Arctic Sea Ice Loss, Long‐Term Trends in Extratropical Wave Forcing, and the Observed Strengthening of the QBO‐MJO Connection

Abstract

A modulation has been identified of the tropical Madden‐Julian oscillation (MJO) by the stratospheric quasi‐biennial oscillation (QBO) such that the MJO in boreal winter is ∼40% stronger and persists ∼10 days longer during the easterly QBO phase (QBOE) than during the westerly phase. A proposed mechanism is reductions of tropical lower stratospheric static stability during QBOE caused by (a) the QBO induced meridional circulation; and (b) QBO influences on extratropical wave forcing of the stratospheric residual meridional circulation during early winter. Here, long‐term variability of the QBO‐MJO connection and associated variability of near‐tropopause tropical static stability and extratropical wave forcing are investigated using European Center reanalysis data for the 1959–2021 period. During the most reliable (post‐satellite) part of the record beginning in 1979, a strengthening of the QBO‐MJO modulation has occurred during a time when tropical static stability in the lowermost stratosphere and uppermost troposphere has been decreasing and extratropical wave forcing in early winter has been increasing. A high inverse correlation (R = −0.87) is obtained during this period between early winter wave forcing anomalies and wintertime tropical lower stratospheric static stability. Regression relationships are used to show that positive trends in early winter wave forcing during this period have likely contributed to decreases in tropical static stability, favoring a stronger QBO‐MJO connection. As shown in previous work, increased sea level pressure anomalies over northern Eurasia produced by Arctic sea ice loss may have been a significant source of the observed positive trends in early winter wave forcing.

Lagged Response of MJO Convection and Precipitation to Solar Ultraviolet Variations on Intraseasonal Time Scales

“…Although the official OMI record only extends back to 1979, a reconstruction of the index back to 1974 using the NOAA PSL OLR data and the 1979-2012 EOFs was performed using the "mjoindices" Python package developed by Hoffmann et al (2021). For the pre-satellite (1959)(1960)(1961)(1962)(1963)(1964)(1965)(1966)(1967)(1968)(1969)(1970)(1971)(1972)(1973) period, the same package was applied to the ERA5 OLR data (see, e.g., Hood & Hoopes, 2023; their Section 2).…”

“…Positive feedbacks from below including cloudradiative feedbacks (Sakaeda et al, 2020;Son et al, 2017) and QBO influences on the MJO-induced cold cap anomaly ahead of MJO convection (Hendon & Abhik, 2018;Kodera et al, 2023;Lim & Son, 2020) are likely necessary to explain the full amplitude of the observed modulation. Several studies have found that the QBO-MJO connection has strengthened significantly since the early 1980s, coinciding with long-term decreases in tropical static stability (Hood & Hoopes, 2023;Klotzbach et al, 2019;Sakaeda et al, 2020).…”