On The Relationship between the QBO and Tropical Deep Convection

The height and amount of tropical deep convection are examined for a correlation with the stratospheric quasi-biennial oscillation (QBO). A new 23-yr record of outgoing longwave radiation (OLR) and a corrected 17-yr record of the highly reflective cloud (HRC) index are used as measures of convection. When binned by phase of the QBO, zonal means and maps of OLR and HRC carry a QBO signal. The spatial patterns of the maps highlight the QBO signal of OLR and HRC in typically convective regions. Spectral analysis of zonal mean OLR and HRC near the equator reveals significant peaks at QBO frequencies. Rotated empirical orthogonal function (REOF) analysis is used to determine if ENSO variations of convection are aliased into the observed QBO signals. Some analyses are repeated using the OLR record after ENSO REOF modes have been removed, yielding very similar results compared to the original analyses. It appears that the QBO signal is distinct from the ENSO signal, although the relative brevity of the OLR and HRC records with respect to the ENSO cycle makes assessing the impact of ENSO difficult. Three mechanisms that can link the QBO with deep convection are investigated: 1) the QBO modulation of tropopause height may allow convection to penetrate deeper in some years compared to other years; 2) the QBO modulation of lower-stratospheric to upper-tropospheric zonal wind shear may result in cloud tops being “sheared off” more in some years than in other years; 3) the QBO modulation of upper-tropospheric relative vorticity may relax dynamic constraints on cloud-top outflow and thus allow more cloud growth in some years compared to other years. Measures of these mechanisms—tropopause pressure and temperature, 50–200-hPa zonal wind shear (cross-tropopause shear), and 150-hPa vorticity, all from the NCEP reanalyses—are compared to OLR and HRC. QBO fluctuations of convection are generally well correlated with QBO fluctuations of tropopause height. In regions where these height fluctuations are relatively small, convective fluctuations are well correlated with QBO variations of cross-tropopause shear, especially during boreal summer and winter when convection is concentrated away from the equator and the largest tropopause height fluctuations. In fact, during summer the shear mechanism appears to dominate such that QBO-related convective behavior is different than during the other seasons. QBO convective behavior is uncorrelated with vorticity fluctuations near the tropopause. A secondary component of this study is the description of a new, long-term OLR dataset. Using measurements from Nimbus-6, Nimbus-7, and the Earth Radiation Budget Satellite (ERBS), the 23-yr OLR record analyzed in this study was constructed. This record has fewer interannual biases due to satellite differences than the well-known NOAA OLR record and, therefore, is more useful for studies of interannual meteorological variations.