As the polar regions warm faster than the tropics, the jet stream is weakened and tends to meander. This results in more sluggish weather patterns and more extreme temperature/precipitation patterns. I explain what I believe is a high-level driver for this behavior by drawing an analogy to a heat engine.
First, a warning: I am not a climate scientist. It is certain that I am missing many important details. However, I think this is worth writing about because it’s a very intuitive explanation for the recent crazy weather patterns in the middle latitudes and it may be a “good enough” answer—something that is close enough to the truth for use in daily conversation. Actual climate scientists are investigating this and will figure it out.
A few basic concepts need to be explained before proceeding:
Jet Stream: An atmospheric phenomenon consisting of narrow, quickly moving air currents that wrap around the Earth at a modest altitude. You can think of them as rivers of air that primarily flow west to east but also undulate north and south over time. There are two main jets—polar and subtropical. Pockets of high pressure (clear, dry air) and low pressure (storms, rain) are often pushed along by these rivers, making the jet stream’s speed, direction, and location very important for moving rain storms along. The Wikipedia article on the subject gets into much more detail and is a good place to gain a general understanding.
Heat Engines: The most basic heat engine has a reservoir of hot material, a reservoir of cold material, and a connection between them. Heat energy travels over the connection from hot to cold and is extracted to perform work—for example, burn coal to heat steam, which expands and moves a piston, which then turns a wheel and makes a train move forward, with the excess heat rejected to the cool environment. This is a specific instance of more general thermodynamic cycles. Key concept here is that a heat difference turns into physical movement.
Anthropogenic Climate Change: The specific set of changes to the climate that have been caused by human activities, especially combustion of fossil fuels, certain industrial processes, and land use change. These activities all emit long-lived greenhouse gases such as carbon dioxide. These particular gases reflect infrared radiation (heat energy), and as more of them accumulate in the atmosphere, more of the heat from the sun is trapped over time and the Earth’s atmosphere and oceans warm. Note especially that the polar regions are warming much faster than the tropics. This is for several reasons—as air heats up, ice melts, and more light is absorbed by land rather than reflected by the ice; drier arctic air is more sensitive to carbon dioxide levels since it has little water vapor in it (water vapor is the strongest greenhouse gas, so doubling carbon dioxide very nearly doubles the heat trapped at poles while having a much smaller impact on the tropics).
An Explanation for Our Crazy Weather
Put the above pieces together.
- The tropics receive much more direct sunlight and have much more water vapor in the air than the poles. It takes time for the winds and ocean currents to move this heat away from the tropics. Therefore the tropics are much hotter than the poles.
- Heat flows from the tropics to the poles (hot to cold). It’s not a straight line, and there are plenty of important details, but in general this is the direction of flow.
- The heat difference between the tropics and the poles does “work” on the atmosphere between them. One of the products of this work are the jet streams that roughly bracket the mid-latitudes (30 to 60 degrees N or S).
- A larger heat difference leads to more energy transferred into the jet stream, making it faster and generally more west-to-east directed. A smaller heat difference does less work and weakens the jet stream.
- Since climate change is warming the poles much faster than the tropics, the driving heat differential is decreasing. This leads to a weaker jet stream.
- A weaker jet stream will have less west-to-east flow energy and will tend to develop more “kinks,” meander north/south, and often get stuck for extended periods of time. It can’t plow through the atmosphere as forcefully.
- This means that extended periods of drought or rain will become more common as the jet stream doesn’t push storms along as quickly—they stall out. It means that large north/south deviations in the jet streams will results in large and often persistent temperature deviations over land.
The above provides something of an explanation for the extended California and Texas droughts, or for the increasing incidence of strong storms in the mid-Atlantic and Northeast US. Speaking as someone who grew up in and lives on the East Coast, the strength of the storms and the amount of rainy deluges received in recent years seems more typical of the southern US. Historical weather records show that the northeastern US tends to have less sun and more rainy days, but still receives a bit less rain than the southern US—that is, it experiences light to moderate rain and storms that often linger. The southern US gets more sun and fewer rainy days, but more rain than northern areas—heavy rain and strong storms that move through quickly. We may be seeing the southern pattern shift to the north.
And the strange kink in the jet stream that pulled Hurricane Sandy westward, toward the New Jersey coast, while not unprecedented, is a phenomenon that we can expect to see more often.
This by no means captures everything going on in the atmosphere, let alone the climate. However, I think it’s a decent first order approximation for understanding the causes of some of the weird weather we’ve been seeing lately.