Weather: What's behind this summer's wacky weather?

Monday, October 5, 2009

Thanks to a meandering jet stream, Vancouver got typical central Canada weather and we got cool, damp, Vancouver-type weather, while the south-western Arctic set new highs

by HENRY HENGEVELD

This past summer, the global atmospheric circulation system appears to have misfired. It delivered the wrong weather to almost everyone across Canada. Residents in the North seemed to get what those in the South expected, Canadians in the East received what the Westerners were to have gotten, and vice versa.

Take Iqaluit, capital of Nunavut, for example. Normally, its average daily maximum temperatures in July rise to 11.6 C.  In August, the average drops again to 10.3 C. This year, July daily maximum temperatures in Iqaluit averaged 3.1 C above the norm, with nine of the 30 days exceeding 20 C. Some regions of the eastern high Arctic experienced even greater temperature anomalies.

Ditto for the southwestern Arctic. Whitehorse posted average July daily maximum temperatures of 24 C, about 3.5 C above the norm. Towards the end of July, much of the western Arctic experienced an intense heat wave that pushed the thermometer in Whitehorse above 30 C for three consecutive days, and set new temperature records for a number of weather stations in the region.

Canadians in British Columbia were not to be outdone by their Arctic neighbours. Vancouver is normally awash with cool Pacific Ocean air that keeps daily maximum temperatures in summer in the range of about 20 C and provides lots of rain days.

This year, hot winds coming from the dry American west kept flowing up the west coast, providing heat and lots of sun for British Columbia. Average temperatures in Vancouver during June and July were about 2-3 C above the norm, and exceeded 34 C for two days in late July. During the entire June-July period, there were only eight rain days.

In sharp contrast, a broad swath of central North America, from the high central Arctic down to the U.S. border and far beyond, was unusually cold and wet during June and July. While Vancouver appeared to be getting typical Central Canada weather, Central Canada was getting the cool, damp Vancouver-type weather. June temperatures across Ontario, were about 1 C below the norm. Those for July dropped to more than 2 C below normal.

For some southern Ontario weather stations, this was the second coldest July on record. While the Ottawa region also set a new record for total July rainfall amount, most other regions of Ontario experienced total rainfalls that were well within the range of past summers. In fact, some regions, such as the area around Chatham, were very dry during most of June, causing concerns about stunted crop growth. However, across most of Ontario, particularly in central regions, much of the precipitation that did fall occurred as frequent light rains.

During June and July, North Bay had 35 days with rain and Peterborough experienced 32 wet days (four times that of Vancouver). Southern Ontario fared somewhat better, but also had to cope with rain, on average, about every third day. The cool, wet conditions helped ensure that smog days were virtually non-existent. However, it also meant that many farmers found it very difficult to find a few dry days for cutting and baling their hay crops. 

So why this inverted weather pattern? Blame it on the upper atmospheric jet stream, a high-speed air flow that moves around the Earth at mid-latitudes, guiding along the movement of air masses below.

The position and meandering of the Northern Hemispheric jet stream is largely controlled by the position of large stable, high-pressure systems in mid-latitudes. Normally, these push the summer jet stream well north of us and keep it in a fairly linear west-to-east flow. This inundates British Columbia with air directly off the Pacific, but leaves much of the rest of Canada east of the Rockies basking in warm tropical and temperate air masses.

This year, starting in mid-June, the western high-pressure system normally positioned over the central mid-latitude North Pacific moved north-eastward towards the Gulf of Alaska.  Meanwhile, as part of a shift in the North Atlantic Oscillation, an unusual high-pressure system began to build in the east over Greenland and the northern Labrador Sea. 

These two high-pressure systems caused a double block of the normal smooth west-east flow across the country. A strong low-pressure system, positioned over James Bay, became ensconced between the two high-pressure systems. As a rule of thumb, air flows clockwise around high-pressure systems and counter-clockwise around lows. Hence, this western high-central low-eastern high-pressure pattern caused large meanders in the jet stream.

In the West, the jet stream bulged far to the north, over Alaska, bringing the warm dry air from the South with it into British Columbia and the western Canadian Arctic. The combined egg-beater effect of the western high and the central low, pushed the jet stream over central North America far to the south, pumping cool Arctic air and lots of moisture over central Canada.

To the east of the James Bay low (and west of the eastern high), the jet stream pushed back up towards the Arctic, bathing the eastern Arctic in heat and sunshine. Presto, British Columbia and parts of Arctic Canada got the summer of summers, while Ontarians got a summer many would rather forget.

This unusual circulation pattern persisted until early August. As the western high-pressure system moved back to its normal positions and the eastern system weakened, the jet stream re-asserted its more northerly position across Canada. Warm weather returned to central Canada, and the west coast again got its rain – a major relief for the water resource managers and wild fire fighters in the region. Now Ontarians could complain about the heat and humidity again. BF

Henry Hengeveld  is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.