After a particularly nasty winter, Alaskans usually hope for a nice summer to erase the memories. Actually, a lot of Alaskan’s do that after every winter. Well, we are just finishing up (or still waiting for the end of, depending on what part of Alaska you live in) a very tough winter in most parts of Alaska, so hope runs high for the summer. Will it soothe or disappoint the weather weary?
The truthful answer? I don’t know. Seasonal forecasts for the warm season have little skill in Alaska. Worse than winter forecasts, which are far from reliable.
There are two points pertinent to this issue:
- Temperatures vary much less in summer than in winter. While that might make it easier by reducing the potential forecast error, in reality the forecast needs to show skill within the context of that variability. This is the technical explanation. From a human standpoint these smaller temperature variations seem to be relatively more important in summer, perhaps because there is more outside activity. There is about as much grumbling about a 3 degree colder than normal summer month as there is about a 6 degree colder than normal winter month.
- Correlations of summer weather in Alaska with climate systems such as the El Nino/La Nina are either too weak or as yet too poorly understood to be a very useful predictor of a particular summer. Frankly most people have been trying to figure the links with winter weather, probably because winter weather is seen as more important (dangerous, costly, etc.) and I suppose that is right to an extent.
So I’m not putting much confidence in my prediction, and asking you not to as well. This is more of a theory to be played out over the next 3 months than something to make decisions on.
A cold sea can’t help much
The ocean is a big driver of the weather and climate, so most effort to understand trends and connections focus there. Water holds heat much better than land and most locales in Alaska are either bordered or surrounded by the sea.
Take a look at where we stand this year. The following maps from NOAA’s ESRL depict the anomaly (difference from the 1971-2000 average) for sea surface temperatures. The global maps (for April and–hot off the press– May of this year) show most oceans areas are not far from average for the time of year (April), but there are a couple significant deviations in the Northern Hemisphere: the NW Atlantic is much warmer than usual (red colors) and the Bering Sea and Northeast Pacific are quite a bit below average (blue colors). The warm area off Japan and the cool equatorial mid-Pacific are characteristic of the negative phase of the Pacific Decadal Oscillation, which I’ll touch on in bit. The contrast between the warm area off Japan and the cold water to the north provides fertile conditions for low pressure development. More or stronger lows can wreck a summer by bringing both rain and ocean-cooled air onshore, not to mention clouds, which keep the sun from heating things up.
Here are the same maps zoomed in on Alaska (the color shading is very slight different).
The Gulf and especially the Bering Sea are up to 2 degrees C colder than average for this time of year. May looks worse than April. Click on the maps for more detail. If you are not adept with Celsius, remember that 2 degrees C is 3.6 degrees F. Very cool water, but everything is relative. Two degrees colder than “normal” is how it is often put.
The new normal
What is normal? In this case it is the average (mean) of the temperature measurements from 1971-2000. (There are now available 1981-2010 normals, at least for land stations, which might be called the new normals, but that’s not what I’m talking about.) Who’s to say that is normal? And who is to say that certain deviations from the average are abnormal? They happen all the time. That’s why I do not like the term normal and usually use average instead. We just need to keep track of what average we are talking about, since there are so many averages floating around. I think the term normal is used by the NWS mainly as a shorthand way of indicating a 30-year average used for climatic comparison. Unfortunately the term implies too much.
In this case, the 1971-2000 period was a relatively warm period, and the current “colder than normal temperatures” may well be the new normal. I’m not just talking about these few maps…I’m talking about summer and winter, land and water…Alaska has been markedly cooler the past few years and this has a good chance of being the new reality for the next 10 to 20 years, with thankful exceptions sprinkled about.
Let me try to put things in perspective with this graph: It attempts to visually correlate our summer highs with the water temperature over the Bering Sea and Gulf of Alaska, and also with the Pacific Decadal Oscillation, which is calculated from sea surface temperatures and pressure patterns from various Pacific locations. The PDO is not a temperature, per say, so its scale is separate from the rest, and I placed it in the middle of the graph, between 1965 and 1970. The PDO ranges between around -2 and +2 on its index. This graph displays a 24 month running average. Click on the graph to get the big version…much easier to see the detail.
Several things are evident in this graph:
- As stated early in this piece, summer temperatures do not vary that much from year to year, at least not compared to winter temperatures.
- The PDO and the two SST traces correlate well (no surprise).
- There are some eye catching correlations between the ocean data (SST & PDO) and the summer temperatures, but it is not without exception.
- There is little overall trend to summer temperatures when taken over the entire 70 years. Shorter term trends follow the PDO, mostly.
Lets look a little close at two recent standout years: the very warm and dry 2004 and the very cool and sort of wet 2008.
The warm summer of 2004 was preceded by very warm water in the Bering Sea and somewhat warm in the Gulf of Alaska in April. This moderated somewhat in May, especially in the Gulf. The cool summer of 2008 had cold water in both the Gulf and Bering. It looks a lot like this year (see maps above), except this year the Bering is relatively colder and in 2008 the Gulf had more cold water. (I did not look further into the summer since this effort is aimed at the predictive value of the water temperatures, so months leading up to summer are what count.)
The cold sea temperatures can’t change very quickly so I’d call it pretty likely that June will end up cool, even if the monthly flip-flop of temperatures that has been consistent since October says we should flip back to warm. For late summer, there is little confidence for anything specific, except to reiterate that with the cool phase of the PDO in full swing, more summers than not are going to be on the cool side.
What about precipitation?
Honestly I have less to go on for precipitation right now, and I’ve come across little from others. With the SST pattern mentioned earlier that favors low pressure development, I’d give a nudge toward wetter than usual, at least while this pattern exists… more storms mean more precipitation, but again, confidence is low. For the official government outlook on precipitation and temperature, look at the maps from the Climate Prediction Center. After June it is basically a blank map…”Equal Chances” for it to be above or below average in both temperature and precipitation, for all of Alaska. Yet another perspective is contained in the 2012 fire danger seasonal outlook from the Alaska Fire Service. The authors state that during the cold phase of the PDO (which we are in) a wet winter tends to followed by a dry summer and vice-versa. If that holds, it would point toward a drier summer, since most of Alaska was on the wet side this winter.
wrap it up
This was a long post, and I realize what little I wrote that amounted to an actual forecast was pretty negative, but I hope the background information was interesting. So, what do you think? Warm or cool summer ahead? Wet or Dry? I’d like to hear.
NCEP server for SST plots This is where I went to make the SST maps.
National Weather Service Anchorage research papers There are a number of papers dealing with the ocean-Alaska climate question, mostly with regard to winter, of course. Most of these are pretty easy to digest.