highest summer temperature (75.3F in 1968 and 1986) was about a half-degree colder than average.
- Richmond’s average over the seven years is 76.4F, or nearly 1 degree colder than average. The lowest summer temperature (74.6F in 1963) was 2.8 degrees colder than average, while the highest summer temperature (77.5F in 2009) was one-tenth of a degree warmer than average.
The seven years produce a composite temperature anomaly map that is cooler than normal across nearly all of the nation:
Now, let’s look at the field of all other summer El Niños (10 in total). These include events that started before June (and lingered through the summer) as well as events that reached moderate intensity by July. El Niños reach “moderate” intensity when SST anomalies warm higher than 1C. Here are the results:
- D.C. was colder than normal in 7 of the 10 summers, with an average temperature about a half-degree below normal. The coldest anomaly occurred in 1972, with temperatures slightly more than 3 degrees below normal.
- Philadelphia was also colder than normal in 7 of the 10 summers. Here, the average temperature was 0.8 degree below normal. The coldest anomaly occurred in 1965, with temperatures 3.5 degrees below normal.
- Richmond, as you might have guessed, was colder than normal in 7 of the 10 summers, with an average temperature 1 degree below normal. Like Philadelphia, it was coldest in 1965 and, like Philadelphia, the average temperature was around 3.5 degrees below normal.
The ten years produce a composite temperature anomaly map that features a little more warmth – albeit limited – across the U.S.:
The data show that El Niño favors cooler than normal summer temperatures over the Mid-Atlantic, but this outcome is somewhat dependent on event intensity. Weak El Niño events are consistently more supportive of relatively cool summers. Stronger El Niño events have a more volatile relationship with summer temperatures, but, on average, favor a cool summer.
This is the first part of a two-post series. The next post will look at El Niño’s impacts on winter temperatures and snowfall in the Mid-Atlantic.