For at least the past one or two decades the adjective extreme has increasingly become used in describing unusual weather. It’s virtually impossible now to escape news of extreme drought, excessive rainfall and floods, record breaking heat waves, cool spells and severe weather outbreaks, etc. which seem to recur year after year around the Northern Hemisphere. This summer was no different except that the behavior and configuration of the polar jet stream, the river of high altitude winds marking the divide between warm and cool air, were rare and mind-boggling.
Instead of meandering as a single stream like it normally does, it transformed into a “dual” jet stream configuration, sometimes transitioning from this dual setup back into a single more coherent stream, back and forth.
The rarity of dual polar jets was highlighted by Professor John Nielsen-Gammon (Texas A&M University) in an article in Popular Mechanics. He pointed out they are something one might see once per decade. From an independent assessment myself, it appears that there are no other polar jet examples comparable to this summer at least as far back as 2000 (the furthest back I’ve looked).
Mostly, the perplexing behavior of the polar jet has been described in befuddling terminology such as weird, mangled, and wobbly. Some have described the jet in a state of disarray, not playing by the so-called rules. Jeff Masters said that in his 30 years doing meteorology, the jet stream has been doing things he’s not seen before.
What follows is a rather technical discussion of how this jet stream pattern evolved and some of the weather characteristics associated with it. Although some terms may not be familiar, the included parenthetical notes and illustrations should help guide you along.
As a general overview I’ve subjectively identified three periods I call Regime 1, 2, and 3. To illustrate associated weather characteristics, I present 500mb zonal wind (representative of upper level jet stream) anomalies and 850mb temperature (low-level temperature) regimes over the June-July-August (JJA) meteorological summer. In these time vs. latitude (30-90N) charts, color coded values are daily means longitudinally averaged (0-360 degrees) at each latitude. Jet streams coincide with the green to red colorization. The three regimes are separated by notably shorter periods of transition from one regime to the next.
Regime 1 (R1) appeared following a regime change at the end of May (not shown) to a dual polar jet which persisted through most of June. Around the beginning of July, R1 transitioned to a single jet mode which characterized Regime 2 (R2). During the third week in July, there was a rapid change to another dual jet configuration in Regime 3 (R3), which subsequently transitioned to a single jet during the middle of August.
It’s important to add that changes in the zonal wind at any given latitude conform directly (via basic meteorological principles, “thermal wind”) to the largest north-south and south-north differences (gradient) in the lower level temperature field (winds adjust to temperature changes, not vica versa, except in the Tropics).
Most significantly, each regime reflects notably different background fields in the three dimensional wind and temperature structure of the atmosphere from the mid-latitudes to the North Pole (NP). Although there is considerable variability within a given regime, each appears to have predominant signatures in observed weather events that differ from those characterizing the other regimes. Some examples appear deeper down.
To further describe aspects of regime transition, I’ll focus on that from R2 to R3. See first the time/lat chart for the period July 1 to Aug 9.
The major difference between R2 and R3 zonal wind anomalies is obvious. Specifically, R2 is characterized by a single maximum in zonal wind speed (single polar jet) centered between 55-65N. Following a relatively short period of transition, two maxima (dual jets) are evident (in R3), the strongest immediately surrounding the NP (80-90N) while the second is seen initially far to the south but migrating slowly towards mid-latitudes.
The zonal wind profiles are directly tied to evolution of the lower level temperature field. R2 is characterized by very warm weather immediately surrounding the NP (80-90N), cool in the 60-75N latitudinal band, and warm centered between 45-55N.
After the relatively short transition period, R3 is virtually a mirror image with very cold around NP, warmth between 65-75N, and cool further south. Close inspection, if you are so inclined (presuming you have very good eyesight), will reveal that zonal wind speed maxima occur precisely where temperature decreases most rapidly from S-N, while minima are found where temperature increases most rapidly from N-S.
So what does all this have to do with extreme weather events?
Almost invariably extreme summer weather of late is discussed in context of anomalies (differences from average) in the polar jet. The anomalies are commonly attributed directly or indirectly to global warming (aka climate change) as manifest in warming occurring faster in the Arctic than latitudes further south (Arctic amplification). Temperatures, therefore, decrease less rapidly than the climatological norm, and the zonal component of the winds at jet levels adjust by weakening relative to “normal”. The response generally speaking is for atmospheric waves to amplify in their meridional (N-S) extent and lead to more frequent occurrences of unusually high amplitude ridges (and/or blocking highs) and troughs (and/or cut-off lows) along with the respective weather associated with these systems. In combination with slowing progression of weather systems, this translates to enhancing prospects for persistent spells of extreme heat, and extended periods of unusually cool and/or wet conditions.
As illustrated in the figures above, the N-S differential heating adjustments in the zonal wind component are considerably more complex with regard to details in the spatial and temporal variability within as well as between regimes. In particular, note variability in details over time and latitude in the blue areas where zonal winds are least strong and thus favorable for high amplitude circulations and possible extreme weather.
Nevertheless, as mentioned earlier, it is possible to discern the principle unique expression of each regime. By way of example, this figure displays those for R2 and R3.
The distinct differences between the two regimes are abundantly clear (the 500mb height anomalies are closely related to the low level temperatures.) Note especially the dramatic transition from relatively cool conditions to extreme warmth over Alaska (influence of high amplitude ridge), the cooling trough in R3 over the Northeast U.S., and dominantly warm (R2) to dominantly cool (R3) over extreme northern Europe.
The figures below exemplify regional differences corresponding to heavy rainfall events (precipitable water – total atmospheric water content above location – is used as an approximation for relative differences in precipitation).
The transition form R2 to R3 brings in flooding rains to Western Europe.
Especially interesting for the U.S. are alternating regions of dominantly dry and dominantly wet conditions in the sequence of regimes transitions over the course of the entire summer, shown below.
Finally, there is no basis at this time (if ever) to determine whether the transitions to and from regimes with dual polar jets made this summer any more or less unusual in occurrences of extreme weather events than over the past 10-15 years, which have been presumed to be less complicated by dual jets.
Scientists tend to believe the increase in extreme weather is tied somehow to the diminishing Arctic ice cover and perhaps more rapid melting of snow cover over Siberia. The “somehow”, especially when coupled to interactions with other plausible and not yet identified factors, remains an open question. No individual or set of observational studies to date and no existing models and modeling strategies are adequate for garnering some insights when dealing with details in regional domains. This is especially true when dual polar jets are added to the mix of complexities. As far as I know, there has not even been a single investigation of the why’s and wherefore’s of this aspect of the problem (or even whether it has been given much thought).