Climate dynamics

Abstract:

The Quasi‐Biennial Oscillation initiative (QBOi) is a model intercomparison programme that specifically targets simulation of the QBO in current global climate models. Eleven of the models or model versions that participated in a QBOi intercomparison study have upper boundaries in or above the mesosphere and therefore simulate the region where the stratopause semiannual oscillation (SAO) is the dominant mode of variability of zonal winds in the tropical upper stratosphere. Comparisons of the SAO simulations in these models are presented here. These show that the model simulations of the amplitudes and phases of the SAO in zonal‐mean zonal wind near the stratopause agree well with the information derived from available observations. However, most of the models simulate time‐average zonal winds that are more westward than determined from observations, in some cases by several tens of m·s–1. Validation of wave activity in the models is hampered by the limited observations of tropical waves in the upper stratosphere but suggests a deficit of eastward forcing either by large‐scale waves, such as Kelvin waves, or by gravity waves.

Abstract:

The extratropical teleconnection from the tropical Pacific in boreal summer exhibits a significant shift over the past 70 years. Cyclonic circulation anomalies over the North Atlantic and Eurasia associated with El Niño in the later period (1978‐2014) are absent in the earlier period (1948‐1977). An initialised atmospheric model ensemble, performed with prescribed sea surface temperature (SST) boundary conditions, replicates some key features of the shift in the teleconnection, providing clear evidence that this shift is not simply due to internal atmospheric variability or random sampling. Additional ensemble simulations, one with detrended tropical SSTs and another with constant external forcing are analysed. In the model, the teleconnection shift is associated with climatological atmospheric circulation changes, which are substantially reduced in the simulation with detrended tropical SSTs. These results demonstrate that the climatological atmospheric circulation and associated teleconnection changes are largely forced by tropical SST trends.

Abstract:

From observations, we identify a wave‐like pattern associated with northwestern European seasonal precipitation events. These events are associated with tropical precipitation anomalies, prompting us to investigate if there are any tropical–extratropical teleconnections, in particular the role of tropical anomalies in driving extratropical dynamics through Rossby wave propagation. Using a hierarchy of models from ray tracing to barotropic and baroclinic models, we investigate the Rossby wave mechanism and test potential tropical drivers and yield qualitative results. Using a barotropic model, we identify potential Rossby wave source regions which are consistent between the observations and the model. These regions include the tropical western and eastern Atlantic, the subtropical eastern Atlantic and, to a smaller degree, the subtropical eastern Pacific. Zonal wavenumber 2 and 3 components of the barotropic model responses match well with the observations and ray tracing supports the importance of these components. We use a baroclinic model to investigate the link between the observed Rossby wave source anomalies and the observed tropical precipitation anomalies. The reduced precipitation observed in the tropical Atlantic just north of the Equator can generate some of the observed Rossby wave source anomalies in the tropical Atlantic, while the increased precipitation observed in the tropical eastern Pacific can generate some of the observed Rossby wave source anomalies in the subtropical eastern Pacific. Our results can also be applied to European drought events because of the qualitative linearity in the observations and in our linear methods.

Abstract:

The closure of pipelines to tackle fuel-theft in central Mexico caused an unexpected fuel shortage that disrupted transport systems in Mexico City in January of 2019. Fuel sales in the Metropolitan Area and CO emissions from reanalysis showed a significant decrease during the fuel shortage of 7% and 6%, respectively. This study analyses the air quality and meteorological conditions during this period to evaluate whether these measures indirectly affected air quality in Mexico City. During the shortage, mean-ambient concentrations of nitric oxide (NO), nitrogen dioxide (NO2) and carbon monoxide (CO) were significantly lower than normal whereas levels of particulate matter (PM) were only modestly lower than usual. Daily-mean NO and CO had record-low anomalies of −10 ppb and −0.5 ppm from typical days, respectively. In contrast, ozone mean-levels were not significantly different than average. The percentage of days with PM mean concentrations above the World Health Organisation guidelines (5 and 19% for particles smaller than 2.5 and 10 μm, respectively) and the percentage of ozone 8-h rolling means above Mexican law (0.5%) were record lows. Meteorological factors, such as wind speed or the mixed-layer height were not significantly different than average. The anomalously low pollution levels were accentuated when each day was compared to days of similar flow patterns. This episode of better than usual air quality showcases how strategies addressing transport emissions could control air quality in Mexico City and highlights that improving ozone mean levels require comprehensive strategies that reduce emissions from all sectors.