4. Discussion
Several highly elaborated conjectures have been advanced on the physical processes that would produce decadal and interdecadal fluctuations in the SST in the equatorial
Pacific. Although these conjectures were constructed from analyses of numerical simulations or simplified models of uncoupled and coupled ocean–atmosphere pro-
cesses, it has yet to be presented empirical evidence supporting these conjectures. Findings in this research describe some properties of decadal and interdecadal fluctua-
tions in equatorial SST and the SOI that ocean–atmosphere models would have to reproduce in support of those conjectures. In this section, we indicate some implications
of these findings.
The regularity of the interfluctuation time of decadal and bidecadal components of SOI and SSTI suggests that some kind of wave mechanism could be the time regulator
of the fluctuations. Results from some modeling studies indicate that fluctuations with a Ž
decadal timescale or longer can be generated in the tropical Pacific Garreaud and .
Battisti, 1999, Munnich et al., 1998, Zhang et al., 1998, and references therein . In this
¨
Ž .
research, peak values of positive negative SOI are unrelated to interfluctuation period. This latter result suggests that advection processes produced by wind stress may not be
controlling the generation of decadal and bidecadal fluctuations. Peaks of SOI fluctuations and associated peaks of opposite sign of SSTI fluctuations
at decadal, bidecadal, and interdecal timescales are strongly and negatively correlated. This results support the prevalent postulate that at these long timescales a major control
of anomalies of SST is the heat flux modulated by the wind speed anomalies. The SOI represents the average surface pressure gradient over the tropical Pacific, and thus it is
also an index of surface wind speed. Immediate response of anomalies of SST to anomalies of wind speed would be expected. However, only after mid-1930s, decadal
and bidecadal components of SOI and SSTI are completely out of phase. Before
Ž .
mid-1930s, fluctuations in SOI leads to fluctuation of opposite sign of SSTI at decadal and bidecadal timescale. This difference in relative phase before and after mid-1930s
suggests that the relative importance of processes involved in the generation of decadal and bidecadal components of SOI and SSTI might have changed around that time.
Evidence was found on the occurrence of a joint amplitude modulation of the decadal component of the SOI and SSTI taking place along a period of about 70-year and
attaining its minimum about 1957. After that time, decadal components are amplifying. These amplifying components contributed to the exceptional negative values of the SOI
and to the high equatorial SST in early 1980s. This joint amplitude modulation of the decadal components of SOI and SSTI explains the large variations of those indexes
since the start of the century. The question arises whether the change in relative phase in mid-1930s in the decadal and bidecadal components of SSTI and SOI produced changes
in some surface atmospheric variables. Preliminary results from ongoing research indicate an affirmative answer. Contemporary to the above-described change of relative
phase in mid-1930s, in central Argentina started a strong positive trend in annual rainfall
Ž .
amount Lucero and Rodrıguez, 1999 . This positive trend, which increased in early
´
1960s, is produced by amplifying waves of annual rainfall perturbation with a decadal and bidecadal timescales. In early 1960s, the positive trend in annual rainfall amount
Ž . increases simultaneously to
1 the entrance in stage of the increasing branch of
Ž amplitude modulation of decadal component of Southern Oscillation, AMDecSO that
. Ž .
produces the amplification of decadal fluctuations , and 2 the amplification of the bidecadal component of the SOI. This topic is currently being analyzed in another
research. A century of data is not enough to determine if AMDecSO and the change of relative
phase between decadal SOI and decadal SSTI are intrinsic to decadal processes in this atmospheric–oceanic system, or they are phenomena occurring only in this century.
However, in the latter case, the question would arise about its possible link to global climate change. AMDecSO changes into a new type of oceanic–atmospheric coherent
interaction at mid-1930s. The setting of an out-of-phase locking between decadal SOI and associated decadal SSTI component is not simultaneous to the start of amplification
of decadal components. The change in relative phase occurs in mid-1930s, while amplitudes of SOI and SSTI signals keeps decreasing until about late 1950s. Early 1960s
starts the amplification of SOI and SSTI signals. The perspective of a constructive interaction of SSTI and SOI signals leading to amplification of oscillation, in a new
resilient decadal mode of oceanic–atmospheric interaction, is of concern.
5. Conclusion
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