[1] In this paper, we propose a new method based on the detection of jet cores with the aim to describe the climatological features of the jet streams and to estimate their trends in latitude, altitude, and velocity in the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) and 20th Century reanalysis data sets. Due to the fact that the detection method uses a single grid point to define the position of jet cores, our results reveal a greater latitudinal definition allowing a more accurate picture of the split flow configurations and double jet structures. To the best of our knowledge, these results provide the first multiseasonal and global trend analysis of jet streams based on a daily-resolution 3-D detection algorithm.
The seasonal mean extra-tropical atmospheric response to El Niño/Southern Oscillation (ENSO) is assessed in the historical and pre-industrial control CMIP5 simulations. This analysis considers two types of El Niño events, characterized by positive sea surface temperature (SST) anomalies in either the central equatorial Pacific (CP) or eastern equatorial Pacific (EP), as well as EP and CP La Niña events, characterized by negative SST anomalies in the same two regions. Seasonal mean geopotential height anomalies in key regions typify the magnitude and structure of the disruption of the Walker circulation cell in the tropical Pacific, upper tropospheric ENSO teleconnections and the polar stratospheric response. In the CMIP5 ensembles, the magnitude of the Walker cell disruption is correlated with the strength of the mid-latitude responses in the upper troposphere i.e., the North Pacific and South Pacific lows strengthen during El Niño events. The simulated responses to El Niño and La Niña have opposite sign. The seasonal mean extra-tropical, upper tropospheric responses to EP and CP events are indistinguishable. The ENSO responses in the MERRA reanalysis lie within the model scatter of the historical simulations. Similar responses are simulated in the pre-industrial and historical CMIP5 simulations. Overall, there is a weak correlation between the strength of the tropical response to ENSO and the strength of the polar stratospheric response. ENSO-related polar stratospheric variability is best simulated in the “high-top” subset of models with a well-resolved stratosphere
[1] The polar vortex in the Northern Hemisphere exhibits high intraseasonal and interannual variability which, to some degree, may be controlled by the quasi-biennial oscillation (QBO) in the tropical stratosphere. Here we analyze the QBO signal in the Northern Hemisphere polar vortex in a model simulation using the general circulation model MAECHAM5 (Middle Atmosphere European Center Hamburg Model), which simulates the QBO as an internal mode of variability. Composites have been computed for the westerly and easterly QBO phases from early winter to late winter (NovemberDecember, December-January and January-February) of a 50-year experiment containing 20 complete QBO cycles. This method identifies tropical and midlatitude patterns in wind and temperature that are related to the secondary meridional circulation of the QBO extending towards the winter pole. In the tropics, significant QBO signature is observed in the mesosphere up to 0.05 hPa only in early winter and mainly in the easterly QBO phase forced by the parameterized gravity waves-mean flow interaction. At high latitudes, MAECHAM5 shows a significantly warmer (colder) polar stratosphere in the easterly (westerly) QBO phase at 30 hPa accompanied by a weaker (stronger) polar vortex in the late winter months, from December to February (December to January) in the easterly (westerly) phase of the QBO. In early winter there is no significant change in temperature and zonal mean zonal wind in the polar stratosphere. The analysis of EP fluxes shows a different behavior between QBO phases, with changes in the waveguide. Wave propagation occurs upwards and polewards in the easterly QBO phase at 30 hPa, while waves are refracted equatorward in the westerly phase. No relationship has been found between the tropical QBO and the final warming at the end of the boreal winter.Citation: Calvo, N., M. A. Giorgetta, and C. Peña-Ortiz (2007), Sensitivity of the boreal winter circulation in the middle atmosphere to the quasi-biennial oscillation in MAECHAM5 simulations,
The Northern Hemisphere (NH) stratospheric signals of eastern Pacific (EP) and central Pacific (CP) El Niño events are investigated in stratosphere-resolving historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), together with the role of the stratosphere in driving tropospheric El Niño teleconnections in NH climate. The large number of events in each composite addresses some of the previously reported concerns related to the short observational record. The results shown here highlight the importance of the seasonal evolution of the NH stratospheric signals for understanding the EP and CP surface impacts. CMIP5 models show a significantly warmer and weaker polar vortex during EP El Niño. No significant polar stratospheric response is found during CP El Niño. This is a result of differences in the timing of the intensification of the climatological wavenumber 1 through constructive interference, which occurs earlier in EP than CP events, related to the anomalous enhancement and earlier development of the Pacific-North American pattern in EP events. The northward extension of the Aleutian low and the stronger and eastward location of the high over eastern Canada during EP events are key in explaining the differences in upward wave propagation between the two types of El Niño. The influence of the polar stratosphere in driving tropospheric anomalies in the North Atlantic European region is clearly shown during EP El Niño events, facilitated by the occurrence of stratospheric summer warmings, the frequency of which is significantly higher in this case. In contrast, CMIP5 results do not support a stratospheric pathway for a remote influence of CP events on NH teleconnections.
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