Displacements and transformations of nitrate-rich and nitrate-poor water masses in the tropical Pacific during the 1997 El Niño

Abstract

sur l'évolution de la langue d'eau verte équatoriale enrichie en chlorophylle dans le Pacifique après le déclenchement d'un El Niño : retard à l'oligotrophie tant qu'il subsiste des nitrates dans les eaux HNLCA Lagrangian analysis was applied to the outputs of a coupled physical-biogeochemical model to describe the redistribution of nitrate-rich and nitrate-poor surface water masses in the tropical Pacific throughout the major 1997 El Niño. The same tool was used to analyze the causes of nitrate changes along trajectories and to investigate the consequences of the slow nitrate uptake in the high nutrient low chlorophyll (HNLC) region during the growth phase of the event. Three patterns were identified during the drift of water masses. The first mechanism is well known along the equator: oligotrophic waters from the western Pacific are advected eastward and retain their oligotrophic properties along their drift. The second concerns the persistent upwelling in the eastern basin. Water parcels have complex trajectories within this retention zone and remain mesotrophic. This study draws attention to the third process which is very specific to the HNLC region and to the El Niño period. During the 1997 El Niño, horizontal and vertical inputs of nitrate decreased so dramatically that nitrate uptake by phytoplankton became the only mechanism driving nitrate changes along pathways. The study shows that because of the slow nitrate uptake characteristic of the tropical Pacific HNLC system, nitrate in the pre-El Niño photic layer can support biological production for a period of several months. As a consequence, the slow nitrate uptake delays the gradual onset of oligotrophic conditions over nearly all the area usually occupied by upwelled waters. Owing to this process, mesotrophic conditions persist in the tropical Pacific during El Niño events

Impacts of ENSO on air‐sea oxygen exchange: Observations and mechanisms

“…On the other hand, the central equatorial Pacific (130°W–180°W) exhibits significant weakening of the easterlies during El Niño (Figure e) and is thus sensitive to the localized effects of winds on upwelling rates. The central equatorial Pacific anomaly also likely contains small contributions from the eastward propagation of warm pool waters [ Radenac et al , ], since these waters were not exposed to intense upwelling and are well equilibrated with the atmosphere. The simulated surface ΔO 2 response is in general agreement with the observed surface O 2 supersaturation reported by Stephens [] along 120°W–140°W during the El Niño of 1997–1998, suggesting the elimination during this event of the typically high O 2 undersaturation observed along the equator [ Wanninkhof et al , ].…”

A global analysis of ENSO synchrony: The oceans' biological response to physical forcing

“…This suggests that negative chlorophyll anomalies are driven by horizontal advection of nitrate‐poor waters from the west rather than generated locally [ Radenac et al , 2001; Christian et al , 2002]. These waters originate not only from the warm pool but also from the even lower chlorophyll northwest subtropical region [ Radenac et al , 2005]. The advection of low‐chlorophyll waters does not invalidate other processes in the central Pacific (e.g., light limitation and/or barrier layer formation, see below) which act to maintain oligotrophic conditions.…”

Water masses as a unifying framework for understanding the Southern Ocean Carbon Cycle

“…The method is based on a analytical solution of the water parcel trajectory within the model gridbox, and has been used in about one hundred studies, mostly focussing on tracing the thermohaline circulation in GCM. It was first applied to the interpretation of passive tracers in the ocean by Rodgers et al (2004) and Radenac et al (2005). When applying Lagrangian trajectory analysis to ocean models, one typically chooses a vertically-oriented surface in the ocean interior and releases many thousands of individual particles, with the understanding that each particle represents Blanke and Raynaud, 1997).…”

“…It will be important to develop this capacity at a later stage, and this could be accomplished by building on the volume-following quantitative approach here used together with the preliminary qualitative steps in this direction that were explored in the works of Radenac et al (2005) and Verdy et al (2007).…”