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. Get the paper in pdf format as it appears in J.Climate
A more recent manuscript on this topic: "Are EOF-based descriptions of the Madden-Julian Oscillation relevant to its role in ENSO?"
Dr. William S. Kessler
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA
