Oh No! Climate Change Might Not Drive Ever Stronger Hurricanes After All!

h/t Judith Curry

Alarmists are going apocalyptic crazy about the sea surface temperatures in the North Atlantic. Tobis hints that we have probably not seen such wild heating since the last interglacial, over a hundred thousand years ago!

Nuts. I didn’t actually realise that humans have been recording N Atlantic sea surface temperatures with accurate instruments since the Eemian! Or maybe those Lost Civilisations we keep hearing about were doing just that and some archaeologist has recently happened upon a repository of sea surface temperatures from one hundred thousand years ago right up to the end of the last Ice Age and the downfall of Atlantis? Yes, that must be it.

Back to reality. Here is what the study says:

Moist convection in an unsaturated atmosphere is associated with substantial irreversible entropy production, which detracts from the energy that the TC can use to power its winds. The increasing moisture content in a warmer atmosphere predicted by Clausius-Clapeyron scaling leads this irreversibility to increase in an unsaturated atmosphere, presenting a larger penalty on 𝑊𝐾𝐸 and decreasing the mechanical efficiency. Our results highlight the importance of giving full consideration to the effects of moisture on the TC heat engine in studies of how climate affects TCs.

There is a satisfying irony here. Climate alarmists and extreme weather catastrophists are inordinately fond of citing Clausius-Clapeyron (basically, the simple physical law which says warmer air can absorb more moisture) in defence of their hyperventilating opinions masquerading as ‘extreme weather attribution science’ re. the association between more intense and prolonged rainfall and the dreaded ‘climate crisis’. But now it seems that Clausius-Clapeyron is putting a brake on their aspirations of claiming that hurricane winds are becoming ever more fearsome and intense because of sea surface heating allegedly due to climate change. Oh, what a shame!

The authors go on to say:

The mechanical efficiency of a TC (Pauluis and Zhang 2017; Fang et al. 2019) is a property of the TC heat engine, defined as the fraction of the total heat input that is available to drive the full 3D wind field, and may vary with climate change. Changes in the mechanical efficiency are set by the combined variability of several energetic terms that reflect the impact of the distributions of temperature and moist entropy in a TC on its energetics, the effects of irreversibility associated with moisture on the kinetic energy produced by the TC heat engine, and the total heat input to the TC heat engine. Although some studies have examined aspects of the variability of the TC mechanical efficiency and related factors (e.g. Pauluis 2016; Pauluis and Zhang 2017; Fang et al. 2019; Wang and Lin 2021), much about this remains poorly understood. In this paper, we investigate how and why sea surface temperatures (SSTs), which are expected to generally increase with climate change, affect the mechanical efficiency of mature TCs.

Note, the ‘settled science’ strikes again (or doesn’t)!

The authors find:

In spite of the increase in 𝑊𝐾𝐸 [total kinetic energy of the tropical cyclone] with SST, the mechanical efficiency decreases at -2.1 % K−1 (Fig. 5, Table 1). This is due to the increase in the heat input 𝑄𝑖𝑛, which grows at 6.3 % K−1. Although the increase in 𝑊𝐾𝐸 acts to increase the mechanical efficiency, the increase in 𝑄𝑖𝑛 acts to decrease it, as can be seen from equation (8). Here, the effect of the increase in the heat input dominates, resulting in a decreased mechanical efficiency.

In other words, as sea surface temperatures become warmer and warmer and more heat is transferred into developing tropical cyclone systems, the increase in heat uptake does not translate efficiently into an increase in kinetic energy (wind speed), because of moist convection processes. This throws cold water on the simplistic formula beloved of climate alarmists, i.e. climate change = higher SSTs = more heat uptake of hurricanes = stronger hurricanes. The authors of the current study point this out. Again, it would seem that the ‘settled science’ is not quite as settled and consensual as the hyperventilating alarmists e.g. at the Guardian and the BBC would have us believe:

We have investigated how the mechanical efficiency, 𝜂𝑚𝑒𝑐ℎ, of mature tropical cyclones is affected by sea surface temperature (SST). We found that 𝜂𝑚𝑒𝑐ℎ decreases with SST, at a rate of -2.1 % K−1 (Table 1). This decrease was driven by a 6.3 % K−1 increase in the total heating of the TC, 𝑄𝑖𝑛, that dominates over the 3.4 % K−1 increase in the kinetic energy of the wind field, 𝑊𝐾𝐸. The increase in 𝑊𝐾𝐸 is smaller than the 7.0 % K−1 increase in its maximum bound, the net heat energy gained by the TC 𝑊𝑀𝑎𝑥. The majority of the increase in the net heat energy is instead reflected by the strong increase in the moist processes in the TC. This increase occurs in part because of an increase in the atmospheric moisture content that is expected from Clausius-Clapeyron scaling.

The related larger irreversibility at higher SST is shown by the increase in the associated moisture penalty, 𝑊𝑀𝑜𝑖𝑠𝑡 , of 9.6 % K−1. 𝑊𝑀𝑜𝑖𝑠𝑡 represents a larger portion of 𝑊𝑀𝑎𝑥 at higher SST, which reduces the magnitude of the increase in 𝑊𝐾𝐸 compared to the increase in 𝑊𝑀𝑎𝑥. This is true regardless of the subset of the TC circulation that we consider (Table 1, Figure 5).

In contrast to our results, Pan et al. (2017) find, using primarily the NCEP-DOE R2 (Kalnay et al. 1996; Kistler et al. 2001; Kanamitsu et al. 2002) and ERA-Interim (Uppala et al. 2005; Berrisford et al. 2011; Dee et al. 2011) reanalysis datasets, that the mechanical efficiency of the global atmosphere increased over the period 1979-2013. However, reanalysis products are not well suited to evaluating climate trends (Thorne and Vose 2010; Chemke and Polvani 2019). Together with the fact that the global atmospheric heat engine and the TC heat engine may behave quite differently in response to warming, this may help explain the discrepancy between our results and theirs.

This perfectly illustrates the difference between scientists who really know about complex thermodynamic interactions in actual storm systems and those who just assume that they know because they’re looking at reanalysis datasets which tell them that storms are getting more intense, so it must be climate change.