Abstract. Natural aerosol emission represents one of the largest uncertainties in our understanding of the radiation budget. Sulfur emitted by marine organisms, as dimethyl sulfide (DMS), constitutes one-fifth of the global sulfur budget and yet the distribution, fluxes and fate of DMS remain poorly constrained. This study evaluates the Australian Community Climate and Earth System Simulator (ACCESS) United Kingdom Chemistry and Aerosol (UKCA) model in terms of cloud fraction, radiation and precipitation, and then quantifies the role of DMS in the chemistry–climate system. We find that ACCESS-UKCA has similar cloud and radiation biases to other global climate models. By removing all DMS, or alternatively significantly enhancing marine DMS, we find a top of the atmosphere radiative effect of 1.7 and −1.4 W m−2 respectively. The largest responses to these DMS perturbations (removal/enhancement) are in stratiform cloud decks in the Southern Hemisphere's eastern ocean basins. These regions show significant differences in low cloud (-9/+6 %), surface incoming shortwave radiation (+7/-5 W m−2) and large-scale rainfall (+15/-10 %). We demonstrate a precipitation suppression effect of DMS-derived aerosol in stratiform cloud deck regions due to DMS, coupled with an increase in low cloud fraction. The difference in low cloud fraction is an example of the aerosol lifetime effect. Globally, we find a sensitivity of temperature to annual DMS flux of 0.027 and 0.019 K per Tg yr−1 of sulfur, respectively. Other areas of low cloud formation, such as the Southern Ocean and stratiform cloud decks in the Northern Hemisphere, have a relatively weak response to DMS perturbations. We highlight the need for greater understanding of the DMS–climate cycle within the context of uncertainties and biases of climate models as well as those of DMS–climate observations.
ABSTRACT:Extreme precipitation over the eastern Australia can be significantly enhanced by topographic interaction with the westerly flow. These extreme events can cause severe flooding, damage and disruption to human activities, yet in some areas they are also an invaluable source of water and snow. In this study, we use rain gauge and snowfall accumulation data to investigate connections between extreme rain and snow in alpine Australia and the large-scale climate. These data have been divided into three geographical locations: the west of the mountains, the ridgeline and the east of the mountains in order to explore the nature of precipitation in each region. The results confirm previous synoptic patterns found earlier in the literature for the western slopes (namely embedded and cutoff lows), and add new insights into the different processes conducive to extreme precipitation on the eastern side of the ranges, which we find to be mostly associated with a blocking structure connected to polar latitudes. Interestingly, our analyses suggests that while a La Niña pattern accompanied by enhanced meridional sea surface temperature (SST) gradients over mid-latitudes is associated with extreme events in the western and high regions, the extreme precipitation composites for the eastern region are associated with a SST pattern that resembles the predominant signal associated with global warming in the Australian region. We hypothesize that while the western and high regions rely on SST gradients, which enhance the westerly jet, the extreme events over the eastern side rely on blocking patterns (anomalous easterlies), which are at least partially responding to the global SST warming with the respective shift of the westerlies to the south. These results help explain why the precipitation over the western side of the Alps is declining more rapidly than the total precipitation observed on the eastern side.
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