The Near Endless Summer of 2022
After a very wet and cool spring, the Pacific Northwest had an unusually long stretch of warm and dry weather in 2022. The lack of rain was especially pronounced in western Washington, with many locations recording record low totals for the months of July through September. Perhaps even more surprising was how dry it remained until the latter portion of October. As an inveterate viewer of weather maps and numerical weather prediction model output, I was struck by the seemingly ubiquitous ridge of high pressure aloft over the Pacific Northwest extending into western Canada, especially late in the past summer and into fall when we usually get rewarded with more active weather, and at least brushed by disturbances coming into our region off the Pacific Ocean. Therefore, I thought it would be interesting to delve into the regional circulation associated with the unusual weather of the past few months, and the extent to which there is historical precedent for the pattern that prevailed.
The circulation pattern for the period of interest is illustrated here using, what else, 500 hPa geopotential height (Z) anomaly maps, separately for the months of July through September and for the 4-month long interval from the summer solstice to 20 October in its entirety (Figure 1). Systematically positive Z anomalies occurred north of the US-Canada border, accompanied by warm air aloft and implied flow anomalies from slightly south of east, and sinking motion for the lower-middle troposphere over WA and BC. The weaker flow from the Pacific also meant the delivery of relatively warm continental air to a greater extent than normal but not especially dry air; the relative humidity was typical and the specific humidity was actually greater than usual at 925 hPa in the atmospheric boundary layer (not shown). As a consequence, the minimum as well as maximum temperatures were well above normal.
The summer of 2022 featured few days of rain, but does that mean that the ridge was present day in and day out? Figure 2 is a time series of daily 500 hPa Z anomalies at a point at the center of the box shown in Figure 1d. This time series features a preponderance of positive anomaly days, as expected. The hottest weather of the period considered here was during an interval near the end of July, which included positive anomalies of about 100 meters. It does not require as much of a perturbation to get it hot at that time of year. Not surprisingly, the greatest deviations from normal occurred in October given how unusually warm and dry the weather was during that interval. Naturally there were some short periods during which the ridge was absent, but based again on experience, they seem to have been mostly minor and fleeting.
In order to put the results shown above in an historical context, a time series of the average 500 Z for the box encompassing primary anomaly in 2022 shown in Figure 1d was constructed for the months of July through September. Dating back to 1948, the year of 2022 had the greatest heights for the region (Figure 3). A selection bias is recognized; the area over which the 500 hPa Z was averaged was picked on the basis of it being highly anomalous and so 2022 is liable to stand out by construction. Nevertheless, a 75-year record means there were plenty of opportunities to match it. It is noted that 2012 represents a recent year with the 2nd highest 500 hPa Z in the selected area. It also had a long stretch of dry weather; for example, KSEA recorded only about 1” of rain from July through September versus the normal total of about 3.5”. It turns out that 2017 was even drier, and also had abnormally high 500 hPa Z north of the border, but the anomalous ridge in this case did not extend westward over the Pacific and so that year was unremarkable in terms of the index plotted in Figure 3.
We close with somewhat of an aside, and that is with a comment on the overall positive trend in 500 hPa Z shown in Figure 3, particularly after the early 1980s. It turns out that sea level pressure (SLP) and 1000 hPa Z trends for the same area are flat to slightly decreasing. The rise in 500 hPa Z is consistent with an increasing – but not monotonic – trend in lower tropospheric temperatures. In other words, heating has meant expansion of the atmosphere and a corresponding increase in 500 hPa Z. An increase of 1°C averaged over the lowest 500 mb is equivalent to an increase in 1000-500 thickness or 500 hPa Z – if 1000 hPa is unchanged – of about 20 meters. Over the interval shown in Figure 3 the rise in 500 hPa Z is about 25 meters, based on a linear fit, implying a temperature increase of slightly greater than 1°C. Presumably we will observe a continuation of overall rises in 500 hPa Z in the area, and for that matter the Pacific Northwest as a whole, but it might be a while until we surpass that of 2022.