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<h1>Report 53:&nbsp; Predictability and the Relationship

Between Subseasonal and Interannual Variability During the Asian Summer

Monsoon</h1>

<br>&nbsp;

<ul><b>Sperber,</b> Kenneth R., Julia M. Slingo and H. Annamalai

<p>July 1999, 53 pp.&nbsp;

<p>The relationship between subseasonal and interannual variability of

the Asian Summer Monsoon has been investigated through analysis of the

dominant modes of variability in the 40-year NCEP/NCAR Reanalysis, with

complementary satellite and surface based precipitation data. The hypothesis

that the characteristics of monsoon subseasonal variability (i.e. weather

regimes) are modulated on interannual times-cales in a systematic and therefore

predictable manner has been tested. The null hypothesis is that predictability

of the seasonal mean monsoon behaviour requires only that the effects of

the slowly varying components of the climate system be correctly simulated.

<p>An interannual mode of monsoon variability has been identified which

is closely related to the observed seasonal mean all-India Rainfall (AIR).

A counterpart of this mode has also been identified at subseasonal timescales

which projects strongly on to the daily AIR, confirming that a common mode

of monsoon variability exists on seasonal and subseasonal timescales.

<p>It has been shown that the temporal behaviour of this subseasonal mode,

as de-scribed by the probability distribution function (PDF) of the principal

component timeseries, does not show any evidence of bimodality, the shape

of the PDF being Gaussian. Further, it has been shown that the mean of

the PDF is systematically and significantly perturbed towards negative

(positive) values in weak (strong) monsoon years as categorized in terms

of the seasonal mean AIR. This translation in the mean of the PDF, rather

than a change in shape of the PDF suggests that anomalous monsoons are

not associated with changes in weather regimes. Further analysis has con-firmed

that low frequency modulation of the basic state is primarily responsible

for these shifts in the subseasonal PDFs, supporting the null hypothesis

that predictability of the seasonal mean monsoon requires only that the

effects of the slowly varying components of the climate system be correctly

simulated. Thus model improvements to reduce systematic errors in the mean

simulation and the response to low frequency boundary forcing may improve

the prospects for dynamical seasonal prediction.

<p>However, the results have also shown that only a subset of the subseasonal

modes are systematically perturbed either by ENSO or in weak vs. strong

monsoon years, suggesting that predictability is likely to be limited by

the chaotic, internal variability of the monsoon system.<i><a href="pdf/53.pdf">(pdf

file)</a></i>

<p><font size=-1>UCRL-MI-123395</font></ul>