. component of Southern Oscillation, AMDecSO . This amplitude modulation attains maximum
amplitude in 1913–1917. Thereafter, amplitude decreases until about 1959. In early-1960s, starts the growing branch of this amplitude modulation of decadal fluctuations. The state of AMDecSO
and contributions from the bidecadal component of SOI and SSTI account for the evolution observed in this century in the values of these two indexes. The modulation of the decadal
component of SSTI is clearly seen in summer and winter averages. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Interdecadal fluctuations; Southern Oscillation; Equatorial Pacific
1. Introduction
Decadal and interdecadal climate variability affects water resources and agricultural production in many regions of the world. Often, this climate variability is wrought in the
Ž . form of quasi-periodic fluctuations in atmospheric variables. A few examples are 1 a
rainfall climate change of multidecadal extent starting in mid-1930s in the southern Ž
. mid-latitudes
Lucero and Rodrıguez, 1999 , formed by 12- and 20-yearr period
´
Ž . amplifying fluctuations. 2 Rainfall variations spanning cycles of 12 and 20 years in
Ž .
central United States Hu et al., 1998 ; and interdecadal fluctuations in precipitation in Ž
. US Pacific coast Chen et al., 1996 .
Ž .
Decadal and interdecadal fluctuations in the Southern Oscillation SO and in the sea Ž
. surface temperature SST of the equatorial Pacific are suspected to contribute to the
generation in several parts of the world of rainfall fluctuations with the same timescales. Observational verification of this influence requires first to know in more detail some of
the statistical characteristics of the SO and the SST in the equatorial Pacific.
We report on some of the characteristics of decadal and interdecadal fluctuations in Ž
. the equatorial Pacific SST and in the Southern Oscillation index SOI . The approach is
statistical and observational. These fluctuations are embedded in the interdecadal variability originating in the Tropics, which is one of the four major types of atmo-
Ž .
spheric decadal and interdecadal variability Latif, 1998 . The data description and methodology is described in Section 2. The rationale and the
Ž . blueprint of the paper are the following. 1 The probability distributions of the time
Ž between consecutive fluctuations in SOI and SSTI sea surface temperature index of
. Wright, 1989
at decadal and interdecadal timescales are unknown. This topic is Ž .
analyzed in Sections 3.1 and 3.2. 2 A search in the literature shows no observational quantitative evidence on the relationship between amplitudes of decadal and interdecadal
components of the SOI and the SSTI. The SOI is a measure of the average gradient of surface pressure across the tropical Pacific, thus it is also a measure of the average
surface wind stress on that region. Therefore, a relationship between amplitudes of decadal and interdecadal components of SOI and of SSTI is tantamount to a relationship
Ž between wind stress and sea surface temperature on the equatorial Pacific Clarke and
. Lebedev, 1996 . That relationship between fluctuations of SOI and SSTI is described in
Ž . Section 3.4. 3 A topic insufficiently studied is the relative phase between simultaneous
fluctuations of SOI and SSTI at decadal and interdecadal timescales. Changes in the Ž .
relative phase of these fluctuations of SOI and SSTI are documented in Section 3.5. 4
Long-term modulation of decadal and interdecadal fluctuations in annual rainfall amount has been found by other authors. For finding the explanation of that modulation, it is of
interest to know if there is a joint amplitude modulation of decadal and interdecadal fluctuations of SOI and of SSTI. Although indication of long-term modulation of time
series of SST is found in the literature, it has not been documented a joint amplitude modulation of the decadal and interdecadal components of the SOI and the SST in the
equatorial Pacific during this century. The existence and characteristics of a joint amplitude modulation is described in Section 3.6. Finally, Section 4 contains a discus-
sion of results, and Section 5 contains a summary of findings.
2. Data and methodology
