Bojan Škerlak

On the linkage between the Asian summer monsoon and tropopause fold activity over the eastern Mediterranean and the Middle East

A climatology of tropopause folds occurring over the Eastern Mediterranean and the Middle East (EMME) has been established using the ERA-Interim reanalyses for the years 1979–2012. The methodology employs an algorithm that detects folds at grid points where the vertical profile features multiple crossings of the dynamical tropopause and allows their classification according to their vertical extent. Our results confirm the findings of an earlier 1 year climatology that recognized a global “hot spot” of summertime fold activity between the eastern Mediterranean and central Asia, in the vicinity of the subtropical jet. Two distinct maxima of activity are identified over Turkey and Iran-Afghanistan where fold frequency exceeds 25%. Occasionally, medium and deep folds form over the two regions at surprisingly low latitudes. This summertime peak in fold activity diverges from the zonal mean seasonal cycle over the subtropics and is driven by the South Asian Monsoon. Starting in late spring, the EMME is gradually brought under the influence of the zonally asymmetric background state induced by the monsoon. As areas of sharply sloping isentropes develop especially over the eastern Mediterranean and Iran-Afghanistan, subsidence and fold formation are favored. Further investigation of the reanalysis data provided empirical evidence that the monsoon also drives the interannual variability of EMME fold activity. An upward trend in fold activity is identified, especially in May, attributed to the recent advanced monsoon onset and the deepening convective activity throughout summer, which promotes upper-level baroclinicity over the EMME and favors folding.

Tropopause folds in ERA‐Interim: Global climatology and relation to extreme weather events

Tropopause folds are intimately linked to upper level frontogenesis and jet stream dynamics. They play an important role for stratosphere-troposphere exchange, the dynamical coupling of upper and lower tropospheric levels, and for generating severe weather events. This study presents a global climatology of tropopause folds using ERA-Interim reanalysis data from 1979 to 2012 and a refined version of a previously developed 3-D labeling and fold identification algorithm. This algorithm objectively separates stratospheric and tropospheric air in complex situations, e.g., in regions with strong low-level inversions, and in extratropical cyclones where diabatically generated potential vorticity anomalies typically occur. Three classes of tropopause folds are defined (shallow, medium, and deep), and their geographical distribution, vertical extent, and seasonal cycle are investigated. Most shallow folds occur along the subtropical jet stream, in agreement with previous studies. Hot spots of medium and deep tropopause folds are found west of Australia and along the coast of Antarctica in the Southern Hemisphere and around the east coast of North America in the Northern Hemisphere. Seasonal cycles show maxima in winter for all fold classes. Medium and deep folds are frequently associated with surface wind gust and precipitation extremes, as quantified for folds over the southern Indian Ocean. Wind gust extremes occur mainly in an elongated band upstream and equatorward of folds, whereas precipitation extremes occur mainly east and poleward of folds. Overall, in the considered region, about 20% of medium folds and 33% of deep folds are associated with surface wind or precipitation extremes in the vicinity of the fold.

Global Climatologies of Eulerian and Lagrangian Flow Features based on ERA-Interim

AbstractThis paper introduces a newly compiled set of feature-based climatologies identified from ERA-Interim (1979–2014). Two categories of flow features are considered: (i) Eulerian climatologies of jet streams, tropopause folds, surface fronts, cyclones and anticyclones, blocks, and potential vorticity streamers and cutoffs and (ii) Lagrangian climatologies, based on a large ensemble of air parcel trajectories, of stratosphere–troposphere exchange, warm conveyor belts, and tropical moisture exports. Monthly means of these feature climatologies are openly available at the ETH Zurich web page (http://eraiclim.ethz.ch) and are annually updated. Datasets at higher resolution can be obtained from the authors on request. These feature climatologies allow studying the frequency, variability, and trend of atmospheric phenomena and their interrelationships across temporal scales. To illustrate the potential of this dataset, boreal winter climatologies of selected features are presented and, as a first applicati...

Tropopause Folds Over the Eastern Mediterranean and the Middle East in EMAC Simulations: Implications for Summertime Tropospheric Ozone

A 35 year climatological analysis (1979–2013) of tropopause fold frequency has been performed over the eastern Mediterranean and the Middle East, focusing on the implications for tropospheric ozone. The analysis is based on simulations with the EMAC (ECHAM5-MESSy) atmospheric chemistry climate model using a T42L90MA resolution, nudged tropospheric meteorology towards ERA-Interim reanalysis data and a tracer for stratospheric ozone (O3s). We implement a 3-D labeling algorithm, in order to detect tropopause folding events in EMAC simulations as areas of multiple crossings of the dynamical tropopause. A clear hot spot of tropopause folds occurs over the broader eastern Mediterranean and Middle East region during the summer season. A significant contribution of tropopause folds to the summertime pool of high tropospheric ozone over the eastern Mediterranean is identified during the selected fold events. Finally, a year-to-year analysis indicates a significant positive correlation between the observed near surface ozone and EMAC simulated middle/lower tropospheric ozone over the eastern Mediterranean.