Extratropical age of air trends and causative factors in climate projection simulations
FECHA:
2019-06-07
IDENTIFICADOR UNIVERSAL: http://hdl.handle.net/11093/4289
VERSIÓN EDITADA: https://acp.copernicus.org/articles/19/7627/2019/
TIPO DE DOCUMENTO: article
RESUMEN
Climate model simulations show an acceleration of the
Brewer–Dobson circulation (BDC) in response to climate change. While the
general mechanisms for the BDC strengthening are widely understood, there
are still open questions concerning the influence of the details of the wave driving. Mean age of stratospheric air (AoA) is a useful transport
diagnostic for assessing changes in the BDC. Analyzing AoA from a subset of
Chemistry–Climate Model Initiative part 1 climate projection simulations, we
find a remarkable agreement between most of the models in simulating the
largest negative AoA trends in the extratropical lower to middle
stratosphere of both hemispheres (approximately between 20 and 25 geopotential kilometers (gpkm) and 20–50∘ N and S). We show that the occurrence of AoA trend minima in those regions is directly
related to the climatological AoA distribution, which is sensitive to an
upward shift of the circulation in response to climate change. Also other
factors like a reduction of aging by mixing (AbM) and residual circulation
transit times (RCTTs) contribute to the AoA distribution changes by widening
the AoA isolines. Furthermore, we analyze the time evolution of AbM and RCTT trends in the extratropics and examine the connection to possible drivers
focusing on local residual circulation strength, net tropical upwelling and
wave driving. However, after the correction for a vertical shift of pressure
levels, we find only seasonally significant trends of residual circulation
strength and zonal mean wave forcing (resolved and unresolved) without a
clear relation between the trends of the analyzed quantities. This indicates
that additional causative factors may influence the AoA, RCTT and AbM
trends. In this study, we postulate that the shrinkage of the stratosphere
has the potential to influence the RCTT and AbM trends and thereby cause
additional AoA changes over time.