Friday, November 10, 2017

Utqiaġvik Wind Events

Back in late September, Alaska's northernmost community of Utqiaġvik (formerly Barrow) experienced a damaging coastal flood event that washed away some lengthy sections of road and caused significant erosion to the coast.  The following article provides detail on the damage:

As noted in the article, a similar event happened in 2015; here's a blog post from the time:

Some of the coastal flooding that occurred in the latest event is evident in this shot from UAF's sea ice webcam:

Later that day the berms were repaired along the beach, but obviously the city's defences are rather fragile.

In view of these events, it's worth taking a look at some historical data to see if high wind events have become more common, and if so, by how much.  To do this I used gridded reanalysis data to estimate the sea-level pressure gradient at 6 hour intervals since 1950 for a grid point close to Utqiaġvik.   Of course the pressure gradient is closely related to the wind speed, so the idea is to use the pressure data as a proxy for estimating changes in wind conditions.

But why not just use the historical wind speed data from Barrow?  The reason is that there are too many uncertainties related to changing measurement practices over time - i.e. it's likely that changes in instrumentation, measuring height, and measuring procedure (such as averaging time), as well as a changing urban environment, have created artificial changes in the long-term wind speed data.  In contrast, the sea-level pressure field in a reanalysis should (by design) provide a consistent estimate over time, although not all reanalysis techniques are equally good, and of course a reanalysis is just a model.  Nevertheless I think the reanalysis data has more potential to provide a useful answer.

The chart below shows annual series derived from the 6-hourly pressure gradient from the NCEP/NCAR reanalysis at a grid point near Barrow.  The two horizontal dashed lines show 1957-2016 mean values and help to highlight subtle changes.  Note that data prior to 1957 is a bit suspect because the number of balloon soundings around the Northern Hemisphere was far smaller in earlier years.

It's interesting to see that there seems to have been a subtle upward shift in pressure gradient since about 2000, and for the middle series (the annual mean of the 12 monthly maxima) most years since 2000 have been above the long-term mean.  The 1957-2016 linear trend in the middle series is statistically significant (p=0.03).

Here's a similar analysis from the 20th Century Reanalysis, which actually goes all the way back to 1851.  Note the suspiciously high pressure gradient values prior to 1957 - this supports the idea that the early estimates are less good.

The annual mean pressure gradient values in the two data sets are correlated at R=0.77, which isn't too bad, although obviously there can be large differences in how the reanalyses handle individual events; the "annual maximum" series are correlated at only R=0.58, and the 20th Century Reanalysis seems to have a very different assessment of the event in 2013 (a blizzard in mid-January).

The 20th Century Reanalysis shows less of an indication of higher pressure gradient in recent years; there is still an upward trend, but it's not really statistically significant (p=0.08 for the middle series).

How about individual wind events that might tend to produce storm surge flooding in Barrow?  To look at this I pulled out all 6-hourly instances when the pressure gradient was above a certain threshold AND Barrow reported a wind direction between 250° and 360°, i.e. winds from west-southwesterly through northerly; we'll call these "flooding winds".  Strong winds from this direction would tend to pile up water on the coast.  Of course there is much more to creating storm surge than just instantaneous wind speed and direction, but this is a quick look.

The chart below shows the number of "flooding wind" hours each year in which the pressure gradient exceeded 3mb per 100km, which is about the 95th percentile year-round.  There was a notable lull in these strong wind events for about 15 years starting in 1976, and this year has been windy, but overall there seems to be little trend for the annual number of hours.

Looking just at the autumn season when shore protection from sea ice is reduced, the picture is a little different - see below.  Interestingly for this time of year it does appear that there has been a long-term increase in strong pressure gradient events that could cause coastal flooding, although the last decade or so has not been notably worse than say the 1990s.

The 20th Century Reanalysis again shows a slightly different result.  Here I've used a slightly lower pressure gradient threshold, because the values are systematically lower, but other than a couple of high totals in 1993 and 2002, the past few decades have not seen an unusual number of "flooding wind" hours.

In summary, the well-known NCEP/NCAR reanalysis provides some evidence of an increasing trend in pressure gradient and therefore presumably increasing winds year-round near Barrow, and the data also suggest that somewhat more strong westerly/northwesterly wind events have occurred in autumn since around 1990.  However, the 20th Century Reanalysis provides only marginal support for the idea, so in the absence of more information it's difficult to draw a firm conclusion.  Perhaps the most useful thing we can say is that there's certainly no evidence of a slackening in pressure gradients, and given the profound reduction in sea ice extent, the rate of shore erosion is likely to be higher than in the past.

As an aside, there's one other interesting aspect of the data that I noted: a pronounced increase in pressure gradient in February, see below.  The other months with the most notable increases were June and July, but November has seen a downward trend.

1 comment:

  1. It's sort of dated to expect the piles of beach sand to hold back the ravages of an active sea. Might be time to spend some $ and install gabions or similar structures like these: