3.3.5 Consistency and Relationships between Temperature and Precipitation
Observed changes in regional temperature and precipitation can often be physically related to one another. This section assesses the consistencies of these relationships in the observed trends. Significant large-scale correlations between observed monthly mean temperature and precipitation (Madden and Williams, 1978) for North America and Europe have stood up to the test of time and been expanded globally (Trenberth and Shea, 2005). In the warm season over continents, higher temperatures accompany lower precipitation amounts and vice versa. Hence, over land, strong negative correlations dominate, as dry conditions favour more sunshine and less evaporative cooling, while wet summers are cool. However, at latitudes poleward of 40° in winter, positive correlations dominate as the water-holding capacity of the atmosphere limits precipitation amounts in cold conditions and warm air advection in cyclonic storms is accompanied by precipitation. Where ocean conditions drive the atmosphere, higher surface air temperatures are associated with precipitation, as during El Niño events. For South America, Rusticucci and Penalba (2000) showed that warm summers are associated with low precipitation, especially in northeast and central-western Argentina, southern Chile, and Paraguay. Cold season (JJA) correlations are weak but positive to the west of 65°W, as stratiform cloud cover produces a higher minimum temperature. For stations in coastal Chile, the correlation is always positive and significant, as it is adjacent to the ocean, especially in the months of rainfall (May to September), showing that high SSTs favour convection.
This relationship of higher warm-season temperatures with lower precipitation appears to apply also to trends (Trenberth and Shea, 2005). An example is Australia, which exhibits evidence of increased drought severity, consistent with the observed warming during the latter half of the 20th century (Nicholls, 2004). Mean maximum and minimum temperatures during the 2002 Australian drought were much higher than during the previous droughts in 1982 and 1994, suggesting enhanced potential evaporation as well (see Section 3.8.4, Box 3.6). Record-high maximum temperatures also accompanied the dry conditions in 2005.