VHF Radar Observations in the Stratosphere and Mesosphere During a Stratospheric Warming

The SOUSY-VHF-radar was used to carry out measurements during minor and a major stratospheric warming in February and March 1980, respectively. Echoes have been received from the stratosphere up to an altitude of about 30 km continuously during day and night, whereas echoes from the mesosphere were restricted to the daytime and occurred sporadically at different heights within the altitude range from 60 to 90 km. The three dimensional velocity vector was derived from Doppler measurements made in three different antenna beam directions with a height resolution of 1.5 km. In particular, the results obtained during disturbed conditions show the change of the zonal winds at mesospheric heights from westerly to easterly. A spectral analysis reveals a diurnal and a weaker semidiurnal tide of the zonal wind component

Eleven-year solar cycle variations in the atmosphere: observations, mechanisms and models

The understanding of natural and anthropogenic climatic change is an important issue in recent studies. The influence of the Sun (11-year solar cycle) as a natural variability factor on the atmosphere is discussed. Statistical studies with observational data (NCEP/NCAR re-analyses) covering four solar cycles show high correlations between the 11 -year solar signal and meteorological parameters, e.g., the geopotential heights and temperatures, in the lower stratosphere and troposphere. Studies with general circulation models (GCM) have discussed the possibility of an indirect dynamical response to direct changes in solar irradiance and ozone in the stratosphere. A physical mechanism explaining the solar influence on the atmosphere is still missing. Part of the mechanism understood so far and ideas from model and observational studies are presented

Middle Atmosphere Program. Handbook for MAP. Volume 16: Atmospheric Structure and Its Variation in the Region 20 to 120 Km. Draft of a New Reference Middle Atmosphere

A draft of a new reference atmosphere for the region between 20 and 80 km which depends largely on recent satellite experiments covering the globe from 80 deg S to 80 deg N is given. A separate international tropical reference atmosphere is given, as well as reference ozone models for the middle atmosphere

Climatological features of stratospheric streamers in the FUB-CMAM with increased horizontal resolution

International audienceThe purpose of this study is to investigate horizontal transport processes in the winter stratosphere using data with a resolution relevant for chemistry and climate modeling. For this reason the Freie Universität Berlin Climate Middle Atmosphere Model (FUB-CMAM) with its model top at 83 km altitude, increased horizontal resolution T42 and the semi-Lagrangian transport scheme for advecting passive tracers is used. A new approach of this paper is the classification of specific transport phenomena within the stratosphere into tropical-subtropical streamers (e.g. Offermann et al., 1999) and polar vortex extrusions hereafter called polar vortex streamers. To investigate the role played by these large-scale structures on the inter-annual and seasonal variability of transport processes in northern mid-latitudes, the global occurrence of such streamers was calculated based on a 10-year model climatology, concentrating on the existence of the Arctic polar vortex. For the identification and counting of streamers, the new method of zonal anomaly was chosen. The analysis of the months October-May yielded a maximum occurrence of tropical-subtropical streamers during Arctic winter and spring in the middle and upper stratosphere. Synoptic maps revealed highest intensities in the subtropics over East Asia with a secondary maximum over the Atlantic in the northern hemisphere. Furthermore, tropical-subtropical streamers exhibited a higher occurrence than polar vortex streamers, indicating that the subtropical barrier is more permeable than the polar vortex barrier (edge) in the model, which is in good correspondence with observations (e.g. Plumb, 2002; Neu et al., 2003). Interesting for the total ozone decrease in mid-latitudes is the consideration of the lower stratosphere for tropical-subtropical streamers and the stratosphere above ~20 km altitude for polar vortex streamers, where strongest ozone depletion is observed at polar latitudes (WMO, 2003). In the lower stratosphere the FUB-CMAM simulated a climatological maximum of 10% occurrence of tropical-subtropical streamers over East-Asia/West Pacific and the Atlantic during early- and mid-winter. The results of this paper demonstrate that stratospheric streamers e.g. large-scale, tongue-like structures transporting tropical-subtropical and polar vortex air masses into mid-latitudes occur frequently during Arctic winter. They can therefore play a significant role on the strength and variability of the observed total ozone decrease at mid-latitudes and should not be neglected in future climate change studies

Decadal-scale periodicities in the stratosphere associated with the solar cycle and the QBO

An interactive two-dimensional model is used to analyze the response of the stratosphere to the 11-year solar cycle in the presence of a quasi-biennial oscillation (QBO). The purpose of the paper is to demonstrate how the solar cycle response of stratospheric ozone and temperature diagnosed from model simulations depends on the QBO. The analyses show that (1) the simulated response to the solar flux when no QBO is imposed is very similar in different periods, despite differences in the magnitude and variability of the solar forcing; (2) the apparent solar response of temperature and ozone is modified by the presence of an imposed QBO; and (3) the impact of the QBO on the derived solar response is greatly reduced when the observed QBO forcing is replaced by an idealized sinusoidal forcing. The impact of the QBO on the solar cycle analysis is larger when only two solar cycles are analyzed but is not negligible even for analysis of four solar cycles. Differences in the QBO contribution account for most of the differences in analyses of separate 22-year periods. The statistical significance is not always a reliable indicator that the QBO effect has been separated

Modeling the whole atmosphere response to solar cycle changes in radiative and geomagnetic forcing

The NCAR Whole Atmosphere Community Climate Model, version 3 (WACCM3), is used to study the atmospheric response from the surface to the lower thermosphere to changes in solar and geomagnetic forcing over the 11-year solar cycle. WACCM3 is a general circulation model that incorporates interactive chemistry that solves for both neutral and ion species. Energy inputs include solar radiation and energetic particles, which vary significantly over the solar cycle. This paper presents a comparison of simulations for solar cycle maximum and solar cycle minimum conditions. Changes in composition and dynamical variables are clearly seen in the middle and upper atmosphere, and these in turn affect terms in the energy budget. Generally good agreement is found between the model response and that derived from satellite observations, although significant differences remain. A small but statistically significant response is predicted in tropospheric winds and temperatures which is consistent with signals observed in reanalysis data sets

Dynamics, stratospheric ozone, and climate change

Dynamics affects the distribution and abundance of stratospheric ozone directly through transport of ozone itself and indirectly through its effect on ozone chemistry via temperature and transport of other chemical species. Dynamical processes must be considered in order to understand past ozone changes, especially in the northern hemisphere where there appears to be significant low-frequency variability which can look “trend-like” on decadal time scales. A major challenge is to quantify the predictable, or deterministic, component of past ozone changes. Over the coming century, changes in climate will affect the expected recovery of ozone. For policy reasons it is important to be able to distinguish and separately attribute the effects of ozone-depleting substances and greenhouse gases on both ozone and climate. While the radiative-chemical effects can be relatively easily identified, this is not so evident for dynamics — yet dynamical changes (e.g., changes in the Brewer-Dobson circulation) could have a first-order effect on ozone over particular regions. Understanding the predictability and robustness of such dynamical changes represents another major challenge. Chemistry-climate models have recently emerged as useful tools for addressing these questions, as they provide a self-consistent representation of dynamical aspects of climate and their coupling to ozone chemistry. We can expect such models to play an increasingly central role in the study of ozone and climate in the future, analogous to the central role of global climate models in the study of tropospheric climate change

The role of the stratosphere in Iberian Peninsula rainfall: a preliminary approach in February

This paper attempts to establish a connection between stratospheric anomalies in the North Pole and rainfall on the Iberian Peninsula through the occurrence of major midwinter warmings (MMWs) and cold events (CEs), taking February as a preliminary approach. We define the MMWs as the warmings which break down the polar vortex, whereas the CEs are the episodes in which the polar vortex remains cold and undisturbed. Both anomalies lead to a wind anomaly around the north polar stratosphere, which is connected with a shortly lagged tropospheric anomaly through a stratosphere-troposphere coupling in winter. A T-mode principal component analysis (PCA) was used as an objective pattern classification method for identifying the main daily surface-level pressure (SLP) patterns for February for the 1961-1990 reference period. Subsequently, those February months with an MMW or a CE influence in the troposphere are identified in the whole study period (1958-2000) by means of the Arctic Oscillation Index (AOI). Thus, performing the same analysis for the selected February months, new principal patterns for detecting changes in surface circulation structure and morphology are obtained. The results show a significant decrease in the westerlies and a southward shift of the storm tracks in Western Europe some weeks after an MMW occurrence, leading to an increase in precipitation in western Iberia and a slight decrease on the eastern Mediterranean fringe. The results are quite the opposite under a CE influence: the westerlies are strengthened and shifted northwards due to the displacement of the Atlantic anticyclone towards Central Europe; dry conditions are established throughout Iberia, except for the Mediterranean fringe, where precipitation shows a considerable increase due to the greater frequency of the northeasterly winds. Finally, an 11-year sunspot cycle-quasibiennial oscillation (QBO) modulation might be demonstrated in Iberian rainfall in February through the occurrence of these stratospheric anomalies