We find that the onset of increased SPCZ activity, in November, originates from the subtropics or from the southeast. It is not until January-February, the height of the SPCZ active season, that the tropical SPCZ is at maximum activity. SPCZ activity declines in March and April as the SPCZ active season comes to an end. Based on these results, the initial build up of SPCZ activity is not a result of increased sea surface temperature (SST) since, in the subtropics, SPCZ activity anomalies are uncorrelated with SST anomalies. These results also bring into question the practice of using a DJF mean to characterize the SPCZ active season, which is quite common. A DJF mean will only capture the active season in the tropics and will miss the most active time for the subtropical SPCZ.
Interannual variability of the SPCZ is closely related to ENSO variability. We find a systematic shift in SPCZ activity to the northeast in El Nino years and to the southwest in La Nina years. Three strong El Nino years (1982, 1991, and 1997), show an extreme change in the location of SPCZ activity. In these years, the mean SPCZ is located very near the equator with a zonal orientation, across much of the western and central Pacific. Once the ENSO signal is removed (or reduced) from the SPCZ activity, there are no trends in SPCZ activity or SPCZ location over the thirty years. The MJO has a strong signature in SPCZ activity that reaches beyond the tropics. Our current research effort is focused on distinguishing between tropical and more extratropical processes in the variability of the SPCZ and we will report on new findings in our presentation.
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