Lagged Strengthening of the Tropical Madden-Julian Oscillation Following Early Winter Sudden Stratospheric Warmings: Observational Analyses and CMIP6 Model Comparisons

In addition to the well-known warming at high latitudes, sudden stratospheric warmings (SSWs) produce cooling and reduced static stability in the tropical lower stratosphere.  Based on 40 years of ERA5 reanalysis data and MJO amplitude data compiled at the U.S. National Oceanic and Atmospheric Administration, if these events occur in early winter (prior to ~ mid-January), the reduced static stability is sufficient to produce a statistically significant, lagged strengthening of the MJO peaking about 25 days after the SSW central date. Late winter SSWs produce no detectable strengthening.  This may be due to the timing of the SSW in early winter when tropical lower stratospheric temperatures and static stabilities are approaching their climatological minima.  This produces lower static stabilities than can occur following a late winter SSW when lower stratospheric temperatures are already higher and rising.  Positive feedbacks from MJO convection-induced temperature anomalies, cloud-radiative effects, and increased tropospheric Rossby wave amplitudes acting to further increase tropical upwelling rates, may further enhance MJO amplitudes.

The lagged strengthening of the MJO following early winter SSWs is also found in at least one climate model simulation in the CMIP6 archive (MRI-ESM-2.0).  We have so far analyzed in detail three full ensemble members (453 model years) of the 4xCO₂ forcing version, which has relatively low climatological static stabilities in the tropical lower stratosphere.  Because of the large number of model years analyzed, the lagged strengthening is statistically robust but is weaker and occurs at a shorter time lag of 10-15 days than is estimated from the available observations.  Using the large number of available early winter SSWs, it is found that those SSWs that produce the largest reductions in static stability in the tropical lower stratosphere (70 to 100 hPa) also produce the largest lagged strengthenings of the MJO.  This supports a top-down static stability mechanism for producing the strengthening. Analyses of data from other climate models in the CMIP6 archive are in progress.

Because early winter SSWs occur primarily under easterly quasi-biennial oscillation (QBO) conditions and late winter SSWs occur most often under westerly QBO conditions, these results have implications for the origin of the observed modulation of the MJO by the stratospheric QBO.  Extratropical wave forcing events (including minor as well as major warmings) are typically stronger in early winter under easterly QBO conditions, as was originally reported by Holton and Tan (1980).  These events will reduce tropical lower stratospheric static stability primarily in boreal winter when the QBO-MJO connection is observed.  While most MJO convection extends only to lower altitudes in the troposphere, it is mainly the strongest MJO events that are modulated by the QBO.  These latter events may extend to higher altitudes and be more affected by stability conditions in the lowermost stratosphere.