William Kininmonth essentially says that it’s possible that the trade winds have changed the climate, but asks why the winds themselves changed. Kininmonth explains that the ocean is much larger and holds much more heat than the atmosphere, and that the ocean drives the winds rather than the other way around. He points out again (as he did before here so eloquently in more detail) that what the paper describes is what we’ve known for a long time about the ENSO patterns and the Pacific Decadal Oscillation (PDO): when an El Nino Strikes, trade winds fall, ocean surface doesn’t turn over as much, the ocean surface is warmer, and the air stays hot above. When La Nina’s occur, trade winds speed up, the ocean stirs, and the cold deep water takes the heat out of the surface of the ocean and the air above.
His points are:
- “Natural variability” is hardly a credible, useful scientific explanation.
- The IPCC said natural variability was small, so if it is larger now, then it was also larger during the rest of the 20th Century? This reduces the effect CO2 had earlier (and the effect it will have in future).
Guest post by William Kininmonth
Chief of Australia‘s National Climate Centre at the Bureau of Meteorology from 1986 to 1998.
Winds of change?
An international research team has published an explanation for the recent stasis in global temperature rise but leaves essential questions unanswered. Variation of Pacific Trade Wind speeds may well be linked to changing global temperature, as argued. Why then did the wind strength vary? The given explanation of ‘natural variations’ is neither scientific nor convincing.
If, as claimed, natural variation has dominated the warming effect of increasing atmospheric carbon dioxide concentration over recent decades then surely natural variation has also been important in determining the 20th century temperature rise. The science of climate change and the role of carbon dioxide are far from settled.
Trade Winds over the Pacific
I have read the England et. al. paper and made some independent checks using data available from the US NOAA National Centre for Environmental Prediction (NCEP) since 1948 (from the international reanalysis project). The basic data in the paper checks out:
The Pacific Trade Winds have increased then decreased over recent decades. (Note easterly trade winds have a negative sign and hence increasing negative is a strengthening trade wind – surface stress is approximately a function of the square of the wind speed and is amplified with increasing wind speed.
The sea surface temperatures of the eastern equatorial Pacific have slightly warmed then cooled although there is significant year-to-year variability that tends to mask the evolution.
England et. al. noted that data prior to the satellite era (about 1979) should be treated with caution but nevertheless the essential form of the records fits with their description, especially post-1979.
ENSO patterns, El Nino and La Nina
The explanation given by England et. al. for the link between trade wind strength and SST over the eastern equatorial Pacific is an extension of the classical interpretation of El Nino: Trade Winds slacken, upwelling over the eastern Pacific ceases, and SST over the eastern Pacific rises. The warmer waters of the eastern Pacific locally exchange heat and latent energy with the overlying atmosphere to modify atmospheric circulations generally, with global impact on weather systems and temperature of the lower atmosphere. We see this from a comparison of the UAH lower troposphere satellite temperature anomalies for the tropics and globe.
Note that with the major El Nino events of 1982/83, 1987/88 and 1997/98 there is significant warming over the tropics and the globe with that of the tropics exceeding the global value (That is, the additional heat imparted to the tropics is distributed globally). Similarly for the cold excursions of the La Nina events there is cooling over the tropics that is translated globally.
The Ocean drives the Air
The issue with the England et. al. paper is the claim that the strengthening of the Trade Winds is the cause for the cooling over the eastern Pacific. The inter-annual ENSO (El Nino/Southern Oscillation) events have their origins in the ocean circulations and the effect of these changes is transmitted to the atmospheric circulation; it is the atmosphere that responds to the ocean and not vice versa.
The cold water of the ocean interior is maintained by: 1) sinking cold saline water under growing winter sea ice in polar regions to form Bottom Water; and 2) Ekman turning associated with the high latitude westerly winds that forces cold surface water under the warmer tropical waters to form Intermediate Water. Both these processes result in gently rising water under the tropical thermocline. As the water mixes upward across the thermocline it takes heat out of the tropical surface layer at a rate of about 20-25 W/m2, a significant fraction of the solar radiation absorbed over the tropical surface. It would be surprising if the overturning circulations to form Bottom and Intermediate Water were at a constant rate, given the complexity of ocean circulations. Changes in ocean overturning circulation of about 5% on multi-decadal timescales could easily explain the slowly changing surface temperatures that have been observed, and hence the changes in the Trade Wind strength.
The question is, does the changing atmospheric circulation regulate the ocean circulation on multi-decadal timescales as proposed by England et. al.? Or is it more plausible that changing ocean overturning on multi-decadal timescales regulate the atmospheric circulation to produce the observed oscillations (such as the Pacific Decadal Oscillation – PDO)? Given that the heat content of the atmosphere is equivalent to that of the top 4 metres of the ocean and the mass of the atmosphere is equivalent to the top 10 metres of the ocean it is unlikely that the atmosphere tail wags the ocean dog!
What “internal variability” gives, it can take away
The IPCC metaphorically painted itself into a corner when, in its 2001 3rd assessment, it claimed that based on computer models the climate system had only limited internal variability (ocean-atmosphere interactions producing decadal and longer oscillations, including of global temperature). If there is little internal variability then the reproduction of 20th century using computer models with only carbon dioxide and known natural forcings is a seemingly plausible explanation for the warming trend of the 20th century; it also provides an estimate of the sensitivity of global temperature to carbon dioxide forcing. In contrast, if there is significant internal variability on all timescales from the inter-annual ENSO to multi-decadal (and possibly centennial) then the influence of carbon dioxide is much less than the models show – the reproduction of 20th century temperature is as much judicial curve fitting and is a misleading explanation of what was driving climate and global temperatures during the 20th century.
Without acknowledging same, England et. al. are implicitly drawing on internal variability to explain the recent temperature pause – varying Trade Wind strength is claimed as the driver but no explanation is given as to what is causing the Trade Wind variation. Inadvertently they are opening the possibility of an important role for internal variability as a basis for global temperature variations. This is at odds with the IPCC claim of limited internal variability and further suggests that much of the global warming of the 20th century was not due to increasing carbon dioxide but due to internal variability. It follows that the sensitivity of global temperature and climate to increasing carbon dioxide concentrations is much less than IPCC claims based on computer models.
The England et. al. paper underscores that the science of climate change is far from settled.
UPDATE: I’ve posted my thoughts on this paper here: http://joannenova.com.au/2014/02/global-wind-excuse-monkey-modeling-shows-global-warming-theory-is-still-not-wrong/
England, M.H., S McGregor, P. Spence, G.A. Meehl, A Timmermann, W. Cai, A.S. Gupta, M.J. McPhaden, A. Purich and A. Santosos, 2014. Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus. Nature Climate Change (online: 9 February 2014) DOI: 10.1038/nclimate2106