Simon’s Stratosphere Watch #3

Apologies for the long gap between blog updates. But since today’s weather in NYC has descended into a snowy, sleety mess, I thought today would be a good day for a blog.

Saturday 12 March 2022

Summary:

It is increasingly likely that the final stratospheric warming will occur soon, but forecast uncertainty remains large. A component of this uncertainty is likely to be stemming from the tropospheric evolution over Scandinavia and Greenland in the next 5 days arising from the track and evolution of a deep extratropical cyclone. Immediate tropospheric impacts from the final warming appear to be muted.

Discussion:

Medium-term

At this point in the season, a sudden stratospheric warming — a reversal of the zonal-mean zonal winds at 10 hPa 60°N — is likely to go on to become the final stratospheric warming (i.e., not followed by a sustained recovery of the westerly winds, indicating the transition to the summertime state). As such, I’m considering forecasts of U10-60 reversals to be forecasts of the final warming. Table 1 in Butler and Domeisen (2021) provides a very useful list of final warming dates and classifications.

All forecasts have been vacillating wildly in recent weeks. The GFS has been particularly keen on run-to-run variability. As a single deterministic forecast, the run-to-run variability of the GFS suggests to me that this is fundamentally a deterministically unpredictable situation. Fig. 1 shows recent 00Z forecasts. Beyond about 10 days, the forecasts become much less coherent, which suggests medium-range tropospheric uncertainty is generating stratospheric uncertainty. One could also imagine the stratospheric vortex as a cricketer facing the great Shane Warne. Yes, Warne could bowl some unplayable balls that would get you out no matter what you did, but to bowl someone out you need both a great ball and the batsman to try to play it in a certain way. That’s probably a useful way of thinking about this coupling mechanism: the uncertainty is multiplied by the way the stratosphere tries to play the ball from the troposphere. In Fig. 1 you can see the vortex has only been bowled out in 3 recent 00Z runs. But will Vortex (not out) continue?

Figure 1: ‘Chiclet’ plot of recent 00Z GFS forecasts of U10-60.

Today’s 00Z suite (Fig. 2) has jumped to much greater confidence of an earlier final warming than most other recent forecasts. The GFS goes for 20 March, the GEFS ensemble mean goes for 21 March, and all 31 members reverse the winds within 16 days. The previous day’s 35-day GEFS shows all 31 members producing the final warming before 15 April, suggesting confidence in an early final warming. Earlier final warmings are more likely in winters where there was no major mid-winter SSW. There are a couple of reasons for that: the vortex tends to recover strongly after a mid-winter SSW, prolonging the decay, whilst if an SSW has not already occurred during midwinter then it is effectively more likely to occur later in early spring (i.e., the final warming becomes just a very late SSW). This year we seem to be in the latter category.

Figure 2: GEFS & GFS forecasts of U10-60 from 00Z 12 March 2022.

The GFS shows the vortex splitting — a wavenumber-2 final warming. This seems to be related to amplification of wavenumber-2 from the troposphere, at least partly through amplification of the Scandinavia-Greenland pattern. This is particularly interesting to me as I wrote two papers (in 2019 and 2020) arguing that the S-G pattern was important for weakening the stratospheric vortex and is both poorly represented in, and poorly predicted by, current forecast models. Other recent studies (e.g. Wang and Tan 2020, Pang et al. 2021) have also demonstrated the importance of this type of Scandinavian variability for stratospheric impacts.

Figure 3: Forecast MSLP anomalies (left) and 100 hPa eddy heat flux (right) for 00Z 17 March from 00Z 12 March GFS.

But what’s driving this S-G pattern, and why does it seem to have such low predictability this time? The cyclonic ‘G’ part is associated with a deep cyclone which is exiting the US east coast today (the same cyclone which is bringing miserable weather to NYC, leading me to write this blog). This cyclone is expected to drop to <930 hPa in the northwest Atlantic, and is associated with anticyclonic wave breaking downstream building the ridge over Scandinavia. Fig. 4 shows 2PVU maps forecast for 14 March 2022 and the GFS analysis from 5 February 2018, during the ‘original’ S-G dipole event which we argued was important for driving the 12 February 2018 SSW. Not identical, but pretty similar.

Figure 4: 2PVU maps for (left) 18Z 14 March 2022 (GFS 54h forecast) and (right) 00Z 5 Feb 2018 (analysis). Maps from Alicia Bentley.

If indeed the this cyclone is a key player in driving the enhanced tropospheric wave activity which leads to the final warming, then what we wrote about the forecasts of the 2018 SSW (“…this forecast evolution was associated with the track and intensity of a cyclone in the northeast Atlantic, with an associated anticyclonic Rossby wave break, which was not well forecast.“) may also be true for the 2022 FW.

Thus, as the tropospheric forecast likely becomes more confident as of today — when the cyclone starts its rapid intensification phase — it’s possible that the stratospheric forecast uncertainty will also diminish. That seems to be the case with today’s GFS forecasts, at least, which have been consistently reversing U10-60 next weekend. But, to keep my cricket analogy going, this is confidence in the bowler only.

Longer-term

The longer-term specifics are swamped by the medium-range uncertainty (the batsman is at the crease and struggling for form against Warne’s impressive bowling repertoire, and we’re trying to forecast whether or not they will still be in after tea — but we don’t know how many balls Warne has left this session. Am I stretching the analogy a bit far?).

What we do know, though, is that the vortex will be severely damaged by what happens over the next week. What is left won’t be scoring a double-century. But there does not, yet, appear to be a large signal for a substantial weakening of the circulation in the lower stratosphere (Fig. 5), where temperatures had become very cold during the strong vortex this winter. This strong circulation will likely take longer to decay and thus reduce the extent to which the troposphere initially feels this change aloft. As a result, it does not look comparable to what happened after the dramatic early final warming of March 2016, for example. Nevertheless, a shift toward a greater risk of -NAO conditions is possible compared with where we have been, but that does not mean this will be a spring-killer.

Figure 5: 100 hPa 60°N zonal-mean zonal wind forecast from GEFS 00Z 11 March.

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