The Building Blocks of Northern Hemisphere Wintertime Stationary Waves

Abstract

An intermediate-complexity moist general circulation model is used to investigate the forcing of stationary waves in the Northern Hemisphere boreal winter by land–sea contrast, horizontal heat fluxes in the ocean, and topography. The additivity of the response to these building blocks is investigated. In the Pacific sector, the stationary wave pattern is not simply the linear additive sum of the response to each forcing. In fact, over the northeast Pacific and western North America, the sum of the responses to each forcing is actually opposite to that when all three are imposed simultaneously due to nonlinear interactions among the forcings. The source of the nonlinearity is diagnosed using the zonally anomalous steady-state thermodynamic balance, and it is shown that the background-state temperature field set up by each forcing dictates the stationary wave response to the other forcings. As all three forcings considered here strongly impact the temperature field and its zonal gradients, the nonlinearity and nonadditivity in our experiments can be explained, but only in a diagnostic sense. This nonadditivity extends up to the stratosphere, and also to surface temperature, where the sum of the responses to each forcing differs from the response if all forcings are included simultaneously. Only over western Eurasia is additivity a reasonable (though not perfect) assumption; in this sector land–sea contrast is most important over Europe, while topography is most important over western Asia. In other regions, where nonadditivity is pronounced, the question of which forcing is most important is ill-posed.

The Generic Nature of the Tropospheric Response to Sudden Stratospheric Warmings

“…0 south of ;508N. Such precursors have been found in observations (e.g., Black and McDaniel 2004;Cohen and Jones 2011;Garfinkel et al 2010) although their representation in models are generally model dependent (Gerber et al 2010). The vertical tripole of tropical wind anomalies represent the winds of the quasi-biennial oscillation, and in particular, with easterlies in the lower-middle stratosphere, provide favorable conditions for SSWs to occur (Holton and Tan 1980).…”

“…(e.g., Matsuno 1970) where q u is the meridional gradient of quasigeostrophic potential vorticity (PV), N 2 is the static stability, k is the zonal wavenumber, c is the phase speed, H is the density scale height, and all remaining variables are as in earlier equations. Even though strictly speaking, the refractive index can only predict wave behavior in the Wentzel-Kramers-Brillouin (WKB) limit, many previous studies have shown that the refractive index can provide useful information despite the fact that their experiments may not satisfy the underlying assumptions (e.g., Chen and Robinson 1992;Simpson et al 2009;Garfinkel et al 2012). It is expected in this framework that waves tend to preferentially propagate away from regions of small n 2 toward regions of larger n 2 .…”

“…To explain the second stage (i.e., the persistent jet shift at longer lags), the general consensus is that synoptic wave feedbacks are necessary (Limpasuvan et al 2004;Kushner and Polvani 2004;Song and Robinson 2004;Garfinkel et al 2013;Hitchcock and Simpson 2014). Indeed, Domeisen et al (2013) employed a dry dynamical core, to show that in the absence of synoptic wave feedbacks in the troposphere, the tropospheric response to an SSW would be a poleward-shifted jet, opposite to what is observed.…”

“…However, as they also relaxed the tropospheric temperatures, the downward impact was very sensitive to the details of the tropospheric climatology (e.g., Gerber and Polvani 2009). In fact, the magnitude of the tropospheric response to an identical stratospheric perturbation can differ by more than a factor of 3 depending on the tropospheric state (Garfinkel et al 2013).…”

“…While many studies have imposed thermal perturbations to the stratosphere to explore changes in stratospheric variability (e.g., Taguchi et al 2001;Jucker et al 2013), the focus has been on the climatological (steady or seasonally evolving) modifications by applying the heating continuously. As SSWs are associated with a sudden onset of a high-latitude warming, we take a novel approach in this study by imposing a warming for only a few days to initiate an SSW, before switching it off and examining the coupled stratosphere-troposphere response.…”