Transport analysis and source attribution of the tropical CO seasonal and interannual variability

in the UT/LS

Junhua Liu and Jennifer LoganSchool of Engineering and Applied Sciences, Harvard University, Cambridge,

Massachusetts, USA

Dec 17th 2010, AGU

Acknowledgements: Thanks to the MLS Science team

Gases exchanged between the UT/LS

Modified from Holton et al. (1995)

H2O tape recorder

Jiang et al. (2010)

CO Tape RecorderFirst identified by Schoeberl et al. (2006) using MLS data, GCM model analysis by Duncan et al. (2007).

Objective:

• To understand the processes that determine observed temporal and spatial variability in CO signal in the UT/LS.

• Advection of the CO seasonal signal near the tropopause. (Schoeberl et al., 2006)

• Seasonal cycle in upwelling acting on the background vertical gradient in CO (Randel et al., 2007).

Two mechanisms hypothesed:

MLS (Microwave Limb Sounder)

MLS V3.3 level 2 data• Prelim. validation with MOZAIC aircraft transects at 215 hPa

• the 100% positive bias in MLS v2.2 has been eliminated

MLS

Mozaic

Courtesy of Inna Megretskaia

GEOS-Chem Model

Standard chemistry run:• 4o × 5o horizontal resolution

• Two runs driven by archived GEOS-4 (2005-06) or GEOS-5 (2005-08) assimilated met. fields – diff. convection parameterization

• Model profiles sampled along the MLS orbit track at the obs. time, and vertically smoothed with the MLS AK.

Tagged CO run:• 2o × 2.5o horizontal resolution• OH fields from chemistry run• CO (P-L) rate in stratosphere from GMI model

CO tape recorder (12ºN- 12ºS)MLS

GEOS-4

GEOS-5

4-yr mean is subtracted from time series

CO from Indonesian fires in late 2006

The GEOS-Chem models capture the main features. Inter-annual variation (IAV) driven by CO fire emissions, especially from Indonesia.

ppb

IAV in emissions in NH fire season apparent (Jan.-Apr.).

Initial conditions of CO and transport at 226 hPa

Tagged CO (12N-12S) source contributions

S. America

S. Africa

Indonesia

N. Africa

Isoprene Left: temporal overlapping of surface BB from different continents generates the semi-annual cycle ~200 hPa.

Biomass Burning

max. in Apr-May, min. in Aug-Sep. GEOS-4 stronger than GEOS-5

70 hPa: CO maximum from SH and NH fires add - shift to annual cycle.

100 hPa: MLS : semi-annual cycle Models: • Fall peak damped in 07,

08. • Matches phase of MLS but

amplitude too small. Vert. transport too weak?

215 &147 hPa:: Semi-annual cycle

N.Af, Indo. S.Af, S.Am Indo.

CO

an

om

aly

(pp

b)

Tropical CO - Seasonality and interannual variation

MLS GEOS-4 GEOS-5

4-yr mean is subtracted from time series

CO peak at 70 hPa from NH fires lags peak at 147 hPa by ~0-2 mon.; lag in peak ~3-4 mon. for CO from SH fires.

215 hPa

147 hPa

100 hPa

70 hPa

Jan 05 Jan 06 Jan 07 Jan 08 Jan 09

Velocity (cm/s)

Winter-spring(Dec-May) Summer-fall (Jun-Nov)

Pre

ssu

re(h

Pa)

Seasonality and vertical profile of upward transportin assimilated met. fields

Transport minimum at ~80 hPa, causing the maximum lag of CO peak from 100 hPa to 70 hPa.

Much slower transport of SH burning signal.

Max. in winter-springMin. in summer-fall

Vel

oci

ty (

cm/s

)

CO mixing ratio and anomaly at 215 & 70 hPa

MLSGEOS-4GEOS-5

• Much less CO has been lofted in GEOS-5.

• CO anomalies show comparable results in GEOS-4 and GEOS-5; Models match the seasonal cycles of MLS - CO tape recorder pattern depends more on phase of vertical transport.

• The model amplitudes are ~10-16 ppb smaller than the obs. at 70 hPa.

CO

mix

ing

rat

io (

pp

b)

CO mixing ratio (ppb)

CO

an

om

aly

(pp

b)

CO anomaly (ppb)

GEOS-5: vertical transport too weak

GEOS-5: 100-75 hPa - Transport too low in all seasons.

GEOS-4:Comparable vertical transport in fall below 90 hPa.

Weaker vertical transport in winter and spring below 90 hPa – possibly causing the damped seasonal cycle in the model

Yang et al., 2008

GEOS vertical velocities computed from mass divergence (zonal mean of 15°S-10°N)

Vertical velocity deduced from heating rates (obs. H2O and O3

Radiative transfer model)P

ress

ure

(h

Pa)

NH spring (MAM)NH summer (JJA)NH fall (SON)NH winter (DJF)

GEOS-4

GEOS-5Pre

ssu

re (

hP

a)

Conclusion• Semi-annual cycles of CO below 80 hPa in the TTL -

determined by the temporal overlapping of surface BB emissions from different continents.

• Transition from semi-annual to annual cycles around 80 hPa – induced by a combination of the signal at the tropopause and the annual cycle of BDC in the LS

• Strong interannual variation with a CO maximum in boreal fall 2006, mainly caused by fires in the Indonesian region.

• GEOS-5: too weak vertical transport, but matches the phase shift in transport, so similar tape recorder pattern.

• GEOS-4: possibly too weak vertical transport in winter and spring - decreasing the amplitude of the seasonal cycle.