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Summertime free-tropospheric ozone pool over the eastern Mediterranean/middle east

Atmospheric Chemistry and Physics (2014) 14(1) 115-132
DOI: 10.5194/acp-14-115-2014
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Abstract

Observations show that the Mediterranean troposphere is characterized by a marked enhancement in summertime ozone, with a maximum over the eastern Mediterranean. This has been linked to enhanced photochemical ozone production and subsidence under cloud-free anticyclonic conditions. The eastern Mediterranean is among the regions with the highest levels of background tropospheric ozone worldwide. A 12 yr climatological analysis (1998-2009) of free-tropospheric ozone was carried out over the region based on the ECMWF (European Centre for Medium-Range Weather Forecasts) ERA-Interim reanalysis data and simulations with the EMAC (ECHAM5-MESSy) atmospheric chemistry-climate model. EMAC is nudged towards the ECMWF analysis data and includes a stratospheric ozone tracer. A characteristic summertime pool with high ozone concentrations is found in the middle troposphere over the eastern Mediterranean-Middle East (EMME) in the ERA-Interim ozone data, Tropospheric Emission Spectrometer (TES) satellite ozone data and simulations with EMAC. The enhanced ozone over the EMME during summer is a robust feature, extending down to lower free-tropospheric levels. The investigation of ozone in relation to potential vorticity and water vapour and the stratospheric ozone tracer indicates that the dominant mechanism causing the free-tropospheric ozone pool is the downward transport from the upper troposphere and lower stratosphere, in association with the enhanced subsidence and the limited horizontal divergence observed over the region. The implications of these high free-tropospheric ozone levels on the seasonal cycle of near-surface ozone over the Mediterranean are discussed. © 2014 Author(s) .

Figures

  • Fig. 1. Map of the Mediterranean Basin with the locations of the four EMEP stations marked: Cabo de Creus, Spain (ES10); Gozo, Malta (MT01); Finokalia, Greece (GR02); and Ag. Marina, Cyprus (CY02). Keep in mind that the positions of the cross sections of Figs. 5 and 7 are indicated in the map by the 35◦ N meridian and the 25◦ E parallel crossing the island of Crete, Greece.
  • Fig. 2. Monthly average ozone fields at 500 hPa based on ERA-Interim ozone (ppbv) over the period 1998–2009 for (a) January, (b) February, (c) March, (d) April, (e) May, (f) June, (g) July, (h) August, (i) September, (j) October, (k) November, (l) December.
  • Fig. 3. July– ugust average ozone anomaly fields (p bv) over the period 2005–2009 based on (a) ERA-Interim ozone at 450 hPa, and on (b) TES observed ozone at 464 mb. Ozone anomalies were calculated as the differences between the July–August mea at each grid point and the July–August mean of the whole area of the domain.
  • Fig. 4. ERA-Interim July–August average fields of ozone (ppbv), potential vorticity (pvu), specific humidity (g kg−1) and vertical velocity (Pa s−1) over the period 1998–2009 at 700 hPa (a, d, g, j), at 500 hPa (b, e, h, k) and at 250 hPa (c, f, i, l), respectively. The vectors in (j), (k) and (l) denote the horizontal vectors based on the zonal and meridional wind components, while the contour lines indicate the respective average values of horizontal divergence (10−5 s−1).
  • Fig. 4. Continued.
  • Fig. 5. Left column: ERA-Interim July–August latitude–pressure cross sections at longitude 24.5◦ E of (a) ozone (ppbv), (c) potential vorticity (pvu), (e) specific humidity (g kg−1) and (g) vertical velocity (Pa s−1) averaged over the period 1998–2009. Right column: ERA-Interim July–August longitude–pressure cross sections at 35.25◦ N latitude of (b) ozone (ppbv), (d) potential vorticity (pvu), (f) specific humidity (g kg−1) and (h) vertical velocity (Pa s−1) averaged over the period 1998–2009. The solid black line in (c) denotes the dynamical tropopause at 2 pvu. The vectors in (g) and (h) denote the horizontal vectors based on the zonal and meridional wind components.
  • Fig. 6. EMAC July–August average fields of simulated ozone (ppbv) and the stratospheric ozone tracer O3s (ppbv) over the period 1998–2009 at 486 hPa (a, c) and at 714 hPa (b, d), respectively.
  • Fig. 7. Upper plates: EMAC July–August latitude–pressure cross sections at 25◦ E longitude of simulated (a) ozone (ppbv) and (b) stratospheric ozone tracer O3s (ppbv) averaged over the period 1998–2009. Bottom plates: EMAC July–August longitude–pressure cross sections at 35◦ N latitude of (c) ozone (ppbv) and (d) stratospheric ozone tracer (ppbv) averaged over the period 1998–2009.

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CITATION STYLE

APA

Zanis, P., Hadjinicolaou, P., Pozzer, A., Tyrlis, E., Dafka, S., Mihalopoulos, N., & Lelieveld, J. (2014). Summertime free-tropospheric ozone pool over the eastern Mediterranean/middle east. Atmospheric Chemistry and Physics, 14(1), 115–132. https://doi.org/10.5194/acp-14-115-2014

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