Friday, January 31, 2014

Questioning the Bering Sea Rule: Part 2

In my last blog post I used the ERA-Interim reanalysis to look at a long-range forecasting technique called the "Bering Sea Rule".  This has been used by many bloggers to attempt to forecast the general weather for various parts of the eastern US several weeks in advance.  In its general form, it claims that whatever happens in the Bering Sea (weatherwise) will happen in the eastern US 2.5-3 weeks later.  However, when I tried to correlate the storminess of the Bering Sea over a 30 year period with the storminess of the eastern US at various lead times, I couldn't find any strong correlations at all in the 2.5 to 3 week (17-23 day) window purported by the Bering Sea Rule.  There were several other interesting correlations that have relations to well-known, larger scale circulation patterns but nothing special about that particular time window.

I heard back from several people after publishing part 1 of this blog with comments about my findings.  Apparently there are other groups numerically trying to validate this rule.  I'm really looking forward to seeing what these researchers come up with.  There are a few additional things I wanted to quickly test that were supported by the comments I received.  These include:
  1. Instead of looking at storminess, try to compare the temperature anomalies in the Bering Sea to temperature anomalies over the eastern US.
  2. The idea that the periodicity of the Bering Sea Rule changes over time---each year the period between what happens in the Bering Sea and what happens in the eastern US is different---not always at the 2.5-3 week lead time.
Temperature anomalies are pretty easy to correlate.  Over the 30-year period the pattern is pretty simple:
Strong anti-correlations at short lag times less than 10 days, implying that if it's warmer than normal in the Bering Sea it's more likely colder than normal in the eastern US and vice-versa.  For large, standing wave patterns this makes a fair bit of sense.  However, beyond 10 days there are only extremely weak positive correlations and, again, there is nothing special about the 2.5-3 week lead time.

What about this idea that the periodicity of this pattern changes every year?  We can test this by going back to one of our storminess metrics (say, mean SLP in each region) and only do these lagged correlations for one year.  Since we expect our strongest teleconnections to phenomena like the Madden-Julian Oscillation during the winter half of the year, I'll limit the time periods to Sept-March.  Let's look at the decade from 2001-2010 and compare the lagged correlation pattern each year.  I'm including small versions of these photos inline; click them to get a larger view.








The correlation pattern changes pretty drastically on a year-to-year basis.  When we look at these shorter time periods, much stronger correlation magnitudes come out.  For some years, there is a particular lead time where there are much stronger correlations than at other lead times (for instance, 35-40 days in 2002-2003, 5-10 days in 2004-2005, 45-65 days in 2005-2006 or 17-23 and 48-52 days in 2006-2007).  But in other years there are numerous peaks of similar magnitude at various lead times.  Furthermore, none of these correlation magnitudes ever gets stronger than 0.6.  Also, in many cases, the largest magnitude correlation is actually of the opposite sign (for instance, 2000-2001,  2001-2003, 2005-2006, 2007-2008).

So what does this all mean?  Looking at individual years, there are actually stronger correlations than I expected to find.  Within a particular year, there are particular wave patterns that probably do repeat a few times at reasonably regular intervals, particularly with large-scale blocking patterns setting up and whatnot.  But, at least with respect to the Bering Sea Rule, in the far majority of years it is very hard (using this metric) to identify one particular time period of enhanced predictability.  Even if you agree on multiple time periods where this might apply, the magnitudes of these correlations are still not very high.  Even in particularly good years, you could hand-wavingly interpret the maximum 0.6 correlation at some lead times to indicate that, at best, 60% of the time when there is a low in the Bering Sea, there will be a low in the eastern US a certain number of days later. And even then it's a range of days where these correlations are strong, usually 5-10 days long.  Given the frequency of troughs moving through the eastern US in a normal winter, it seems rather likely that there will be a low-pressure center sometime during a 1-week long period several weeks from now.  There also is this problem of some of the strongest correlations being negative---implying that the opposite of what happens in the Bering Sea would actually be a better prediction.

So that was just some followup on a few more Bering Sea Rule ideas.  There is ongoing research into larger-scale patterns of predictability and they'll do a far better and more thorough job than anything I would do here.  It would be interesting to try and refine the Bering Sea Rule ideas further---maybe this only applies to the strongest storms or the biggest cold-snap events?  At present, though, it seems difficult to quantify the Bering Sea Rule as it stands (certainly not with the 2.5-3 week window that seems to be widely used).

1 comment:

  1. I love your analysis. After years of using the NCEP/NCAR reanalysis data, it's taking some practice to figure out the EC data. Your analysis is motivation for me to keep at it. Great job!