| Previous studies have discussed the interannual variability of a meridional
seesaw of dry and wet conditions over South America (SA) associated to the
modulation of the South Atlantic Convergence Zone (SACZ). However, they did
not explore if the variability inter ENSO (El Niño Southern Oscillation)
can be related to the phase changes of this dipole. To answer this question,
an observational work was carried out to explore the atmospheric and Sea
Surface Temperature (SST) conditions related to the same ENSO signal and to
opposite dipole phases.
Rotated Empirical Orthogonal Function (REOF) analysis was applied over
normalized Chen precipitation seasonal anomalies in order to find the dipole
mode in the Austral Summer (December to February). The fourth rotated mode,
explaining 6.6% of the total variance, consists of positive loading over
the SACZ region and negative loading over northern Argentina. Extreme events
were selected and enhanced activity of SACZ during the Summer season (SACZ+)
was identified in nine years: five during La Niña events (LN) and two
in El Niño episodes (EN). On the other hand, inhibited manifestations of
this system (SACZ-) were identified in seven years: four in EN and two
during LN.
Power spectrum analysis indicated that the interannual variability of the
precipitation dipole seems to be related to the low frequency and to the
quasi-biennial part of ENSO variability. The ENSO events with the same
signal can present opposite phases for the dipole. The results suggest that
the displacement of the convection over Indonesia and western Pacific can
play an important role to modulate the seesaw pattern. |