El Niño Winters In Charleston, WV
On September 13, 2006, the Climate Prediction Center announced that El Niño conditions have developed in the equatorial Pacific, with a significant increase in ocean temperatures. There is the potential for this event to strengthen to a moderate event by winter.
The El Niño-Southern Oscillation Cycle is a naturally occurring phenomenon, in which the ocean temperatures of the equatorial Pacific fluctuate between two states. The El Niño state is associated with warmer than normal water in the equatorial eastern and central Pacific, whereas the La Niña state is associated with cooler than normal water in these same regions. These conditions can contribute to changes in the position and intensity of the jet stream, affecting storm tracks and weather in the United States.
Jet Stream Configurations during El Niño and La Niña
So, what has occurred during previous El Niño winters at Charleston, WV?
Since 1950, based upon the Oceanic Niño Index, there have been 16 winters within an El Niño event. The average winter temperature (December/January/February) of these years, was 34.7F degrees, which is 1.2F degrees below the normal at Charleston of 35.9F degrees. Winter liquid precipitation was also below normal at 8.86, with normal for these three months being 9.76. Snowfall during these months came in above normal at 26.5, with normal December/January/February snowfall being 24.4. None of these values fall within any of the top ten lists at Charleston.
These four maps detail the change in average winter temperature, relative to normal, during all El Niño events in comparison to moderate/strong El Niño events. The Trend Adjusted maps integrate what has occurred over the past 10 years, to what has happened during only El Niño events. Because the past 10 years have been above normal, the adjusted maps are even more above normal than just the El Niño maps would suggest by themselves. The Oceanic Niño Index is used operationally to declare an El Niño or La Niña event, and also to compare present and past events. This value is calculated by measuring the Sea Surface Temperature departures from average in the Niño 3.4 region, a picture of this part of the Pacific Ocean is shown below.
Because there is a difference between all El Niño events and the moderate/strong cases, lets look closer at the data, and find El Niño events that match up better with what is currently happening. We need to find years in which a weak El Niño begins late summer or early fall, and following the Climate Prediction Centers forecast, becomes moderate into winter. Another value used to measure the El Niño/Southern Oscillation, is called the Multivariate El Niño Index, or the MEI. This value is more robust than the ONI, since it not only contains Sea Surface Temperatures, but other influenced parameters such as wind and pressure. Also, a value is computed every two months, unlike the ONI which is computed every three months. The smaller timeframe of the MEI will allow us to compare previous El Niño events more specifically, than if we just looked at the ONI.So, looking at MEI values for August/September and September/October lets find 4 years during these couplets which were above the 2006 Aug/Sep-Sep/Oct MEI, and 4 years which were below. This methodology fits in with what is described by Wolter, concerning MEI ranks, at the webpage referenced below. Lets get a picture of the average winter temperature anomalies during all of these years.
This indicates that the average winter temperature was about 1C below normal for these years at Charleston, or about 34.1F. This is colder than the average winter temperature computed by averaging all El Niño winters together. But, this temperature does not fall within the top ten list for coldest winters in Charleston. Also, it may not be fair to include the winters of 1976 and 1977 into the above graphic, as those are the two coldest winters on record in Charleston. Lets see what the picture looks like taking out those two historic seasons.
This is more similar to the four El Niño maps above, although the warmer anomalies are shifted more to the west. Also, Charleston now is represented by near normal winter temperatures.
So, which scenario are we to believe? The historic cold of 1976 and 1977, or the normal winters forecasted by the rest of the dataset? There are a few other important tools that can help us.
The North Atlantic Oscillation or the NAO is the fluctuation of the subtropical high in the Azores and the subpolar low near Iceland. The positive phase of the NAO, means higher pressures in the central North Atlantic, with the negative phase allowing higher pressure in the high latitudes of the North Atlantic. A stronger negative phase of the NAO usually goes hand-in-hand with a Greenland block, with a strong ridge centered across that island. When a significantly negative NAO is in place, and cold air has had time to build in Canada, the ridging in the higher latitudes forces that air south, into the northern and eastern United States.
The above shows average winter temperature anomalies in which the NAO index averaged less than or equal to -1.0 for December, January and February. As one can see, the eastern United States was below normal.
Another helpful index, the Pacific-North American pattern, can help us determine height (trough or ridge) tendencies through the central Pacific to the western coast of the United States. A positive PNA indicates ridging near Hawaii and the western United States, whereas the negative phase details higher pressures across the north Pacific and troughing near the west coast. Similar to the NAO, a positive PNA not only helps to force Canada air farther south, but also disallows relatively warmer Pacific air from moderating the continental polar/arctic airmass through Canada.
The above shows average winter temperature anomalies with the PNA index less than or equal to -0.5.
Much of the eastern half of North America was above normal, with northwestern Canada and Alaska, much below normal.
The state of the NAO and MEI indices leading up to the winter of 2006-7 present a dichotomy. Since 1950, no other August/September/October NAO average has been as negative as that in 2006, whereas the value of the MEI for September/October indicates that the current El Niño event is the tenth most significant since 1950. Thus, the latter would indicate a warmer than normal winter, but the previous would suggest a much colder than normal winter.
Can the PNA pattern this fall lead us in one direction or the other?
Hard to say, but it is important to note that the PNA index is itself affected by El Niño, which doesnt help matters. Looking back at past data, 1993 and 1994 best match the fall MEI and PNA values of this year. Considering the NAO, the best match available was the fall of 1968. So, if we wanted to create an outlook for the upcoming winter, just for fun, we should find the average winter temperature anomalies for all of these years. Using these three years will perhaps capture the anomalous fall NAO average, while also taking into consideration the strengthening El Niño.
Using our 3 past seasons, the above was produced. The picture on the left is the average winter temperature anomaly, on the right is the 500mb height anomaly. The light blue shading over our region indicates average winter temperatures about 0.5C below normal, or 35F. Both of these maps actually fit the duality of the fall season thus far. In other words, both a height and temperature anomaly is in the neighborhood or where it would be if an NAO was present for part of the winter. But, the negative PNA also shows itself with the lower heights and cold temperature anomaly in northwestern Canada and off the Northwest coast, respectively. What this may be trying to convey is that every so often, the NAO will overcome the warmer contributions of El Niño and the negative PNA, providing us with a chilly airmass, but only for a few days at a time. Effectively, the conflicting factors spoken of earlier may cancel one another out.
Also of particular interest, is snow cover across North America. A cold airmass will retain its intensity for a longer period through Canada, if snow cover builds earlier in the season. The blue colors indicate above normal snow cover, with white indicating normal, and brown showing below normal snow.
The Oct 2006 map shows more areas of normal or above normal snow cover, than the previous 3 years we have been focusing on. This could mean that the beginning of winter 2006-7 could start out colder. However, the most recent warm pattern in the eastern half of the United States and southern Canada seems to be turning the snow cover factor in the other direction. It will be interesting to watch the snow cover anomaly for Nov.
Here are the winter (December/January/February) snowfall totals for the years we have been focusing on:
As previously stated, the information provided here is by no means a forecast, but simply a look at what has happened during previous winters, with somewhat similar initial factors. NOAAs official winter forecast can be found at www.noaanews.noaa.gov/stories2006/s2742.htm.
Climate Prediction Center http://www.cpc.noaa.gov
Snow Cover Graphics from Rutgers University Global Snow Labhttp://climate.rutgers.edu/snowcover/
Wolter, K., and M.S. Timlin, 1998: Measuring the strength of ENSO - how does 1997/98 rank? Weather, 53, 315-324.
Multivariate ENSO Index http://www.cdc.noaa.gov/people/klaus.wolter/MEI/mei.html
NOAAs Earth system Research LaboratoryNCEP/NCAR Reanalysishttp://www.esrl.noaa.gov/