1. Changes in the tropical belt and their potential effect on trace gas distributions in the UTLS
Sean Davis1,2, Karen Rosenlof1
1NOAA Earth System Research Laboratory, Chemical Sciences Division
2Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado at Boulder

2. Overview 1. How is the tropical belt changing?
Update of previous work
Tropopause-based metrics
Post-2000 slowdown in tropical belt growth
Scherer et al., ACP, 2009
2. How do these changes impact UTLS tracer distributions?
E.g., H2O balloon record at Boulder

3. Tropical belt metrics
| TPP|
∆TPP=20 mb

4. Tropical belt trends Trends ~ 0.5-2º dec-1
Observational trends > model trends
(Johanson and Fu, J. Clim., 2009)
Trends largest in summer
Change in SH > NH
(Lu et al., GRL, 2009, others)
Trends on the order of deg decade have been reported in the total width of the tropical belt
Seidel et al., Nature Geosc., 2008

5. Tropical belt: ∆TPP=20 mb
Trend (° dec-1 ± 2)
NCEP/NCAR
ERA-40
JRA
1.1 ± 0.5
1.1 ± 0.6
0.8 ± 0.7
0.7 ± 0.4
0.3 ± 0.4

6. Tropical belt: ∆TPP=20 mb

7. Tropopause pressure gradient: | TPP|

8. Latitude of maximum | TPP|
NOTE, no statistically significant trends in any of the data sets in terms of TPP.

9. Maximum | TPP| trends
This is a plot of the magnitude of the maximum tropopause pressure gradient. Note that there appear to be trends in the SH and not in the NH, with the SH gradient becoming stronger in the NCEP and JRA reanalysis

10. Boulder WV: seasonal cycle
| TPP|
∆TPP=20 mb
Between K, air is in the troposphere at Boulder for part of the year when the ‘tropical belt’ is to the north of Boulder. Moving tropical belt more to the north would presumably lead to an increase in WV over Boulder at these levels.

11. Boulder WV: seasonal cycle
Effective diffusivity -- here is a proxy for mixing. High values = higher mixing, low values = barrier regions/low mixing. Note the tropopause barrier is evident here.

12. Tropical belt relative to Boulder

13. Conclusions Using ∆TPP=20 mb, tropical belt growth is
~ 1° dec-1 ( )
~ .5° dec-1 (1979-present)
Slower post-2000
Larger in SH than in NH
Preliminary results using maximum | TPP|
No change in latitude of max | TPP|
Increase in SH | TPP|
Seasonal cycle in Boulder WV and tropical belt related
Poleward tropical belt displacement would presumably increase WV at some levels (~ K), although changes in mixing need to be taken into account
No obvious trends in tropical belt at Boulder