Mixed Layer Physics Research Articles

Multidecadal trend of increasing iron stress in Southern Ocean phytoplankton.

Southern Ocean primary productivity is principally controlled by adjustments in light and iron limitation, but the spatial and temporal determinants of iron availability, accessibility, and demand are poorly constrained, which hinders accurate long-term projections. We present a multidecadal record of phytoplankton photophysiology between 1996 and 2022 from historical in situ datasets collected by Biogeochemical Argo (BGC-Argo) floats and ship-based platforms. We find a significant multidecadal trend in irradiance-normalized nonphotochemical quenching due to increasing iron stress, with concomitant declines in regional net primary production. The observed trend of increasing iron stress results from changing Southern Ocean mixed-layer physics as well as complex biological and chemical feedback that is indicative of important ongoing changes to the Southern Ocean carbon cycle.

The Irrawaddy River Jet in the Andaman Sea During the Summer Monsoon

The Irrawaddy (IR) is the largest river discharging into the Andaman Sea and plays an important role in the salinity distribution and the mixed layer physics of the Andaman Sea. This study presents the first report of the IR plume pathways in the Andaman Sea during the summer monsoon and the mechanisms behind them. An ocean circulation model is employed to conduct idealized experiments in which the freshwater forcing, due to rivers other than IR as well as precipitation, are ignored. Our simulations reveal that, during the summer monsoon, the discharge from Irrawaddy spreads as a freshwater jet oriented towards southeast and accumulates over the shelf at the eastern coast of the Andaman Sea. Climatology of Chlorophyll-a concentration measured by satellite and surface currents from global ocean model reanalysis indicates the presence of the Irrawaddy freshwater jet during the summer monsoon. The evolution of surface salinity and currents along the jet suggests that the IR freshwater traps momentum imparted by winds. The momentum balance in the Irrawaddy jet is between Coriolis and wind friction term, indicating that the freshwater jet is completely driven by winds during the summer monsoon. Surface distribution of wind friction term also shows that the northwest-southeast orientation of the Irrawaddy jet is due to the southwesterly orientation of the summer monsoon winds. Further experiments with three different wind forcing scenarios (no winds, winds over the equator only, and winds over the Bay of Bengal only) reveal that the flow of Irrawaddy jet during the summer monsoon is completely controlled by the local winds.

Revisiting Near-Inertial Wind Work: Slab Models, Relative Stress, and Mixed Layer Deepening

AbstractThe wind generation of near-inertial waves is revisited through use of the Pollard–Rhines–Thompson theory, the Price–Weller–Pinkel (PWP) mixed layer model, and KPP simulations of resonant forcing by Crawford and Large. An Argo mixed layer climatology and 0.6° MERRA-2 reanalysis winds are used to compute global totals and explore hypotheses. First, slab models overestimate wind work by factors of 2–4 when the mixed layer is shallow relative to the scalingH* ≡u*/(Nf)1/2, but are accurate for deeper mixed layers, giving overestimation of global totals by a factor of 1.23 ± 0.03 compared to PWP. Using wind stress relative to the ocean currents further reduces the wind work by an additional 13 ± 0.3%, for a global total wind work of 0.26 TW. Second, the potential energy increase ΔPE due to wind-driven mixed layer deepening is examined and compared to ΔPE computed from Argo and ERA-Interim heat flux climatology. Argo-derived ΔPE closely matches cooling, confirming that cooling sets the seasonal cycle of mixed layer depth and providing a new constraint on observational estimates of convective buoyancy flux at the mixed layer base. Locally and in fall, wind-driven deepening is comparable in importance to cooling. Globally, wind-driven ΔPE is about 11% of wind work, implying that >50% of wind work goes to turbulence and thus not into propagating inertial motions. The fraction into this “modified wind work” is imperfectly estimated in two ways, but we conclude that more research is needed into mixed layer and transition-layer physics. The power available for propagating near-inertial waves is therefore still uncertain, but appears lower than previously thought.

Physical and Biological Drivers of Biogeochemical Tracers Within the Seasonal Sea Ice Zone of the Southern Ocean From Profiling Floats

AbstractHere we present initial findings from nine profiling floats equipped with pH, O2, , and other biogeochemical sensors that were deployed in the seasonal ice zone (SIZ) of the Southern Ocean in 2014 and 2015 through the Southern Ocean Carbon and Climate Observations and Modelling (SOCCOM) project. A large springtime phytoplankton bloom was observed that coincided with sea ice melt for all nine floats. We argue this bloom results from a shoaling of the mixed layer depth, increased vertical stability, and enhanced nutrient and light availability as the sea ice melts. This interpretation is supported by the absence of a springtime bloom when one of the floats left the SIZ in the second year of observations. During the sea ice covered period, net heterotrophic conditions were observed. The rate of uptake of O2 and release of dissolved inorganic carbon (derived from pH and estimated total alkalinity) and is reminiscent of biological respiration and is nearly Redfieldian for the nine floats. A simple model of mixed layer physics was developed to separate the physical and biological components of the signal in pH and O2 over one annual cycle for a float in the Ross Sea SIZ. The resulting annual net community production suggests that seasonal respiration during the ice covered period of the year nearly balances the production in the euphotic layer of up to 5 mol C m−2 during the ice free period leading to a net of near zero carbon exported to depth for this one float.

Assessment of a NEMO-based hydrodynamic modelling system for the Great Lakes

Environment Canada recently developed a coupled lake–atmosphere–hydrological modelling system for the Laurentian Great Lakes. This modelling system consists of the Canadian Regional Deterministic Prediction System (RDPS), which is based on the Global Environmental Multiscale model (GEM), the MESH (Modélisation Environnementale Surface et Hydrologie) surface and river routing model, and a hydrodynamic model based on the three-dimensional global ocean model Nucleus for European Modelling of the Ocean (NEMO). This paper describes the performance of the NEMO model in the Great Lakes. The model was run from 2004 to 2009 with atmospheric forcing from GEM and river forcing from the MESH modelling system for the Great Lakes region and compared with available observations in selected lakes. The NEMO model is able to produce observed variations of lake levels, ice concentrations, lake surface temperatures, surface currents and vertical thermal structure reasonably well in most of the Great Lakes. However, the model produced a diffused thermocline in the central basin of Lake Erie. The model predicted evaporation is relatively strong in the upper lakes. Preliminary results of the modelling system indicate that the model needs further improvements in atmospheric–lake exchange bulk formulae and surface mixed layer physics.

A multi-model study of the restratification phase in an idealized convection basin

The representation of baroclinic instability in numerical models depends strongly upon the model physics and significant differences may be found depending on the vertical discretization of the governing dynamical equations. This dependency is explored in the context of the restratification of an idealized convective basin with no external forcing. A comparison is made between an isopycnic model including a mixed layer (the Miami Isopycnic Coordinate Ocean Model, MICOM), its adiabatic version (MICOM-ADIAB) in which the mixed layer physics are removed and the convective layer is described by a deep adiabatic layer outcropping at the surface instead of a thick dense mixed layer, and a z-coordinate model (OPA model). In the absence of a buoyancy source at the surface, the mixed layer geometry in MICOM prevents almost any retreat of this layer. As a result, lateral heat exchanges in the upper layers are limited while mass transfers across the outer boundary of the deep convective mixed layer result in an unrealistic outward spreading of this layer. Such a widespread deep mixed layer maintains a low level of baroclinic instability, and therefore limits lateral heat exchanges in the upper layers over most of the model domain. The behavior of the adiabatic isopycnic model and z-coordinate model is by far more satisfactory although contrasted features can be observed between the two simulations. In MICOM-ADIAB, the more baroclinic dynamics introduce a stronger contrast between the surface and the dense waters in the eddy kinetic energy and heat flux distributions. Better preservation of the density contrasts around the dense water patch maintains more persistent baroclinic instability, essentially associated with the process of dense water spreading. The OPA simulation shows a faster growth of the eddy kinetic energy in the early stages of the restratification which is attributed to more efficient baroclinic instability and leads to the most rapid buoyancy restoring in the convective area among the three simulations. Dense water spreading and warm surface capping occur on fairly similar time scales in MICOM-ADIAB although the former is more persistent that the latter. In this model, heat is mainly transported by anticyclonic eddies in the dense layer while both cyclonic and anticyclonic eddies are involved in the upper layers. In OPA, heat is mainly brought into the convective zone through the export of cold water trapped in cyclonic eddies with a strong barotropic structure. Probably the most interesting difference between the z-coordinate and the adiabatic isopycnic model is found in the temperature distribution ultimately produced by the restratification process. OPA generates a spurious volume of intermediate water which is not seen in MICOM-ADIAB where the volume of the dense water is preserved.

Impact of circulation on export production, dissolved organic matter, and dissolved oxygen in the ocean: Results from Phase II of the Ocean Carbon‐cycle Model Intercomparison Project (OCMIP‐2)

Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon‐cycle Model Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse‐resolution ocean circulation models. The model mean (±1σ) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr−1. Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation‐based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep‐ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep‐ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross‐model means of several biogeochemical metrics show better agreement with observation‐based estimates when restricted to those models that best simulate deep‐ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.

Mechanisms controlling primary and new production in a global ecosystem model – Part I: Validation of the biological simulation

Abstract. A global general circulation model coupled to a simple six-compartment ecosystem model is used to study the extent to which global variability in primary and export production can be realistically predicted on the basis of advanced parameterizations of upper mixed layer physics, without recourse to introducing extra complexity in model biology. The "K profile parameterization" (KPP) scheme employed, combined with 6-hourly external forcing, is able to capture short-term periodic and episodic events such as diurnal cycling and storm-induced deepening. The model realistically reproduces various features of global ecosystem dynamics that have been problematic in previous global modelling studies, using a single generic parameter set. The realistic simulation of deep convection in the North Atlantic, and lack of it in the North Pacific and Southern Oceans, leads to good predictions of chlorophyll and primary production in these contrasting areas. Realistic levels of primary production are predicted in the oligotrophic gyres due to high frequency external forcing of the upper mixed layer (accompanying paper Popova et al., 2006) and novel parameterizations of zooplankton excretion. Good agreement is shown between model and observations at various JGOFS time series sites: BATS, KERFIX, Papa and HOT. One exception is the northern North Atlantic where lower grazing rates are needed, perhaps related to the dominance of mesozooplankton there. The model is therefore not globally robust in the sense that additional parameterizations are needed to realistically simulate ecosystem dynamics in the North Atlantic. Nevertheless, the work emphasises the need to pay particular attention to the parameterization of mixed layer physics in global ocean ecosystem modelling as a prerequisite to increasing the complexity of ecosystem models.

Interannual Variability of Indian Ocean Heat Transport

Abstract The work in this paper builds upon the relatively well-studied seasonal cycle of the Indian Ocean heat transport by investigating its interannual variability over a 41-yr period (1958–98). An intermediate, two-and-a-half-layer thermodynamically active ocean model with mixed layer physics is used in the investigation. The results of the study reveal that the Indian Ocean heat transport possesses strong variability at all time scales from intraseasonal (10–90 days) to interannual (more than one year). The seasonal cycle dominates the variability at all latitudes, the amplitude of the intraseasonal variability is similar to the seasonal cycle, and the amplitude of the interannual variability is about one-tenth of the seasonal cycle. Spectral analysis shows that a significant broadband biennial component in the interannual variability exists with considerable coherence in sign across the equator. While the mean annual heat transport shows a strong maximum between 10° and 20°S, interannual variability is relatively uniform over a broad latitudinal domain between 15°N and 10°S. The heat transport variability at all time scales is well explained by the Ekman heat transport, with especially good correlations at the intraseasonal time scales. The addition of the Indonesian Throughflow does not significantly affect the heat transport variability in the northern part of the ocean.

Impact of interannual rainfall anomalies on Indian Ocean salinity and temperature variability

A nonlinear reduced gravity model with four active layers and mixed layer physics is used to investigate how precipitation anomalies affect salinity and temperature variability in the Indian Ocean. In one experiment the model is forced by observations of monthly varying winds and rainfall for the period 1980–2000. In another it is forced by the same winds and climatological rainfall. Comparison between the two experiments is focused on the 1992–2000 period, during which rainfall variations attain extrema in the southeastern Indian Ocean with two deficits close to zero in 1994 and 1997 and a maximum of 16 mm/d in 1998. Rainfall anomalies significantly affect model sea surface salinity (SSS), but SSS is also highly governed by winds. Indeed, SSS is at its absolute minimum in 1994 despite the local rainfall deficit. Sea surface temperature (SST) is strongly affected along the equator east of 80°E and along Indonesia where the mixed and the barrier layers become thin in 1994 and 1997. There the precipitation deficits affect SST primarily by entraining more subsurface water to the surface. In 1994 the subsurface waters are anomalously cool because of eastward advection of colder water from the west. In 1997 they are so because of northward advection of water cooled by coastal upwelling farther south. The precipitation excess does not affect SST because the temperature difference between the mixed layer and the barrier layer is very little then. Including interannual rainfall anomalies makes SST off Sumatra colder in 1994 and 1997, in much better agreement with observations.

Impact of 4D-variational assimilation of WOCE hydrography on the meridional circulation of the Indian Ocean

Abstract World Ocean Circulation Experiment (WOCE) hydrographic sections and a sea-surface climatology are combined with a ocean general circulation model through a 4D-variational method to analyze the meridional overturning of the Indian Ocean. The regional model is run with realistic surface forcings over year 1995 for which most of WOCE Indian Ocean sections were made. The assimilation controls the initial temperature and salinity fields, surface forcings and open-boundary velocities, temperature and salinity. When no observations are assimilated, the model shows that the deep (below 1000 m ) meridional overturning is weak compared to observation-based estimates. This is a common feature of general circulation models. In contrast, after the assimilation, the model develops a deep overturning of 17×10 6 m 3 s −1 at 32°S when a 10×10 6 m 3 s −1 Indonesian Throughflow (ITF) is prescribed. The mass flux of bottom waters that moves northward below 3200 m is balanced by a southward mass flux of deep waters between 1000 and 3200 m . The deep overturning carries 10% of the total southward energy flux of 1.2 PW at 32°S. The intensity of this deep overturning changes only by ±2×10 6 m 3 s −1 when the annual mean ITF is zero or 30×10 6 m 3 s −1 . The upper circulation is less constrained by the assimilation because of the large temporal and spatial variability of this ocean and also because of limitations in the representation of the mixed layer physics during the assimilation process. Limitations in the physics of the model also are thought to be the source of the slow erosion of the deep overturning when the model is run for several years from its optimal state.

Improved Representation of Upper-Ocean Dynamics and Mixed Layer Depths in a Model of the North Atlantic on Switching from Eddy-Permitting to Eddy-Resolving Grid Resolution

Two configurations of a primitive-equation model of the North Atlantic are analyzed with respect to the simulated cycling of energy, mass, and heat in the upper ocean. One model is eddy-permitting (1/3° horizontal resolution), the other one is eddy-resolving (1/9° resolution), with both models using identical topographies and identical forcing fields at the surface and lateral boundaries. Besides showing some improvement in the simulated mean circulation and heat budgets, the eddy-resolving model reaches good agreement with satellite altimeter measurements of sea surface height variability. An unexpected finding of the model intercomparison is that simulated winter mixed layer depths in mid and high latitudes turn out to be systematically shallower by some 50 to 500 m in the higher resolution run, thereby agreeing better with observations than the 1/3° model results. This model improvement is related to enhanced levels of baroclinic instability leading to a decrease in potential energy and an associated increase in stratification. In the high-resolution model, shear-induced tilting of lateral density gradients generates stratification within the mixed layer itself, at a rate sufficient to set off an average surface heat loss of 5 W m–2 in mid and high latitudes. Although this is small compared to present uncertainties in surface heat fluxes, the resulting reduction in mixed layer depths may be important for an accurate simulation of water mass formation, air–sea gas exchange, and marine biological production. With traditional formulations of mixed layer physics assuming that properties are set by purely vertical mixing, and parameterizations of lateral subgrid-scale mixing often being tapered to zero in the mixed layer, present mixing schemes would have to be modified in order to account for eddy-induced generation of stratification in the surface mixed layer in noneddy-resolving ocean models.

Forcing Mechanisms of Sea Level Interannual Variability in the Bay of Bengal

A nonlinear, 4½-layer reduced-gravity ocean model with active thermodynamics and mixed layer physics is used to investigate the causes of sea level interannual variability in the Bay of Bengal, which may contribute to flooding and cholera outbreaks in Bangladesh. Forcing by NCEP–NCAR reanalysis fields from 1958 to 1998 yields realistic solutions in the Indian Ocean basin north of 29°S. Controlled experiments elucidate the roles of the following forcing mechanisms: interannual variability of the Bay of Bengal wind, equatorial wind, river discharges into the bay, and surface buoyancy flux including precipitation minus evaporation (heat fluxes + P − E). Sea level changes in the bay result largely from wind variability, with a typical amplitude of 10 cm and occasionally 10–25 cm at an interannual timescale. Near the eastern and northern boundaries, sea level anomalies (SLAs) are predominantly caused by equatorial wind variability, which generates coastal Kelvin waves that propagate into the bay along the eastern boundary. Near the western boundary the bay wind has a comparable influence as the equatorial wind, especially during the southwest monsoon season, owing to the counterclockwise propagation of coastal Kelvin waves forced by the large-scale alongshore wind stress in the bay. In the bay interior, SLAs are dominated by the equatorial wind forcing in the central bay, result almost equally from the equatorial and the bay wind in the southwestern bay, and are dominated by the bay wind forcing in the southwestern bay during the southwest monsoon. The westward intensification of the bay wind influence is associated with the westward propagation of Rossby waves forced by the large-scale wind curl in the interior bay. The effect of heat fluxes + P − E is generally small. Influence of interannual variability of river discharges is negligible. SLAs caused by the equatorial wind at the equator and that caused by the bay wind along the northern and western boundaries as well as in the southwestern bay are significantly correlated, reflecting the anomalous wind pattern associated with the dipole mode event in the tropical Indian Ocean. Given the dominance of equatorial wind forcing near the northern bay boundary, SLAs (or alternatively westerly wind anomalies) in the equatorial ocean may serve as a potential index for predicting Bangladesh flooding and cholera.

Seasonal and interannual modulation of mixed layer variability at 0°, 110°W

Long, high resolution time series from the 0°, 110°W tropical atmosphere ocean mooring in the eastern equatorial Pacific are used to analyze how warm and cold phases of El Niño-Southern Oscillation (ENSO) and the annual cycle modulate the near-surface stratification and sea-surface temperature (SST) diurnal cycle. During the annual warm season (February–April), when solar warming is large and wind mixing weak, the isothermal-mixed layer depth ( MLD T ) is shallow (typically 10 m deep) and the 1 m SST diurnal cycle amplitude is large (typically up to 0.4°C). Likewise during the remainder of the year when SST is generally cool, typically the diurnal cycle amplitude is less than 0.2°C and the isothermal-mixed layer is deeper than 20 m. Thus, annual variations in wind and insolation, which lead to an annual cycle in SST, also cause annual modulation of the SST diurnal cycle and near-surface stratification, consistent with one-dimensional mixed layer physics. However, on interannual time scales, mixed layer physics are more complicated. In particular, the diurnal cycle amplitude and MLD T anomalies are out of phase with the SST anomalies. MLD T is anomalously deep and the SST diurnal cycle amplitude is anomalously low during the warm phase of ENSO. On these longer timescales, MLD T tends to be strongly influenced by thermocline-depth variability. In addition, salinity stratified barrier layers large enough to support temperature inversions were often observed at 0°, 110°W during the final stage of El Niños. As SST rose above 28.5°C during the final stage of the 1997–1998 El Niño, a regime shift was observed, with large temperature inversions, a relative increase in SST diurnal cycle amplitude, and large variability in the mixed-layer depth. It is likely that barrier layers (inferred from temperature inversions) allowed warm conditions to remain, even as the thermocline and mixed-layer depths shoaled.

Influence of precipitation minus evaporation and Bay of Bengal rivers on dynamics, thermodynamics, and mixed layer physics in the upper Indian Ocean

A 4½‐layer model with active thermodynamics and mixed layer physics is used to examine how salinity distributions forced by precipitation P minus evaporation ε and by river runoff in the Bay of Bengal affect dynamics, thermodynamics, and mixed layer physics in the upper Indian Ocean. Each of the four active layers represents a distinct water mass type: the surface mixed layer, the seasonal thermocline (barrier layer in the tropics), the thermocline, and upper intermediate water. Waters are allowed to transfer between layers by interfacial velocities w1, w2, and w3. Velocity w1 parameterizes entrainment and detrainment from the surface mixed layer, and it is determined largely by Kraus and Turner [1967] physics. Velocity w2 is primarily a parameterization of subduction. In regions where precipitation is strong enough for P − ε > 0, forcing by P − ε thins the surface mixed layer (layer 1) because of decreased entrainment, and thus thickens the seasonal thermocline (layer 2, a barrier layer). Additionally, surface currents generally strengthen, T2 warms considerably, and sea surface temperature (SST) increases somewhat, resulting in temperature inversions at some locations in the southern bay and eastern equatorial ocean. This forcing also causes large temperature changes in the thermocline (layer 3), primarily because of heating or cooling by anomalous subduction. During the Southwest Monsoon, forcing by inflow from Bay of Bengal rivers increases SST by 0.5°–1°C along the northeast coast of India. This is because coastal Kelvin waves driven by the Ganges‐Brahmaputra River inflow suppress coastal upwelling there. During the Northeast Monsoon, fresh river water is carried southward by the East India Coastal Current (EICC), raising sea level along the coast and strengthening the EICC by 10 cm s−1. The river water decreases entrainment around the perimeter of the bay during winter, thereby thinning the surface mixed layer, increasing T2 and resulting in temperature inversions in the northwestern bay. River inflow also causes significant temperature anomalies in layer 3 by affecting subduction.

Modeling microzooplankton and macrozooplankton dynamics within a coastal upwelling system

A simple nutrient–phytoplankton–zooplankton (NPZ) pelagic ecosystem model coupled to a two-dimensional primitive equation circulation model with explicit mixed-layer physics is configured in a coastal setting to study the biological response to idealized wind-driven upwelling conditions. Conventional ecosystem model parameterization, which assumes macrozooplankton as the target grazers, leads to upwelling-induced phytoplankton blooms that exhaust available nutrient supply and whose zonal scale increases with wind duration. Offshore zooplankton maxima result from upwelled water with greater total nitrogen concentrations than initial ambient surface water. Substantial vertical mixing in the surface boundary layer sets the vertical scale of the productivity. Phytoplankton sinking contributes to a nearshore accumulation of total nitrogen, and enhances the magnitude and duration of the phytoplankton bloom. The system responds differently when the zooplankton are parameterized to represent microzooplankton. The phytoplankton and zooplankton maxima have more limited zonal extent, are more independent of the duration of wind forcing, and near-surface nutrient levels remain high over most of the domain. When winds are relaxed, the diminished offshore transport reveals the underlying biological oscillations in the microzooplankton-parameterized ecosystem, and reduced vertical mixing decouples surface from subsurface dynamics. In contrast, the macrozooplankton system relaxes to a steady state supporting limited phytoplankton and large zooplankton levels in the upwelling region.

Eddy Subduction in a Model of the Subtropical Gyre

Abstract Subduction is the process by which fluid transfers from the mixed layer to the interior of the ocean. South of the Gulf Stream extension, 18° mode water is formed in a region of high subduction rates. In this region there is high mesoscale eddy activity. In the present study the role of eddies in modifying the large-scale subduction into mode water is investigated. An eddy-resolving isopycnic ocean model with mixed layer physics included and coupled to an atmospheric anomaly model is used for this purpose. The geometry and forcing of the model are idealized. Annual mean subduction rates into mode water up to 200 m yr−1 are found south of the Gulf Stream extension. The eddy contribution to the annual subduction is estimated by comparing the annual mean subduction rates, obtained from monthly mean quantities, with the total subduction rates (i.e., eddy plus mean contributions). The latter are determined by integrating the detrainment rates over the period when fluid is irreversibly detrained. Eddy ...

Dynamics of the Eastern Surface Jets in the Equatorial Indian Ocean*

An hierarchy of ocean models is used to investigate the dynamics of the eastward surface jets that develop along the Indian Ocean equator during the spring and fall, the Wyrtki jets (WJs). The models vary in dynamical complexity from 2½-layer to 4½-layer systems, the latter including active thermodynamics, mixed layer physics, and salinity. To help identify processes, both linear and nonlinear solutions are obtained at each step in the hierarchy. Specific processes assessed are as follows: direct forcing by the wind, reflected Rossby waves, resonance, mixed layer shear, salinity effects, and the influence of the Maldive Islands. In addition, the sensitivity of solutions to forcing by different wind products is reported. Consistent with previous studies, the authors find that direct forcing by the wind is the dominant forcing mechanism of the WJs, accounting for 81% of their amplitude when there is a mixed layer. Reflected Rossby waves, resonance, and mixed layer shear are all necessary to produce jets with realistic strength and structure. Completely new results are that precipitation during the summer and fall considerably strengthens the fall WJ in the eastern ocean by thinning the mixed layer, and that the Maldive Islands help both jets to attain roughly equal strengths. In both the ship-drift data and the authors’ “best” solution (i.e., the solution to the highest model in the authors’ hierarchy), the semiannual response is more than twice as large as the annual one, even though the corresponding wind components have comparable amplitudes. Causes of this difference are as follows: the complex zonal structure of the annual wind, which limits the directly forced response at the annual frequency;resonance with the semiannual wind; and mixed layer shear flow, which interferes constructively (destructively) with the rest of the response for the semiannual (annual) component. Even in the most realistic solution, however, the annual component still weakens the fall WJ and strengthens the spring one in the central ocean, in contrast to the ship-drift data; this model/data discrepancy may result from model deficiencies, inaccurate driving winds, or from windage errors in the ship-drift data themselves.

Stochastically Forced Mode Water Variability

Substantial interannual to decadal variability is observed in the properties of subtropical mode water of the North Atlantic. In this study the response of mode water to stochastic atmospheric forcing is investigated in a numerical model. In a series of experiments the response is studied to different components of stochastic atmospheric forcing, such as wind stress, freshwater flux, and heat flux. The numerical model consists of an isopycnal ocean model with explicit mixed layer physics. The stochastic forcing is superimposed on the climatological forcing. The stochastic forcing function has an idealized form, but the amplitude, the spatial, and the temporal variability are based on observations. When a stochastic heat flux is applied, an atmospheric anomaly model is coupled to the ocean model. The geometry of the model is idealized and mimics the subtropical gyre of the North Atlantic. The stochastic wind stress forcing excites an internal mode in the mode water layer of the model. The response is characterized by the propagation of baroclinic waves. The spectrum of the response to stochastic freshwater flux is red. In the coupled model the stochastic heat flux forcing generates variability characterized by a dipole pattern in the mode water. The spectrum of the response is red and dominates the response to the stochastic wind stress and freshwater flux. The response is damped by an atmospheric feedback that consists of anomalous heat fluxes, depending on the SST anomalies generated by the stochastic forcing itself. Only stochastic heat flux forcing can generate mode water variability of the observed amplitude. A preferred timescale in mode water variability should be contained in the forcing itself or it may result from modes that could not be simulated by the present model.

An eddy‐permitting coupled physical‐biological model of the North Atlantic: 1. Sensitivity to advection numerics and mixed layer physics

Physical influences on biological primary production in the North Atlantic are investigated by coupling a four‐component pelagic ecosystem model with a high‐resolution numerical circulation model. A series of sensitivity experiments demonstrates the important role of an accurate formulation of upper ocean turbulence and advection numerics. The unrealistically large diffusivity implicit in upstream advection approximately doubles primary production when compared with a less diffusive, higher‐order, positive‐definite advection scheme.This is of particular concern in the equatorial upwelling region where upstream advection leads to a considerable increase of upper ocean nitrate concentrations. Counteracting this effect of unrealistically large implicit diffusion by changes in the biological model could easily lead to misconceptions in the interpretation of ecosystem dynamics. Subgrid‐scale diapycnal diffusion strongly controls biological production in the subtropical gyre where winter mixing does not reach the nutricline. The parameterization of vertical viscosity is important mainly in the equatorial region where friction becomes an important agent in the momentum balance.