Seasonal Modulation of Dissolved Oxygen in the Equatorial Pacific by Tropical Instability Vortices

Abstract

Tropical instability vortices (TIVs) have a major influence on the physics and biogeochemistry of the equatorial Pacific. Using an eddy‐resolving configuration of the Community Earth System Model (CESM‐HR) and Lagrangian particle tracking, we examine TIV impacts on the three‐dimensional structure and variability of dissolved oxygen (O2) in the upper equatorial Pacific water column. In CESM‐HR, the simulated generation and westward propagation of TIVs from boreal summer through winter lead to the seasonal oxygenation of the upper northern equatorial Pacific, exhibited as a deepening of hypoxic depth west of 120°W. TIV effects on the equatorial Pacific oxygen balance are dominated by eddy‐advection and mixing, while indirect TIV effects on O2 consumption play minor roles. These advective effects reflect the transient displacements of isopycnals by eddy pumping as well as vortex transport of oxygen by eddy trapping, stirring, and subduction. TIVs influence on the upper equatorial Pacific O2 distribution and variability has important implications for understanding and modeling marine ecosystem dynamics and habitats, and should be taken into consideration in designing observation networks in this region.

Strong Habitat Compression by Extreme Shoaling Events of Hypoxic Waters in the Eastern Pacific

“…Fittingly, the phase of other THREEs detected around 5°N west of 120°W (April-June) corresponds with the weak TIV activity during boreal spring. We expect the role of absent TIVs, however, to be secondary in driving THREEs, as we find increased numbers of THREEs predominantly during La Niña periods, which are marked by increased TIV activity (Eddebbar et al, 2021;Zheng et al, 2016).…”

“…For example, a transient weakening of the eastward flowing subsurface currents in the tropical Pacific that ventilate the upper ODZ (i.e., the Equatorial Undercurrent, the North/South Equatorial Countercurrents and the subsurface Countercurrents or "Tsuchiya jets", Stramma et al (2010)), can also lead to a shoaling of the HI. Similarly, tropical instability vortices (TIVs), that is, anticylonic eddies that form along the Equatorial Pacific (centered at around 5°N), usually oxygenate the oxycline west of 120°W during boreal summer to winter (Eddebbar et al, 2021). In contrast, the absence of TIVs during boreal spring goes along with a significant shoaling of the HI (Eddebbar et al, 2021).…”

“…Similarly, tropical instability vortices (TIVs), that is, anticylonic eddies that form along the Equatorial Pacific (centered at around 5°N), usually oxygenate the oxycline west of 120°W during boreal summer to winter (Eddebbar et al, 2021). In contrast, the absence of TIVs during boreal spring goes along with a significant shoaling of the HI (Eddebbar et al, 2021). Fittingly, the phase of other THREEs detected around 5°N west of 120°W (April-June) corresponds with the weak TIV activity during boreal spring.…”

“…On subseasonal to seasonal time scales, subsurface hypoxic volumes are further affected by variability in wind-driven upwelling (Adams et al, 2016;Prakash et al, 2012), up-or downwelling Rossby and Kelvin waves (Graco et al, 2017;Lüdke et al, 2020;Vergara et al, 2016) or subsurface current variability (Pizarro-Koch et al, 2019). On even shorter time scales, mesoscale eddy variability (Auger et al, 2021;Bettencourt et al, 2015;Frenger et al, 2018;Vergara et al, 2016) is complemented by tropical instability vortices (Eddebbar et al, 2021), submesoscale variability (Bertrand et al, 2010;Grados et al, 2016), synoptic wind variability (Duteil, 2019) or internal gravity waves (Bertrand et al, 2014;Booth et al, 2012;Grados et al, 2016) in inducing HI variability. While all of the above processes go along with changes in circulation or mixing, they can also be associated with changes in biological oxygen production and consumption.…”

Modulation of the Eastern Equatorial Pacific Seasonal Cycle by Tropical Instability Waves

“…This reduces the volume of warm subsurface water brought from the western Pacific into the cold tongue, leading to a rectified cooling effect that partly counterbalances the direct TIW warming through NDH (Maillard et al., 2022). By modulating the heat and momentum budgets, TIWs have a complex impact on the mean flow of active and passive tracers, which together with their strong effect on the vertical oceanic mixing (Holmes & Thomas, 2015; Inoue et al., 2012; Lien et al., 2008; Moum et al., 2009) can substantially influence biogeochemical properties (Eddebbar et al., 2021) in this key region for ecosystems and the global carbon cycle (Takahashi et al., 2009).…”