Kessler and Kleeman 1999 Abstract

Rectification of the Madden-Julian Oscillation into the ENSO cycle

Kessler, W.S. and R. Kleeman

Journal of Climate, 13, 3560-3575.

An ocean general circulation model, forced with idealized, purely oscillating winds over the western equatorial Pacific similar to those observed during the Madden-Julian Oscillation, developed rectified low-frequency anomalies in SST and zonal currents, compared to a run in which the forcing was climatological. The rectification in SST resulted from increased evaporation under stronger-than-normal winds of either sign, from correlated intraseasonal oscillations in both vertical temperature gradient and upwelling speed forced by wind stress, and from zonal advection due to nonlinearly-generated equatorial currents. The net rectified signature produced by the MJO-like winds was SST cooling (about 0.4°C) in the west Pacific, and warming (about 0.1°C) in the central Pacific, thereby flattening or reversing the background zonal SST gradient. It is hypothesized that, in a coupled system, such a pattern of SST anomalies would tend to spawn additional westerly wind anomalies as a result of SST-induced changes in the low-level zonal pressure gradient. This was tested in an intermediate coupled model initialized to 1 January 1997, preceding the 1997-98 El Niño. On its own, the model hindcast a (relatively weak) warm event, but when the effect of the rectified SST pattern was imposed, a coupled response produced the hypothesized additional westerlies and the hindcast El Niño became about 50% stronger (measured by east Pacific SST anomalies), suggesting that the MJO can interact constructively with the ENSO cycle. This implies that developing the capacity to predict, if not individual MJO events, then the conditions that affect their amplitude, may enhance predictability of the strength of oncoming El Niños.

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A more recent manuscript on this topic:  "Are EOF-based descriptions of the Madden-Julian Oscillation relevant to its role in ENSO?"

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