CMIP6 Models Underestimate the Holton‐Tan Effect

Abstract

The Quasi-Biennial Oscillation (QBO) is a pattern of descending and alternating easterly and westerly winds in the tropical stratosphere that influences the atmospheric circulation globally (Gray et al., 2018). The QBO can enhance or suppress tropical convection (Son et al., 2017), change the strength and location of the jet-stream (Wang et al., 2018), and change the velocity of the polar stratospheric wind in the southern (Yamashita et al., 2018) and northern hemispheres (Lu et al., 2020). These large-scale remote atmospheric responses to the QBO are sensitive to its structure, its meridional extent (Hansen et al., 2013), its vertical extent (Andrews et al., 2019), how deep the QBO extends into the lower stratosphere (Collimore et al., 2003), and the phase of its easterly and westerly jets (Gray et al., 2018). Now with the Coupled Model Intercomparison Project 6 (CMIP6) models spontaneously generating the QBO, there is variability in how the models represent its structure (Richter et al., 2020), suggesting that there is variability in how the models represent the QBO teleconnections. This presents an opportunity to use multiple models to study one of the most extensively researched, yet still enigmatic, teleconnections associated with the QBO, the Holton-Tan effect (HTE, Holton & Tan, 1980).During early winter, if QBO westerlies are located in the middle stratosphere (10 hPa) and QBO easterlies in the lower stratosphere (50 hPa), hereafter referred to as QBOE, the polar stratospheric westerlies (polar vortex) are weaker than average (Holton & Tan, 1980). One explanation of how this happens emphasizes the QBO mean meridional circulation (QBO-MMC). The QBO-MMC acts as a residual mean meridional circulation that maintains the dynamically forced temperature response to the QBO against radiative relaxation (Hitchman et al., 2021;Pahlavan et al., 2021;Plumb & Bell, 1982). During QBOE, the westerly vertical wind shear below the 10 hPa QBO westerlies coincides with tropical warming, generated by descending motion.

Non‐Linear Response of the Extratropics to Tropical Climate Variability

“…Although the magnitude of the QBO generated by our model is less than observations, the model still generates a realistic QBO extratropical teleconnection which is critical for our study (Menary et al., 2018). However, it is likely that the results found here are model dependent, as it has been found that some models involved in CMIP6 are deficient at representing the QBO and hence its teleconnections (Elsbury et al., 2021; Richter et al., 2020).…”

North American cooling signature of strong stratospheric wave events depends on the QBO phase

“…Since the QBO in climate models has a weak amplitude at 50 hPa compared to reanalysis, here the QBO is indexed using the standardized seasonal 10 hPa zonal wind that is averaged between 5 • S and 5 • N (Elsbury et al 2021a). This works because of the anticorrelation between 10 hPa and 50 hPa QBO winds (Richter et al 2020, Anstey et al 2022b.…”

QBO/Solar Influences on the Tropical Madden‐Julian Oscillation: A Mechanism Based on Extratropical Wave Forcing in Late Fall and Early Winter

“…This would concentrate wave activity in the northern polar region (Figure S1a in Supporting Information S1), thereby weakening the vortex and strengthening the residual meridional circulation. There has been some support for this mechanism but much debate and the exact origin of the Holton-Tan effect continues to be the subject of ongoing research (Elsbury et al, 2021;Lu et al, 2020;Rao et al, 2020;Silverman et al, 2021;Watson & Gray, 2014;White et al, 2016). It has further been recognized since the work of Labitzke (1982Labitzke ( , 1987 and Labitzke and van Loon (1988) that major SSWs do not usually occur in early winter under SMAX conditions, but tend to be delayed until late winter, implying a secondary solar influence on the strength of extratropical wave forcing.…”