Decadal variability in the northeast Pacific in a physical-ecosystem model: Role of mixed layer depth and trophic interactions

Abstract This paper presents a simple diagnostic model for estimating mixed layer depth based solely on the one-dimensional heat balance equation, the surface heat flux, and the sea surface temperature. The surface fluxes drive heating or cooling of the upper layer whereas the surface temperature acts as a “thermostat” that regulates the vertical extent of the layer. Daily mixed layer depth estimates from the diagnostic model (and two standard bulk mixed layer models) are compared with depths obtained from oceanic profiles collected during the 1956–80 Canadian Weathership program at Station P and more recent (2001–07) profiles from the vicinity of this station from Argo drifters. Summer mixed layer depths from the diagnostic model agree more closely with observed depths and are less sensitive to heat flux errors than those from bulk models. For the Weathership monitoring period, the root-mean-square difference between modeled and observed monthly mean mixed layer depths is ∼6 m for the diagnostic model and ∼10 m for the bulk models. The diagnostic model is simpler to apply than bulk models and sidesteps the need for wind data and turbulence parameterization required by these models. Mixed layer depths obtained from the diagnostic model using NCEP–NCAR reanalysis data reveal that—contrary to reports for late winter—there has been no significant trend in the summer mixed layer depth in the central northeast Pacific over the past 52 yr.

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Influences of initial plankton biomass and mixed layer depths on the outcome of iron-fertilization experiments

Biogeosciences Discussions ◽

10.5194/bgd-4-4411-2007 ◽

2007 ◽

Vol 4 (6) ◽

pp. 4411-4441 ◽

Cited By ~ 1

Author(s):

M. Fujii ◽

F. Chai

Keyword(s):

Mixed Layer ◽

Phytoplankton Community ◽

Mixed Layer Depth ◽

Marine Ecosystem ◽

Ecosystem Model ◽

Layer Depth ◽

Iron Enrichment ◽

Mesozooplankton Biomass ◽

Marine Ecosystem Model ◽

Enrichment Experiments

Abstract. Several in situ iron-enrichment experiments have been conducted, where the response of the phytoplankton community differed. We use a marine ecosystem model to investigate the effect of iron on phytoplankton in response to different initial plankton conditions and mixed layer depths. Sensitivity analysis of the model results to the mixed layer depths reveals that the modeled response to the same iron enhancement treatment differed dramatically according to the different mixed layer depth. The magnitude of the iron-induced biogeochemical responses in the surface water, such as maximum chlorophyll, is inversely correlated with the mixed layer depth, as observed. The significant decrease in maximum surface chlorophyll with mixed layer depth results from the difference in diatom concentration in the mixed layer, which is determined by vertical mixing. Sensitivity of the model to initial mesozooplankton (as grazers on diatoms) biomass shows that column-integrated net community production and export production are strongly controlled by the initial mesozooplankton biomass. Higher initial mesozooplankton biomass yields high grazing pressure on diatoms, which results in less accumulation of diatom biomass. The initial diatom biomass is also important to the outcome of iron enrichment but is not as crucial as the mixed layer depth and the initial mesozooplankton biomass. This modeling study suggests not only mixed layer depth but also the initial biomass of diatoms and its principle grazers are crucial factors in the response of the phytoplankton community to the iron enrichments, and should be considered in designing future iron-enrichment experiments.

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Exploration of the critical depth hypothesis with a simple NPZ model

ICES Journal of Marine Science ◽

10.1093/icesjms/fsv016 ◽

2015 ◽

Vol 72 (6) ◽

pp. 1916-1925 ◽

Cited By ~ 11

Author(s):

Marina Lévy

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Ecosystem Model ◽

Model Solution ◽

Critical Depth ◽

Phytoplankton Blooms ◽

Layer Depth ◽

Seasonal Evolution ◽

Physical Forcing ◽

Surface Irradiance

Abstract The critical depth hypothesis (CDH) is a predictive criteria for the onset of phytoplankton blooms that comes from the steady-state analytical solution of a simple mathematical model for phytoplankton growth presented by Sverdrup in 1953. Sverdrup's phytoplankton-only model is very elementary compared with state-of-the-art ecosystem models whose numerical solution in a time-varying environment do not systematically conform to the CDH. To highlight which model ingredients make the bloom onset deviate from the CDH, the complexity of Sverdrup's model is incrementally increased, and the impact that each new level of complexity introduced is analysed. Complexity is added both to the ecosystem model and to the parameterization of physical forcing. In the most complete experiment, the model is a one-dimensional Nutrient-Phytoplankton-Zooplankton model that includes seasonally varying mixed layer depth and surface irradiance, light and nutrient limitation, variable grazing, self-shading, export, and remineralization. When complexity is added to the ecosystem model, it is found that the model solution only marginally deviates from the CDH. But when the physical forcing is also changed, the model solution can conform to two competing theories for the onset of phytoplankton blooms—the critical turbulence hypothesis and the disturbance recovery hypothesis. The key roles of three physical ingredients on the bloom onset are highlighted: the intensity of vertical mixing at the end of winter, the seasonal evolution of the mixed-layer depth from the previous summer, and the seasonal evolution of surface irradiance.

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On the Emergence of the Atlantic Multidecadal SST Signal: A Key Role of the Mixed Layer Depth Variability Driven by North Atlantic Oscillation

Journal of Climate ◽

10.1175/jcli-d-19-0283.1 ◽

2020 ◽

Vol 33 (9) ◽

pp. 3511-3531

Author(s):

Ayako Yamamoto ◽

Hiroaki Tatebe ◽

Masami Nonaka

Keyword(s):

Heat Flux ◽

North Atlantic Oscillation ◽

Mixed Layer ◽

North Atlantic ◽

Mixed Layer Depth ◽

Vertical Component ◽

Surface Heat ◽

Ocean Dynamics ◽

Layer Depth

AbstractDespite its wide-ranging potential impacts, the exact cause of the Atlantic multidecadal oscillation/variability (AMO/AMV) is far from settled. While the emergence of the AMO sea surface temperature (SST) pattern has been conventionally attributed to the ocean heat transport, a recent study showed that the atmospheric stochastic forcing is sufficient. In this study, we resolve this conundrum by partitioning the multidecadal SST tendency into a part caused by surface heat fluxes and another by ocean dynamics, using a preindustrial control simulation of a state-of-the-art coupled climate model. In the model, horizontal ocean heat advection primarily acts to warm the subpolar SST as in previous studies; however, when the vertical component is also considered, the ocean dynamics overall acts to cool the region. Alternatively, the heat flux term is primarily responsible for the subpolar North Atlantic SST warming, although the associated surface heat flux anomalies are upward as observed. Further decomposition of the heat flux term reveals that it is the mixed layer depth (MLD) deepening that makes the ocean less susceptible for cooling, thus leading to relative warming by increasing the ocean heat capacity. This role of the MLD variability in the AMO signature had not been addressed in previous studies. The MLD variability is primarily induced by the anomalous salinity transport by the Gulf Stream modulated by the multidecadal North Atlantic Oscillation, with turbulent fluxes playing a secondary role. Thus, depending on how we interpret the MLD variability, our results support the two previously suggested frameworks, yet slightly modifying the previous notions.

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