The Quasi-Biennial Oscillation in Model-Simulated and Observed Equatorial Temperature of the Lower Stratosphere
In an otherwise thorough survey of metrics for models that simulate the Quasi-Biennial Oscillation, Schenzinger et al. say “there is no comparable observational dataset” to compare with model-simulated stratospheric temperatures (Geosci. Model Dev. 10, 2157-2168, 2017). They compare model temperatures only with reanalysis datasets, which come from numerical weather prediction models and therefore are not direct observations. But stratospheric brightness temperatures have been directly observed by microwave radiometers on board a series of satellites since 1979. Christy and Drouilhet (J. Climate 7, 106-120, 1994) found a QBO signal in 1979-1992 data from the Microwave Sounding Unit’s lower stratospheric Channel 4 (with peak weighting function at the 75hPa pressure level and half-power values at 120 and 40hPa, about 20±5 km altitude). To detect the QBO signal, Christy and Drouilhet used a latitude weighting that matches the QBO’s anti-correlation between the Equator and ±15°. Fernandez et al. also found a QBO signal in 1979-2000 MSU data, using EOF analysis (J. Climate 17, 3934-3946, 2004). Recently, Christy and Covey extended Christy and Drouilhet’s analysis through 2017 (“Stratospheric Temperature,” Section 2b5 in State of the Climate in 2017. Bull. Amer. Meteor. Soc. 99, S150–S152; Figure 2.10c). The QBO signal was again clearly visible. Although temperature trends for the lower atmosphere are sensitive to inter-satellite calibration and other processing details, giving rise to some controversy, data for the lower stratosphere are more robust. Christy and Covey’s Figure 2.9 shows that data processed separately by the National Oceanic and Atmospheric Administration (NOAA) and by Remote Sensing Systems (RSS) gives a QBO index that is virtually identical to that of Christy’s group at the University of Alabama at Huntsville (UAH).