Wednesday, May 21, 2014

Upper-Level Blocking Highs

The strong high pressure observed aloft over Alaska early this month led me to wonder about the climatological distribution of these events, both in space and across the annual cycle, as well as long-term changes in the frequency of blocking highs (and lows) aloft.  From the standpoint of casual observation, it seems like recent years have seen a large number of strong high-latitude blocking events over Alaska, but it would be nice to see if the data actually shows that the frequency has been higher of late.  Another motivation is that there is speculation in the scientific community that high-latitude blocking is becoming more common over time because of warming in the Arctic; but again it would be nice to see what the data have to say.

To answer this I turned again to the truly wonderful NCEP/NCAR global reanalysis, which provides a long-term consistent gridded history of upper-level analysis fields.  Beginning in 1951, I calculated the mean 500 mb height for each day and then counted all the occasions on which each grid point had a higher height than nearby grid points to the north, south, east, and west.  When this occurred, the grid point was considered to be the location of a "cut-off" high pressure system or "anticyclone".  I adjusted the east-west spacing of the calculation at different latitudes to account for differing longitude-spacing of the grid points and eventually arrived at a complete history of the central locations of 500 mb high pressure systems.

The map below shows the mean annual frequency of occurrence of 500 mb high-pressure centers for three periods: 1951-1980, 1981-2010, and 2001-2013.  The main features of all three maps are very similar, which illustrates the stability of the climate system; all three show that the highest frequency of blocking high pressure systems over the entire northern extratropics is over the Arctic Ocean north of the Chukchi Sea.

Interestingly the 2001-2013 map shows a marked increase in frequency over the Arctic Ocean and also over Greenland.  The map below shows the change in frequency between 1951-1980 and 2001-2013.  There is a small increase in frequency evident over most of Alaska, but the more striking differences are located farther north and over Greenland.  Overall, it appears the data do support the idea of increased blocking in the high-latitudes, though the changes are highly regionally dependent and appear to be modest over Alaska.

There are lots of other interesting aspects of the data, including the seasonal dependence of blocking frequency near Alaska, but for today I'll just post one more map that shows how Fairbanks daily high temperatures in June vary with the location of blocking highs.  (June is the month with highest blocking frequency over Fairbanks.)  For each grid point, I found all the dates on which a cut-off high was observed at that location, and then I extracted the daily maximum temperature anomaly (relative to 1981-2010 normals) for Fairbanks on these occasions.  The map below shows that Fairbanks daily high temperatures average 4-5 °F above normal in June when a 500 mb blocking high is located over southeastern Alaska, around 2-3 °F above normal when a high is over Fairbanks itself, and 1-3 °F below normal when a high is over the Bering Sea.  This is not at all surprising, but it's interesting to see the shape and magnitude of the patterns.  No doubt the anomalies would be larger in magnitude if I had selected only the more intense blocking events, rather than all the occurrences of isolated high pressure aloft.


  1. Talk about number crunching! How long did you have to run your computer?

    On the maps there seems to be "fingers" coming from the poles that appear unrealistic. Could it be that your algorithim to take into account longitude is introducing artifacts?

    1. I forget exactly but I think I ran six threads for perhaps six hours to crunch all the numbers. It was not exactly optimized code so could probably be made much faster.

      I am sure there are indeed a few artifacts near the pole. The adjustment for longitude spacing maintains a stable frequency per grid point, rather than going to near-zero frequency for each of the densely-packed points; but the calculation is still performed on each grid point, so there are some variations on much smaller scales in the east-west direction. The north-south grid spacing is fixed at 2.5 degrees.

  2. There's been a reported increase in Greenland glacial runoff. I wonder if additional high pressure has anything to do with it? Why is it that only Greenland and north of Alaska affected? Is this why Barrow is having higher temps - something blocking cold air? So many more questions than answers. Perhaps you could do a follow-up with low pressure?

    1. Eric,

      High pressure aloft is associated with higher temperatures aloft, so you could well be right about a connection to Greenland melt. It would be interesting to look at regional temperature trends aloft.

      Greenland and north of Alaska are favored locations for blocking, mostly because of the Northern Hemisphere distribution of topography and land/ocean contrasts (I believe); so the increase in blocking frequency shows up in these locations. The percentage increase also has the most pronounced maximum over Greenland; this is associated with the tendency for negative North Atlantic Oscillation in recent years (and some colder winters in western Europe).

      Regarding Barrow... interesting question! I have the sense that Barrow has warmed more than many other Arctic locations, and presumably this is partly due to circulation changes in the vicinity. It makes sense that Barrow would be colder with a cold low to the north, so you might be on to something. Worth investigating.

      I intend to make another post showing the low pressure changes.