The regular occurrence of enhanced intraseasonal variability over the western equatorial Pacific during the onset stage of El Niño has fostered a wide-ranging debate over the possible role of the Madden-Julian Oscillation (MJO; Madden and Julian 1972, 1994) in the El Niño-Southern Oscillation (ENSO) cycle. There are at least two viewpoints currently existing on this subject. First, since coupled models without the MJO are capable of skillful ENSO forecasts, one school of thought holds that there is little influence (e.g., Zebiak 1989). This viewpoint notes that some level of MJO activity occurs over the west Pacific nearly every year, El Niño or not, and therefore argues that the spatial-temporal characteristics of the MJO are not of fundamental importance. On the other hand, since the surface wind and flux pattern of the MJO (giving rise to zonal gradients along the equator) suggests an efficient projection onto low-frequency ENSO-like modes, another school argues that the MJO can act as a disruptive or stochastic influence on an otherwise regular ENSO cycle and thereby contribute to the observed irregularity (Moore and Kleeman 1998).
The present paper examines physical processes that might produce coupling between the MJO and the ENSO cycle. Many observers have pointed out that the propagation of oceanic Kelvin waves leads to prominent MJO signatures in thermocline depth in the east, where SST is highly sensitive to the vertical temperature gradient (Kessler and McPhaden 1995; McPhaden 1999). However, while the propagating thermocline signals can be impressively large, a nonlinear process would be necessary to couple intraseasonal and interannual frequencies, but this has been difficult to demonstrate (see Kessler et al. 1995 for one example). The purpose of the present study is to investigate nonlinear mechanisms by which the oscillating winds of the MJO could have a rectified effect on the ocean-atmosphere system.
The longest time series showing the spatial-temporal evolution of intraseasonal variability is the regular satellite measurements of outgoing longwave radiation (OLR) that have been made continuously since 1979. OLR is a measure of tropical deep convection, and numerous studies have used coherent aspects of its intraseasonal variability to indicate the MJO (Knutson and Weickmann 1987; Rui and Wang 1990; Hendon and Salby 1994; Zhang and Hendon 1997; Shinoda et al. 1998; Hendon et al. 1999). We constructed an index of intraseasonal activity over the west Pacific warm pool by bandpassing OLR (approximately between 25- and 120-day periods), averaging within 2°S-2°N, 155°E-175°E, then squaring these values and plotting the square root of the 1-yr running mean of the resulting time series (Fig. 1). A similarly constructed index was made from the only long time series of in situ winds in the western equatorial Pacific, at the Tropical Atmosphere-Ocean (TAO) mooring at 0°, 165°E (see section 2c), and is also plotted in Fig. 1. Comparing these indices with the Southern Oscillation Index (SOI) shows that warm pool intraseasonal rms roughly doubled during each of the El Niño events of the past 18 years; the lag correlation between OLR and SOI is 0.74, with OLR leading by about 70 days (the correlation is significant above the 95% level; see appendix A). The zonal wind index is also highly correlated with the SOI (r = 0.80), showing the clear association of intraseasonal zonal winds with the onset of recent El Niño events. The high correlation among the three quantities is due to the eastward spread of intraseasonal activity over the west Pacific early in El Niño events, reflecting the fact that as warm SST expands eastward during the growth stage of El Niño, convection tends to follow (Fink and Speth 1997). However, we note that only perhaps half the intraseasonal variance is associated with the eastward-propagating, spatially coherent global mode (often measured by 200-mb zonal winds) that defines the MJO (Slingo et al. 1999; Hendon et al. 1999). In contrast to the present warm pool intraseasonal index, the overall level of global MJO activity is not well correlated with the ENSO cycle. Although the low-zonal-wavenumber MJO does not alter its overall character due to the ENSO cycle, convection associated with individual MJO events occurs over warm SST and therefore can extend farther east with the onset of El Niño, without necessarily producing a large change in the global MJO mode. In any case, Fig. 1 shows that the early stages of El Niño events are associated with a significant increase in intraseasonal activity over the warm pool. Of course, the high correlation seen in Fig. 1 does not imply causality, and it remains possible that the enhanced intraseasonal variance is an incidental symptom of advancing El Niño conditions but not an essential feature of it. Nevertheless, the regular association between these two signals at different frequencies raises the question of whether nonlinear interaction might occur. In the rest of this paper we evaluate potential mechanisms through which the correlations seen in Fig. 1 might in fact represent an active and constructive element of the ENSO cycle.
Figure 1. Time series of the SOI (line), OLR intraseasonal rms, averaged in the region 2°S-2°N, 155°E-175°E (dash, W m-2), and zonal wind intraseasonal rms at the TAO mooring at 0°, 165°E (dotted line, m s-1). The SOI is inverted (so El Niño events are positive on the plot).
The focus here is on rectified nonlinear changes in the ocean in response to oscillating winds, and how this response could feed back to modify the lower-frequency coupled system. From another point of view, there is a rich literature on air-sea interactions associated with the atmospheric thermodynamics of the MJO (Emanuel 1987; Neelin et al. 1987; Lau et al. 1989; Webster 1994; Hendon and Glick 1997; Jones et al. 1998), and a useful schematic overview of several theories is given in Flatau et al. (1997).
In the present research, two separate models were used in series to understand the effect on the coupled system of intraseasonal winds. First, an ocean general circulation model (OGCM) was used to investigate the (nonlinear) effects on the ocean of imposed intraseasonal zonal winds, then a much simpler intermediate coupled model was used to evaluate the effects of the resulting SST pattern on the coupled system during the onset of El Niño. While this procedure is somewhat indirect and risks missing coupled feedbacks that might occur on the intraseasonal timescale itself, it makes diagnosis of the oceanic physical mechanisms more straightforward than would be possible in a coupled GCM context. A posteriori, the subtlety of some aspects of these mechanisms bears out the utility of isolating the two systems, at least for the initial examination of these phenomena as conducted here.
Section 2 of this paper discusses the two models used to study these phenomena, and how the idealized forcing fields were imposed. Section 3 describes the rectified signals in the ocean due to imposed intraseasonal winds, and diagnoses the mechanisms responsible for the rectification. Section 4 shows results from the coupled model experiments, and section 5 discusses the implications with regard to the ENSO cycle. Note that here we use the term "low frequency" to refer to periods of more than a few months, unlike much of the atmospheric literature in which intraseasonal periods themselves are considered low frequency.
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