Sudden Stratospheric Warmings (SSW's)
Jeff Masters forwarded me an e-mail from one of our readers inquiring about sudden stratospheric warming (there's a dramatic SSW ongoing over Asia...see loop of 10-mb temperature anomalies below). As a weather forecaster, most of the sudden warmings that catch my attention are related to tropopause folds (not SSW's). A tropopause fold is a process by which air from the lower stratosphere intrudes into the troposphere. To give you an idea of the spatial scale of tropospheric folds, their horizontal domain is on the order of a few hundred kilometers and the depth is typically a few kilometers.
Tropopause folds are associated with upper-tropospheric frontogenesis, the formation of jet streaks, and tropopause undulations (these processes are all related). By the way, I wrote about tropopause undulations and their role in cyclogenesis in a blog posted on December 29. At any rate, the subsidence and compressional warming associated with tropopause folds promote warming in the lower stratosphere. Moreover, tropopause folds are marked by relatively high concentrations of ozone at altitudes typically reserved for the troposphere. Back in the 1990s, I taught my students that another tracer of stratospheric air associated with folds is the presence of Strontium-90, which was left over from the above-ground testing of atomic bombs in the early 1960s (Strontium-90 obviously has a very long half-life).
Temperature anomalies at 10 mb from December 8, 2012, to January 6, 2013, show a dramatic and sudden warming over Asia.
But I digress. The reader was interested in the spectacular, sudden stratospheric warmings (SSW's...revisit the lollapalooza above). By way of comparison, the life span of an SSW is noticeably longer than a tropopause fold. Specifically, folds tend to have a lifetime of a day or so (perhaps as long as five days), but SSW's can last for weeks. Moreover, SSW's can have long-lasting effects, sometimes keeping the stratosphere warm for the rest of winter.
So why did I broach the idea of tropopause folds? Easy answer. I just didn't want readers to confuse tropopause folds with SSW's. With my worrisome nature appeased, I'll get back on topic now. For starters, spectacular, mid-winter SSW's are marked by a warming of up to 40 degrees Celsius at 50 mb (lower half of the stratosphere) over a period of just a few days. These sudden warmings can impact weather patterns in the troposphere (SSW's can promote a negative NAO and cold weather in the contiguous states, for example), so they are worthy of study.
I usually interject mathematics to help me explain SSW's, so I'll see if I can accomplish the same thing with words. The usual set-up in the Northern Hemisphere's lower stratosphere is a north-south thermal gradient, with a temperature maximum at roughly latitude 45 degrees north and progressively colder air toward the pole (absorption of ultraviolet radiation by ozone fades with increasing latitude as the intensity of solar radiation fades toward the dark high latitudes...keep in mind that it's winter and polar regions are dark).
If you were a student in my class, I'd drill it into your head that any north-south temperature gradient is associated with vertical wind shear. When it's warm to the south and cold to the north (as it is in the normal winter set-up), the vertical wind profile in the stratosphere is characterized by westerly winds whose speeds increase with increasing altitude. Such a relationship between horizontal temperature gradients and vertical wind shear are stipulated by the thermal wind equation. I don't want to get into math, so I'll just use the term, "thermal wind arguments." At any rate, vertical wind shear associated with the normal north-south temperature gradient culminates with a westerly wind maximum (fast winds blowing from the west) near the top of the stratosphere. This wind maximum near the top of the stratosphere is called the stratospheric polar-night jet (this jet is a feature of the stratosphere's general circulation during winter).
A cross section of average temperatures (in Kelvins) from 1979 to 1998 as a function of pressure (right axis) and altitude (left axis). Average isotachs denoting wind speeds in meters per second are represented by thin white contours (solid corresponds to westerly and dashed represents easterly winds). "J" marks the positions and altitudes of jet streams. Courtesy of NASA.
You can see the stratospheric polar-night jet (marked by a big "J" near 10 mb in the Northern Hemisphere) on the cross section of average January temperatures from 1979 to 1998 above. Note the polar vortex at high latitudes, an intense cyclonic vortex that forms over the winter pole. The polar vortex represents one of the most prominent circulation features in the stratosphere during winter. To the south of the polar vortex in the Northern Hemisphere, note the thin, white isotachs (meters per second) that represent the long-term average strength of the polar-night jet. In this cross section, the westerly polar-night jet blows out of the page right at you, with warmer stratospheric air on your left. By the way, dashed white contours indicate easterly winds (blowing from the east).
SSW's were discovered over Berlin in the early 1950s (the good old days). Subsequent research indicated that upward-propagating, planetary-scale Rossby waves transported energy and momentum into the stratosphere. Without getting into too much atmospheric physics, upward-propagating Rossby waves produce a meridional transverse circulation in the stratosphere, with sinking air over high latitudes and rising air closer to the equator. The dramatic compressional warming associated with the subsidence over high latitudes is the hallmark of SSW's that I mentioned earlier. As you can imagine, dramatic warming in the region of the polar vortex upsets the circulation in the stratosphere. As the normal north-south temperature gradient breaks down, winds associated with the polar-night jet weaken (thermal wind arguments) and sometimes reverse (become easterly). If stratospheric winds at this level become easterly, further upward transfer of wave energy is blocked, and the deceleration of westerly winds and subsidence warming work it way downward.
SSW's occur almost exclusively in the Northern Hemisphere because of the hemispheric differences in topography. As it turns out, the upward-propagating Rossby waves that incite SSW's are topographically forced (planetary waves are generated by the distribution of land and ocean, mountains, etc.). SSW's usually keep the Northern Hemisphere's stratosphere warmer for relatively long periods so that an ozone hole doesn't develop over the polar region. Indeed, an ozone hole, in the strictest sense of the term, only forms over the Southern Hemisphere.
SSW's alter the tropospheric circulation through a process called potential vorticity thinking (pretty odd, I know). PVT essentially acts like a thermal wind adjustment. During an SSW, changes to the stratospheric polar vortex in the lower stratosphere cause an imbalance. Generally speaking, when a flow is out of balance, the adjustment process ripples to regions relatively far from the location where the imbalance occurs. In this case, when there is an imbalance in the lower stratosphere, the adjustment extends all the way down to the earth's surface. Indeed, the adjustment "excites" the negative phase of the Northern Annular Mode (also known as the Arctic Oscillation) and the negative phase of the North Atlantic Oscillation. In turn, this "excitation" can pave the way for an outbreak of cold air over the Northeast States.