Discussion: The July 2018 quasi-biennial oscillation index (QBO) was -29.10 the strongest monthly easterly phase in the 1948-2018 climatology (Table 1). According to the London MET Office the QBO is a “regular variation of the strong winds that blow high above the equator reversing direction entirely every 14 months and completing the full cycle in 28-29 months making the QBO the most regular cycle in the atmosphere outside the cycle of the seasons.”
Table 1: The strongest monthly negative (easterly) quasi-biennial index occurring in the 1948-2018 climatology.
There are varying views on the influence of the QBO on climate. The QBO works in conjunction with other aspects of the climate to produce certain types of weather. However, the easterly QBO is historically attached to diminished hurricanes in the North Atlantic basin and increased susceptibility to high latitude blocking patterns during boreal winter causing increased mid-latitude snow and cold risk.
Climate Impact Company took a look at previous strong –QBO regimes and found that hurricanes were diminished in the North Atlantic if –QBO was accompanied by El Nino (1994) whereas –QBO and neutral ENSO or La Nina presence generally was associated with active hurricane seasons. The strong –QBO does not seem to interfere with the established ENSO relationships to North Atlantic hurricane activity except strong –QBO does have a tendency to push an active hurricane season out of the deep tropics to the subtropics as observed in 2005. The risk of cold winters in the U.S. and Europe increases when strong –QBO is present if the North Atlantic is cooler than normal which was common during the long-term cool cycle of the Atlantic multi-decadal oscillation 1965-1994.
QBO gained recognition as a climate predictor in 1984 by William Gray, renowned seasonal tropical cyclone forecaster at Colorado State University. However, CSU dropped the QBO as a predictor in 2007 as the reliability of this index to tropical cyclone activity weakened. Speculation is the QBO relationship to tropical cyclone activity may have changed due to the equatorial convection patterns adjusting to persistent warmer ocean temperatures during the past 20 years.
Currently, the North Atlantic basin sea surface temperature anomaly (SSTA) is +0.12C which is very close to normal. However, the basin-wide observation is an average of dramatic cooling south of Greenland and in the North Atlantic subtropics with very warm SSTA east of the U.S. Mid-Atlantic/Northeast region. Forecast models predicted a robust warm pattern for the North Atlantic basin this summer season and that regime has failed to develop. The cool SSTA south of Greenland occurring at the same time the tropics are cooler than normal is characteristic of the cool cycle of the Atlantic multi-decadal oscillation (AMO). The long-term cycle of the AMO flipped to the warm phase in the middle 1990’s. Is this decadal oscillation changing phase (Fig. 1)?
Fig. 1: The North Atlantic basin entered the long-term warm cycle in the mid-to-late 1990’s but speculation is developing that a reversal toward the cool phase is possible.
If the cooler North Atlantic hangs on the remainder of summer and continues to cool relative to normal entering winter the reasonable is anticipation of increased risk of high latitude blocking high pressure patterns causing mid-latitude cold and snow. Interestingly, most climate models are currently forecasting a mild El Nino winter for much of the northern hemisphere.