Just Another Number?

Numbers have meaning only in context.  A temperature of 98.6 degrees F is normal body temperature, but only in context of the mean.  This is not normal for every person.  The S&P 500 crossing 1600 is meaningless unless it is in context of what the market is doing.  Is it going up or down?  How far has it gone from previous levels?  Only then does the number have meaning.

A milestone was reached recently, Thursday, May 9^th, at the Mauna Loa Observatory in Hawaii.  Levels of carbon dioxide reached 400 parts per million (ppm) for the first time in human existence.  Daily measurements are made at the observatory of the gases that lead to atmospheric change.  One of the main gases monitored is carbon dioxide (CO2).  Levels of 400.03 and 400.08 ppm were measured independently by the National Oceanic & Atmospheric Administration (NOAA) and Scripps Oceanographic Institute respectively.

Mauna Loa Observatory in Hawaii.  Click on the image for a larger version.  Image Credit: NOAA/ESRL.

This was a daily mean that was measured for the first time.  Hourly measurements exceeded this number quite a few times since mid-April.  The monthly mean of CO2 will likely peak this month, at a level of about 399 ppm, and then fall until October.  It is likely to pass the 400 ppm in the spring of next year.  The yearly mean will not likely cross this level until 2015 or 2016.

A Drought Worth Having

Droughts can be quite nasty with it comes to rainfall.  However, droughts can be a good thing when it comes to severe weather.  The U.S. is currently experiencing such a drought for tornadoes.

The monthly number of tornadoes for 2013 compared to the 1981-2010 average. Click on the image for a larger version.  Image Credit: Climate Central.

The year started off looking above normal, but that quickly turned in February.  Significantly fewer tornadoes were observed in March and April.  May is normally the peak of tornado season for much of the country.  The drought has continued and will likely continue into the middle of May.  It is hard to say if it will continue beyond that, but it is likely.

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 Dramatic Change This Spring

It is amazing how fast the weather pattern can change.  Temperatures were much below normal at the end of March.  There was a brief warm up at the beginning of April, but a brief cool snap followed.  Finally the cold air retreated leaving much of South Carolina with above normal temperatures.  In a matter of five days high temperatures in Columbia went from 55 degrees F on April 4^th to 87 degrees F on April 9^th & 10^th.

A previous post talked about the delay of spring this year.  High pressure centered near Greenland continued to pump cold Arctic air south into North America.  This was part of the negative phase of the Arctic Oscillation which became extreme in March.  This led to colder temperatures in March than in January.

This image shows the sea level pressure anomaly calculated from NCEP climate reanalysis data from February 25 through March 25, 2013. The area of anomalous high pressure over Greenland illustrates the negative phase of the Arctic Oscillation, by which persistent high pressure blocks Arctic air from moving eastward, causing it to drop down into the middle latitudes and is a contributing factor in keeping temperatures unseasonably cool in Canada and the US.  Click on the image for a larger view.  Image Credit: NOAA.