Abstract
A strong relationship between the quasi-biennial oscillation (QBO) of equatorial stratospheric winds and the amplitude of the Madden-Julian oscillation (MJO) during the boreal winter has recently been uncovered using observational data from the mid-1970s to the present. When the QBO is in its easterly phase in the lower stratosphere, it favors stronger MJO activity during boreal winter, while the MJO tends to be weaker during the westerly phase of the QBO. Here we show using reconstructed indices of the MJO and QBO back to 1905 that the relationship between enhanced boreal winter MJO activity and the easterly phase of the QBO has only emerged since the early 1980s. The emergence of this relationship coincides with the recent cooling trend in the equatorial lower stratosphere and the warming trend in the equatorial upper troposphere, which appears to have sensitized MJO convective activity to QBO-induced changes in static stability near the tropopause. Climate change is thus suggested to have played a role in promoting coupling between the MJO and the QBO.
Highlights
The Madden-Julian oscillation[16] dominates tropospheric tropical subseasonal variability and is characterized by a large-scale (~10,000 km) region of enhanced deep convection, coupled to a zonal overturning circulation through the depth of the troposphere, which together propagate eastward along the equator with a local period of 30–70 days and an eastward phase speed of ~5 ms−1
We define the Madden-Julian oscillation (MJO) using the Realtime Multivariate MJO (RMM) index derived by Wheeler and Hendon[36], hereafter WH, which are formed using satellite-observed outgoing longwave radiation (OLR) and zonal winds at 200 and 850hPa from the National Centers for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) Reanalysis[37]
Synthetic data for 60-year periods demonstrates that an upward trend of MJO amplitude during QBOE since 1959, which increases as observed by ~2 standard deviations over the period 1959–2014, can account for the strengthening of the negative correlation from ~−0.2 for the 1959–1988 period to ~−0.7 for 1985–2014 (Supplementary Fig. S5)
Summary
The Madden-Julian oscillation[16] dominates tropospheric tropical subseasonal variability and is characterized by a large-scale (~10,000 km) region of enhanced deep convection, coupled to a zonal overturning circulation through the depth of the troposphere, which together propagate eastward along the equator with a local period of 30–70 days and an eastward phase speed of ~5 ms−1. We explore the boreal winter MJO-QBO relationship by computing the correlation of December-February mean MJO amplitude with the QBO index in a sliding 30-year window (Fig. 2).
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