Thorough Survey of Zonal Mean Influence of the Stratospheric QBO on the Tropospheric Circulations and Convection

This study investigates downward influence of the stratospheric QBO on the tropospheric circulations and convection in the zonal-mean perspective and its statistical significance for neutral ENSO periods (the periods without El Nino or La Nina events) during 1979-2015, with the monthly mean datasets of NOAA/OLR, GPCP precipitation, and ERA-Interim reanalysis datasets (JRA-55 and MERRA as well).

The QBO phase is defined with the first two principal components (PCs) derived from the EOF analysis of the zonal-mean zonal wind in the equatorial stratosphere, in order to obtain two groups of data samples with opposite QBO phase in the two-dimensional PC phase space. The central value, θ_c, and the range, Δθ, of the phase angle in the polar coordinate are introduced to define the two groups with θ_c and θ_c+180°. We perform a composite analysis of any quantity for the two groups and determine statistical significance of the composite difference by a two-sided Student’s t-test. The parameters θ_c and Δθ are swept from 0° to 180° with an 1° interval to conduct a thorough survey of the QBO influence on the troposphere.

During the Austral summer (December-February), statistically significant composite differences exist in the troposphere over six areas, including the ones consistent with some previous findings; (1) the zonal wind and temperature differences associated with the QBO modulation of the polar vortex, known as the “Holton-Tan relationship” and (2) the zonal wind difference extending from the tropical lower stratosphere into the upper troposphere over the northern midlatitudes. The other four significant differences are (3) the vertical wind, OLR, and precipitation differences in the tropics, (4) the temperature differences in both subtropics in the upper troposphere, together with the vertical wind difference beneath the temperature difference in the southern hemisphere, (5) the precipitation difference with the zonal and vertical wind differences near the south pole, and (6) the zonal wind differences associated with the jet shift in the southern midlatitudes, together with the difference of the temperature in the lowermost stratosphere and the vertical wind in the lower troposphere. This study reveals some statistically significant differences of the vertical wind and measures of moist convection in the troposphere together with the corresponding zonal wind differences.

During the Boreal summer (June-August), when little literature has reported the QBO-related modulations in the troposphere, statistically significant composite differences in the troposphere are found over four areas, together with the statistically significant differences of the precipitation. The most noticeable difference is (1) the zonal wind differences associated with the jet shift in the southern midlatitudes, together with the temperature differences in the lowermost stratosphere and the vertical wind differences extending from the lower stratosphere into the surface. The other differences are (2) the zonal and vertical wind differences below the midlatitude jet core in the northern hemisphere, together with the temperature difference in the lowermost stratosphere, (3) the zonal and vertical wind differences near the south pole, and (4) the zonal and vertical wind differences near the north pole extending from the lower stratosphere into the surface, together with the temperature differences in the lowermost stratosphere.