Abstract
AbstractBarotropic variability plays an important role in a variety of extratropical atmospheric processes, such as annular modes, teleconnections, and baroclinic life cycles, which occur on a wide range of time‐scales. Extratropical dynamics is dominated by high‐frequency (periods shorter than 10 days) transient waves, which drive barotropic variability through baroclinic life cycle events. However, other types of waves (e.g. low‐frequency, with periods longer than 10 days, and stationary waves) also play an important role in shaping extratropical dynamics on various time‐scales. This study uses reanalysis data in the context of the zonal momentum budget to address the relative importance of stationary, low‐frequency and high‐frequency waves in driving barotropic variability at high (synoptic) and low (subseasonal‐to‐seasonal) frequencies both locally in storm‐track regions and in the zonal mean in both hemispheres. The analysis reveals that the eddy forcing of barotropic variability on synoptic time‐scales is dominated by the interaction between low‐frequency (and stationary) and high‐frequency waves, and not by high‐frequency self‐interactions. On longer (subseasonal‐to‐seasonal) time‐scales the picture is more complex, with increased importance of low‐frequency self‐interactions but still largely negligible high‐frequency self‐interactions. A better understanding of the mechanisms driving barotropic variability on subseasonal‐to‐seasonal time‐scales may help advancing predictability on these time‐scales.
Highlights
Barotropic variability plays an important role in a variety of extratropical atmospheric processes, such as annular modes, teleconnections, and baroclinic life cycles, which occur on a wide range of time-scales
The analysis reveals that the eddy forcing of barotropic variability on synoptic time-scales is dominated by the interaction between low-frequency and high-frequency waves, and not by high-frequency self-interactions
This study has examined the forcing of barotropic zonal-wind variability on synoptic and S2S time-scales, from both a zonal mean and a local perspective
Summary
Barotropic variability in the atmosphere is a long-studied topic as it plays an important role in many atmospheric processes, such as annular modes (Thompson and Wallace, 2000; Lorenz and Hartmann, 2001; 2003), baroclinic life cycles (Simmons and Hoskins, 1978), and Rossby-wave teleconnections (Hoskins and Karoly, 1981; Wallace and Gutzler, 1981; Barnston and Livezey, 1987; Sardeshmukh and Hoskins, 1988; Wallace et al, 1988; Hoskins and Ambrizzi, 1993). Whitaker and Sardeshmukh, 1998), i.e. incorporating linear stationary–transient interactions and treating high-frequency waves as white noise forcing, while neglecting other nonlinear wave–wave interactions (such as low-frequency self-interactions) While this approximation provides a very simple model and may be used for predicting the system on various time-scales, the nonlinear processes play an important role in weakening the variability on longer time-scales (Vallis et al, 2004), which may limit the predictability.
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