Variability of zonal mean tropical temperatures derived from a decade of GPS radio occultation data

Abstract

Variability in tropical zonal mean temperatures over 10-30 km is analyzed based on high-quality, high-vertical-resolution GPS temperature measurements covering 2001-13. The observations are used to quantify variability spanning time scales of weeks to over a decade, with focus on behavior of the tropopause region and coupling with the upper troposphere and stratosphere. Large variations associated with the seasonal cycle, quasi-biennial oscillation (QBO), and El Niño-Southern Oscillation (ENSO) are isolated and removed, and residual time series are analyzed using principal components and spectrum analysis. The residual temperature exhibits maximum variance in the lower stratosphere, with a vertical structure similar to the seasonal cycle. Residual temperatures exhibit two dominant modes of variability: a "deep stratosphere mode" tied to high-latitude planetary wave forcing and a shallow "near-tropopause mode" linked to dynamically forced upwelling near the tropopause. Variations in the cold point tropopause (and by inference in global stratospheric water vapor) are closely tied to the near-tropopause mode. These coherent temperature patterns provide further evidence of distinct upper and lower branches of the tropical Brewer-Dobson circulation. Zonal mean temperatures in the lower stratosphere and near the cold point are most strongly coupled to the upper troposphere on time scales of ~(30-60) days, probably linked to the Madden-Julian oscillation (MJO). Enhanced temperature variance near the tropopause is consistent with the long radiative relaxation time scales in the lower stratosphere, which makes this region especially sensitive to low-frequency dynamical forcing.