Why So Cold?

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I’m going to continue the conversation from Chief Meteorologist Jason Simpson’s blog post last night about the colder weather (Snow Miser Just Won’t Let Go), in a new post explaining why February and March have both been unseasonably cool months, compared to January, which was unusually warm. This latest cold spell, or rather continuation of the unseasonably cold weather we’ve been having is thanks to a blocking pattern, which is weakening and finally allowing for a warming trend to start with high pressure building in.

Seeing two days of flurries/light snow in late March is extremely rare, especially if you compare it to March 25th and 26th of 2012, where the high temperature at the Huntsville International Airport was 74 and 79 degrees respectively.  The average high temperature for this time of year across the Tennessee Valley is in the upper 60s. It’s not just the end of March that has been unusually cold; most of March has been that way with only seven days of the month so far at or above average. Even the above-average temperatures have been only greater by a margin of five degrees!

These below average temperatures are due in part to the negative phase of the North Atlantic Oscillation (NAO) which is the large scale fluctuation of pressure between the subtropical high near the Azores in the Atlantic Ocean and the Icelandic low pressure. When the phase is negative, both the high pressure and low pressure are weaker and the reduced pressure gradients between the two allows the eastern United States to experience more cold air intrusions and therefore snowier weather conditions. Below is a graph from the Climate Prediction Center showing that the NAO has been mostly negative since mid February. Though it is forecast (red lines) to trend closer towards neutral at least through April 1st.

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The Arctic Oscillation (AO) is the fluctuation of atmospheric pressure between the polar and middle latitudes.  It is also in a negative phase, which allows cold air to plunge into the United States. Below is a graph from the Climate Prediction Center showing that the AO has been negative since early February, with a more recent trend towards neutral.

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February and March were unusually cooler months, but January ended up being warmer than average. As expected in January, we had several cold air intrusions from Canada, but temperatures never dipped quite as low as expected.  Dr. Roy Spencer, a climatologist, author, former NASA scientist and the Principal Research Scientist at the University of Alabama in Huntsville (UAH) wrote a phenomenal blog post on February 20th explaining why, titled: ‘Apparent Reason for January 2013 Tropospheric Warmth’. In a prior blog post on February 5th,  Dr. Roy Spencer explained that “the most common cause of such warm spikes (when there is no El Nino to blame) is a temporary increase in convective heat transfer from the ocean to the atmosphere.”

Below is a graph from Roy Spencer’s blog of the Sea Surface Temperature (SST)  Anomalies from the TRMM (Tropical Rain Measuring Mission) Microwave Imager (TMI) that flies on TRMM. It is clear by looking at the graph that there was a drop in the SST.

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Here is another graph from Dr. Spencer’s blog that shows the monthly rainfall anomalies during the same time frame, which shows that rainfall was definitely above average for January.

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So what does this all mean? You have to think about the water cycle: evaporation of water from the ocean surface into the air above it is a cooling process. That is likely why SSTs showed a cooling trend in January.  With more moisture in the air, it led to condensation, more clouds, and less direct sunlight warming the ocean (which is also a likely reason why the SSTs were lower).  With more moisture in the atmosphere it also increased the potential for more rain and is in part a reason why there was above-average rainfall in January.

Condensation is a warming process in which latent heat is released, and it warms the air around it.  The latent heat released during condensation caused the atmosphere to warm and cold air intrusions to not be blunted.  In other words, temperatures did not get as cold as they could have due to the warmer atmosphere they were moving into.

So in summary, the evaporative cooling from the evaporation of ocean water into the air above it caused the SSTs to cool. The condensation that occurred due to increased moisture in the air, caused the release of latent heat, warming the air around it and the atmosphere due to the large expanse of condensation over a large area.

Though the atmosphere didn’t stay warmer for long, eventually that energy is released into space, which is why after briefly warming in January, the forecast of a cold February verified.  It looks like we may have to wait until mid April for a return to normal temperatures, for more, check out Jason’s blog post above.

- Jennifer Watson

Twitter: @JWatson_Wx

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