Predictability of weather and climate

Abstract:

Circumglobal teleconnections from wave-like patterns in the mid-latitude jets can lead to synchronized weather extremes in the mid-latitudes of Northern and Southern hemispheres. The simultaneous occurrence of record breaking and persistent northern hemisphere temperature anomalies in Summer 2018 were previously discussed in the context of a persistent zonally elongated wave-7 pattern that stretched over large parts of the northern hemisphere over an extended time and let to considerable societal impacts. Various diagnostics have been put forward to quantify and detect such wave patterns, many of which incorporate low-pass time filtering to separate signal from noise. In this response we argue that advancing our understanding of the large-scale circulation’s response to anthropogenic climate change and reducing associated uncertainties in future climate risk requires a diverse range of perspectives and diagnostics from both the climate and weather research communities.

Abstract:

Model representations of the stratospheric semi-annual oscillation (SAO) show a common easterly bias, with a weaker westerly phase and stronger easterly phase compared to observations. Previous studies have shown that both resolved and parameterized tropical waves in the upper stratosphere are too weak. These waves propagate vertically through the underlying region dominated by the stratospheric quasi-biennial oscillation (QBO) before reaching the SAO altitudes. The influence of biases in the modelled QBO on the representation of the SAO is therefore explored. Correcting the QBO biases helps to reduce the SAO easterly bias through improved filtering of resolved and parameterized waves that contribute to improving both the westerly and the easterly phases of the SAO. The time-averaged zonal-mean zonal winds at SAO altitudes change by up to 25 % in response to the QBO bias corrections. The annual cycle in the equatorial upper stratosphere is improved as well. Most of the improvements in the SAO occur during the QBO easterly phase, coinciding with the period when the model's QBO exhibits the largest bias. Nevertheless, despite correcting for the QBO bias, there remains a substantial easterly bias in the SAO, suggesting that westerly wave forcing in the upper stratosphere and lower mesosphere is still severely under-represented.