Late 20th century atmospheric circulation changes Robert J. Allen

ISBN: 9781109206029

Published:

NOOKstudy eTextbook

193 pages


Description

Late 20th century atmospheric circulation changes  by  Robert J. Allen

Late 20th century atmospheric circulation changes by Robert J. Allen
| NOOKstudy eTextbook | PDF, EPUB, FB2, DjVu, audiobook, mp3, ZIP | 193 pages | ISBN: 9781109206029 | 8.61 Mb

Climate models and theoretical expectations have predicted that the upper troposphere should be warming faster than the surface. Direct temperature observations from radiosonde and satellite data have often not shown this expected trend- butMoreClimate models and theoretical expectations have predicted that the upper troposphere should be warming faster than the surface.

Direct temperature observations from radiosonde and satellite data have often not shown this expected trend- but non-climatic biases have been found in such measurements. An alternative approach is used to infer tropospheric temperature trends from wind measurements, using the thermal wind equation (TWE).

Based on radiosonde stations in the western tropical Pacific, observed long-term mean and seasonal temperature variations closely obey geostrophic balance- incorporating additional (ageostrophic) terms yields negligible improvement. The observed winds, therefore, offer a useful constraint on the horizontal structure of monthly and longer temperature variations (although the reverse is not true close to the equator where f → 0).

This conclusion is also supported by general circulation model output and Microwave Sounding Unit (MSU) temperature data.-These results support the use of the wind field as a way of overcoming heterogeneities in the temperature records for the monitoring of climate change. This technique is extended to derive global estimates of temperature trends for the upper troposphere to the lower stratosphere since 1970. Over the period of observations, a maximum warming trend of 0.65 +0.47K decade-1 exists near the 200 hPa pressure level, below the tropical tropopause.

Warming patterns are consistent with model predictions except for small discrepancies close to the tropopause. These findings are inconsistent with the trends derived from radiosonde temperature datasets and from NCEP/NCAR reanalyses of temperature and wind fields. The agreement with models increases confidence in model-based predictions of future climate change.-Similar to the discrepancy in tropical tropospheric warming rates, disagreement exists in large-scale atmospheric circulation changes, with models underestimating the late 20th century trend in the North Atlantic Oscillation/Northern Annular Mode (NAO/NAM).

The Community Atmosphere Model (CAM), along with a newly developed satellite-based aerosol data set, is used to investigate the effects of absorbing aerosols on high latitude circulation. These experiments show that atmospheric heating near the surface is capable in principle of driving significant NAO/NAM-like changes, but the specific pattern of aerosol forcing change thought to have occurred in the late 20 th century does not seem to account for the observed circulation changes over the same interval. This suggests some other forcing, or internal feedback mechanism, must have been responsible.

These findings are consistent with coupled climate models, which show negligible differences in NAO/NAM trends between models with black carbon (the primary absorbing aerosol) and models without. However, these CAM experiments show that the type of aerosol (i.e. anthropogenic versus natural) leads to significantly different circulation changes, suggesting aerosol type enhances the variability of high-latitude circulation, in agreement with observations.

A mechanism is proposed to explain the observed signals, involving wave-mean flow interaction and troposphere-stratosphere coupling.



Enter the sum





Related Archive Books



Related Books


Comments

Comments for "Late 20th century atmospheric circulation changes":


e-skauto.pl

©2010-2015 | DMCA | Contact us