M7 Low Frequency Variability

Topic

Low-frequency variability encompasses all atmospheric fluctuations persisting over periods ranging from several weeks to several decades—timescales far longer than the few days a typical weather system lasts. Four distinct phenomena coexist within this low-frequency variability: atmospheric blocking, which halts the jet stream for days or weeks; annular modes, which shift the jet stream and polar vortex north or south; stationary wave teleconnections, which link widely separated regions; and sudden stratospheric warmings, which couple the stratosphere and troposphere on weekly timescales. It is precisely this diversity of timescales and mechanisms that makes low-frequency variability far more difficult to predict than day-to-day weather.

Within this low-frequency variability, atmospheric blocking is the phenomenon operating on the shortest timescale: an anticyclone settles into an anomalous position and stalls the jet stream there for a period of five to thirty days, causing persistent weather anomalies such as heatwaves or floods. What holds this anticyclone in place is a non-linear interaction between the mean flow and Rossby waves; when these Rossby waves become sufficiently large in amplitude, they trigger the formation of a persistent dipole circulation—a pair of opposing circulation centers that collectively block the zonal flow that would normally pass through the region.

Annular modes represent a different type of fluctuation within this same low-frequency variability: rather than blocking the flow at a single location, they cause the jet stream and the polar vortex to shift in unison toward the north or the south. The Arctic Oscillation and the North Atlantic Oscillation are the versions of this annular mode that govern the Northern Hemisphere, while the Antarctic Oscillation does the same in the Southern Hemisphere; in all three cases, when the corresponding annular mode enters its negative phase, it causes the jet stream and the polar vortex to contract toward lower-than-usual latitudes.

In addition to these annular modes, there are other types of teleconnections formed by Rossby waves that remain stationary rather than propagating; the Pacific-North American pattern and the Pacific-South American pattern are the best-known examples, and both link tropical Pacific convective activity to pressure anomalies appearing far away over North America or South America.

The final component of low-frequency variability consists of sudden stratospheric warming events, which disrupt the stratospheric polar vortex within the stratosphere itself and cause the resulting disturbance to propagate downward, reaching the troposphere within two to six weeks. Upon reaching the troposphere, this disturbance forces the Arctic Oscillation into its negative phase, and it is this negative phase that increases the frequency of Arctic air outbreaks into mid-latitudes. Thus, ranging from atmospheric blocking—which may resolve within a couple of weeks—to annular modes and teleconnections that can persist in the same phase for months or even years, low-frequency variability encompasses mechanisms of widely varying durations under a single common denominator: they all stem from the behavior of Rossby waves—whether stalling during a blocking event, shifting into an annular mode, remaining stationary within a teleconnection, or propagating vertically to trigger a sudden stratospheric warming. It is this diversity of behaviors that causes the atmosphere to "remember," weeks or decades later, the disturbance that originally set them in motion.

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