Thursday 25 August 2005

P2

PTH0099
Is the decadal variability of the Atlantic SST dipole a periodic feature?
Clauzet, Gabriel¹, Wainer, Ilana¹
1 University of Sao Paulo, Brazil
Author email: gclauzet@usp.br
The variability of the tropical Atlantic Ocean is a superimposition of several time scales with respect to two main modes: the equatorial mode, similar to the ENSO phenomenon, and the north-south inter-hemispheric SST gradient (also know as the Atlantic Dipole mode (AD)). In the past two decades research with respect to the AD pointed out marked decadal-scale variability. Wavelets analysis technique is applied on historical SST time-series from 1870 to 2000 to evaluate the changes in the low frequency variability of this mode. The results show that the inter-hemispheric SST mode presents significant inter-decadal changes in its variance during the last 130 years. The AD mode does not present a "natural oscillation" at decadal time-scale along all the record, as found in the literature. The analysis of the filtered time-series for the 8-12 years period shows significant variance during three distinguished periods: 1870 to 1890, 1910 to 1930 and 1965 to 1985. Besides the decadal oscillation, which is restricted to these three periods, the AD spectrum presents oscillations of higher frequency (1-4 years) along the remaining record. Two global climate indexes are analyzed in order to examine possible cause for these changes: NINO3.4 and NAO. The analysis of the filtered time-series for the 8-12 years period for the NAO index shows a similar spectrum to the AD, with an increase in the variance at the same periods. For the ENSO index analysis the predominant variance occurs at the 2-7 years period, but there is also a remarkable increase of the variance for the same periods with respect to the decadal variability of the AD mode. Therefore one concludes that the decadal variability of the AD is directly influenced by the NAO and possibly under remote influence of ENSO, characterizing it as an episodic and not a period oscillation.

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