CGD 2008 Profiles in Science: Julie Arblaster

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

Meehl, G.A., J.M. Arblaster, and W.D. Collins, 2008: Effects of Black Carbon Aerosols on the Indian Monsoon. Journal of Climate, 21, 2869-2882.

Abstract: A six-member ensemble of twentieth-century simulations with changes to only time-evolving global distributions of black carbon aerosols in a global coupled climate model is analyzed to study the effects of black carbon (BC) aerosols on the Indian monsoon. The BC aerosols act to increase lower-tropospheric heating over South Asia and reduce the amount of solar radiation reaching the surface during the dry season, as noted in previous studies. The increased meridional tropospheric temperature gradient in the premonsoon months of March-April-May (MAM), particularly between the elevated heat source of the Tibetan Plateau and areas to the south, contributes to enhanced precipitation over India in those months. With the onset of the monsoon, the reduced surface temperatures in the Bay of Bengal, Arabian Sea, and over India that extend to the Himalayas act to reduce monsoon rainfall over India itself, with some small increases over the Tibetan Plateau. Precipitation over China generally decreases due to the BC aerosol effects. There is a weakened latitudinal SST gradient resulting from BC aerosols in the model simulations as seen in the observations, and this is present in the multiple-forcings experiments with the Community Climate System Model, version 3 (CCSM3), which includes natural and anthropogenic forcings (including BC aerosols). The BC aerosols and consequent weakened latitudinal SST gradient in those experiments are associated with increased precipitation during MAM in northern India and over the Tibetan Plateau, with some decreased precipitation over southwest India, the Bay of Bengal, Burma, Thailand, and Malaysia, as seen in observations. During the summer monsoon season, the model experiments show that BC aerosols have likely contributed to observed decreasing precipitation trends over parts of India, Bangladesh, Burma, and Thailand. Analysis of single ensemble members from the multiple-forcings experiment suggests that the observed increasing precipitation trends over southern China appear to be associated with natural variability connected to surface temperature changes in the northwest Pacific.

Figure caption: Distributions of black carbon optical depth for 1999 by season: (a) DJF, (b) MAM, (c) JJA, and (d) September-November (SON). This pattern was scaled back in time based on globally averaged human population.

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Meehl, G.A., J.M. Arblaster, G. Branstator, and H. van Loon, 2008: A Coupled Air-Sea Response Mechanism to Solar Forcing in the Pacific Region. Climate Change, Journal of Climate, 21, 2883-2897.

Abstract: The 11-yr solar cycle [decadal solar oscillation (DSO)] at its peaks strengthens the climatological precipitation maxima in the tropical Pacific during northern winter. Results from two global coupled climate model ensemble simulations of twentieth-century climate that include anthropogenic (greenhouse gases, ozone, and sulfate aerosols, as well as black carbon aerosols in one of the models) and natural (volcano and solar) forcings agree with observations in the Pacific region, though the amplitude of the response in the models is about half the magnitude of the observations. These models have poorly resolved stratospheres and no 11-yr ozone variations, so the mechanism depends almost entirely on the increased solar forcing at peaks in the DSO acting on the ocean surface in clear sky areas of the equatorial and subtropical Pacific. Mainly due to geometrical considerations and cloud feedbacks, this solar forcing can be nearly an order of magnitude greater in those regions than the globally averaged solar forcing. The mechanism involves the increased solar forcing at the surface being manifested by increased latent heat flux and evaporation. The resulting moisture is carried to the convergence zones by the trade winds, thereby strengthening the intertropical convergence zone (ITCZ) and the South Pacific convergence zone (SPCZ). Once these precipitation regimes begin to intensify, an amplifying set of coupled feedbacks similar to that in cold events (or La Niña events) occurs. There is a strengthening of the trades and greater upwelling of colder water that extends the equatorial cold tongue farther west and reduces precipitation across the equatorial Pacific, while increasing precipitation even more in the ITCZ and SPCZ. Experiments with the atmosphere component from one of the coupled models are performed in which heating anomalies similar to those observed during DSO peaks are specified in the tropical Pacific. The result is an anomalous Rossby wave response in the atmosphere and consequent positive sea level pressure (SLP) anomalies in the North Pacific extending to western North America. These patterns match features that occur during DSO peak years in observations and the coupled models.

Figure caption: (a) The average anomalies of SST (°C) in the 11 solar peak years for DJF computed relative to all other years—1883, 1893, 1905, 1917, 1928, 1937, 1947, 1957, 1968, 1979, and 1989—from the NOAA ERSST dataset (available online at http://www.cdc.noaa.gov/cdc/data.noaa.ersst.html). (b) The average tropical rainfall anomalies (mm day-1) for January-February (GPCP gridded precipitation dataset) in the solar peaks in 1979, 1989, and 2000, in comparison to all other years. Dashed line is the 6 mm day-1 contour from the long-term mean climatology. (c) As in (a) but for the average anomalies of SLP (hPa) (Hadley Centre SLP dataset); shading indicates significance at or above the 95% level, indicating the relative magnitude of the anomalies compared to the noise. For further details regarding observed data sources, see van Loon et al. (2007).

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Meehl, G. A., J. M. Arblaster, and C. Tebaldi (2007), Contributions of natural and anthropogenic forcing to changes in temperature extremes over the United States. Geophys. Res. Lett., 34, L19709, doi:10.1029/2007GL030948.

Abstract: Observations averaged over the U.S. for the second half of the 20th century have shown a decrease of frost days, an increase in growing season length, an increase in the number of warm nights, and an increase in heat wave intensity. For the first three, a nine member multi-model ensemble shows similar changes over the U.S. in 20th century experiments that combine anthropogenic and natural forcings, though the relative contributions of each are unclear. Here we show results from two global coupled climate models run with anthropogenic and natural forcings separately. Averaged over the continental U.S., they show that the observed changes in the four temperature extremes are accounted for with anthropogenic forcings, but not with natural forcings (even though there are some differences in the details of the forcings). This indicates that most of the changes in temperature extremes over the U.S. are likely due to human activity.

Figure caption: Three temperature-related extremes indices available for nine models in the WCRP CMIP3 multi-model dataset at PCMDI averaged for the continental U.S., annual means, anomalies from 1951-99. The models are interpolated to the HadEX grid and only grid points with valid observations are included in the area-weighted average: (a) frost days (in days), (b) growing season length (in days), and (c) warm nights (in %).