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Abstract

The "Principal Oscillation Pattern" technique is used to derive an index of
the tropical 30— to 60—day oscillation. In the 200 mb equatorial velocity
potential field one dominant pair of POPs is found. Its properties compare
very Well with the properties of the oscillation identified in previous
studies. In particular, a good correlation between the time evolution of the
POP coefficients and area averaged OLR is found.

The POPS are derived from a two year subinterval of the whole five year data
set. This leaves independent data for subsequent verification. The patterns
and their characteristic numbers are almost unchanged if the whole data set is
analysed. Also, the analysis is insensitive to changes of the analysis area:
If the analysis is limited to 900 longitude equatorial sectors the signal is
identified also and its patterns are consistent with the patterns derived from
the full data set. Interestingly, the signal is best defined in the eastern
hemisphere.
The POPs may be used to derive "associated correlation patterns" of other
quantities in winter and summer separately. The path of the oscillation has a
marked annual cycle: In northern winter it migrates from the Indian Ocean
across Northern Australia into the region of the SPCZ and in northern summer
it moves from the Indian Ocean across South Asia along the ITCZ to South
America.

The POP coefficient may be seen as a bivariate index of the state (phase and
strength) of the 30— to 60—day oscillation. Since the POP technique
incorporates a prediction equation for the phase of the POP coefficients our
POP model allows for the prediction of the complex amplitude of the
oscillation. In a sequence of forecast experiments, of which about two thirds
used independent data, the POP forecasts were found to be useful in about half
of all cases for lead times of several days.

The correlation and RMS skills were calculated for the POP forecast and for
persistence. The POP forecast appears to be considerably better with respect
to both measures. The correlation skill scores 60% after 7 days. The POP
forecast is most skillful in northern winter and if strong signals are present
with minima of velocity potential in the eastern hemisphere.