Physical and biogeochemical forcing of oxygen and nitrate changes during El Niño/El Viejo and La Niña/La Vieja upper-ocean phases in the tropical eastern South Pacific along 86° W

[摘要] Temporal changes in the water mass distribution and biogeochemical signals inthe tropical eastern South Pacific are investigated with the help of anextended optimum multi-parameter (OMP) analysis, a technique for inversemodeling of mixing and biogeochemical processes through a multidimensionalleast-square fit. Two ship occupations of a meridional section along85°50' W from 14° S to 1° N are analysed duringrelatively warm (El Niño/El Viejo, March 1993) and cold (La Niña/LaVieja, February 2009) upper-ocean phases. The largest El Niño–SouthernOscillation (ENSO) impact was found in the water properties and water massdistribution in the upper 200 m north of 10° S. ENSO promotes thevertical motion of the oxygen minimum zone (OMZ) associated with the hypoxicequatorial subsurface water (ESSW). During a cold phase the core of the ESSWis found at shallower layers, replacing shallow (top 200 m) subtropicalsurface water (STW). The heave of isopycnals due to ENSO partially explainsthe intrusion of oxygen-rich and nutrient-poor antarctic intermediate water(AAIW) into the depth range of 150–500 m. The other cause of the AAIWincrease at shallower depths is that this water mass flowed along shallowerisopycnals in 2009. The shift in the vertical location of AAIW reaching theOMZ induces changes in the amount of oxygen advected and respired inside theOMZ: the larger the oxygen supply, the greater the respiration and the lowerthe nitrate loss through denitrification. Variations in the intensity of thezonal currents in the equatorial current system, which ventilates the OMZfrom the west, are used to explain the patchy latitudinal changes of seawaterproperties observed along the repeated section. Significant changes reachdown to 800 m, suggesting that decadal variability (Pacific decadaloscillation) is also a potential driver in the observed variability.

[学科分类] 地球化学与岩石