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Summary of achievements

Julie Arblaster has concentrated on the analysis of twentieth century climate model experiments in order to understand the climate response to various external forcings. Black carbon aerosols, which have been accumulating over Asia in recent decades, were found to lead to increased warming in the lower troposphere over the Asian monsoon region and an enhancement of pre-monsoon rainfall. In accordance with that observed, a cold-event response to solar maximum, the peak of solar forcing from the Sun that occurs on an approximate 11-year cycle, was found in two NCAR climate models. Using these same models, temperature extremes over the United States were attributed for the first time to anthropogenic forcing using specialised runs where the models were forced with anthropogenic and natural forcings separately. Future research will investigate the ability of climate models to simulate observed changes in extremes at the regional scale, emerging signals of climate change in the first half of the 21st Century and additional experiments aimed at understanding the response of the climate system to variations in solar forcing.

Publications

Alexander, L.V. and J.M. Arblaster. 2009: Assessing trends in observed and modelled climate extremes over Australia in relation to future projections. International Journal of Climatology, 29, 417-435, doi:10.1002/joc.1730.

Figure 3: High resolution figure

Abstract: Multiple simulations from nine globally coupled climate models were assessed for their ability to reproduce observed trends in a set of indices representing temperature and precipitation extremes over Australia. Observed trends over the period 1957-1999 were compared with individual and multi-modelled trends calculated over the same period. When averaged across Australia, the magnitude of trends and interannual variability of temperature extremes were well simulated by most models, particularly for the index for warm nights. The majority of models also reproduced the correct sign of trend for precipitation extremes although there was much more variation between the individual model runs. A bootstrapping technique was used to calculate uncertainty estimates and also to verify that most model runs produce plausible trends when averaged over Australia. Although very few showed significant skill at reproducing the observed spatial pattern of trends, a pattern correlation measure showed that spatial noise could not be ruled out as dominating these patterns. Two of the models with output from different forcings showed that the observed trends over Australia for one of the temperature indices was consistent with an anthropogenic response, but was inconsistent with natural-only forcings. Future projected changes in extremes using three emissions scenarios were also analysed. Australia shows a shift towards warming of temperature extremes, particularly a significant increase in the number of warm nights and heat waves with much longer dry spells interspersed with periods of increased extreme precipitation, irrespective of the scenario used.

Figure caption: Changes in mean temperature (left column) and precipitation (right column) for observations (a, b), 20th century simulations (c, d) and 21st century SRES A1B simulations (e, f). Twentieth century changes are represented as trends from 1957 to 1999, while future changes are differences of 2080–2099 minus 1980–1999. Stippling in (e and f) indicates regions where the multi-model mean change divided by the intermodel standard deviation of the change is greater than one, a measure of the consistency of the multi-model response. The same nine models for which extremes indices were analysed are used to form the multi-model means here.

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Meehl G.A. and J.M. Arblaster, 2009: A lagged warm event-like response to peaks in solar forcing in the Pacific region. J. Climate, doi:10.1175/2009JCLI2619.1.

Figure 2: High resolution figure

Abstract: The forced response coincident with peaks in the 11 year decadal solar oscillation (DSO) has been shown to resemble a cold event or La Niña-like pattern during DJF in the Pacific region in observations and two global coupled climate models. Previous studies with filtered observational and model data have indicated that there could be a lagged warm-event or El Niño-like response following the peaks in the DSO forcing by a couple of years. Here we examine observations and two climate model simulations, and show that dynamical coupled processes initiated by the response in the tropical Pacific to peaks in solar forcing produce wind-forced ocean Rossby waves near 5N and 5S. These reflect off the western boundary, producing downwelling equatorial Kelvin waves that contribute to transitioning the tropical Pacific to a warm event or El Niño-like pattern that lags the peaks in solar forcing by a couple of years.

Figure caption: Composite differences, peak solar years (denoted year 0) minus climatology from the SODA ocean reanalysis data, as well as the year prior (year -1), and the years after (denoted year+1, year+2) for a) zonal wind stress at 10N, b) zonal wind stress at the equator, c) zonal wind stress at 10S, d) upper ocean heat content at 5N, e) upper ocean heat content at the equator, and f) upper ocean heat content at 5S. Units are on color bar and stippling indicates significance at the 90% level from a t test. Solid lines highlight ocean Rossby waves (slanting down from right to left), and equatorial Kelvin waves (slanting down from left to right).

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Meehl, G.A., J.M. Arblaster, K. Matthes, F. Sassi and H. Van Loon. 2009: Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing. Science, 325, 1114-1118, doi:10.1126/science.1172872.

Figure 3: High resolution figure

Abstract: One of the mysteries regarding Earth's climate system response to variations in solar output is how the relatively small fluctuations of the 11-year solar cycle can produce the magnitude of the observed climate signals in the tropical Pacific associated with such solar variability. Two mechanisms, the top-down stratospheric response of ozone to fluctuations of shortwave solar forcing and the bottom-up coupled ocean-atmosphere surface response, are included in versions of three global climate models, with either mechanism acting alone or both acting together. We show that the two mechanisms act together to enhance the climatological off-equatorial tropical precipitation maxima in the Pacific, lower the eastern equatorial Pacific sea surface temperatures during peaks in the 11-year solar cycle, and reduce low-latitude clouds to amplify the solar forcing at the surface.

Figure caption: Composite averages for DJF for peak solar years. (A) Observed SSTs for 11 peak solar years (2) (°C). (B) Same as (A) except for precipitation for three available peak solar years (2) (mm day^-1). (C) Same as (A) except for CCSM3 average of five ensemble members for 20th-century climate (19) (°C). (D) Same as (C) except for precipitation (19) (mm day^-1). (E) Same as (A) except for WACCM with specified nonvarying SSTs, for 10 peak solar years. (F) Same as (E) except for precipitation (mm day^-1). (G) Same as (A) except for WACCM coupled to the dynamical ocean, land, and sea ice components of CCSM3, for 11 peak solar years (°C). (H) Same as (G) except for precipitation (mm day^-1). Stippling indicates significance at the 5% level, and dashed lines indicate position of climatological precipitation maxima.