Previous studies have identified an association between temperature anomalies in the Northern Hemisphere and the strength of stratospheric polar westerlies. Large regions in northern Asia, Europe and North America have been found to cool during the mature and late stages of weak vortex events in the stratosphere. A substantial part of the temperature changes are associated with changes in the Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) pressure patterns in the troposphere. The apparent coupling between the stratosphere and the troposphere may be of relevance for weather forecasting, but only if the temporal and spatial nature of the coupling is known. Using 51 winters of re-analysis data, we show that the development of the lower-tropospheric temperature relative to stratospheric weak polar vortex events goes through a series of well-defined stages, including the formation of geographically distinct cold air outbreaks. At the inception of weak vortex events, a precursor signal in the form of a strong high-pressure anomaly over northwest Eurasia is associated with long-lived and robust cold anomalies over Asia and Europe. A few weeks later, near the mature stage of the weak vortex events, a shorter-lived cold anomaly emerges off the east coast of North America. The probability of cold air outbreaks increases by more than 50% in one or more of these regions during all phases of the weak vortex events. This shows that the stratospheric polar vortex contains information that can be used to enhance forecasts of cold air outbreaks. As large changes in the frequency of extremes are involved, this process is important for the medium-range and seasonal prediction of extreme cold winter days. Three-hundred-year pre-industrial control simulations by 13 coupled climate models corroborate our results.

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BackgroundClimate change is expected to have large impacts on health at low latitudes where droughts and malnutrition, diarrhea, and malaria are projected to increase.ObjectivesThe main objective of this study was to indicate a method to assess a range of plausible health impacts of climate change while handling uncertainties in a unambiguous manner. We illustrate this method by quantifying the impacts of projected regional warming on diarrhea in this century.MethodsWe combined a range of linear regression coefficients to compute projections of future climate change-induced increases in diarrhea using the results from five empirical studies and a 19-member climate model ensemble for which future greenhouse gas emissions were prescribed. Six geographical regions were analyzed.ResultsThe model ensemble projected temperature increases of up to 4°C over land in the tropics and subtropics by the end of this century. The associated mean projected increases of relative risk of diarrhea in the six study regions were 8–11% (with SDs of 3–5%) by 2010–2039 and 22–29% (SDs of 9–12%) by 2070–2099.ConclusionsEven our most conservative estimates indicate substantial impacts from climate change on the incidence of diarrhea. Nevertheless, our main conclusion is that large uncertainties are associated with future projections of diarrhea and climate change. We believe that these uncertainties can be attributed primarily to the sparsity of empirical climate–health data. Our results therefore highlight the need for empirical data in the cross section between climate and human health.

Polar lows (PLs) are small-scale and intense low-pressure systems that form at high latitudes in both hemispheres. Due to their limited spatial scale and brief lifetimes, weather and climate models are often unable to resolve these systems. One way to overcome this problem is to define a suitable proxy for PLs, with which the likelihood of PL formation can be assessed even in coarse-resolution datasets. This study draws on previous studies and an empirical database of 63 PLs to quantify the respective influences of low-level static stability and upper-level forcing on PL formation, as both of these factors are known to favour PL development. Little redundancy between the two parameters is found. After defining threshold values for the two parameters, climatological properties of favourable conditions for PLs are computed for the North Atlantic, the North-West Pacific and the Southern Hemisphere. The low-level static stability, which is strongly modified during marine cold-air outbreaks, puts important constraints on where PLs can form, while the upper-level forcing determines whether or not they will form. As a result of the climatologically lower tropopause in the Labrador Sea region, favourable conditions for PLs occur more often there than in the Nordic Seas, which has long been believed to be the main PL region in the Northern Hemisphere (NH). In the Southern Hemisphere, favourable conditions for PLs occur substantially less often than in the NH. The PL index defined here is suitable for other climatological studies and PL forecasting.