June was another in a series of cool months for much of the northwestern US. Since about Valentines Day we have struggled mightily to beat climatology in terms of temperature, especially for daily high temperatures (see below climograph for 2011 for Spokane, WA and Boise, ID). Using my trusty eyeballs, the number of days with high temperatures below climatological “normals” outnumbers those over by a huge ratio for the past 140 days, with a similar but less emphasized signal for minimum temperatures.
Overall for June temperatures across much of the inland northwest were about 3F cooler than 1971-2000 normals, which BTW are in the process of being changes to reflect “new” normals per 1981-2010. The cool streak has furthered delays for agricultural and recreational activities, and snowmelt in the mountains. Last week the mountains in central Idaho were still holding on to snow above 6500ft with the culmination of a very wet cool season and cool spring temperatures resulting in some ludicrous 1000+% of “normal” SWE numbers week ago – with “normal” used lightly in this case.
While we had a cool June here in the NW, much of the rest of the country was warmer than normal, with particularly unusual warmth across northern Texas partially a result of the extremely dry land surface conditions associated with the ongoing drought in the south.
June was not particularly wet and featured more of the convective variety of precipitation with a proclivity for western Montana.
It’s again tough to attribute the cool/wet winter/spring to any single facet of the climate system. One thing worth noting is that we had previously mentioned the La Nina this winter as shaping up to be one of the strongest in 30 years. Now that she has retracted we can assess how this years La Nina stacked up to previous La Ninas.
We can quantify the El Nino-Southern Oscillation through both the influence it has in terms of modifying sea surface temperatures (SST) in the tropical Pacific using a sea surface temperature index, as well as using an indicator that represents its manifestation in terms of modifying the Walker circulation, or atmospheric pressure patterns across the Pacific. The former uses a fixed SST pattern, while the latter examines the relative pressure difference between Darwin and Tahiti. Let’s take a look at how they played out this year.
First the SST signal from one of the primary El Nino tracking areas in the Pacific :
Low values of SST in the Nino 3.4 region correspond to increased upwelling in the tropical eastern Pacific, stronger trade winds and more of the warm water remaining in the tropical West Pacific, or a La Nina. Here we can see a strong, but not historical La Nina. Clearly 2008 and 1998/99 and 2000 La Nina were of equal strength as viewed here.
Now the SOI, or atmospheric view:
Here a larger SOI corresponds to a stronger west-east difference in pressure between Darwin (generally in a region of low pressure in the warm pool) and Tahiti (in a region of subsidence). Not only was the SOI the largest its been in 20 years, but in some view the strongest, but one of the strongest La Nina’s per the atmospheric side of the coin in the past 50-100 years. The Australian Bureau of Meteorology has a nice page chalked full of information on El Nino. So while the ocean and atmosphere were talking to one another this winter, the atmosphere was a bit louder. There are a host of potential explanations for this, but it also brings up a bigger discussion of how we should monitor ENSO as it relates to impacts for areas well removed from the tropical Pacific that rely on atmospheric teleconnections.