The South West Tropical Pacific (SWTP) is generally considered as an N-limiting oligotrophic area with parts characterized by permanent deep thermocline and nutricline as the Melanesian Archipelago. However, ocean color imagery shows in this region a strong seasonal chlorophyll cycle, with large blooms which have for long been attributed to nitrogen-fixing cyanobacteria as Trichodesmium. SeaWiFS surface chlorophyll analysis of the area between 5°S-25°S, 150°E-170°W has been conducted from 1997 to 2004. A strong seasonal and interannual variability is described, which relates well with the sea-truth measured chlorophyll concentrations. An empirical orthogonal function analysis helped in determining principal explicative factors of this variability, and in identifying different patterns, i.e., seasonality (austral winter and summer blooms), which may be driven by different fertilization processes, and a unexplained interannual variability. This work represents part of the French PROOF-IRD DIAPAZON (DIAzotrophy in a PAcific ZONe) program aiming at estimating nitrogen and carbon fixation by cyanobacteria at the SWTP scale