Wintertime influence of the Madden-Julian Oscillation on mid-latitude precipitation via changes in the thermodynamic balance

The Madden-Julian Oscillation (MJO) has a strong influence on daily precipitation in Southwest Asia during winter. Here, the influence is considered in terms of changes in vertical velocity as diagnosed through calculating the terms of the thermodynamic energy equation from observational data.

The MJO has its largest effect on tropical rainfall in the eastern Indian Ocean. When the tropical rainfall is affected in this region, the associated changes in jet-level winds extend to the northwest over the Southwest Asia region, exerting an opposite influence on the synoptic storms moving into that region during the cold season. In a seven-year daily station record for Afghanistan, precipitation was 55% greater during the negative phase of the Madden Julian Oscillation than during the positive phase. The occurrence of extreme events was also affected, with the 10 wettest days all occurring during the negative phase, and an apparent relationship noted with flooding events.

To better understand this out-of-phase relationship between the MJO-related tropical convection and the changes in precipitation over Southwest Asia, changes in vertical velocity are examined within the context of the thermodynamic balance as the circulation and temperature anomalies of the MJO intersect the wintertime subtropical jet stream over Asia. As expected, vigorous upward motion occurs in the region of enhanced tropical convection in the eastern Indian Ocean when the MJO is in its positive phase. Interestingly, however, a broad region of equally vigorous subsidence occurs over Southwest Asia. This strong subsidence can be expected to suppress local precipitation and thus is a key dynamical aspect of the MJO influence. The forcing of the subsidence, however, which is considerably in excess of what might be expected by “Hadley”-type forcing from the tropical convection, is not immediately obvious.

To examine the factors associated with the vigorous anomalies in subsidence over Southwest Asia, the terms of the hydrostatic thermodynamic equation (tendency, horizontal temperature advection, diabatic heating, and the product of static stability and vertical velocity) are calculated from the NCEP reanalysis data and composited by the phase of the MJO. When the tendency term is small and static stability is approximately constant (as is the case here) vertical velocity is balanced by diabatic heating and temperature advection. The MJO variability is associated with vigorous tropical convection in the eastern Indian Ocean which results in first-baroclinic, Rossby-like wind and temperature anomalies extending over Southwest Asia. The interaction of the MJO circulation and the mean flow result in both advection of the MJO temperature anomalies by the mean wind and advection of the mean thermal gradient by the MJO wind anomalies. Via the thermodynamic energy equation, both these temperature advection terms are primary contributors to subsidence over Southwest Asia. Thus, a dynamical link may be identified between MJO-related tropical convection in the eastern Indian Ocean and suppressed precipitation over Southwest Asia: the Rossby-like wind and temperature response to the MJO tropical convection impinges on the westerlies over Southwest Asia, the resultant changes in the thermodynamic balance result in strong subsidence over Southwest Asia, and this subsidence suppresses local precipitation.