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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 L

L

H

H

H

Azores High

Thermal low – West African Monsoon

Saharan High – Subtropical High

SLP

GPH – 500 hPaGPH – 850 hPa

GPH – 700 hPa

Summer synoptic conditions over the WM (LTM 1981-2010)

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Hadley and Walker circulations affecting the Med. Basin

African Monsoon Asian

Monsoon

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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).

The WM is further away from the descending branch of the Asian Monsoon cell leading to weaker subsidence

Walker Cell - Asian Monsoon

Inversion in the winds (upper level subsidence vs. low-level rising air) is distinctly different for both basins!

Western Mediterranean

Eastern Mediterranean

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Subsiding air masses over the WM is tenfold weaker as compared to the EM! (0.1 – 1 cm s-1 )

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.

DowndraftUpdraft

Updraft generated by the West African Monsoon

1.0

0.1

Jun-Aug. LTM Omega vertical motion (Pa s-1) at 850 hPa (NCEP/NCAR LTM (1981-2010)

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Air mass transport – Climatology JJA 1981-2010Wind 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/s10 – 13 m/s

6 – 8 m/s

Stronger mid-tropospheric flow over the WM

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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.

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NAO + NAO -

• 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)

Possible impacts of the NAO on atmospheric pollution over the WM.

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Composite means and anomalies of meteorological parameters characterizing extreme NAO+ and NAO- years over the WM over the 65-yr record

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SLP composite means and anomalies for JJA NAOI extreme summers (1950-2014)

NAOI 83 = 1.26NAOI 94 = 1.07

Mean (1983,1994)

SLP slightly higher over the WM

NAOI 2012 = - 1.61NAOI 2009 = - 1.18

SLP slightly lower over the WM

NAO+

NAO-

Anomaly = Mean83,94 – LTM1981-2010

Mean (2009, 2012) Anomaly = Mean09,12 – LTM1981-2010

11(From: Allan, R. J., and T. J. Ansell, 2006: J. Climate, 19, 5816–5842.)

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-

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U925 composite means and anomalies for JJA NAOI extreme summers (1950-2014)

NAO+

NAO-

Slightly weaker zonal wind over the WM

Slightly stronger zonal wind over the WM

Mean (1983,1994)

Mean (2009, 2012)

Anomaly (1983,1994)

Anomaly (2009, 2012)

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T925 composite means and anomalies for JJA NAOI extreme summers (1950-2014)

NAO+

NAO-

Slightly warmer air over the WM

Much warmer air over the WM

Mean (1983,1994)

Mean (2009, 2012)

Anomaly (1983,1994)

Anomaly (2009, 2012)

New results from Pierre Nabat

Dust outbreaks and weather regimes

NAO- Atlantic Low / NAO +

BlockingAtlantic Ridge

DustAOD

Sea level pressure

Summer (JJA) weather regimes (anomalies) / Climatologies 1980/2012

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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

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SLP Mean: Jun-Aug 2012

SLP Anom: Jun-Aug 2012

Anomaly = Mean2012 – LTM1981-2010

NAO = - 1.61Deepening of the Icelandic low and withdrawal west and weakening of the Azores High, whole Med. Basin more cyclogenetic.

Negative SLP anomalies over the Med. Basin.

Summer 2012

Sea-Level Pressure

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SLP Mean: Jun-Aug 2013

SLP Anom: Jun-Aug 2013

Anomaly = Mean2013 – LTM1981-2010

NAO = 0.72Intensification and progression eastward of the Azores High and less cyclogenetic conditions over WM.

Positive SLP anomalies over the Western Med. Basin.

Summer 2013

Sea-Level Pressure

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SLP Mean: Jun-Aug 2014

SLP Anom: Jun-Aug 2014

Anomaly = Mean2014 – LTM1981-2010

NAO = - 0.81

Near normal strength and position of the Azores High.

Slight negative SLP anomalies over the Western Med. Basin.

Summer 2014Sea-Level Pressure

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Stronger zonal wind comp.

(3 km) during NAO- summers (2012, 2014)

Weaker zonal wind comp. (3 km) over the basin during

NAO+ summer (2013)

700 hPa Mean - 2012

700 hPa Mean - 2014

700 hPa Mean - 2013

U700 composite means for JJA NAOI for summer 2012/13/14

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925 hPa Temp Anom. 2012

925 hPa Temp Anom. 2014

925 hPa Temp Anom. 2013

Colder air temperature over

the WM during NAO+ summer (2013)

Warmer air temperature over

the WM during NAO- summer (2012)

Air Temperature

Daily weather regimes in 2012

Summer 2012

NAO –BlockingAtlantic RidgeAtlantic Low / NAO +

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• 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.

Very preliminary conclusions….

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The following slide exposes the possible issues for the discussion

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• 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?

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.