This paper suggests that the CMIP5 models’ (which IPCC relies upon) predicted US SW temperature sensitivity to the GHG has been significantly overestimated by about a factor of two.

Imprint of the Atlantic multi-decadal oscillation and Pacific

decadal oscillation on southwestern US climate: past, present,

and future

Petr Chylek • Manvendra K. Dubey • Glen Lesins • Jiangnan Li • Nicolas Hengartner

Abstract

The surface air temperature increase in the southwestern United States was much larger during the last few decades than the increase in the global mean. While the global temperature increased by about 0.5 °C from 1975 to 2000, the southwestern US temperature increased by about 2 °C. If such an enhanced warming persisted for the next few decades, the southwestern US would suffer devastating consequences. To identify major drivers of southwestern climate change we perform a multiple-linear regression of the past 100 years of the southwestern US temperature and precipitation. We find that in the early twentieth century the warming was dominated by a positive phase of the Atlantic multi-decadal oscillation (AMO) with minor contributions from increasing solar irradiance and concentration of greenhouse gases.

The late twentieth century warming was about equally influenced by increasing concentration of atmospheric greenhouse gases (GHGs) and a positive phase of the AMO. The current southwestern US drought is associated with a near maximum AMO index occurring nearly simultaneously with a minimum in the Pacific dec- adal oscillation (PDO) index. A similar situation occurred in mid-1950s when precipitation reached its minimum within the instrumental records. If future atmospheric concentrations of GHGs increase according to the IPCC scenarios (Solomon et al. in Climate change 2007: working group I. The Physical Science Basis, Cambridge, 996 pp, 2007), climate models project a fast rate of southwestern warming accompanied by devastating droughts (Seager et al. in Science 316:1181–1184, 2007; Williams et al. in Nat Clim Chang, 2012).

However, the current climate models have not been able to predict the behavior of the AMO and PDO indices. The regression model does support the climate models (CMIP3 and CMIP5 AOGCMs) pro- jections of a much warmer and drier southwestern US only if the AMO changes its 1,000 years cyclic behavior and instead continues to rise close to its 1975–2000 rate. If the AMO continues its quasi-cyclic behavior the US SW temperature should remain stable and the precipitation should significantly increase during the next few decades.

Discussion and conclusion

A multiple linear regression analysis of the twentieth century US SW climate suggests a strong oceanic influence on both the southwestern US temperature (from the AMO) and precipitation (from the PDO and AMO). About a half of the recent (post 1975) US SW warming trend can be attributed to the anthropogenic influences of increasing atmospheric concentration of greenhouse gases and aerosol variability (GHGA), with the remaining half being due to a positive phase of the AMO. The US SW precipitation has been dominated by oceanic influences (PDO and AMO) with no direct effect due to anthropogenic greenhouse gases and aerosols (GHGA). This of course does not exclude a possibility that the GHGA affects the AMO and PDO.

To estimate the future US SW climate evolution using the regression model we need to make an assumption concerning the future AMO behavior. The situation that we consider most likely is the repetition of a cyclic behavior that was observed during the twentieth century (Schle- singer and Ramankutty 1994) as well as during the previous hundreds of years (Delworth and Mann 2000; Gray et al. 2004; Chylek et al. 2011, 2012). The regression model with a continuing AMO cyclic behavior suggests a stable temperature close to its present level and increasing precipitation within the next two to three decades.

A rising AMO index at the rate comparable to its 1975–2005 increase would bring harsh climatic conditions to the southwestern US. Projected temperature would increase by 2050 by about 2 °C above the current level (a warming similar to that predicted by the ensemble mean of the CMIP5 simulations) and precipitation would decrease by an additional 30 % compared to the current conditions. A strong warming and severe drought predicted on the basis of the ensemble mean of the CMIP climate models simulations (Seager et al. 2007; Williams et al. 2012) is supported by our regression analysis only in a very unlikely case of the continually increasing AMO at a rate similar to its 1970–2010 increase. There is substantial evidence to support future AMO cyclic behavior. Instrumental records of central England temperature (Tung and Zhou 2013), tree rings (Delworth and Mann 2000; Gray et al. 2004) and ice core analysis (Meeker and Majewski 2002; Chylek et al. 2011, 2012; Henriksson et al. 2012) demonstrate the existence of the AMO cycles for many hundreds and possibly thousands of years when anthropogenic influences were negligible. Ice core analysis suggests a shorter AMO quasi-period- icity (about 20 years) during the Little Ice Age and a longer periodicity in the Medieval Warm Period (Chylek et al. 2012). Atmosphere–Ocean coupled climate models (Metha and Delworth 1995; Griffies and Bryan 1997; Delworth and Knutson 2000; Dong and Sutton 2001; Wei and Lohmann 2012; Mahajan et al. 2011; Henriksson et al. 2012; Yang et al. 2013; Escudier et al. 2013; Zanchettin et al. 2013) as well as simplified conceptual ocean models (Frankcombe and Djikstra 2011), or sta- tistical harmonic models (Humlum et al. 2011; Mazzarella and Scafetta 2012; Scafetta 2012) suggest a future per- sistent AMO like multi-decadal oscillation. Based on this evidence of the past behavior we expect the AMO to retain its cyclic behavior during the twenty-first century with a cycle length of 60–70 years.

It seems that the AMO index may have reached its peak around 2005 and started to turn downward (Fig. 4) but still in a positive AMO phase. Within a few years we should be able to see more clearly if this was a real turning point or only a temporary pause.

Fig. 4 a Instrumental era AMO (black), and three considered cases of its future projection: [1] repetition of the 65 year cycle (blue), [2 ] a constant at the present AMO value (green), and [3] continuation of the 1975–2010 increasing trend (red). b Regression reconstruction of the US SW temperature (black), and three different projections of the US SW temperature ([1], [2], [3]) based on the three considered AMO future projections. The observed US SW temperature (SWT gray) and the CMIP5 ensemble mean (yellow) with the RCM4.5 pathway are also shown. All data are the 5 year moving means. c 95 % confidence level (thin lines) for regression model and cases [1] and [3] of the temperature projection

The US SW temperature and precipitation are strongly influenced by the AMO and PDO. The fact that the CMIP simulations ensemble mean can reproduce the 1970–2010 US SW temperature increase without inclusion of the AMO (the AMO is treated as an intrinsic natural climate vari- ability that is averaged out by taking an ensemble mean of individual simulations) suggests that the CMIP5 models’ predicted US SW temperature sensitivity to the GHG has been significantly (by about a factor of two) overestimated.

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Received: 21 February 2013 / Accepted: 26 August 2013

© The Author(s) 2013. This article is published with open access at Springerlink.com Paper: Chylek et all ClimDyn_US_SW (PDF)

h/t to Dr. Roger Pielke Sr.

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