Additive effect of two solar forcing mechanisms and influences on. tropical Pacific climate. National Center for Atmospheric Research

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1 Additive effect of two solar forcing mechanisms and influences on tropical Pacific climate Gerald A. Meehl Julie Arblaster, Fabrizio Sassi, Katja Matthes, and Harry van Loon National Center for Atmospheric Research

2 Observations Peaks in the 11 year solar cycle show a response pattern similar to a low amplitude La Nina event also present a year or so before and after the peaks van Loon, H., G. A. Meehl, and D. J. Shea, 2007, Journal of Geophysical Research Meehl, Arblaster and Branstator, 2008, J. Climate van Loon, H., and G.A. Meehl, 2008: J. Atmospheric. and Solar- Terrestrial Physics

3 The most well-documented mechanism proposed to explain the climate system response to solar forcing: top-down stratospheric ozone mechanism

4 The top-down stratospheric ozone mechanism Increased solar increased ozone heating/increased ozone amount modified temperature and zonal wind altered wave propagation changed equator to pole energy transport and circulation enhanced tropical precipitation (e.g. Haigh, 1996; Shindell et al., 1999; Balachandran et al., 1999; Kodera and Kuroda, 2002)

5 For increased solar input during the first half of the 20 th century, model simulations indicate enhanced ITCZ and SPCZ precipitation and a La Niña-like (or cold event-like) response (Meehl, G.A., W.M. Washington, T.M.L. Wigley, J.M. Arblaster, and A. Dai, 2003: J. Climate, 16, )

6 (Meehl, G.A., J.M. Arblaster, G. Branstator, and H. Van Loon, 2008: J. Climate) 11 peak solar years, 4 member ensemble for PCM, 5 member ensemble for CCSM3 Two global coupled climate models show a similar La Niñalike response to peaks in the 11 year solar forcing in multimember ensembles

7 A new mechanism proposed to explain this response to solar forcing: bottom-up coupled air-sea mechanism

8 The bottom-up coupled air-sea mechanism: increased solar over cloud-free regions of the subtropics translates into greater evaporation, and moisture convergence and precipitation in the ITCZ and SPCZ (and south Asian monsoon), stronger trades, and cooler SSTs in eastern equatorial Pacific Meehl, G.A., W.M. Washington, T.M.L. Wigley, J.M. Arblaster, and A. Dai, 2003, J. Climate Van Loon, Meehl and Arblaster, 2004, JASTP Meehl, G.A., J.M. Arblaster, G. Branstator, and H. Van Loon, 2008, J. Climate

9 Could the two mechanisms add together to boost the climate response to solar forcing? 1. top-down stratospheric ozone mechanism 2. bottom-up coupled air-sea mechanism

10 top-down only resolved stratosphere fixed SSTs, ozone chem both mechanisms together resolved stratosphere fully coupled, ozone Bottom-up coupled air-sea mechanism (left) and top-down stratospheric ozone mechanism (center) are additive to strengthen convection in tropical Pacific and produce a stronger La Niña-like response to peaks in solar forcing (right) Composites for simulated peaks in the 11 year solar cycle bottom-up only fully coupled stratosphere, no chem Model simulations

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12 To warm the ocean surface layer ~0.1 C, it takes about 0.5 Wm -2 (White, 1998) Warming for peak solar is C (Shindell et al. 1999), but peak solar forcing is only about 0.2 Wm -2 (Lean et al., 2005) How can the solar forcing at the surface be amplified to produce the observed warming of ~0.1 C?

13 To warm the ocean surface layer ~0.1 C, it takes about 0.5 Wm -2 (White, 1998) Warming for peak solar is C (Shindell et al. 1999), but peak solar forcing is only about 0.2 Wm -2 (Lean et al., 2005) How can the solar forcing at the surface be amplified to produce the observed warming of ~0.1 C? In CCSM and coupled WACCM, some values of net solar in the tropical and subtropical Pacific are over 1 Wm -2 Tropical Pacific average (30N-30S, 150E-90W) peak solar for coupled WACCM is C, with net solar at the surface of +0.9 Wm -2 (Standard WACCM net solar is +0.1 Wm -2 )

14 To warm the ocean surface layer ~0.1 C, it takes about 0.5 Wm -2 (White, 1998) Warming for peak solar is C (Shindell et al. 1999), but peak solar forcing is only about 0.2 Wm -2 (Lean et al., 2005) How can the solar forcing at the surface be amplified to produce the observed warming of ~0.1 C? In CCSM and coupled WACCM, some values of net solar in the tropical and subtropical Pacific are over 1 Wm -2 Tropical Pacific average (30N-30S, 150E-90W) peak solar for coupled WACCM is C, with net solar at the surface of +0.9 Wm -2 (Standard WACCM net solar is +0.1 Wm -2 ) The magnification of net solar at the surface to provide values above 0.5 Wm -2 to warm the ocean surface ~0.1 C comes from cloud feedbacks from coupled air-sea response in the Pacific

15 Meehl and Arblaster, 2009, J. Climate, in press La Nina-like response gives way to El Nino-like response a couple years after peaks in solar forcing in observations (top) and models (bottom)

16 Increased solar forcing around the peaks of the 11 year solar cycle produces a La Niña-like response in SSTs with easterly equatorial wind stress anomalies Poleward shifts of the ITCZ and SPCZ produce westerly wind stress anomalies near 10N and 10S Negative wind stress curl anomalies form near 5N and 5S Wind-driven downwelling Rossby waves propagate to the western boundary, reflect as downwelling equatorial Kelvin waves, and result in El Niño-like SST anomalies lagging the peaks in solar forcing by a couple of years

17 Conclusions 1. Peaks of the 11 year solar cycle forcing produce SST and precipitation anomalies with a La Nina-like pattern in the Pacific 2. Bottom-up coupled air-sea mechanism and top-down stratospheric ozone mechanism add to strengthen tropical convection more than either one alone, and leads to amplifying cloud feedbacks 3. Initial La Nina-like response gives way to El Nino-like response a couple years after peak solar years

18 The solar mechanism should work in a similar way for volcanoes, producing a warming in the equatorial eastern Pacific

19 Joint MTM-SVD spectra show Nino3.4 solar response in all-forcings 20 th century simulations but not control run

20 Are these just La Niña events? Are there any differences in the response to solar to differentiate it from La Niña? Troposphere Stratosphere Peak solar La Niña (Cold Events) (van Loon and Meehl, 2008,JASTP)