Revisiting The Exceptional Pacific North West Heatwave Part 1

NASA's Gavin Schmidt Ridiculed Meteorologist Cliff Mass On Twitter - Big Mistake

Before the event was even over, Gavin Schmidt took to Twitter to criticise meteorologist Cliff Mass for stating on his blog that the proximate cause of the forecast extreme heatwave was atmospheric dynamics and that global warming would have only a minor influence on the predicted extremely anomalous temperatures. Cliff said:

The proximate cause of this event is a huge/persistent ridge of high pressure, part of a highly anomalous amplification of the upper-level wave pattern. 

There is no evidence that such a wave pattern is anything other than natural variability (I have done research on this issue and published in the peer-reviewed literature on this exact topic).

So without global warming,  a location that was 104F would have been 102F.  Still a severe heat wave, just slightly less intense.

Let me end with the golden rule of temperature extremes:  the bigger the temperature extreme the SMALLER the contribution of global warming.  Think about that.

Schmidt was having none of it and he responded with a series of very condescending tweets:

Then this final tweet where he claims that the hotter it gets, the more certain will be the attribution to global warming; completely the opposite of Cliff’s ‘golden rule’.

Schmidt, as it turns out, was proved dead wrong by subsequent research (see below) and Cliff Mass was proven 100% correct. I was also correct in doubting that the Pacific North West heatwave could be attributed to global warming - even before the subsequent ridiculous World Weather Attribution analysis was published claiming that the event would have been ‘virtually impossible’ without climate change. On July 1st 2021 I wrote this post:

I stated back then:

In the Northwest heatwave for instance, previous records have been completely obliterated. A regional secular warming trend of a degree or two cannot explain these extremes. It has to be down to weather and that means meteorology: dynamics, not thermodynamics (global warming). In order to attribute these extreme events to global warming (thermodynamical influence), you also have to explain how dynamical influences have been so altered by generalised global warming that they are capable of producing such 'unprecedented' extremes in temperature. You also have to be very careful to eliminate other possible contributory factors to extreme temperatures such as preceding drought conditions, land use changes and urbanisation. So, I'll wait until the formal attribution study is published by WWA and then comment.

This study was published in November 2022, entitled

Drivers and Mechanisms of the 2021 Pacific Northwest Heatwave

It finds the following:

Based on our model experiments with a nudged atmosphere, the effect of soil moisture amounts to 2.8°C during the peak of the event, while the circulation effect reaches >13.4°C. This soil moisture contribution is solely based on simulated (one-way) land-atmosphere interactions—but not (two-way) feedbacks—for actual (aFsF) and climatological (aFsC) soil moisture in CESM. In addition, the surface only affects winds in the ABL and lower troposphere due to enforcing the ERA5 large-scale circulation further aloft. Despite the inability of the atmosphere to influence soil moisture in these experiments, we emphasize that the prescribed soil moisture has been calculated in offline CLM simulations using ERA5 meteorological forcing and is hence consistent with the near-surface conditions arising from actual land-atmosphere feedbacks and other interactions. We also provide an alternative estimate of the circulation effect by first comparing the base simulation (aFsF) to the ensemble mean of simulations with interactive atmosphere and constrained soil moisture (aIsF); this yields an even stronger contribution of up to 14.7°C. By design, however, this estimate includes effects from both circulation as well as soil moisture, since the ERA5-constrained middle and upper tropospheric winds in aFsF can contain imprints of actual land-atmosphere and other interactions or feedbacks that shaped the large-scale flow. Land-atmosphere interactions were found to be particularly strong during other record-shattering midlatitudinal heatwaves (e.g., Fischer et al., 2007; Miralles et al., 2014), and strongly depend on the prevailing large-scale circulation. As such, temperature impacts of land-atmosphere interactions during the 2021 PNW heatwave tend to be underestimated in simulations with fully interactive atmospheres. Therefore, we consider the corresponding soil moisture effect (aIsF-aIsI) of at most 1.5°C to represent a “generic” estimate that likely fails to capture the interactions between anomalous soil moisture and event-specific circulation. This approach hence differs notably from the soil moisture contribution based on aFsF and aFsC, for which the same actual large-scale circulation is prescribed. The same limitation applies to the two remaining drivers in our additive framework; the effects of anomalous SSTs (w.r.t. the mean state of 2015–2020), and of our changing climate (compared to 1982–2008). Averaged over 26–30 June, the background warming acted to amplify the event's magnitude by 0.9°C, whereas the surface ocean only contributed 0.1°C. The circulation explains 81.0% of the event anomaly in the same 5-day period, while the event-specific soil moisture effect amounts to 11.7%. Even though the generic soil moisture effect is considerably weaker at the peak of the event, it explains nearly the same fraction of the 5-day mean temperature anomaly with 10.2%, whereas the corresponding circulation contribution accounts for 82.4%. The remaining event magnitude is attributable to anomalous ocean surface state (0.9%) and recent warming (6.4%).

So, atmospheric circulation (dynamics) accounts for 82.4% (>13.4C)of the observed anomalous high temperatures during June 26-30, soil moisture deficits account for 10.2% and recent regional warming (concurrent with the global warming trend) accounts for just 6.2% (0.9C). Cliff Mass was almost spot on with his assessment of a 2F difference (0.9C=1.62F). Climate scientist Gavin Schmidt OTOH just looks plain silly. The event was overwhelmingly attributable to atmospheric circulation (dynamics/meteorology); only a small fraction could be attributed to global warming, precisely because the extreme temperatures were so anomalous and extreme.