Tropical and Stratospheric influences on extratropical short-term climate variability

The relative influences of tropical forcing and pre-existing stratospheric anomalies on short-term climate variability in the extratropical troposphere during winter are investigated by constructing a linear inverse model (LIM) from observed simultaneous and 5-day lag correlation statistics. As a key test of linearity, observed 21-day lag covariability of all the model variables throughout the Northern Hemisphere is compared with that predicted by the LIM. Over most of the hemisphere, notably including the Pacific sector, the reproduction is quite good, supporting the view that the dynamics of short-term climate variability are effectively linear on weekly time scales and are driven by white noise. An important exception occurs within an area extending from the North Atlantic to the Barents Sea, where variability predicted by the LIM is not as persistent as is observed. The fair performance of the linear model in this region suggests that some of the nonlinearity over the North Atlantic may not be merely represented by unpredictable noise.

Investigation of interactions between variables within the LIM shows that while tropical forcing greatly enhances persistent variability globally, particularly throughout the Pacific sector and over North America, stratospheric effects are much more localized to the polar region. Over the North Atlantic, both processes enhance climate variability. Moreover, anomalies that amplify in response to tropical forcing extend throughout the depth of the troposphere and into the stratosphere, whereas anomalies that amplify due to stratospheric forcing tend to have a more shallow response that is largest at the surface and relatively weak in the mid-troposphere, with relatively little effect on tropical anomalies either. Additionally, in the absence of this “external” forcing, some persistent variability still occurs in locations corresponding primarily to the centers of the leading eigenmodes of the unforced portion of the linear operator, one that has a wavenumber-5 circumglobal structure with largest amplitude over the Arabian Sea and the central Pacific, and two that represent north-south dipoles across the North Atlantic jet. Overall, while the forcing, primarily from the Tropics, only somewhat increases overall climate variability, it strongly increases the persistent and predictable portion of that variability.