We have shown that an ocean general circulation model, forced with purely oscillating intraseasonal wind stresses similar to those observed during the Madden-Julian oscillation, developed rectified low-frequency anomalies in its SST and zonal currents, compared to a run in which the forcing was climatological. The rectification resulted from three main processes: first, evaporative cooling under strong winds of either sign [section 3b(1)]; second, an eastward equatorial current generated nonlinearly due to advection of wind input momentum by Ekman convergence/divergence [section 3b(2)]; and third, changes in the vertical temperature profile so that Ekman upwelling is correlated with the vertical temperature gradient and therefore produces a net cooling under oscillating winds [section 3b(3)].
The overall signature of the rectified anomalies in the OGCM was low-frequency SST cooling (about 0.4°C) in the western Pacific under the strong MJO winds due to both evaporation and vertical advection, and weaker warming (up to 0.1°C) in the central Pacific due to zonal advection acting on the background SST gradient (Fig. 3). One might expect that, in a coupled system, such a pattern of zonally contrasting SST anomalies (reducing or reversing the background SST gradient) 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 hindcasts a relatively weak warm event, but when the rectified SST pattern seen in the OGCM was imposed, the hindcast El Niño became about 50% stronger (measured by east Pacific SST anomalies, Fig. 7, bottom) as a coupled response did in fact produce the hypothesized additional westerlies.
A series of model runs examined the sensitivity of the OGCM rectification to intraseasonal forcing of various forms (section 3c). The relative importance of evaporation versus advection was qualitatively estimated by comparison with an additional OGCM run in which only the wind stress (not the speed) was modified by the oscillating high-frequency winds. This suggested that on the equator, the two processes produce about equal rectified SST changes in this model, although since evaporation is due to the wind directly, while advection is due to equatorial dynamics, evaporation in these experiments (and probably in reality as well) affected a wider meridional region and therefore was more important to SST overall. However, it is difficult to go beyond this qualitative assessment given the crudity of the idealized MJO winds and the weaknesses of the OGCM itself. Model runs exploring the parameter space defining the MJO (section 3c) showed that the SST rectification was insensitive to MJO propagation speed and produced similar patterns for all choices of forcing period within the intraseasonal band. Varying the amplitude of the stress forcing showed that the nonlinear terms (especially zonal advection) were much more effective at higher-stress amplitudes (larger than 3 × 10-2 N m-2). On the other hand, evaporation depends on the square root of wind stress and therefore is less sensitive to the forcing amplitude. For these reasons the evaporative cooling under the strong winds is the most robust feature of the rectification found in these experiments. Since 2-3 × 10-2 N m-2 is a typical value for the stress amplitude of the coherent, eastward-propagating part of the intraseasonal signal (Shinoda et al. 1998), this suggests that moderate or stronger MJO events should be sufficient to produce the effects noted here. Finally, it has been noted (e.g., Hendon et al. 1999) that perhaps half the intraseasonal variance over the west Pacific is spatially incoherent, often associated with the convection-favorable phase of the MJO but with smaller scales. Artificial small-scale intraseasonal wind fields were constructed to force the OGCM; the SST fields resulting from these runs had the same character of cooling under the strong winds in the west and weakly warming the central Pacific. Since the actual MJO occurrences over the west Pacific include both the coherent eastward-propagating mode and additional incoherent variability (both of which resulted in similar tendencies in the OGCM), a typical MJO should produce a significant rectified SST signature as found here. Overall, these model experiments suggest that the results are not qualitatively sensitive to the form of the forcing, but that strong intraseasonal winds over the warm pool will always produce this pattern of rectified SST.
The sensitivity of the coupled model used here to external perturbation of SST has been analyzed extensively elsewhere (Moore and Kleeman 1997; Kleeman and Moore 1997). This analysis shows that perturbations with spatial structures similar to those deduced here for the MJO (an east-west dipole along the equator) are particularly efficient in forcing a low-frequency ENSO response in this model. As other models have different optimal forcing patterns (e.g., Xue et al. 1997; Thompson 1998), the sensitivity of our results to the present simplified coupled model physics should be tested. A perhaps more convincing test of these ideas would be in a coupled GCM that simulated the full coupled interaction, including feedbacks at the intraseasonal timescale itself, which are known to occur in nature (Hendon and Glick 1997; Shinoda et al. 1998). A further aspect of the interaction of the MJO with ENSO that could be tested in a coupled GCM is that in reality the entire envelope of enhanced intraseasonal variance moves east with an advancing El Niño (Fink and Speth 1997; McPhaden 1999). Our results suggest that the rectifying SST and consequent westerly winds should also move east, amplifying the signal. Possible effects of this coupled propagation could not be studied by the present two-model, two-separate-calculation combination, but could in a coupled GCM.
The question of rectification of the MJO into the ENSO cycle is relevant to our ability to forecast El Niño events, since increased intraseasonal variability over the western Pacific has been a prominent feature of all the events since we have had the capability to observe these frequencies (with the introduction of satellite OLR in 1979; Fig. 1). If rectification of the MJO resulting in low-frequency SST changes does occur in nature, and if the result of these SST changes is a corresponding enhancement of westerly winds, then the amplification of warm events due to the MJO would need to be accounted for when making ENSO forecasts, which is presently not the case for at least some dynamical model forecasts. However, since the occurrence of individual MJOs is apparently weatherlike and not predictable far in advance, this suggests that it may be difficult to improve our ability to estimate the amplitude of foreseen oncoming El Niños, as was the case in 1997. In general, models that do not well represent the MJO (many present atmospheric GCMs) may be expected to underpredict the amplitude of El Niños for this reason.
In late 1996, several forecast models (including the one used here) correctly predicted that 1997 would see the development of an El Niño event. Yet none of these models forecast the extremely steep rise of central and eastern Pacific SST that took place in March-July 1997, following the strong MJO events of December 1996 and March 1997 (CPC 1996), until the impacts of those events had been assimilated by the models. [After the fact, model hindcasts have been run that do indicate a steep SST rise in early 1997, but these experiments have not been diagnosed (D. Anderson 1998, personal communication).] This suggests that the MJO events played a role in the extreme and unpredicted growth of the 1997-98 El Niño. We hypothesize that SST warming in the central Pacific associated with the onset of El Niño allows MJO-associated winds and convection to extend further out over the Pacific, even if the global MJO does not change. Thus MJO winds during the onset of El Niño can develop a long fetch and consequent large effect on the ocean dynamics and thermodynamics and can enhance the amplitude of the event through the processes discussed here. From this perspective, it is not the low global-wavenumber MJO mode that varies with the ENSO cycle (it does not), but its fetch over the Pacific. This resolves the apparent contradiction that MJO activity occurs every year but El Niño does not. However, it is emphasized that, although the present results suggest that the MJO can interact constructively with the onset of El Niño to amplify a developing warm event, the MJO on its own does not appear to be the cause of El Niño. This distinction must be made because coupled models without anything resembling the MJO generate ENSO-like behavior and have demonstrated forecast skill. Though it seems clear that the ENSO cycle would exist without the MJO, this premise does not in any way preclude a role for rectification of the MJO and coupled feedback leading to enhancement of El Niño events as suggested here.
We have shown plausible mechanisms by which oscillating winds associated with the MJO can produce significant nonlinear effects on SST. Whether or not the model solutions studied here are realistic in detail, the sense of these effects does not appear to be model dependent, and all point in the direction of flattening the west Pacific zonal SST gradient, and therefore, in the direction of enhancing westerly wind anomalies as the SST anomalies feed back to the atmosphere.
The present results suggest that the relatively high-frequency signals of the MJO can interact constructively with the ENSO cycle through nonlinear ocean dynamics and latent heat fluxes producing rectified SST that feeds back to the coupled system. Therefore further work toward understanding these processes and the factors that contribute to variations of the MJO, especially the precursor conditions that affect their amplitude, may enhance our ability to predict the strength of oncoming El Niños.
Acknowledgments. The authors thank David Battisti, Meghan Cronin, Russ Davis, Harry Hendon, Mike McPhaden, Andy Moore, Dennis Moore, Klaus Weickmann, and Bob Weller for illuminating discussions. A reviewer provided us with the opportunity to reconsider these issues in detail for almost a year. Support for this work came from NOAA's Office of Global Programs via the Stanley P. Hayes Center (WSK).
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