Thread by @CColose

Thread time (mildly technical) for some broader context in relation to #PolarVortex

Because it’s popular, let’s dig in, and also is this thing related to global warming? Let’s start with the basics. We’ll get to polar vortex toward the end.

It turns out when you take a “fluid,” like an atmosphere, or a big water tank in a lab, and subject it to temperature gradients (e.g., driven by differential solar heating) and then make the fluid rotate, interesting things happen.

Digression (a): In reality, when you take (under)graduate atmospheric science classes, you bond not over how you will become rich promoting global warming, but rather hours spent writing down equations that describe those interesting things.

If Earth did not rotate, the circulation would be fairly straightforward- air would rise in the tropics, sink in high latitudes, and form a global overturning “Hadley cell.”

A slowly rotating body (Venus shown below) broadly fits into this characterization, with very small horizontal temperature gradients and near pole-to-pole circulation cells. But rotation inhibits the ability of the circulation to equalize horizontal temperature differences.

(That's temperature in the top, wind speed in the middle, and the overturning "streamfunction" at the bottom, showing the global extent of the Hadley cell) sciencedirect.com/science/articl…

The influence of the Coriolis effect on north-south motions will lead naturally to the generation of east–west (or in fancy terms, zonal) flow. The relevance of the rotation is quantified by a so-called “Rossby Number,” which is very small for fast rotating (and larger) planets.

Things that imply smaller Rossby radii typically lead to more jets in a hemisphere. Flow in this regime we call “geostrophic,” so wind (rather than flowing from high to low pressure), is actually perpendicular to horizontal pressure gradients.

To see this you can just pick your favorite daily weather map (like today), look high enough in the atmosphere (away from the effect of surface friction) and just look at the wind direction (the flag-like things) in relation to pressure contours aos.wisc.edu/weatherdata/up…

Geostrophy holds at large-scales not just on Earth, but Mars and the Gas Giants. A jet is implied because the vertical gradient of the zonal winds is related to the north-south temperature gradient. See this post from Patrick:

https://twitter.com/PatrickTBrown31/status/1090350990530772992

But is not purely zonal flow outside the tropics. Jet streams form on fast rotating bodies (Earth & Mars) become baroclinically unstable (driven by temp contrasts/rotation). These planets are characterized by tropical Hadley circulations & baroclinically active mid-latitudes

(Baroclinic instability is a particular turbulence-generating process on rapidly rotating planets, a dynamical instability driven by the extraction of potential energy from a latitudinal temperature contrast.)

The mid-latitude atmosphere is full of eddies, & you see the formation of large-scale weather systems (the vehicle for high-latitude heat transport from low to high latitudes). It is baroclinic instability that generates midlatitude cyclones and anticyclones in the atmosphere.

You can also see eddies form in rotating water tanks, see this video e.g.,

Digression (b): in those same (under)graduate classes, we often played with throwing dye in rotating tanks. Fun!

So the jet stream develops wave-like meanders, and eventually traveling wave patterns (e.g., Rossby waves).

Digression (c): in those same (under)graduate classes- you write down lots of equations with cosines & exponentials and other nonsense that look like they describe waves

But the point is the traveling wave patterns, i.e., the familiar high & low pressure systems associated with day-to-day weather maps you always see. These things show up in simple models too. You don't need fancy things to generate this structure.
gfdl.noaa.gov/blog_held/28-t…

Phew…okay. So what is a polar vortex? It’s actually a planetary-scale west-to-east flow that encircles high latitudes. And there’s a troposphere and stratosphere vortex, as shown in Waugh et al.

It’s also a climatological feature, not "weather." There are vortices on other planets, too! We’ve talked about ridges and troughs in terms of waves propagating along the jet stream— the polar vortex roughly means that.

And weather disturbances occur transiently and along the edge of the “vortex” edge. It's just troughs and ridges. See this helpful review paper on what "polar vortex" means, both in the sci literature context and in the public journals.ametsoc.org/doi/pdf/10.117…

Other planets can be quasi-geostrophic baroclinic dominated, Hadley cell-dominated, day-night circulation-dominated, etc. Earth is full of these eddies and traveling wave-like weather systems in high latitudes. You often see blobs of alternative cold/warm spots in temp anomaly.

and in plots like sea level pressure, height, etc. These waves, climatologically, are acting to moving heat poleward (the net heat transport is not zero because there is a correlation between the north-south wind anomaly & the local temperature anomaly)

climatereanalyzer.org/wx/DailySummar…

So, really, the answer to "it's cold outside, what about global warming" isn't just, "it's weather, not climate." Kind of. There are synoptic disturbances bringing cold air from the Arctic. But that is a climatological mechanism that serves a purpose, especially during winter.

And it's cold during the winter at high latitudes. At least, relatively cold. And cold to us. Remember Earth would probably be nearly frozen over without greenhouse gases.

But there is not really any evidence global warming makes cold more cold or cold more likely.

There are suggestions that changing the pole-to-equator temp gradient changes the surface weather expression of these wave dynamics, etc. These are complicated. not usually robust, and almost certainly hidden (in practice) in the noise of natural variability.