Climate dynamics

Abstract:

Tim Woollings discusses the reasons behind the natural events such as the cold European winter of 2009-10, heatwave in Russia, and devastating floods in Pakistan. The jet streams are literally jets of fast-moving air that are strongest about 10km up in the atmosphere - around the level where airliners fly. Jet streams vary in strength from week to week. One of the most common variations in the North Atlantic jet stream is for the whole jet to shift to the north or the south. While the North Atlantic jet stream generally points roughly due east, straight across the Atlantic, it often meanders north and south. When waves on the ocean surface become too large they overturn and break, resulting in very turbulent motion. When Rossby waves break, the resulting weather situation is known as blocking.

Abstract:

We study the response of the Southern Hemisphere circulation to the 1982 eruption of El Chichón and 1991 eruption of Pinatubo volcanoes in a suite of up-to-date coupled climate models. We find a significant response in austral spring and autumn in the years following the eruptions, which consists of a stronger stratospheric polar vortex and lowered sea-level pressure over the Antarctic, both consistent with the positive phase of the Southern Annular Mode. The seasonality of the response may be explained in terms of zonal flow-planetary wave interactions. This dynamical response is inconsistent with the observational reanalyses in the polar stratosphere in spring, but not in the troposphere where the internal variability is large compared to the magnitude of the response. © 2010 Royal Meteorological Society.

Abstract:

Stratospheric variability is examined in a vertically extended version of the Met Office global climate model. Equatorial variability includes the simulation of an internally generated quasi-biennial oscillation (QBO) and semiannual oscillation (SAO). Polar variability includes an examination of the frequency of sudden stratospheric warmings (SSW) and annular mode variability. Results from two different horizontal resolutions are also compared. Changes in gravity wave filtering at the higher resolution result in a slightly longerQBOthat extends deeper into the lower stratosphere.At the higher resolution there is also a reduction in the occurrence rate of sudden stratospheric warmings, in better agreement with observations. This is linked with reduced levels of resolved waves entering the high-latitude stratosphere. Covariability of the tropical and extratropical stratosphere is seen, linking the phase of the QBO with disturbed NH winters, although this linkage is sporadic, in agreement with observations. Finally, tropospheric persistence time scales and seasonal variability for the northern and southern annular modes are significantly improved at the higher resolution, consistent with findings from other studies. © 2010 American Meteorological Society.