Chapter 2: Atmospheric Circulation

Global Circulations: Planetary-scale winds.

Synoptic Circulations: Macroscale winds, including winds around highs and lows.

Local Circulations: Mesoscale winds due to local topography, including katabatic winds.

Global Circulation

Three-cell Circulation Model:

• 0^o-30^o: Hadley Cell. As air travels towards the pole, it is deflected by the Coriolis Force and turns East at 25^o. The Coriolis Force causes convergence aloft, and air starts to sink between 20^o and 35^o. The subsiding air is relatively dry due to adiabatic heating and moisture release near the equator. The subsidence zone (≈30^o) is the world’s sub-tropical high-pressure zone.
• 30^o-60 ^o: Ferrel Cell. Surface flow splits towards the poles and equator (trade winds). The Coriolis force has greater impact on circulation, and net surface flow is greater towards the poles. Deflection creates sporadic westerly winds aloft.
• 60 ^o-90 ^o: Polar Cell. Subsidence near the poles produce a divergent surface flow towards the equator. This surface flow is deflected by the Coriolis Force to form the polar easterlies (trade winds). The southern boundary of arctic air is the Arctic Front. Where the polar air meets warmer air at mid-latitudes is the Polar Front.

Areas associated with rising will create large zones of low pressure, such as the equatorial low (Doldrums), and subpolar low (50 ^o-60 ^o). These zones do not make a continuous belt due to surface variations, rather they create cell-shaped pressure zones. Two semi-permanent features are most prominent in Western Canada: the Arctic high, and the Aleutian low.

In the Arctic high, subsidence produces clear skies and divergent surface flow, resulting in polar easterly winds. As the Arctic high strengthens during winter, the subtropical high (Pacific high) weakens and moves eastward, lessening its effect on Western Canada.

The Aleutian low is a series of low-pressure systems moving through the Gulf of Alaska, creating a semi-permanent low-pressure centre. This is common during the winter months. Areas affected by this low experience cloudy conditions and heavy precipitation.

An illustration demonstrating global circular patterns

Jet Streams. A pressure gradient from the temperature contrast between the equator and the poles, balanced by the Coriolis force, would result in a theoretical wind flowing East to West parallel to the isobars. This theoretical wind is called the geostrophic wind. While the winds near the surface wouldn’t resemble this theoretical wind due to surface friction, surface friction decreases with altitude, and the resemblance becomes more apparent at higher elevations (approximately 6-9km). The pressure gradient also increases with altitude, which results in higher wind speeds aloft. Wind speeds as often as over 200km/h. Jet streams vary in width from less than 100km to over 500km, and are only a few kilometres thick.

Jet streams are commonly located over areas where there are significant temperature changes over a short distance. These areas are called fronts. For example, the polar jet is situated near the polar front. Essentially, jet streams separate areas of cold air to the north from areas of warmer air to the south.

Jet streams follow a meandering westerly path. Occasionally it can run north-south, or split into two separate jets. The polar jet moves southward during the winter due to seasonal temperature changes and changes in the temperature gradient. Upper level flows that are exactly westerly are referred to as zonal flow, and a highly amplified flow with a north-south pattern is a meridional flow.

Jet streams are important in weather forecasting, as they act as steering currents for weather systems. Weather systems are commonly found underneath jet streams.

Jet streaks, which are very high-speed winds embedded within the jet stream, also influence the weather by deepening troughs of low pressure.

An illustration of the polar and subtropical jet streams.

Waves. Jet streams follow wavelike paths called Rossby waves. Their wavelength are around 4000-7000km. Airflow within the Rossby wave travel in excess of 200km/hr, however the waves move very slowly.

Wave cyclones occur in the middle and upper troposphere. There will initially be a front separating warm air to the South from cold air to the North. A wave on the front will form as an upper-level disturbance embedded in the Jetstream moves over the front. The front develops a ‘kink’. As the wave intensifies, the cold and warm fronts become more organised. The faster moving cold front will eventually catch up and overtake the warm front, creating an occluded front. As the occlusion increases, it will eventually cut off the supply of warm moist air, causing the low-pressure system to gradually dissipate. Pressure systems typically form over oceans, weaken immediately over the coast, and die inland.

An illustration of the evolution of a wave cyclone.

Winds and Ocean Currents. Wind is the main driving force in ocean currents due to friction. There is a strong relationship between atmospheric circulation and ocean circulation. Winds create vertical movement of water called upwelling or downwelling. During upwelling, cold water from deeper ocean layers rise to the surface to replace warmer surface water moved by winds.

Ocean circulation dramatically effects local and global weather. One well-known circulation is the El Nino Southern Oscillation (ENSO). ENSO has the following warm and cold phases:

• El Nino: Warming of the tropical pacific, relatively high on the west
• La Nina: Cooling of the tropical pacific, colder and lower on the west

Both events occur every 3-7 years, and last between 9 months to 2 years. El Nino events cause high freezing levels, rain at higher elevations, and persistent weak layers, whereas La Nina events cause low temperatures, higher precip, and large snowstorm avalanche cycles. Canadian trends show a slight decrease in fatalities during La Nina years, whereas North American trends show an overall increase (30-60 percent) in fatalities during El Nino years, particularly in Maritime Climates.

ENSO Warm Episode (El Nino)

ENSO Cool Episode (La Nina)

Land Circulation

Land and Sea Breezes. Daytime heating will heat the air over land more than over water. The air over land heats and expands, and creates an area of low pressure. Cool air then moves from the sea onto land. The reverse happens overnight as the air over land cools more rapidly than over the sea. They are more pronounced in summer. These sea breezes can begin as early as 10am.

Mountain and Valley Circulation. Daytime heating will heat the air over mountain slopes slower than air at the same elevation in the valley bottoms. The warm air moves upslope, creating a valley breeze (anabatic winds). This leads to cumulus clouds and thundershowers over the peaks. In the evening, this pattern is reversed and a mountain breeze occurs (katabatic winds). Anabatic winds are more pronounced in summer, and katabatic winds more pronounced in winter. Katabatic winds also flow off glaciers into the valleys, and create funky wind loading patters on the snowpack. These are referred as glacier winds.

Gap Winds. Low-level wind that accelerated as it blows through a narrow valley or gap between mountains. These are strongest at the narrowest point. A gap wind typically forms as a result of a pressure gradient across the gap.

Chinook Winds. Chinook winds occur on the East slopes of the Rockies. Moisture is wrung out of the air on the windward side of the Rockies through orographic lifting. The air then descends the leeward side and is heated adiabatically. The winds are large scale and create strong temperature gradients across the mountains. Chinook winds typically occur during winter and sprint, and result in drastic warming.