East Coast versus West Coast: The Continental Weather Seesaw

It often seems that the U.S. east and west coasts are on some kind of weather seesaw.   Recently westerners have been complaining that the East Coast is warm and the West is cool, but sometimes it goes the other way.  Quite frequently, warm records are found on one coast when cool records are occurring on the other.   The coastal weather seesaw some people call it.   And now,  a new index reveals its intimate details.   In this blog you will view the Coastal Contrast Index (CCI), an advanced new diagnostic tool never shown before in public.
  
The CCI is based on the difference between the temperatures of major cites on both coasts.  Specifically, the mean temperatures of eastern cities (TE) minus the mean temperatures of western cities (TW).

CCI= TE-TW

For TE we use Boston, New York, Washington DC and Atlanta.  For TW we use Seattle, Portland, San Francisco, and Los Angeles.  The selection of these particular cities is based on arcane scientific principles that are too complicated to explain in this blog.  Yes, the latitudes don't match exactly, but that is where the people are.  And I should note that the data analysis for the CCI was done by data analyst extraordinaire of the UW:  Neal Johnson.

Ready to see it?  Take a look at this graph (click to enlarge) for January 1 through July 21st.

The blue line shows the daily climatological values based on long-term average temperatures for those dates and cities. 

Notice that from January 1 through roughly 10 April, the West Coast is generally WARMER than the East Coast by about 10F.   And then in late spring and summer the East Coast becomes warmer by roughly 10F again.  The seasonal seesaw...its real.  And the reasons are clear:  the West Coast has the relatively mild Pacific to its west, which keeps us warm in winter and cool in summer.  The East Coast has a large continental area to its west (weather generally comes from the west in the midlatitudes as you know), which is cold in winter and warm in the summer.

But this graph doesn't stop there!  The red line shows the index for THIS YEAR and the information is sobering.  And very different than normal. 

During the winter, the East Coast was WAY cooler than normal compared to the West Coast .  We are talking 20-25F cooler than the West.  You remember that--snowstorm after snowstorm, frigid day after frigid day for the forlorn easterners.  Some folks even blamed than on global warming.

And then in roughly mid-February an atmospheric switch was flipped and on most days the East Coast was warmer than the West--much warmer than it should be.  On many days, particularly this summer,  the East was 15-20F warmer than the West.   And if we included the last few few days, there would be a spike to 25 or more.

The reason for this anomalous behavior has been discussed in the blog several times:  the establishment of a persistent mean trough over the West Coast and a ridge over the central and eastern U.S. during February, a pattern that has just not gone away.

Finally, you will notice several big swings of the index and there is a reason.  Although there has been ON AVERAGE a trough over the west and a ridge over the east, on a daily basis sometimes there are brief excursions to the opposite situation, occasions when there is a ridge over the west and a trough over the east.  Then the seesaw goes the other way.  And westerners love it!

Why is the east and west coasts so frequently out of phase?  The reason is that the flow aloft is usually very wave like and the wavelength of the typical wave is frequently just about right to give us the out-of-phase coasts (see graphic).

The upper level flow pattern is very wave-like and has typical wavelengths.

This wavelength is set by a number of things, such as  the rotation rate of the earth and the typical temperature variations of the planet.    Why did we get this anomalous wave pattern this year?  La Nina is partially to blame, but there is probably more.   Is this a sign of global warming?  I don't anyone can answer this question at this point and keep in mind that while some places are warmer than normal, others are cooler.

One incident says little about long-term climate change, but some in the media are already saying that this incident is "consistent with" or "the kind of events we will experience under" global warming.  Want to see an egregious example of this--check out the NY Times.