Where is the equator?
A definition based on atmospheric composition
and its chemical implications
Christopher Holmes - Florida State University
Michael Prather
- UC Irvine
Methane
?
?
CFC-11
?
HIPPO I, Jan 2009
(Wofsy et al., 2011,Wecht et al., 2012)
?
ITCZ = Atmospheric equator
Textbook picture of ITCZ
• Surface convergence
• Convection & Rainfall
• Divergence in upper troposphere
Real circulations
• Variation across meridians
• Double/Complex ITCZ
• Regions without deep convection
• ITCZ ill-defined over land
(Satymurty et al., 1998)
• Surface convergence ≠ Rainbelt
(Nicholson et al., 2009)
NOAA GridSat,
Knapp et al., 2011
Hemispheric tracers defined
Emissions: e90N, e90S – Constant, 90-day lifetime
“Atmospheric equator”:
where e90N = e90S
Model:
Meteorology:
UCI CTM
ECMWF (cy. 36) T42 (~2.8°)
Sample Days
Surface-layer hemisphere tracers
Atmospheric equator
•
moves with the sun
• Monsoons
(Eq. @ 30°N in S. Asia)
Atmospheric Equator & ITCZ
Atmospheric equator &
IR Brightness temperature (proxy for cloud top height)
Tracer method :
• Corresponds with ITCZ,
where it’s well defined
•
Works over continents &
monsoon regions
•
Large daily variability, esp.
in Asian monsoon
NOAA GridSat,
Knapp et al., 2011
Implications for atmospheric chemistry:
Which hemisphere are emissions in?
SF6 emissions, 2008
Region where emissions may
be miscategorized
TransCom (Patra et al., 2009)
Emission Ratio, NH/SH
Geographic
32
Atmospheric
25
4% slower interhemispheric exchange
Implications for atmospheric chemistry:
CH4+OH oxidation gradient
Background:
Observations of CH3CCl3 imply that CH4 oxidation is faster in the SH than NH (Montzka et al., 2000)
CTMs/GCMs predict +28 ± 10% faster CH4 oxidation in the NH (Naik et al., 2013)
CH4 oxidation by OH (UCI CTM)
In UCI CTM…
•
CH4 oxidation is 25% faster in
geographic NH, but only 5%
faster in the atmospheric NH.
•
MCF obs. constrain the
atmospheric ratio, so CTMs
and GCMs need consistent
tests
GEOS-Chem vs. CH4 oxidation constraints
UCI CTM
GEOS-Chem
Observations
v9-01-02
v9-02
v10-01
RCP
Standard
Emissions
RCP
Resolution
T42 (~2.8°) 2°x2.5°
4°x5°
4°x5°
τCH4+OH, years
8.5
10.1
8.7
9.5
Geographic
25%
27%
23%
23%
Atmospheric
5%
12%
6.8%
6.7%
11.2 ± 1.3 (Prather et al., 2012)
NH/SH kCH4*
-15 ± 10% (Montzka et al., 2000)
-3 ± 12% (Patra et al., 2014)
Model developments in the last 2 years
have improved NH/SH kCH4, but not τCH4.
*Mass-weighted OH ratios differ from kCH4 ratios by ~5%
Conclusions
•
The atmospheric equator is not synonymous
with the ITCZ
•
2-Box models should define hemispheres
consistent with the atmosphere
•
3-D CTMs/GCMs should diagnose
hemispheric properties consistent with the
atmosphere
Vertical structure of the atmospheric equator
Eastern Pacific
The winter hemisphere typically
undercuts the summer hemisphere
India
Filamentary intrusions into opposite
hemisphere
Over India, cause monsoon dry spells
(Krishnamurthi et al., 2010)
CH4 vs. Hemispheric tracers
HIPPO I Southbound, Jan 2009
Hemispheric tracers gradients align
with gradients in CH4
Including slopes in upper
troposphere
Contours: log(e90N/e90S)
Model sampled at 160W,
not along flight track & day
Kort et al., 2011; Wecht et al., 2012
Where does air cross the atmospheric equator?
Smallest fluxes over equatorial continents (Africa, S America) and W. Pacific
Largest fluxes in in S. Asian monsoon & E. Pacific (shallow ITCZ)