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QBO@60 webinar series

In lieu of the QBO@60 workshop in July 2021, an online seminar took place on 6 July 2021 that featured two invited talks celebrating the discovery of the QBO. The invited speakers were Kevin Hamilton and Mike Wallace.

The recording of the seminar can be found here.

The discovery of the QBO and related developments, 1883-1961
Kevin Hamilton, International Pacific Research Center, University of Hawai`i at Mānoa

In the last months of 1883 a global sensation was spawned by the sudden appearance of spectacular sunrises and sunsets throughout the world (e.g. see Simon Winchester’s 2003 bestseller “Krakatoa – the Day the World Exploded”). The effort to demonstrate the connection between the colorful twilight phenomena and the August 1883 eruption of Krakatau in Indonesia led to the discovery of the first atmospheric jet stream – in this case a very strong easterly jet at roughly 25 km altitude and completely circling the equator. At the time almost nothing else was known about the atmosphere much above ground level. However, the 1890’s saw the beginning of the era of systematic scientific exploration of the atmosphere with unmanned balloons. By 1902 the existence of the stratosphere was established. Scattered balloon observations in the tropics in the ensuing years showed that at times the zonal winds in the equatorial lower stratosphere could feature both layers of easterlies and layers of westerlies. These observations were interpreted as reflecting the presence of a very narrow westerly jet within a broad and robust “Krakatoa easterly” regime. Progress in understanding of the stratospheric circulation was boosted in the 1950’s by the observations taken in support of the extensive nuclear weapons tests conducted in the tropical Pacific. These observations demonstrated that even a zeroth order view of the circulation in the tropical stratosphere must include the interannual reversals of the very strong prevailing zonal winds. In 1961 groups in the US and the UK independently published papers that showed the presence of a quasi-repeatable oscillation of period near two years and that had a characteristic downward phase propagation. This discovery was completely surprising – indeed initially utterly mystifying – and inspired many of the most important developments in dynamical meteorology in the ensuing 6 decades. This talk will review the history through 1961 and highlight the contributions from some now rather forgotten amateur and professional scientists.

Impact of the discovery of the QBO upon research in the subsequent decade
Mike Wallace University of Washington

The period following the discovery of the QBO in 1960, was marked by theoretical advances relating to the propagation of planetary waves on an equatorial beta-plane and their interactions with the zonal mean background flow. Another important development was the identification of vertically propagating planetary waves in the equatorial lower stratosphere. Events during the period 1966-68 followed one another in such quick succession that the time sequence is fuzzy. In terms of this symposium, what’s remarkable is that they culminated in Lindzen and Holton’s theoretical explanation of how the QBO works. Their first paper, published in 1968, describes how vertically propagating gravity waves give up their momentum to the basic state zonal flow in easterly and westerly shear zones, causing them to descend. Though it involved what appeared at the time to be some questionable assumptions about equatorially trapped planetary waves and how they interact with the mean flow, not only has the Lindzen and Holton mechanism endured: its validity is more widely acknowledged now than when it was first proposed. With this history as a backdrop, I will share a few reminiscences of the events that transpired back at that time and a few reflections on what we’ve learned about the QBO and related phenomena in the years since then.