Seasonal evolution and interannual-intraseasonal variability of the South Pacific Convergence Zone (SPCZ) during the austral summer half year

The seasonal evolution and interannual-intraseasonal variability of the SPCZ are examined using a new data set of SPCZ labels, indicating SPCZ location, every three hours for the past 30 years. SPCZ labels are based on instantaneous satellite observations from the infrared and visible channels of geostationary satellites and total precipitable water from microwave measurements. All the data are input into a spatiotemporal statistical model that produces the labels. The model is designed to emulate the way a human observer would identify the SPCZ and seeks to maintain the spatial and temporal continuity of the SPCZ. The model takes into account whether neighboring data points (in space and time) belong to the SPCZ and the result is an elongated convection zone. Prior knowledge of typical SPCZ location is also used in the statistical model. The SPCZ labels are used to quantify the seasonal evolution as well as interannual and intraseasonal variability of the SPCZ.

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.