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Natural External Forcing in the Climate System

Searching for the impact of past external forcing is essentially an uncertainty showcase: On one hand the forcings are known only through proxies, not absolute measures and at the same time the climate records, the series containing the response signal to the forcing, are equally restricted to individual proxy records from point sources.


Volcanic and solar forcing: Statistical characteristics

The goal is to identify and describe the temporal evolution of the major external forcings (solar variations and explosive volcanism) using a comprehensive approach based on available independent proxy records. Despite uncertainty in physical basis or proxy-based reconstruction, we attempt to isolate the primary characteristics and statistical properties of the forcing series.

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11-year Sunspot Cycle and observational record back to AD 1610 (Source: NCAR-HAO)

Description of forcings (solar variations and explosive volcanism) using all available independent proxy records for isolation of characteristic statistical properties of the series. Tailored statistical tools were developed and published that allow for flexible and efficient signal detection.

Solar variations that include relatively smooth quasi-oscillations at numerous frequencies are isolated with the multi-resolution wavelet technique. Compared to Fourier analyses, wavelets capture the local characteristics of the time series.

Volcanic forcing is of a very different temporal nature as explosive eruptions instantaneously inject large amounts of aerosol into the stratosphere. The aerosol spreads laterally before it starts to decay. Between eruptions, no aerosol might be present. To appropriately represent this problem, a bi-distribution state-space model is applied. For the first time such a statistical model based ‘spike-detection’ automatically extracts pulse-like signals while providing a measure of confidence through a posterior probability.

Volcanic Click on image to enlarge
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Volcanic signal detection: A state-space approach (B) as compared to various threshold based approaches (A and A') (Details: see Naveau et al. 2003)

The advantages of both techniques include their flexibility, their automatic employment as well as their independence from arbitrary measures of thresholds. We provide both tools through the Web to a wide community dealing with detection of these or similar signals. Currently the two techniques are being combined into a single procedure. The next step is to address spatial signal detection. Along a different line, we evaluate uncertainty characteristics of detecting the 11-year solar cycle in surface climate of the GCMs. We evaluate single to multi-ensemble simulations to test signal to noise relationships.

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Solar signal detection: Wavelet analysis of 85- and 11-year cycles in solar activity and Mann et al. (1998) Northern Hemisphere surface temperature (Details: see Oh et al. 2003)
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Volcanic signal detection: Cooling spike detection with state-space based model in Briffa et al. (1998) and Jones et al. (1998) proxy climate reconstructions. Note both signal and associated posterior probability. (Details: see Naveau et al. 2003)

Team/Collaborators: C. Ammann (NCAR), P. Naveau (University of Colorado), H.-S. Oh (University of Alberta, Canada)


Spatio-temporal fingerprinting

Based on the previously identified forcing series, this component deals with the search for fingerprints in the climate system left by the external forcing. Two steps lead to increasingly comprehensive signal detection: First we restrict the search to temporal dimension in which we analyze hemispheric or global climate time series in both proxy reconstructions as well as climate model output. Second, instead of evaluating single hemispheric series, we then explore the spatial information to see if the noise can be further suppressed and if the spatial patterns are compatible with different potential physical processes that translate the forcing into a response in the climate system.

A sequence of coupled simulations with differently scaled solar forcing has been performed for the last Millennium. While the lack of knowledge on the real world climate sensitivity paired with the unknown magnitude of the natural forcings pose significant problems for model-data intercomparison in general, a climatically based interpretation of the model results is pointing towards a real world solar forcing of only limited magnitude. The sensitivity of the NCAR-Climate System Model (CSM) is at the low end of the spectrum of climate models (2 degrees Celsius for doubling CO2). When forced with different solar irradiance change magnitudes, it is the smaller rather than the larger amplitudes that generate climate variations in the model that are most consistent with reconstructions. If the model's sensitivity would be correct, then no or only a relatively small solar background trend is required to clearly generate the centennial scale climate variations of the past millennium, including the Little Ice Age. If the real world climate sensitivity were larger, then solar variations would be indeed very small. This conclusion was presented at a Solar Workshop (SORCE Science Meeting, Sonoma, Dec. 2003) and found matching results from the solar community using up to 4 independent lines of evidence all suggesting that the 11-year cycle (as observed by satellites since 1978) might contain essentially the full range of solar irradiance variations.

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Coupled model simulation using various solar forcing series to represent the uncertainty in background trend of the solar total irradiance. (Details: see Ammann et al. (in prep))

Implications are substantial for the Climate Change debate. Further studies, particularly including dynamical feedback mechanisms for signal translation into the climate system are under investigation.

Team/Collaborators: C. Ammann (NCAR), F. Joos (University of Bern, Switzerland), H.-S. Oh (University of Alberta, Canada), P. Naveau (University of Colorado), R. Tomas (NCAR)


Climate Mode response to external forcing

The Earth's climate system has a set of preferred modes of variability. Under unforced conditions, it's the inherent behavior in these modes that guide magnitude and spatio-temporal structure of climate variability. A key question for understanding both past and potential future climate is if and how these internal modes of climate respond to external radiative forcing. In the past, primary forcing factors include solar irradiance changes and explosive volcanism. In the future, it is increasingly the anthropogenic greenhouse gas signal that will impact the climate. Only recently have improved and extended proxy reconstructions offered a basis to study spatio-temporal mode responses to forcing. At the same time, coupled Ocean-Atmosphere General Circulation Models are not only ready to run over many centuries, they can now also be realistically forced with external forcing. Therefore, the impact of the regional and climate mode studies is geared both for understanding the past but also to potentially improve regional predictions in the future.

  • Centennial scale ocean variability in CSM

  • A strong ~125-year oscillation in the N-Atlantic ocean was found. Since no similar oscillations are known in the real world (except of about half the frequency), significant effort was put in to understand this variation. The impact is far reaching in atmosphere and ocean, including the thermohaline overturning.

    Team/Collaborators: R. Tomas (NCAR), N. Graham (Scripps and HRC), C. Ammann (NCAR)

  • E-Asia climate response to the AD 1815 Tambora eruption

  • Three selected time windows around significant volcanic or solar forcing periods as well as the drought episode that brought the Ming Dynasty to its knees. GCM (NCAR-CSM and GISS) analysis and Regional Modeling using GCM boundary conditions.

    Team/Collaborators: J. Bradbury and R.S. Bradley (University of Massachusetts), C. Ammann and R. Tomas (NCAR), W.-C. Wang (SUNY Albany), D. Rind (NASA-GISS)

  • European circulation regimes over the last 500 years

  • Evaluation of dominating circulation regimes isolated in historical reconstructions and model. NAO and blocking cases with comparable behavior in control and low-solar case. No blocking in high-solar situation because polar vortex is too strong.

    Team/Collaborators: C. Casty, J. Luterbacher, H. Wanner, E. Fischer (University of Bern, Switzerland), C. Ammann and R. Tomas (NCAR)

  • Tropical climate variability in response to radiative forcing

  • Previously regarded as most independent mode, ENSO variations over the recent past seem to contain significant externally forced response behavior: cooling in E equatorial Pacific during increase solar irradiance, but higher occurrence of warming during low-solar irradiance as well as after large volcanic eruptions.

    Team/Collaborators: N. Graham (Scripps and HRC), C. Ammann, E. Wahl and R. Tomas (NCAR), M. Mann and B. Adams (University of Virginia), K. Cobb (Georgia Tech), M. Hughes (University of Arizona), R.S. Bradley (University of Massachusetts)



Adams J.B., M.E. Mann, C.M. Ammann, 2003: Proxy evidence for an El Niño-like response to volcanic forcing. Nature, 426, 274-278.

Ammann C.M., J.T. Kiehl, C.S. Zender, B.L. Otto-Bliesner and R.S. Bradley: Coupled simulations of the 20th century including external forcing. Journal of Climate, (in press).

Ammann C.M., G.A. Meehl G., W.M. Washington and C.S. Zender, 2003: A monthly and latitudinally varying volcanic forcing dataset in simulations of the 20th century climate. Geophys. Res. Lett., 30(12), 1657, doi:10.1029/2003GL016875.

Ammann C.M., P. Naveau, 2003: Multi-decadal periodicity in tropical explosive volcanism and its influence on climate. Geophys. Res. Lett., 30(5), 1210, doi:10.1029/2002GL016388.

Mann, M.E., C.M. Ammann, R.S. Bradley, K. Briffa, T.J. Crowley, M. Huges, P.D. Jones, M. Oppenheimer, T. Osborn, J.T. Overpeck, S. Rutherford, K.E. Trenberth, and T.M.L. Wigley, 2003: On past temperatures and anomalous late 20th century warmth. EOS, 84(27), 256ff.

Naveau P., C.M. Ammann, H.S. Oh and W. Guo, 2003: An automatic statistical methodology to extract pulse-like forcing factors in climatic time series: Application to volcanic events. In: Robock, A. and C. Oppenheimer (Eds.): Volcanism and the Earth's Atmosphere. Geophysical Monograph.

Naveau P., M. Genton and C.M. Ammann: Time series analysis with a skewed Kalman filter. In: Genton, M. (Ed.), Skew-elliptical distributions and their applications: A journey beyond normality. 19pp., (in press).

Oh, H.-S., C.M. Ammann, P. Naveau, D. Nychka, and B.L. Otto-Bliesner, 2003: Multi-resolution time series analysis applied to solar irradiance and climate reconstructions. Journal of Atmospheric and Solar-Terrestrial Physics, 65, 191-201.

Santer B.D., M.F. Wehner, T.M.L. Wigley, R. Sausen, G.A. Meehl, K.E. Taylor, C.M. Ammann, J. Arblaster, W.M. Washington, J.S. Boyle, W. Brueggemann, 2003: Contributions of anthropogenic and natural forcing to recent tropopause height changes. Science, 301, 479-483.

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