Future extreme Weather Patterns over the Great Lakes region

Understanding changes in synoptic patterns and characteristics is tantamount to understanding regional climate change. To date, the synoptic evaluations that have been done regarding climate change output from General Circulation Models have been restricted mainly to examining changes in storm tracks across large areas - e.g., across the Atlantic Ocean. In this study, output from the Canadian Coupled Climate Model (CGCM1) and the Hadley Coupled Climate Model (HadCM2) are examined relative to present conditions to evaluate potential changes in synoptic-scale patterns over the Great Lakes region towards the end of this century.

The synoptic analyses are motivated by stakeholder interests. For example, precipitation plays a big role in governing lake levels, which are of interest to commercial shippers. Both models show an increase in precipitation for the future - particularly for heavy precipitation (e.g., > 0.5 inches) events. The HadCM2 shows more of an increase with most of it coming from mid-summer through mid-winter. The CGCM1 shows less of an increase with most of it coming from mid-winter through mid-summer. As another example, because the Great Lakes region has a large number of recreational sailboaters, winds in the summer are always a concern. The HadCM2 suggests more variable windspeeds and directions in summer - but with more frequent northerly flow. The CGCM1 also suggests more frequent northerly flow but a shift toward weaker windspeeds. The number of days with winds above 10 ms-1 drops from 25 to 18%.

The wind, precipitation, and temperature changes are driven by changes in high and low pressure systems. Both models exhibit decreases in cyclone numbers (14% for CGCM1 and 18% for HadCM2) for the future. The HadCM2 shows a slight increase in the number of strong cyclones (SLP < 1000 hPa) but a greater decrease in the number of weak cyclones (SLP > 1005 hPa). Additionally, most of the decreases occur during the spring - so that spring and fall are nearly tied for the season with the most cyclones. The CGCM1 shows a general decrease in the number of cyclones with central SLP < 1010 hPa and more or less general decreases for each month.

Both models exhibit slight changes in anticyclone numbers for the future (3% increase for CGCM1 and 3% decrease for HadCM2). The HadCM2 shows a slight increase in the number of weak anticyclones (SLP < 1020 hPa) but a greater decrease in the number of strong anticyclones ( SLP 1025 hPa). Additionally, most of the decreases occur during the summer - so that the seasonal distribution is more uniform. The CGCM1 does not exhibit changes that are as systematic as those in the HadCM2 in terms of intensity - but significant decreases occur in summer and significant increases occur in fall.

The changes in high and low pressure systems are responsible for changes in other aspects like heat/cold waves and severe storms. An understanding of all these synoptic changes provides richness and a more conceptual understanding of how climate change will affect the Great Lakes region.