Tag Archives: ENSO

Record Warm March for the U.S.

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Spring heat dominated the weather pattern across this country last month. Locally in NYC, March 2012 was our second warmest March on record.  Nationally, it was the warmest March ever for the contiguous United States.

As a whole, the country registered an average temperature of 51.1°F, which is 8.6°F above the 20th century average for the month.  According to the NOAA report released this week, every state in the lower forty-eight experienced at least one record warm day.  A total of 15,272 daily high temperature records were broken.

Scientists say the same reasons that caused this year’s record warm winter brought us an unusually mild March.  They include La Nina, the North Atlantic Oscillation, and their influence on the position of the Jet Stream.

NOAA is forecasting a continuation of this warm trend, especially in the eastern and southern parts of the country.

Record Warm Winter in the U.S.

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It’s official! This was one of the warmest winters ever in the United States. According to NOAA, this meteorological winter (December-February) was the fourth warmest on record in the lower forty-eight. Registering an average temperature of 36.8°F, the country was 4°F above its long-term average.

Scientists say that a number of factors played a part in producing this unseasonable warmth.  To begin with, the Jet Stream, the boundary between warm southern air and cold arctic air, stayed well to the north this winter. As a result, mild conditions dominated the season and most of the country received below average snowfall. In fact, it was our third smallest winter snow cover in forty-six years of satellite record keeping.  Without snow to reflect the sun’s rays, the exposed ground absorbed solar energy, helping to perpetuate the warm conditions.

Two other significant factors in this winter’s story are La Nina and the North Atlantic Oscillation (NAO).  These powerful climate systems influence the shape and path of the Jet Stream.  La Nina, part of the larger El Nino Southern Oscillation (ENSO), brings cooler than normal water to the eastern Pacific Ocean. As a result, high pressure builds over the cool water and pushes the Jet Stream northward. NAO, the southern branch of the larger Arctic Oscillation, runs in positive and negative phases, depending on the pressure differences between Iceland and the Azores. A positive phase, like the one that dominated this winter, promotes a fairly straight path for the Jet Stream.  It also brings warmer than normal conditions to the eastern two-thirds of the U.S.

Given these constraints, the Jet Stream only managed to dip south a few times this season.  When it did, we were abruptly reminded that it was, in fact, still winter.  Those few cold blasts, however, never lasted very long. On the whole, it felt like a year without a winter.

A Year of Snow, a Year of Plenty

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There is an old weather proverb that says, “A year of snow, a year of plenty.”  Rooted in agriculture, it refers to the traditional process of cold winters providing snow cover for fields that prevent crops from sprouting too early.  That snow then melts in the spring and provides moisture for a successful growing season.

As we now know, this winter has been anything but traditional with most of the United States experiencing uncommonly dry and mild conditions. In fact, today the U.S. Drought Monitor showed that 38% of the country is experiencing moderate drought.  This is up from 28% in December.  Some of the states most seriously affected by the lack of precipitation include, Texas, Georgia, Florida, and parts of California.

One of the reasons for the quiet weather this winter is the northern position of the Jet Stream.  Also known as the “storm track”, the Jet Stream usually dips south in the winter bringing cold air and snowstorms to the continental U.S.  The current La Niña episode, however, has pushed the Jet Stream north keeping most of the lower forty-eight states warmer and drier than normal.

Forecasters are predicting an end to this pattern in the spring when La Niña is expected to subside.

La Nina Returns

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La Nina has returned for a second year in a row.  According to NOAA’s winter climate outlook, this oceanic-atmospheric phenomenon will strongly shape our upcoming winter season.

La Nina is a climatic episode associated with the larger El Nino-Southern Oscillation (ENSO) climate pattern in the Pacific Ocean.   During a La Nina event, sea surface temperatures in the eastern Pacific are cooler than normal.  This ocean temperature anomaly influences weather around the globe.

In the US,  La Nina will impact both the temperature and precipitation in many parts of the country. The southern states are likely to see conditions that are both warmer and dryer than normal.  This is not good news for the drought-stricken state of Texas.  The northern tier is expected to experience  below average temperatures with the northwest getting above average precipitation.  The northeast and mid-Atlantic states have a 50/50 chance of seeing irregular conditions from La Nina.  In this region, the Arctic Oscillation (AO), a different oceanic-atmospheric pattern, has a stronger influence on winter weather.

Less predictable than La Nina, the AO continually transitions between positive and negative phases. A negative phase will bring cold arctic air and snowy conditions to the eastern US.  The cold snaps and heavy snow we saw last winter in the northeast were influenced by a very strong negative phase of the AO.  These strong phases can last anywhere from a few days to a few weeks and can be difficult to anticipate in long term forecasts.

The current La Nina event is forecast to last through February.

Photo Credit: NOAA