Chlorophyll Bloom in the Western Equatorial Pacific During the 1998 El Niño / La Niña Transition: the Role of Kiribati Islands as Seen From Satellite, in-situ Data, and a High-Resolution Simulation
Monique Messie(1) , Marie-Hélène RADENAC(1) , and Jérôme Lefevre(2)
18 avenue Edouard Belin,
31401 Toulouse cedex 9,
(2) Centre IRD de Nouméa, BP A5, 98848 NOUMEA Cedex, New Caledonia
During the 1998 El Niño / La Niña transition, ocean color observation from the recently launched SeaWiFS sensor showed a dramatic chlorophyll bloom that spread between 160°E and 175°E in the equatorial Pacific. Surface chlorophyll concentrations notably increased from March to June 1998, and reached more than 0.8 mg m-3 in May, in a region usually known as oligotrophic. Just prior to this event, the thermocline and the nutrient pool were shallow across the equatorial Pacific. In such environmental conditions, previous studies attributed this bloom to a wind-driven upwelling and to the shoaling of the Equatorial UnderCurrent, which brought both nitrate and iron to the surface waters. However, they do not explain its particular location, which coincided with the presence of the Kiribati Islands. These small coral atolls form an obstacle both for the surface westward flowing South Equatorial Current and the subsurface eastward flowing Equatorial UnderCurrent. In the context of a strongly uplifted nutrient pool on the equatorial basin scale, we investigated possible dynamical perturbations induced by the islands that could favor the phytoplankton growth. In addition to SeaWiFS chlorophyll data, we used satellite (Topex/Poseidon altimetry, TMI SST, ERS surface winds) and in situ data (TAO/TRITON mooring) to assess the physical context of the bloom. Variations of altimetric sea level anomalies (SLA) are mainly the result of the vertical motions of the thermocline. Therefore, altimetry is useful to gather information about the variations of the depth of the nutrient pool. Data from the equatorial TAO mooring at 165°E provided subsurface variations of temperature and currents. Then, we modeled the event with the Regional Oceanic Modeling System (ROMS) with 1/6° horizontal resolution and 30 vertical levels. The model reproduced the spatial patterns and temporal evolution of SST and SLA, as well as the vertical structure of temperature and velocity. In an ongoing study, we use it to examine possible pathways of nitrate-rich and iron-rich waters, analyze why the chlorophyll bloom was confined west of the Kiribati islands and infer the role they played in the generation and development of the bloom.