“Weather Whiplash”

7 07 2013

Another guest post by Mark Cochrane.  This is an extension on that ABC Catalyst video I posted the other day that explains

Mark Cochrane

Mark Cochrane

how warming the Earth causes freezing weather, and other unexplained extremes.  well, they’re explained now at any rate!

In the northern hemisphere the spring seasons of the last two years have been a study in extreme contrasts. In much of North America and Europe, one year’s spring was unseasonably warm (cool) and the other unseasonably cool (warm). This is why scientists refer to Global Climate Change versus the often used term Global Warming. Although the climate changes that we are experiencing are a function of the warming planet the local effects are quite variable in space and time.

Climate is nothing more than the thermal redistribution of solar energy from the equator towards the poles. If the Earth was not spinning this would result in warm air rising near the equator and flowing poleward at the boundary with the stratosphere until returning to ground level at the poles.  Because the Earth is spinning though, air flow is affected by the Coriolis Force (conservation of the angular momentum of the air mass), air moving north in the northern hemisphere gets bent to the right (east), while air moving south also gets deflected to the right (west). In the southern hemisphere the same principle holds but the deflections are to the left. The result is the formation of three major cells of vertical airflow in each hemisphere.

Wherever the air rises tends to be wetter and cloudier while where the air descends the conditions tend to be drier and less cloudy. In the graphic below 1, 2 and 3 correspond to the Hadley, Mid-latitude (Ferrel), and Polar Cells. At the latitude where cells 1 and 2 descend together many of the world’s major deserts are found. Similarly the poles are also deserts (albeit cold ones!).

Increased solar heating in the tropics has intensified and expanded the Hadley Cell. The result being more intense precipitation in the tropics and a poleward shift of the subtropical dry zones by up to 2 degrees per decade (roughly 120 miles (200 km)) (Zhou et al. 2011). This trend does not bode well for places like Southern Europe which will eventually experience conditions similar to those of North Africa.

Note in part B of the figure that jet streams form at the boundaries of cells. The Polar jet is of particular importance for temperate zone weather patterns. The strength of the jet depends on the temperature differences between the mid-latitude and Polar cells. The jet oscillates north-south (Rossby waves) and as long as the zonal winds (red) dominate the associated weather patterns move along quickly too. However, with slower zonal winds the jet stream begins to meander and meridional flow (North-South) causes the wave amplitude to increase.

This becomes important because it allows both unusually warm air to penetrate to polar zones and polar air masses to dip into southern climes. Once established, meridional flow can result in blocking patterns that effectively lock the system into a state and cause prolonged warm or cold periods with associated weather systems for affected regions. Just where the waves set up can change from year-to-year.

All of this falls back on the rapid melting of the Arctic sea ice. As the ice melts, more energy is trapped as heat in the regional ocean waters because the ice is no longer there to reflect much of the sunlight and the ice is also no longer available to absorb much of the heat as it melts back to fluid form. Both theory and observations tie these changes to Arctic Amplification of temperatures as the planet warms. This rapid Arctic warming reduces the temperature gradient between the polar and mid-latitude regions, slowing the jet stream and allowing for frequent meridional flow patterns to become established.

The recent term for this phenomenon is “weather whiplash”, as described in this 7 minute video interview of Dr. Jennifer Francis as part of the Yale forum on Climate Change & The Media that contrasts the 2012 & 2013 weather patterns.



One response

10 07 2013

Congratulations Mike & Mark. That is the clearest explanation I have ever seen. Clears up a range of difficult questions in one story. The world needs a lot more stuff like this. Keep up the good work.

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