Climate change: the new normal isn't what it used to be

Monday, October 7, 2013

The jet stream drives the weather and defines the climate. But, with the warming of the Arctic, it is weakening, causing atmospheric blocks that make the weather more extreme and difficult to forecast

by PHIL CHADWICK

Climate is what we expect; weather is what we get. Both are undeniably changing, and what is normal is very debatable. Lobby groups from both extremes of the climate change debate cloud the issues and nothing much gets accomplished when no one seems to really know what is going on with our environment. So here are some simple facts to consider. Meteorologists have been talking about these since the 1970s but they are still news to most.

The jet stream drives the weather and in turn defines the climate. Think of the jet stream as an atmospheric river. This river is fuelled by the temperature contrast between the warm tropics and the cold poles.

Both warm and cold air is always in motion. The thermal wind generates the winds high in the atmosphere and is created by temperature differences. If your left hand is in the cold air (north) and your right hand in the warm air then you are looking in the direction that the thermal wind is blowing . . . which leads to the creation of the jet stream. The stronger the temperature contrast, the stronger and straighter the wind. It is exactly like a river flowing down a steep slope.

When I first became a meteorologist in 1977, I learned to love the jet stream. The location of this atmospheric river was predictable and it allowed long-range forecasts of some accuracy. The jet stream was straight, true and fast and I relied on it for my predictions.

Now consider that the Arctic is warming three or four times faster than the rest of the world. There are many scientific reasons for this, but let us leave those for another article. The temperature contrast between the North Pole and the equator is now much less than it was. The atmospheric river is now 14 per cent slower than it was in the 1990s. The jet stream is also not blowing as straight and true – it tends to meander like a stream in a flood plain where the slope of the land is nil. This is something new that meteorologists have never seen before.

Atmospheric blocks develop when a weaker jet stream follows a twisted path across the landscape. The jet stream is more like those oxbow lakes in the flood plains. These patterns have become way more common, when once they were rare.  

Weather systems that get blocked in the stagnant and meandering atmosphere drop their load of moisture over small areas, instead of spreading it along their path as they race along with the historically stronger and straighter jet stream. The atmospheric blocks also create areas of abnormal heat and drought simultaneously with areas of cold and wet.

This aptly describes the weather patterns of the past few years, but is it climate change? I say yes. The lobby groups with their extreme views generate confusion. The political world hopes that any such impacts are beyond the electoral horizon. There is a lot of dithering and not much gets done or understood better through quality science.

For the farmer, the weather patterns of the 21st century are more likely to be feast or famine. Certainly the weather can never please everyone.  The patterns associated with atmospheric blocks are likely to produce too much or too little of whatever you want for your particular application.

The good old days of a stronger and straighter atmospheric river that directs weather systems toward, across and beyond your region are gone with the wind. Atmospheric blocks are becoming the new normal and they are challenging to forecast. One thing is for sure: the new normal is not what it used to be. BF

Phil the Forecaster Chadwick has been a professional meteorologist since 1977, specializing in training, severe weather and remote satellite and radar sensing.