Western WA Heat Waves
As we enter the summer months, it is timely to discuss a research paper that was recently published in the Journal of Applied Meteorology and Climatology (Bumbaco et al. 2013). OWSC collaborated with the Oregon Climate Service at Oregon State University to examine the historical record of heat waves occurring west of the Cascade Mountains in both Washington and Oregon. The record-breaking heat wave that occurred at the end of July 2009 was the impetus into looking into these events in more detail. A summary of a few of the main findings is presented here.
We evaluated western WA and OR together, and used 37 stations to form a regional temperature time series. The heat waves were defined as three consecutive days above the 99th percentile for the maximum and minimum temperature anomalies, separately. This translated into daytime heat waves that were 17.1 F above normal and nighttime heat waves that were 8.3 F above normal for three consecutive days. For Seattle, for example, these anomalies are equivalent to the maximum threshold of 89.4 and minimum threshold of 61.5 F.
The historical daytime heat events are shown in Figure 1a and the nighttime heat events in Figure 1b. For the purposes of this overview, ignore the blue dots, and consider only the red and black symbols. There were 13 daytime events and 15 nighttime events identified in the record. Five of the heat waves (in 1941, 1942, 1981, 2006, and 2009) were in both the daytime and nighttime sets; the other 18 individual events were of just one type. The average start dates of both types were at the end of July: 23 July for the daytime events and 26 July for the nighttime events. The earliest heat event in the record began on 30 June and was both a daytime and nighttime event (in 1942). The latest heat event began on 10 August (1977) for the daytime events and 27 August (1935) for the nighttime events. Note a clustering of nighttime events at the end of the record in Figure 1b. A trend analysis was performed on the events identified and there is a statistically significant increasing trend in the frequency of nighttime events in western WA and OR. This upward trend is consistent along the west coast, as was also found in California (Gershunov et al. 2009). Despite the record-breaking heat event in 2009 that was a daytime event as well as a nighttime event, no trend was found in the frequency or magnitude of the daytime events.
Using the 20th Century Reanalysis product, composite patterns of several atmospheric properties associated with the daytime and nighttime categories of heat waves were also constructed (not shown). Many similarities between the two types of events were found; for example, a prominent ridge at the 500 mb level (mid-troposphere) is needed to help produce downslope warming over the west side of the Cascade Mountains. This mechanism appears to be particularly important for getting the days hot. For the nighttime events, we found higher precipitable water content, particularly at the beginning of an event. This result suggests that high nighttime temperatures are more related to downward longwave radiative fluxes, as mediated by moisture aloft, as opposed to downslope warming like the daytime events. These differences, especially at the onset of a heat event, may be especially valuable in an operational setting for discriminating between daytime (e.g., stronger ridge and lack of moisture) and nighttime (e.g., higher humidity and weaker sea level pressure gradient) events.
More information on this study can be found in the research paper that is available online here: http://journals.ametsoc.org/doi/abs/10.1175/JAMC-D-12-094.1. While some folks may wish on shooting stars for these types of weather events, it is important to keep in mind that they can have significant societal impacts. Residents of western WA and OR are quite vulnerable to heat as they typically do not have air conditioning and are not acclimated to hot weather. A preliminary analysis of western WA hospitalizations shows a 50% increase in heat-related hospitalizations during the summers with heat events defined in our study, and other studies (e.g., Jackson et al. 2010) have shown measurable negative health impacts from heat in the Pacific Northwest. OWSC pans to examine further the events defined here, with a focus on determining the characteristics that are most closely linked to regional human health.
References:
Bumbaco, K.A., K.D. Dello, N.A. Bond, 2013: History of Pacific Northwest heat waves: synoptic pattern and trends. J. Applied Meteorol. Climatol., e-View doi: http://dx.doi.org/10.1175/JAMC-D-12-094.1
Gershunov, A., D.R. Cayan, and S.F. Iacobellis, 2009: The great 2006 heat wave over California and Nevada: Signal of an increasing trend. J. Climate, 22, 6181–6203.
Jackson, J.E., and Coauthors, 2010: Public health impacts of climate change in Washington State: Projected mortality risks due to heat events and air pollution. Climatic Change, 102, 159–186.