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

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

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

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 1 - 2 deg decade have been reported in the total width of the tropical belt Seidel et al., Nature Geosc., 2008

Tropical belt: ∆TPP=20 mb Trend (° dec-1 ± 2) NCEP/NCAR ERA-40 JRA 1979-2001 1.1 ± 0.5 1.1 ± 0.6 0.8 ± 0.7 1979-2008 0.7 ± 0.4 0.3 ± 0.4

Tropical belt: ∆TPP=20 mb

Tropopause pressure gradient: | TPP|

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

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

Boulder WV: seasonal cycle | TPP| ∆TPP=20 mb Between 325 - 370 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.

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.

Tropical belt relative to Boulder

Conclusions Using ∆TPP=20 mb, tropical belt growth is ~ 1° dec-1 (1979-2001) ~ .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 (~320 - 370 K), although changes in mixing need to be taken into account No obvious trends in tropical belt at Boulder