WEAVER ET AL.
21
amplitude. 4. ENSO/MJO interaction MJO variability has the potential to be significantly modulated by interannual variations of the climate system, and in particular by ENSO, given the co-location of the MJO and ENSO climate variability modes over the tropical Indian and Pacific oceans. To separate the impacts of ENSO on the structure of MJO variations we first stratify the daily anomaly fields by positive and negative ENSO phases. The classification is gleaned from the Climate Prediction Center (CPC) historical Niño 3.4 indices from 1982-2006. Figure 3 shows the observed representation (top), and the CFSv1 (middle) and CFSexp (lower) during El Niño (left) and La Niña (right) years respectively. Given our focus on ENSO modulation the lag correlations are performed against a western Pacific precipitation index, calculated as the area averaged precipitation in the 160ºE-185ºE 10ºS-10ºN domain. Striking differences in the propagation features are noted between El Niño and La Niña years. During El Niño the intraseasonal anomalies of precipitation and 850 hPa zonal winds propagate coherently through the Indian Ocean, across the Maritime Continent, and into the western Pacific, separating as they enter the eastern Pacific domain. During La Niña years the propagation of the precipitation and 850 hPa zonal wind anomalies appears to be thwarted as they enter the western Pacific, perhaps a consequence of the negative feedback via coupling of the atmosphere to a cold ENSO SST anomaly. The model responses are mixed. Both versions of the CFS capture the propagation features over the Eastern Hemisphere during El Niño, however tend to speed up the propagation of 850 hPa zonal wind anomalies upon crossing the dateline. The situation during La Niña is not as clear. Neither version of the CFS seems to coherently propagate the 850 hPa zonal wind and precipitation anomalies over the Indian Ocean as in the observed depiction. This indicates that the simulated intraseasonal activities in the western Pacific are less connected to the variability in the Indian Ocean than that in the observed, a manifestation of the model’s failure to propagate across the Maritime Continent. The simulated features east of the Dateline do exhibit some degree of fidelity, mostly in the wind field. The interruption to the eastward propagating intraseasonal anomalies near the Dateline is captured to some degree, more so in the CFSexp. The observed power spectra of 850 hPa zonal winds (15°S-Eq; 165°E-190°E) and OLR (20°S-5°S; 160°E-185°E) over the western
Fig. 4 Western Pacific power spectra as a function of ENSO phase over the domain of 15ºS-Eq and 160ºE-185ºE for 850 hPa zonal winds, and 20ºS-5ºS and 160ºE-185ºE for OLR. The unit is w2 m-4 day for OLR and m2 s-2 day for 850 hPa zonal winds.