The Height of the Midlatitude Tropopause: The Role of Moisture

This study proposes a moist dynamical constraint for the midlatitudes that relates the mean plus two standard deviations of the equivalent potential temperature in the lower troposphere to the (equivalent) potential temperature at the tropopause level. This moist dynamical constraint is derived in the framework of the Statistical Transformed Eulerian Mean, which approximates the zonal mean atmospheric circulation in arbitrary vertical coordinates. It emphasizes the role of the ascending branch of the midlatitude storm tracks that carries the low-level poleward-moving warm moist air parcels from the subtropics into the midlatitude upper troposphere lower stratosphere.

The proposed moist dynamical constraint is first explored using four reanalysis datasets including the ERA-Interim, the NCEP/DOE Reanalysis II, the NCEP Climate Forecast System Reanalysis and the 20th Century Reanalysis version 2. It's found that the proposed moist dynamical constraint is successful in capturing the annual cycle for both the two hemispheres, with very close to one correlation coefficient and close to one linear regression coefficient. The results are robust among different reanalysis datasets except for the 20th Century Reanalysis which tends to over-estimate the tropopause height. Furthermore, the proposed hypothesis also works well in obtaining the inter-annual variability, especially for Northern Hemisphere summer with above 0.6 correlation. On the contrary, the dynamical constraint is rather weak in explaining the inter-annual variability in the Southern Hemisphere and fails to work for Northern Hemisphere winter. This work suggests the important role of the moist dynamics in determining the midlatitude tropopause, especially in Northern Hemisphere summer.

Furthermore, the moist hypothesis is also examined in an ensemble of coupled climate models that participate in the Coupled Model Intercomparison Project phase 5 (CMIP5). The CMIP5 models tend to simulate smaller potential temperature at the tropopause (i.e. cold bias) as compared to reanalysis datasets for both summer and winter for both hemispheres. And this is almost always associated with smaller equivalent potential temperature at low levels (i.e. dry and cold bias), especially in Northern Hemisphere summer. In addition, several models under-estimate the one-to-one relationship in the annual cycle, which suggests possible biases in midlatitude moist convection simulations.