Variability and trend of the north west Australia rainfall: observations and coupled climate modeling

Since 1950, there has been an increase in rainfall over North West Australia (NWA), occurring mainly during the Southern Hemisphere (SH) summer season. A recent study using 20th century multi-member ensemble simulations in a global climate model forced with and without increasing anthropogenic aerosols suggests that the rainfall increase is attributable to increasing Northern Hemisphere aerosols. The present study investigates the dynamics of the observed trend toward increased rainfall and compares the observed trend with that generated in the model forced with increasing aerosols. We find that the observed positive trend in rainfall is projected onto two modes of variability. The first mode is associated with an anomalously low mean sea level pressure (MSLP) off NWA instigated by the enhanced sea surface temperature (SST) gradients towards the coast. The associated cyclonic flows bring high moisture air to northern Australia, leading to an increase in rainfall. The second mode is associated with an anomalously high MSLP over much of the Australian continent; the anticyclonic circulation pattern with northwesterly flows west of 130°E and generally opposite flows in northeastern Australia, determine that when rainfall is anomalously high, west of 130oE, rainfall is anomalously low east of this longitude. The sum of the upward trends in these two modes compares well to the observed increasing trend pattern. The modeled rainfall trend, however, is generated by a different process. The model suffers from an equatorial cold-tongue bias: the tongue of anomalies associated with El Niño-Southern Oscillation extends too far west into the eastern Indian Ocean. Consequently, there is an unrealistic relationship in the SH summer between Australian rainfall and eastern Indian Ocean SST: the rise in SST is associated with an increasing rainfall over NWA. In the presence of increasing aerosols, a significant SST increase occurs in the eastern tropical Indian Ocean. As a result, the modeled rainfall increase in the presence of aerosol forcing is accounted for by these unrealistic relationships. It is not clear whether, in a model without such defects, the observed trend can be generated by increasing aerosols. Thus, the impact of aerosols on Australian rainfall remains an open question

Precipitation changes in a GCM resulting from the indirect effects of anthropogenic aerosols

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94948/1/grl13844.pd

Cloud droplet spectral dispersion and the indirect aerosol effect: Comparison of two treatments in a GCM, Geophys

[1] Two parameterizations of cloud droplet spectral dispersion and their impact on the indirect aerosol effect are compared in a global climate model. The earlier scheme specifies b, the ratio of droplet effective radius to volumemean radius, in terms of N, the cloud droplet number concentration. The new scheme specifies b in terms of mean droplet mass (L/N), where L is liquid water content, to account for the effect of variations in L. For low to moderate N, the new scheme gives a stronger increase of b with increasing N than the old scheme. In a present-climate simulation, the new scheme shows a stronger gradient between remote regions (small b) and polluted/continental regions (large b). The new scheme also offsets the first indirect aerosol forcing (DF) more strongly: DF = À0.65 W

Have Australian rainfall and cloudiness increased due to the remote effects of Asian anthropogenic aerosols?

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94749/1/jgrd13340.pd

Constraining cloud lifetime effects of aerosols using A‐Train satellite observations

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95434/1/grl29404.pd

Projected effects of declining anthropogenic aerosols on the southern annular mode

Declining emissions of anthropogenic aerosols have been shown to contribute to global warming in climate projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5). This study considers the response of the southern annular mode (SAM) in austral summer to declining aerosols in simulations forced by Representative Concentration Pathway 4.5 (RCP4.5) using CSIRO-Mk3.6, a CMIP5-generation model. A ten-member ensemble forced by RCP4.5 for the period 2006–2100 is compared with another experiment, which is identical except that emissions of anthropogenic aerosols are held fixed at their 2005 values. With fixed aerosol emissions, the model simulates a negative (but statistically insignificant) ensemble-mean SAM trend in austral summer, suggesting that the effects of recovering stratospheric ozone slightly outweigh the effects of increasing long-lived greenhouse gases (GHGs). In contrast, the standard RCP4.5 experiment (including additional warming due to declining aerosols) simulates a positive ensemble-mean SAM trend, and the difference between the two trends is significant at 5%. The response of Southern Hemisphere zonal-mean atmospheric circulation and temperature to declining aerosols resembles the response to increasing GHGs; this suggests that the positive SAM trend due to declining aerosols may be driven by mechanisms that are similar to those that cause the positive SAM trend in response to increasing GHGs