A Cool Start To May 2013

Just as we were getting used to hotter springs, Mother Nature pulls a fast one.  It has been unusual to say the least for South Carolina weather since April 26^th.  Cloudy, cool conditions have been the rule with plenty of rain.  The griping has begun as many are tied of conditions we normally attribute to Seattle.

My favorite tweet during the past week has been “Breaking news: This just in – Cleveland wants its weather back.”  I’m sure those in Cleveland might think this an improvement over what they normally get in early May.

So what has been happening?  Why has it been so cloudy and cool for so long?  When will this go away so that we can get on with summer?

The first half of spring was influenced by a weather pattern associated with a negative phase of the Arctic Oscillation.  This was described in the previous post.  However, that has not driving the weather pattern recently.  Instead the jet stream has retreated to the north leaving upper-level low pressure systems isolated from the main flow.  This has resulted in a blocking pattern where weather systems slow to a crawl and can persist for days.

The weather pattern at 500 mb (~18,000 ft) on Saturday, May 4, 2013 at 12z UT.  The jet stream had retreated farther to the north leaving isolated pockets of low pressure over the mid-latitudes.  Click on the image for a larger version.  Image Credit: WLTX-TV.

A Change In The March 2013 Outlook

The Climate Prediction Center (CPC) issues outlooks for the next month in the middle of the current month.  They will often issue an updated outlook near the last day of the current month.  Usually these projections do not vary much, but this month saw a substantial revision.

Computer models designed to forecast daily weather are run out to 15 days, four times each day.  Most are familiar with the 7-day forecast.  The CPC also issues 8-14 day outlooks for the second week of the forecast in additions to the monthly and seasonal outlooks that they make each month.

Beginning with the 8-14 day temperature outlook on February 21 and the one issued on February 28, it seems that the first half of March would be colder than normal for South Carolina as seen in the outlooks below.

The 8-14 day temperature outlooks.  The first one made February 21 and the second one (right) made February 28.  Click on the image for a larger view.  Image Credit: NOAA/CPC.

The reason for this projection is that the Arctic Oscillation (AO) has been in a negative phase with the forecast for it to become strongly negative next week.  This usually produces a cold outbreak for parts of the U.S.  Last year the AO was strongly positive and the U.S. saw the warmest March on record.

The Rise Of Extreme Weather

Extreme weather has always been with us, but the trend over the past few decades has been for it to increase in number.  The Climate Extremes Index (CEI) was developed by the National Climatic Data Center (NCDC) and goes back to 1910.  It looks at the number of events that occur.  The graph below compares the period of January through October so that data from this year can be included.

The CEI for the period of January - October 1910 - 2012.  Image Credit: NOAA/NCDC.

The CEI with the experimental tropical cyclone indicator included.  Image Credit: NOAA/NCDC.

A new report recently released (before Hurricane Sandy) by Munich Re, the world’s largest reinsurance company, re-enforces this observation.  The report is about the increase in extreme weather over North America.  The report states:

The Global Weather Pattern Shifts

The previous post described how changes in the Arctic were causing changes in the weather and climate.  These changes extended down in the mid-latitudes where the impacts included cold, snowy winters.  However, the impacts of a warming earth do not end there.  Research shows that mankind is shifting the entire global atmospheric circulation.

Background

Meteorologists have developed an idealized view of the global atmospheric circulation over the past century.  It has long been recognized that there is too much heat at the equator and too little at the poles.  The atmosphere acts like “the great equalizer “ by transporting heat from the tropics to the poles.  If the earth were not rotating, this would be a simple straight-forward transfer.

The situation is complicated by the fact that the earth does rotate on its axis.  This has given rise to the three-cell global circulation model.  The three cells from the tropics to the poles are named: Hadley, Ferrel, and Polar.  This model is pictured below with the resulting wind patterns.

The three-cell general circulation model of the atmosphere.  This is an idealized model based on observations.  Image credit: NASA.

The Hadley cell (also known as the tropical cell) extends from the equator to 30 degrees latitude.  Many of the world’s major deserts lie at the poleward extent of the Hadley cell where the air descends from upper levels.  This produces a persistent dry climate with little rainfall.  The North American Desert, Sahara, Australian Desert, and Kalahari Desert all are products of this circulation.