Local atmospheric decoupling in complex topography alters climate change impacts

Cold air drainage and pooling occur in many mountain valleys, especially at night and during winter. Local climate regimes associated with frequent cold air pooling have substantial impacts on species phenology, distribution and diversity. However, little is known about how the degree and frequency of cold air drainage and pooling will respond to a changing climate. Evidence suggests that, because cold pools are decoupled from the free atmosphere, these local climates may not respond in the same way as regional-scale climates estimated from coarse-grid general circulation models. Indeed, recent studies have demonstrated that historical changes in the frequency of synoptic conditions have produced complex spatial variations in the resulting climatic changes on the ground. In the mountainous terrain of the Oregon Cascades, we show that, at relatively exposed hill slope and ridge top locations, air temperatures are highly coupled to changes in synoptic circulation patterns at the 700-hPa level, whereas in sheltered valley bottoms, cold air pooling at night and during winter causes temperatures to be largely decoupled from, and relatively insensitive to, 700-hPa flow variations. The result is a complex temperature landscape composed of steep gradients in temporal variation, controlled largely by gradients in elevation and topographic position. When a projected climate warming of 2.5 °C was combined with likely changes in the frequency distribution of synoptic circulation, modelled temperature changes at closely spaced locations diverged widely (by up to 6 °C), with differences equalling or exceeding that of the imposed regional temperature change. Because cold air pooling and consequent atmospheric decoupling occur in many mountain valleys, especially at high latitudes, this phenomenon is likely to be an important consideration in understanding the impacts of climate change in mountainous regions. Copyright  2009 Royal Meteorological Society Supporting information may be found in the online version of this article.