Cool Season Rainfall Research Articles

Commonalities of carbon dioxide exchange in semiarid regions with monsoon and Mediterranean climates

Comparing biosphere–atmosphere carbon exchange across monsoon (warm-season rainfall) and Mediterranean (cool-season rainfall) regimes can yield information about the interaction between energy and water limitation. Using data collected from eddy covariance towers over grass and shrub ecosystems in Arizona, USA and Almeria, Spain, we used net ecosystem carbon dioxide exchange (NEE), gross ecosystem production (GEP), and other meteorological variables to examine the effects of the different precipitation seasonality. Considerable crossover behavior occurred between the two rainfall regimes. As expected in these usually water-limited ecosystems, precipitation magnitude and timing were the dominant drivers of carbon exchange, but temperature and/or light also played an important role in regulating GEP and NEE at all sites. If significant rainfall occurred in the winter at the Arizona sites, their behavior was characteristically Mediterranean whereby the carbon flux responses were delayed till springtime. Likewise, the Spanish Mediterranean sites showed immediate pulse-like responses to rainfall events in non-winter periods. The observed site differences were likely due to differences in vegetation, soils, and climatology. Together, these results support a more unified conceptual model for which processes governing carbon cycling in semiarid ecosystems need not differ between warm-season and cool-season rainfall regimes.

The Synoptic Climatology of Cool-Season Rainfall in the Central Wheatbelt of Western Australia

Abstract Synoptic weather systems form an important part of the physical link between remote large-scale climate drivers and regional rainfall. A synoptic climatology of daily rainfall events is developed for the Central Wheatbelt of southwestern Australia over the April–October growing season for the years 1965–2009. The climatology reveals that frontal systems contribute approximately one-half of the rainfall in the growing season while cutoff lows contribute about a third. The ratio of frontal rainfall to cutoff rainfall varies throughout the growing season. Cutoff lows contribute over 40% of rainfall in the austral autumn and spring, but this falls to about 20% in August when frontal rainfall climbs to more than 60%. The number of cutoff lows varies markedly from one growing season to another, but does not exhibit a significant long-term trend. The mean rainfall per cutoff system is also highly variable, but has gradually declined over the analysis period, particularly in the past decade. The decline in rainfall per frontal system is less significant. Cutoff low rainfall has contributed more strongly in percentage terms to the recent decline in rainfall in the Central Wheatbelt than the frontal component and accounts for more than half of the overall trend. Atmospheric blocking is highly correlated with rainfall in the region where cutoff low rainfall makes its highest proportional contribution. Hence, the decline in rain from cutoff low systems is likely to have been associated with changes in blocking and the factors controlling blocking in the region.

Climate change, Variability and Conservation impacts in Australia.

Australian conservation scientists, managers and decision makers must come to grips with anthropogenic climate change, imposed upon an already variable regional climate system. Pre- and post-instrumental records and climate proxies indicate that Australia has experienced wet and dry cycles over intra-decadal to millennial time scales. Precipitation variation across Australia is correlated with different climate features but reliable tools for seasonal rainfall prediction are still some years away. Atmosphere-Ocean General Circulation Models predict a widening of the Hadley circulation and strengthening of the Southern Annular Mode, which should result in reduced cool season rainfall over southern Australia. Shifts in the Australian climate over the Holocene epoch, most notably increased ENSO variability after 5 000 years ago, are associated with substantial vegetation change and indicate the speed at which ecosystems may be altered. The CO2 fertilization of plant biomes may mitigate increasing aridity to some extent but, in general, climate change is expected to negatively affect native vegetation and agricultural productivity. Sea-level rise is predicted to be substantial over this century and, when coupled with increased storm intensity, poses threats in the form of erosion, salinization and flooding. The best chance of building adaptable ecosystems and preserving ecosystem services requires the extension, integration and possibly optimization of reserve systems, in concert with improved management of other threatening processes (habitat loss, invasive species, overexploitation, pollution and disease). In addition, a price on carbon dioxide emissions would provide incentives for privately funded reforestation schemes, but additional incentives promoting mixed species over monoculture plantings would be required to assure maximum biodiversity benefits.

Climate change impacts on streamflow extremes and summertime stream temperature and their possible consequences for freshwater salmon habitat in Washington State

This study evaluates the sensitivity of Washington State’s freshwater habitat of Pacific Salmon (Oncorhynchus spp.) to climate change. Our analysis focuses on summertime stream temperatures, seasonal low flows, and changes in peak and base flows because these physical factors are likely to be key pressure points for many of Washington’s salmon populations. Weekly summertime water temperatures and extreme daily high and low streamflows are evaluated under multimodel composites for A1B and B1 greenhouse gas emissions scenarios. Simulations predict rising water temperatures will thermally stress salmon throughout Washington’s watersheds, becoming increasingly severe later in the twenty-first century. Streamflow simulations predict that basins strongly influenced by transient runoff (a mix of direct runoff from cool-season rainfall and springtime snowmelt) are most sensitive to climate change. By the 2080s, hydrologic simulations predict a complete loss of Washington’s snowmelt dominant basins, and only about ten transient basins remaining in the north Cascades. Historically transient runoff watersheds will shift towards rainfall dominant behavior, undergoing more severe summer low flow periods and more frequent days with intense winter flooding. While cool-season stream temperature changes and impacts on salmon are not assessed in this study, it is possible that climate-induced warming in winter and spring will benefit parts of the freshwater life-cycle of some salmon populations enough to increase their reproductive success (or overall fitness). However, the combined effects of warming summertime stream temperatures and altered streamflows will likely reduce the reproductive success for many Washington salmon populations, with impacts varying for different life history-types and watershed-types. Diminishing streamflows and higher stream temperatures in summer will be stressful for stream-type salmon populations that have freshwater rearing periods in summer. Increased winter flooding in transient runoff watersheds will likely reduce the egg-to-fry survival rates for ocean-type and stream-type salmon.

Characteristics and variability of synoptic features associated with cool season rainfall in southeastern Australia

AbstractCool season rainfall variability in southeastern Australia is investigated via classification and characterization of the predominant types of synoptic systems occurring in the region. These types are frontal systems, cut‐off low systems, and other systems. Rainfall in the region is dominated by cut‐off systems and these systems are the main influence on the interannual variability of rainfall. Both cut‐off systems and frontal systems display an enhancement of thermal (thickness) gradient as rainfall increases, but the mechanisms for intensification differ. Cut‐off systems intensify in the region in association with local increases in baroclinicity and the subtropical jet, whereas frontal systems tend to intensify via a confluence of subtropical and polar jets. Interannual rainfall variability is examined for groupings of years based on both clustering of continental rainfall patterns and on El Nino/Southern Oscillation (ENSO)/Indian Ocean Dipole (IOD) years. Cut‐off systems exhibit consistent enhancements of thermal gradients for groupings of years in which they produce more rainfall. For ENSO/IOD groupings, the cut‐off thermal gradients are consistent with the underlying sea surface temperature (SST) anomalies. Wet years in southeastern Australia are usually produced by cut‐off systems, but can also be produced by frontal systems. In those cases the mid‐tropospheric flow pattern is reminiscent of the negative Southern Annular Mode (SAM) pattern. The positive SAM pattern is also associated with enhanced rainfall in the southeast via local intensification of blocking and cut‐off systems. Copyright © 2008 Royal Meteorological Society

The Synoptic Decomposition of Cool-Season Rainfall in the Southeastern Australian Cropping Region

Abstract Daily rainfall during the April–October growing season in a major cropping region of southeastern Australia has been related to particular types of synoptic weather systems over a period of 33 yr. The analysis reveals that cutoff lows were responsible for at least 50% of all growing-season rainfall and accounted for 80% of daily rainfall events exceeding 25 mm per station. The proportion of rainfall contributed by cutoff lows varies throughout the growing season. It is highest in austral autumn and spring (55% and 57%, respectively) and falls to a minimum in July (42%). By way of contrast, the total contribution of all types of frontal systems to growing-season rainfall is about 32%, although the monthly value reaches a maximum of 41% in July when mean cutoff rainfall reaches a minimum. Rainfall associated with fronts is strongly concentrated in the lower range of daily falls (less than 10 mm per station). Frontal rainfall is found to be more consistent from year to year than is cutoff rainfall. The number of cutoff lows per season is highly variable, and there is a significant correlation between the number of cutoff days and atmospheric blocking in the region south of Australia in each month of the growing season. The mean amount of rainfall per cutoff day is also variable and has declined by approximately 0.8 mm over the analysis period. An understanding of the mechanisms controlling year-to-year variability of cutoff rainfall is therefore an important step in improving seasonal forecasts for agriculture in southeastern Australia.