Anthony Lupo
Associate Professor of Atmospheric Science in the Department of Soil, Environmental, and Atmospheric Sciences at the University of Missouri Columbia
19 Jun 2006

A recent study by Vecchi, et al titled "Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing"[1], states that there has been a discernable (approximately 3.5 percent) weakening in the Walker Circulation since the mid-1800's. [The Walker Circulation is an atmospheric circulation germane to the tropical latitudes in the Pacific Region, whose strength is linked to El Niño.] The researchers also suggest that the Walker Circulation will weaken by another 10 percent by 2100, and they attribute these changes to anthropogenic climate change. They also suggest that the climate is moving toward a more El-Niño-like state. [The El Niño here refers to the warming of sea surface temperatures that occurs in the tropical central and eastern Pacific every 2 - 7 years, and the associated changes in the atmospheric circulation.] The researchers used historical records dating back to the mid-1800s, as well as a general circulation model (GCM) to study the problem. [A GCM is a dynamic model that uses basic physical principles and mathematical statements to simulate the atmosphere, and these can be used in a predictive or a diagnostic capacity. Thus, a GCM, or any model, is a statement of hypothesis about how a system works.]

If the climate were moving toward a more El Niño-like state, then it would be reasonable to assume that the El Niño phenomenon would occur more often, and/or become stronger with time. This indeed seemed to be the case during the period 1977 - 1998 when three strong El Niños occurred, and several years during the early 1990's were arguably in a prolonged El Niño-like state. By the year 2000, it certainly seemed that El Niño was becoming more common and stronger -- as a paper published by researchers at the Russian Academy of Sciences [2] reveals. However, since 1997-1998, for almost an entire decade, there has not been a strong El Niño. The El Niño of 2002-2003 was very weak, and the El Niño of 2004-2005 is not even classified as an El Niño using some definitions [3]. The Vecchi, et al paper does not account for this fact, as their study only covers the 1861 to 1992 period. The Tropical Pacific has been characterized more by La Niña conditions since the Fall of 1998. [La Niña is simply characterized by cooler than average waters occurring in the same regions where El Niño occurs.]

This recent period mimics the period of 1947-1977, when El Niños were weak and La Niña events were more prominent. This would confirm the results of studies [4], including some by our own research group [5,6,7], that have suggested that a longer-term Pacific Ocean-based cycle, the Pacific Decadal Oscillation (PDO) modulates the occurrence and intensity of El Niño on a very long-term (50-70 year) timescale.

As for the GCMs, it is close to impossible to use them to deduce a small percentage change in the strength of tropical circulations such as the Hadley or Walker Circulations. Comparisons of the different General Circulation Models to observations [8] have consistently shown that the models tend to underestimate and/or overestimate the intensity of these circulations by as much as 33 percent. Since the models, in some cases, cannot even get close to replicating these circulations well, is it possible to discern a 3.5 percent change in the strength of these circulations? One can even question the use of observational data for the tropics going back to the mid-1800's to deduce a 3.5 percent change in the circulation strength, since obviously, the farther back in time the records go, the fewer observations there are. This is especially true for the upper-air circulation, as upper-air observations were not viable before about the 1940's. Even if there were good and plentiful observations going back to the mid-1800 for tropical areas, it is well known that atmospheric pressures and heights from balloon measurements have a 1-2 percent error-margin built in. This would then account for half or more of the purported change in the circulation intensity.

Thus, while recent studies [1] may claim to be able to quantify the anthropogenic climate change signal in the data and using the models, it would be wise to remember the limitations of the tools and data at our disposal. Suddenly, a 3.5 percent change in the strength in the Walker Circulation per 150 years seems not only irrelevant, but dubious as well.

[1] Vecchi G.A., et al., 2006. Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature, 44, 73-76.

[2] Mokhov, I.I., D.V. Khvorostyanov, and A.V. Eliseev, 2004. Decadal and Longer-term Changes in ENSO Characteristics. I. J. Climatol., 24, 401 - 414.

[3] The Center for Ocean and Atmospheric Prediction Studies (http://www.coaps.fsu.edu)

[4] Gershunov, A., and T.P. Barnett, 1998. Interdecadal modulation of ENSO teleconnections. Bull. Amer. Meteor. Soc., 79, 2715 - 2725.

[5] Lupo, A.R., and G. Johnston, 2000. The Interannual Variability of Atlantic Ocean Basin

Original paper in http://www.sepp.org/weekwas/2006/July%208.htm