Atmospheric conditions featuring the summer over the WM Synoptics and dynamics – Long Term Mean Climatological overview. Possible impacts of the NAO on atmospheric pollution over the WM. First evaluation of the climatological conditions during Summer Charmex campaigns – 2012, 2013, and 2014.

H L Summer synoptic conditions over the WM (LTM 1981-2010) SLP GPH – 700 hPa H L Azores High Thermal low – West African Monsoon GPH – 850 hPa GPH – 500 hPa Typical summer conditions at the surface is the eastward extension of the Azores High caped by the subtropical high Saharan High – Subtropical High

Hadley and Walker circulations affecting the Med. Basin African Monsoon Asian Monsoon However, as you will see, the role of the global circulation cells is limited as related to the WM

The WM is further away from the descending branch of the Asian Monsoon cell leading to weaker subsidence Inversion in the winds (upper level subsidence vs. low-level rising air) is distinctly different for both basins! Walker Cell - Asian Monsoon Western Mediterranean Eastern Mediterranean As contrasted to the EM – the WM is further away from the descending branch of the Asian Monsoon cell consequently the downdraft are weaker and cease at a higher altitude. Note that the inversion in the winds (upper level subsidence vs. low-level rising air) is distinctly different. The inversion in the EM is really close to the surface and the one in the WM is in mid-troposphere. Closed cell circulation of the Asian Monsoon depicted by the vertical zonal cross section averaged over the 20-35°N latitudinal band of wind vectors for July-August (NCEP/NCAR LTM 1957-98) (from: Ziv, et al., 2004).

Both combined cell circulations result in a weaker subsiding air masses as compared to the EM possibly acting as lid for dispersion of pollutants over the Med. Basin. Subsiding air masses over the WM is tenfold weaker as compared to the EM! (0.1 – 1 cm s-1 ) 0.1 1.0 Updraft generated by the West African Monsoon Updraft Downdraft Jun-Aug. LTM Omega vertical motion (Pa s-1) at 850 hPa (NCEP/NCAR LTM (1981-2010)

Stronger mid-tropospheric flow over the WM Air mass transport – Climatology JJA 1981-2010 Wind Vector – 1.5 km Wind Vector – 5.5 km Wind Vector – 3.0 km Wind Vector – 9.0 km 1 – 4 m/s 13 – 16 m/s 10 – 13 m/s 6 – 8 m/s Stronger mid-tropospheric flow over the WM

Atmospheric dispersion and transport conditions over both Med Atmospheric dispersion and transport conditions over both Med. Basins during summer (NCEP/NCAR Reanalysis LTM) Subsiding air mass bases Western Med. Eastern Med. Asian Monsoon – Walker Cell 4.5 km 1.0 km Omega850hPa Downdraft motion 0.1 cm s-1 1.0 cm s-1 Wind Vector 3-5.5 km 8 – 11 m s-1 6 – 8 m s-1 The WM is characterized by weaker subsiding conditions and stronger winds at mid-tropospheric layers.

Possible impacts of the NAO on atmospheric pollution over the WM. This winter type oscillation controls the strength and direction of westerly winds and storm tracks across the north Atlantic. Beside climatic conditions NAO affects climatic derived conditions (i.e., air pollution transport) NAO + NAO - Since we believe that the NAO affects not only climatic conditions but also climatic derived conditions such as air pollutant transport and dispersion, let’s focus on the role of the NAO’s circulation and its impact on the environment. But first few words about the NAO: The NAO is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of SLP between the Icelandic Low and the Azores High. The NAO controls the strength and direction of westerly winds and storm tracks and varies over time with no particular periodicity. In it positive phase the 2 barometric systems are prominent deflecting the storms to NW Europe leaving the Med. Basin in dry conditions. During its negative phase, the 2 systems are weaker and the Med. Basin is more cyclonic and wetter.

Composite means and anomalies of meteorological parameters characterizing extreme NAO+ and NAO- years over the WM over the 65-yr record First let us see how extreme NAO positive and negative years affect some meteorological parameters over the WM

SLP slightly higher over the WM SLP composite means and anomalies for JJA NAOI extreme summers (1950-2014) Mean (1983,1994) NAO+ Anomaly = Mean83,94 – LTM1981-2010 NAOI 83 = 1.26 NAOI 94 = 1.07 SLP slightly higher over the WM NAO- Mean (2009, 2012) Anomaly = Mean09,12 – LTM1981-2010 For this matter, the 2 highest years of positive and negative NAOI were identified. NAOI 2012 = - 1.61 NAOI 2009 = - 1.18 SLP slightly lower over the WM

The positions of SLP anomalies for the 2 highest NAO+ and NAO- years derived are consistent with the study of Allan and Ansell based on 122-y (1881-2003) NAO+ NAO- NAO+ NAO- The positions of the SLP anomalies …. (From: Allan, R. J., and T. J. Ansell, 2006:  J. Climate, 19, 5816–5842.) 

Slightly weaker zonal wind over the WM U925 composite means and anomalies for JJA NAOI extreme summers (1950-2014) Anomaly (1983,1994) Mean (1983,1994) NAO+ Slightly weaker zonal wind over the WM Anomaly (2009, 2012) Mean (2009, 2012) NAO- Slightly stronger zonal wind over the WM

Slightly warmer air over the WM T925 composite means and anomalies for JJA NAOI extreme summers (1950-2014) Anomaly (1983,1994) Mean (1983,1994) NAO+ Slightly warmer air over the WM Mean (2009, 2012) Anomaly (2009, 2012) NAO- Much warmer air over the WM

New results from Pierre Nabat Dust outbreaks and weather regimes Summer (JJA) weather regimes (anomalies) / Climatologies 1980/2012 Sea level pressure Atlantic Low / NAO + Atlantic Ridge NAO- Blocking Dust AOD

Conditions during Summer ChArMEx campaigns – JJA Composite Means and Anomalies for 2012, 2013, and 2014. Sea-level pressure 700 hPa Zonal Wind 925 hPa Air Temperature After we identified the summer conditions characterizing the WM during extreme positive and negative NAO years, let’s check if these derived conditions are consistent for the specific summer Charmex campaigns

Summer 2012 NAO = - 1.61 Sea-Level Pressure SLP Mean: Jun-Aug 2012 Sea-Level Pressure Summer 2012 NAO = - 1.61 Deepening of the Icelandic low and withdrawal west and weakening of the Azores High, whole Med. Basin more cyclogenetic. SLP Anom: Jun-Aug 2012 Anomaly = Mean2012 – LTM1981-2010 The summer of 2012 had an NAO negative characterized by a deepening of the Icelandic Low and negative SLP anomalies over the WM Negative SLP anomalies over the Med. Basin.

Summer 2013 NAO = 0.72 Sea-Level Pressure SLP Mean: Jun-Aug 2013 Sea-Level Pressure Summer 2013 NAO = 0.72 Intensification and progression eastward of the Azores High and less cyclogenetic conditions over WM. SLP Anom: Jun-Aug 2013 The summer of 2013 displays an almost opposite picture: we do see an intensification of …. Positive SLP anomalies over the Western Med. Basin. Anomaly = Mean2013 – LTM1981-2010

Summer 2014 NAO = - 0.81 Sea-Level Pressure SLP Mean: Jun-Aug 2014 Sea-Level Pressure Summer 2014 NAO = - 0.81 Near normal strength and position of the Azores High. SLP Anom: Jun-Aug 2014 Anomaly = Mean2014 – LTM1981-2010 The last summer campaign with a moderate negative NAOI is manifested by slight negative SLP anomalies over the WM Slight negative SLP anomalies over the Western Med. Basin.

U700 composite means for JJA NAOI for summer 2012/13/14 700 hPa Mean - 2012 U700 composite means for JJA NAOI for summer 2012/13/14 700 hPa Mean - 2013 700 hPa Mean - 2014 Weaker zonal wind comp. (3 km) over the basin during NAO+ summer (2013) The zonal wind component is stronger for both NAO negative phase years 2012 and 2014. Stronger zonal wind comp. (3 km) during NAO- summers (2012, 2014)

Air Temperature 925 hPa Temp Anom. 2012 925 hPa Temp Anom. 2013 925 hPa Temp Anom. 2014 Colder air temperature over the WM during NAO+ summer (2013) And warmer air temperature at shallow atmospheric layers. Warmer air temperature over the WM during NAO- summer (2012)

Daily weather regimes in 2012 NAO – Blocking Atlantic Ridge Atlantic Low / NAO + Daily weather regimes in 2012 Summer 2012

Very preliminary conclusions…. Large-scale atmospheric dispersion and transport conditions are favored over the WM contrary to the EM. In spite of the summer NAO (SNAO) smaller variability and spatial extent, extreme SNAO suggest that: SNAO- : WM more cyclogenetic, stronger winds, warmer as compared to SNAO+ Summer 2012 (NAOI = -1.6) Weak Azores High, Med. Basin more cyclogenetic than its summer LTM as indicated by stronger zonal winds at mid-tropos. and warmer air temp. Summer 2013 (NAOI = 0.72) Eastward penetration of Azores High, less cyclogenetic conditions over W. Med., weaker winds at mid-tropos. and colder air temp. Summer 2014 (NAOI = -0.81). Slightly lower SLP, slightly stronger zonal winds over the W. Med. The very preliminary conclusions that can be drawn are: …. It would be interesting to see whether the pollutant concentrations show a systematic different spatial distribution for the summers of 2012 and 2013 which were of opposite NAOI.

The following slide exposes the possible issues for the discussion

1. Large-scale atmospheric dispersion and transport conditions are favored over the WM contrary to the EM: The WM is characterized by weaker subsiding conditions and stronger winds. 2. Summer NAO- years favor dispersion and transport over the WM: More cyclogenetic and stronger winds. Is the first statement true and does it hold for all summers? Is the second statement true ? If indeed true, can this rule of thumb be turned upside down by internal climate variability (e.g. NAO)? Can we point on the responsible for the unexpected low pollution concentration (emitting sources/ diluting conditions) in 2012,13 and 14? Do we have enough pollutants data to assess if these conditions differ much for 2012, 2013 and 2014? First statement: Probably yes. A warmer climate might push the Hadley cell (African Monsoon) further north but not expected to change the position of the Walker cell. Second statement: Changes in the phases of the NAO due exist on a yearly basis as shown here. But do not necessarily reflect changes in air pollution concentrations since 1) NAO reflects transient perturbations, not LTM transport as shown 2) the vertical zonal cross section of the circulation cell (Asian Monsoon) is an Eulerian diagram because it shows the LTM not lagrangian trajectory of polluted air masses