Interannual changes of the intraseasonal fluctuations in the eastern boundary current off South America

Recent work has shown that strong intraseasonal fluctuations (40-70 day period) of sea level and currents over the continental slope along the west coast of South America are associated mainly with free, coastal trapped waves forced by equatorial Kelvin waves. Wind events in the western tropical Pacific Ocean associated with Madden-Julian Oscillations (MJO) force the equatorial waves. Here we take a closer look at the structure of such interseasonal fluctuations off South America based on eight years (1991-1999) of direct current observations over the slope at 220, 485 and 750 m depths at 30°S off Chile as well as sea level data from the same period from Callao (12°03'S), Arica (18°29'S), Caldera (27°03'S) and Valparaiso (33°02'S). This period was characterized by moderate, weak, and strong El Niño events in 1991-1992, 1994 and 1997-1998, respectively, and by La Niña events in 1996 and 1998-1999.

Our results based on Wavelet and other analyses show significant seasonal and interannual modulation of the intraseasonal fluctuations off Peru and Chile. Intraseasonal variability was greatest during the austral summer whereby this seasonal modulation was more apparent in the 220 m depth current meter than in deeper current meters or in sea level (the MJO is known to be most active in the western tropical Pacific at this time of year). The strongest intraseasonal variability at all current meter depths and all sea level stations was associated with the initial phase of the moderate and strong El Niño events. During the 1991-1992 event, sea level and current variability was centered at a period of about 50 days. On the other hand, the intraseasonal variability exhibited a more complex, bimodal distribution during the 1997-1998 event with periods near 40 and 70 days whereby the 40 day waves tended to follow the 70 day waves. Furthermore, the 40 day waves tended to dissipate more rapidly than the 70 day waves as they traveled southward along the coast of South America.

Comparisons with current and wind series from the western and central equatorial Pacific (TOGA-TAO array data) confirm that most of the intraseasonal variability throughout our eight year period of observations arrived as free, coastal trapped waves, forced originally along the equator. However, our data from the coast of Chile from Caldera southward also indicate that some of the intraseasonal variability in that region may be forced by local winds of intraseasonal periods which may be associated with a more direct atmospheric teleconnection from the western tropical Pacific to subtropical latitudes off South America