sea salt over the southern ocean - harvard...
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NOAA NWS Collection
Sea salt over the Southern Ocean
Sea salt aerosols are a major contributor to aerosol mass, but emissions remain highly uncertain
Global source (Tg/yr) Reference 5,000 (1,200-20,000) Lewis & Schwartz, 2004
5,400 Alexander et al., 2005: Monahan et al. (1986) 2 size bins (0.1-0.5 and 0.5-10µm); GEOS-STRAT u
3.41 10m
8,000 GEOS-4 (2003-2006) 13,000 Based on MODIS AOD
MODIS AOD: 20°S-80°S (2003-2006)
Coarse AOD QuikSCAT Total AOD MODIS Aqua
m/s Collection 5 (cloud fraction<50%)
60-80% of AOD from coarse aerosols
Ifremer/CERSAT
GEOS-Chem AOD GEOS-4 winds Sea-salt + dust
Surface winds
Model underestimates MODIS coarse AOD by 30% but good corr. (r2=0.74)
Aqua AOD Aqua AOD Model AOD
Windspeed dependence 2003-2006 daily AOD grouped in 1 m/s wind bins (620,000 points)
Eliminate cloud fraction>50% + dust>10% AODcoarse
AOD co
arse
AODcoarse = f(u10)
Infer higher sea salt emissions for winds <12 m/s but lower emissions at very high winds (>16 m/s)
Windspeed (m/s) Se
a salt
emiss
ions
(kg
km-2
d-1)
Emis(sea salt) = f(u10) Aqua emis ∝1.87u2.00 (r=0.62) Model emis ∝0.06u3.27 (r=0.97)
Windspeed (m/s)
20°S-80°S
τss~12h
AODcoarse(obs) x AODcoarse(model) Emis (model)
Implications Let’s apply this empirical f(u2) relationship to daily QuikSCAT winds calculate global sea salt emissions
Southern Ocean (20º-80ºS)
Global
GEOS-Chem 4,800 8,000 MODIS Aqua 6,200 12,900
Sea salt emissions (Tg/year) Sea salt emissions
Inferred global sea salt emissions 60% larger than modeled Largest discrepancy for tropical latitudes with weak winds (4-7m/s). Is
there a dependence on factors other than wind?
MODIS Aqua
GEOS-Chem
Global distribution of emissions
Top-down sea salt emissions GEOS-Chem sea salt emissions
Model/Satellite comparison
Coarse AOD Windspeed
• Model underestimates observed AOD by ~30% on average • Good agreement GEOS-4/QuikSCAT winds
20°S-80°S