Western Boundary Current Regions Research Articles

Averaging-Related Biases in Monthly Latent Heat Fluxes

Abstract Seasonal-to-multidecadal applications that require ocean surface energy fluxes often require accuracies of surface turbulent fluxes to be 5 W m−2 or better. While there is little doubt that uncertainties in the flux algorithms and input data can cause considerable errors, the impact of temporal averaging has been more controversial. The biases resulting from using monthly averaged winds, temperatures, and humidities in the bulk aerodynamic formula (i.e., the so-called classical method) to estimate the monthly mean latent heat fluxes are shown to be substantial and spatially varying in a manner that is consistent with most prior work. These averaging-related biases are linked to nonnegligible submonthly covariances between the wind, temperature, and humidity. To provide additional insight into the averaging-related bias, the methodology behind the third-generation Florida State University monthly mean surface flux product (FSU3) is detailed to highlight additional sources of errors in gridded datasets. The FSU3 latent heat fluxes suffer from this averaging-related bias, which can be as large as 90 W m−2 in western boundary current regions during winter and can exceed 40 W m−2 in synoptically active portions of the tropics. The regional impacts of these biases on the mixed layer temperature tendency are shown to demonstrate that the error resulting from applying the classical method is physically substantial.

Characterization of Turbulent Latent and Sensible Heat Flux Exchange between the Atmosphere and Ocean in MERRA

Abstract Turbulent fluxes of heat and moisture across the atmosphere–ocean interface are fundamental components of the earth’s energy and water balance. Characterizing both the spatiotemporal variability and the fidelity of these exchanges of heat and moisture is critical to understanding the global water and energy cycle variations, quantifying atmosphere–ocean feedbacks, and improving model predictability. This study examines the veracity of the recently completed NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) product in terms of its turbulent surface fluxes. This assessment employs a large dataset of directly measured turbulent fluxes as well as other turbulent surface flux datasets. The spatial and temporal variability of the surface fluxes are examined in terms of their annual-mean climatologies, their seasonal covariability of near-surface bulk parameters, and their representation of extremes. The impact of data assimilation on the near-surface parameters is assessed through evaluation of the incremental analysis update tendencies. It is found that MERRA turbulent surface fluxes are relatively accurate for typical conditions but have systematically weak vertical gradients in moisture and temperature and a weaker covariability between the near-surface gradients and wind speed than found in observations. This results in an underestimate of the surface latent and sensible heat fluxes over the western boundary current and storm-track regions. The assimilation of observations generally acts to bring MERRA closer to observational products by increasing moisture and temperature near the surface and decreasing the near-surface wind speeds.

Probability Distribution Characteristics for Surface Air–Sea Turbulent Heat Fluxes over the Global Ocean

Abstract To analyze the probability density distributions of surface turbulent heat fluxes, the authors apply the two-parametric modified Fisher–Tippett (MFT) distribution to the sensible and latent turbulent heat fluxes recomputed from 6-hourly NCEP–NCAR reanalysis state variables for the period from 1948 to 2008. They derived the mean climatology and seasonal cycle of the location and scale parameters of the MFT distribution. Analysis of the parameters of probability distributions identified the areas where similar surface turbulent fluxes are determined by the very different shape of probability density functions. Estimated extreme turbulent heat fluxes amount to 1500–2000 W m−2 (for the 99th percentile) and can exceed 2000 W m−2 for higher percentiles in the subpolar latitudes and western boundary current regions. Analysis of linear trends and interannual variability in the mean and extreme fluxes shows that the strongest trends in extreme fluxes (more than 15 W m−2 decade−1) in the western boundary current regions are associated with the changes in the shape of distribution. In many regions changes in extreme fluxes may be different from those for the mean fluxes at interannual and decadal time scales. The correlation between interannual variability of the mean and extreme fluxes is relatively low in the tropics, the Southern Ocean, and the Kuroshio Extension region. Analysis of probability distributions in turbulent fluxes has also been used in assessing the impact of sampling errors in the Voluntary Observing Ship (VOS)-based surface flux climatologies, allowed for the estimation of the impact of sampling in extreme fluxes. Although sampling does not have a visible systematic effect on mean fluxes, sampling uncertainties result in the underestimation of extreme flux values exceeding 100 W m−2 in poorly sampled regions.

Mean-flow and topographic control on surface eddy-mixing in the Southern Ocean

Surface cross-stream eddy diffusion in the Southern Ocean is estimated by monitoring dispersion of particles numerically advected with observed satellite altimetry velocity fields. To gain statistical significance and accuracy in the resolution of the jets, more than 1,5 million particles are released every 6 months over 16 years and advected for one year. Results are analyzed in a dynamic height coordinate system. Cross-stream eddy diffusion is highly inhomogenous. Diffusivity is larger on the equatorward flank of the Antarctic Circumpolar Current (ACC) along eddy stagnation bands, where eddy displacement speed approaches zero. Along such bands, diffusivities reach typical values of 3500 m2 s-1. Local maxima of about 8-12.103 m2 s-1 occur in the energetic western boundary current systems. In contrast, diffusivity is lower in the core of the Antarctic Circumpolar Current with values of 1500-3000 m2 s-1, and continues to decrease south of the main ACC system. The distribution of eddy diffusion is set at three scales: at circumpolar scale, the mean flow reduces diffusion in the ACC and enhances it on the equatorward side of the current; at basin scale, diffusion is enhanced in the energetic western boundary current extension regions; at regional scale, diffusion is enhanced in the wake of large topographic obstacles. We find that the zonally average structure of eddy diffusion can be explained by theory which takes mean flow into account; however, local values depend on eddy propagation, not simply described by a single wave speed, and topography.

Numerical study on the interactions between a mesoscale eddy and a western boundary current

A reduced-gravity primitive equation eddy resolving model has been used to study the decay of a mesoscale eddy as it migrates toward a western boundary current (WBC) region. The model results indicated that the gradient of the relative vorticity to the east of the WBC is an important factor in the interaction between an eddy and a WBC. A circular eddy is deformed into an elliptical form during the eddy–WBC interaction with the major axis of a cyclonic/anticyclonic eddy aligning in the NW/NNE direction, respectively. Because of the difference in the major axes orientations for the cyclonic and anticyclonic eddies, the kinetic energy transfer between a WBC and a particular eddy has very different behavior. A cyclonic eddy loses its energy to the mean field, whereas an anticyclonic eddy can obtain energy from the mean flow during the WBC–eddy interaction. An anticyclonic eddy, however, still decayed from losing its water and friction dissipation during the interaction period.

Coupled Ocean–Atmosphere Responses to Recent Freshwater Flux Changes over the Kuroshio–Oyashio Extension Region

Abstract Observations have indicated a trend of freshwater loss in the global western boundary current extension regions over several recent decades. In this paper, the coupled ocean–atmosphere response to the observed freshwater flux trend [defined as evaporation minus precipitation (EmP)] over the Kuroshio–Oyashio Extension (KOE) region is studied in a series of coupled model experiments. The model explicitly demonstrates that the positive EmP forcing in the KOE region can set up a cyclonic gyre straddling the subtropical and the subpolar gyre, which induces anomalous southward cold advection in the west and northward warm advection in the interior. This leads to the formation of a temperature dipole in the midlatitudes with a cooling in the west and a warming in the east. With the positive EmP forcing in the KOE, the response of the extratropical atmospheric circulation in the North Pacific sector is characterized by an equivalent barotropic low originating primarily from the western tropical Pacific changes and countered by the extratropical SST forcing. The positive EmP forcing also strengthens the tropical zonal SST gradient and thus ENSO through several competing processes including the surface-coupled wind–evaporative–SST (WES) mechanism, subduction of extratropical warm anomalies, and spinup of the density-driven meridional overturning circulation. Applications to recent Pacific climate changes are discussed.

An Enhancement of Low-Frequency Variability in the Kuroshio–Oyashio Extension in CCSM3 owing to Ocean Model Biases

Abstract Enhanced decadal variability in sea surface temperature (SST) centered on the Kuroshio Extension (KE) has been found in the Community Climate System Model version 3 (CCSM3) as well as in other coupled climate models. This decadal peak has higher energy than is found in nature, almost twice as large in some cases. While previous analyses have concentrated on the mechanisms for such decadal variability in coupled models, an analysis of the causes of excessive SST response to changes in wind stress has been missing. Here, a detailed comparison of the relationships between interannual changes in SST and sea surface height (SSH) as a proxy for geostrophic surface currents in the region in both CCSM3 and observations, and how these relationships depend on the mean ocean circulation, temperature, and salinity, is made. We use observationally based climatological temperature and salinity fields as well as satellite-based SSH and SST fields for comparison. The primary cause for the excessive SST variability is the coincidence of the mean KE with the region of largest SST gradients in the model. In observations, these two regions are separated by almost 500 km. In addition, the too shallow surface oceanic mixed layer in March north of the KE in the subarctic Pacific contributes to the biases. These biases are not unique to CCSM3 and suggest that mean biases in current, temperature, and salinity structures in separated western boundary current regions can exert a large influence on the size of modeled decadal SST variability.

Western Pacific coastal sources of iron, manganese, and aluminum to the Equatorial Undercurrent

We present results from the first zonal transect of iron, aluminum, and manganese conducted from the western source region of the Equatorial Undercurrent (EUC) to the central equatorial Pacific. Trace metals were elevated along the slope of Papua New Guinea and within the New Guinea Coastal Undercurrent (NGCU), which is the primary Southern Hemisphere entry path of water to the EUC. Subsurface maxima in total acid‐soluble iron, aluminum, and manganese were evident in the EUC. These maxima were generally greatest in the western equatorial Pacific and decreased in magnitude eastward. Maxima in iron and aluminum persisted to 140°W; maxima in manganese extended to 175°W. Iron and manganese maxima were deeper (25–75 m) than aluminum maxima and located in the lower EUC, which undergoes less interior ocean mixing than shallower waters. The depth of the aluminum subsurface maxima correlated strongly (r = 0.88) with the depth of the EUC velocity maximum. Surface waters were enriched in aluminum and manganese offshore of Papua New Guinea. Surface metal concentrations decreased eastward throughout the western warm pool up to the longitude (∼180°W) of the salinity front. Detrital sediment input from either direct riverine input or sediment resuspension appeared to be the primary mechanism of supplying metals to the NGCU. We estimated eastward fluxes of metals in the EUC and found greatest fluxes in the western equatorial Pacific between 160°E and 165°E, except for aluminum. Fluxes of aluminum and, to a lesser extent, manganese increased concurrently with water volume transport in the central equatorial Pacific. Iron transport in the EUC remained constant east of the dateline, apparently due to the combined effects of dilution by meridional entrainment and scavenging. Iron was mobilized in a highly active western boundary current region and transported eastward in the lower EUC.

Role of the Gulf Stream and Kuroshio–Oyashio Systems in Large-Scale Atmosphere–Ocean Interaction: A Review

Abstract Ocean–atmosphere interaction over the Northern Hemisphere western boundary current (WBC) regions (i.e., the Gulf Stream, Kuroshio, Oyashio, and their extensions) is reviewed with an emphasis on their role in basin-scale climate variability. SST anomalies exhibit considerable variance on interannual to decadal time scales in these regions. Low-frequency SST variability is primarily driven by basin-scale wind stress curl variability via the oceanic Rossby wave adjustment of the gyre-scale circulation that modulates the latitude and strength of the WBC-related oceanic fronts. Rectification of the variability by mesoscale eddies, reemergence of the anomalies from the preceding winter, and tropical remote forcing also play important roles in driving and maintaining the low-frequency variability in these regions. In the Gulf Stream region, interaction with the deep western boundary current also likely influences the low-frequency variability. Surface heat fluxes damp the low-frequency SST anomalies over the WBC regions; thus, heat fluxes originate with heat anomalies in the ocean and have the potential to drive the overlying atmospheric circulation. While recent observational studies demonstrate a local atmospheric boundary layer response to WBC changes, the latter’s influence on the large-scale atmospheric circulation is still unclear. Nevertheless, heat and moisture fluxes from the WBCs into the atmosphere influence the mean state of the atmospheric circulation, including anchoring the latitude of the storm tracks to the WBCs. Furthermore, many climate models suggest that the large-scale atmospheric response to SST anomalies driven by ocean dynamics in WBC regions can be important in generating decadal climate variability. As a step toward bridging climate model results and observations, the degree of realism of the WBC in current climate model simulations is assessed. Finally, outstanding issues concerning ocean–atmosphere interaction in WBC regions and its impact on climate variability are discussed.

Interannual variability of CFC-11 absorption by the ocean: an offline model study

The global ocean Chlorofluorocarbon (CFC-11) was simulated in an offline model driven by re-analysis ocean currents in order to identify the mechanisms of interannual to interdecadal variability of air–sea CFC fluxes. The model was forced with the observed anthropogenic perturbations of atmospheric CFC-11 from the post industrial period (1938) following the OCMIP-II flux protocols along with the observed winds from 1960 to 1999 in the formulation of surface gas exchanges. The model ocean CFC-11 inventories, at the end of 1990s, accounted approximately 1% of the total atmospheric CFC-11, which is consistent with the corresponding observations. The mid-to-high latitude oceans were venue for strong (weak) oceanic sinks (sources) of CFC-11 during the winter (summer) months. The Southern Ocean (south of 40°S) and the North Atlantic (north of 35°N) provided two largest sinks of CFC-11, through which 31.4 and 14.6% of the global ocean CFC-11 entered, respectively. The eastern tropical Pacific Ocean exhibited large interannual variability of CFC-11 flux with a strong (weak) sink during La Nina (El Nino) years and represented 36% of the global CFC-11 flux variability. The North Atlantic and Southern Ocean were found as regions of large sink efficiency: a capacity to sink more CFC than outsource, although it reduced by 80 and 70%, respectively, in the last 40 years compared to 1960. The sink to source ratio of global ocean CFC-11 fluxes were reduced from 90 to 50% in the last 40 years. This indicates a saturation of CFC in the above-thermocline subsurface that makes the upper ocean less efficient in absorbing CFC in recent decades. A positive trend in CFC sink is now limited to the Southern Ocean, central tropical Pacific and western boundary current regions which possess active upwelling of old water with long time since last atmospheric contact. However, a globally averaged trend was a reduced CFC-11 sink, by emitting 30% of the total ocean CFC-11 that was absorbed during last 40 years.

Observations of Entry and Exit of Potential Vorticity at the Sea Surface

Abstract Although potential vorticity (PV) is central to many theories of the oceanic circulation, the entry–exit of PV at the sea surface has not been thoroughly discussed from an observational perspective. After clarifying the notion of “PV entry and exit,” and the mechanisms responsible for it, a climatology of this quantity for the Northern Hemisphere is presented. It is found that surface PV loss over western boundary current regions and their interior extension is a robust feature over the North Pacific and Atlantic basins. At high latitudes, mechanical and diabatic effects act in concert in the North Atlantic to drive the net PV exit. In the Pacific, however, these effects oppose each other and the net entry–exit of PV is more uncertain. At low latitudes, surface winds are found to be particularly important in setting the surface PV exit in the Pacific, equatorward of the intertropical convergence zone.

Impact of the subtropical mode water biogeochemical properties on primary production in the North Atlantic: New insights from an idealized model study

An idealized biophysical model of the North Atlantic was designed to investigate the setting and variability of the subtropical mode water (STMW) biogeochemical properties and its impact on surface primary production in the North Atlantic. The model solution first emphasizes that the exact timing of STMW formation versus the timing of the spring bloom is of primary importance for setting the STMW biogeochemical properties. The surface primary production reaches its maximum in March in the STMW formation region just before its subduction. Thus the spring bloom consumes nitrate at the surface before STMW subducts, and STMW leaves the upper layers depleted in nutrients and fueled in organic matter. This spring consumption explains the low nutrient content of STMW observed near its source region by J. B. Palter et al. (2005). Furthermore, the model suggests that STMW plays a key role in exporting dissolved organic matter (DOM) at subsurface. The spring bloom produces a significant amount of DOM sequestrated in the mode waters after its subduction. This large pool of DOM is then remineralized with time along the transit of STMW through the subtropical gyre. Consequently, the nutrient content of STMW increases as it moves away from its source region. Finally, the model shows also that STMW is very important in controlling primary production in the western boundary current (WBC) region. Indeed, STMW remains isolated from the surface along its trajectory within the subtropical gyre. It joins the mixed layer by obduction in the WBC region only. This nutrient‐rich old STMW irrigates and fertilizes the euphotic zone primarily in the WBC and then spreads along the boundary between the two gyres by advection.

Disentangling the upwelling mechanisms of the South Brazil Bight

This article presents a suite of long-term numerical simulations that investigate the dynamical mechanisms controlling the circulation in the South Brazil Bight (SBB). The overarching goal of these simulations is to quantify the relative contributions of local wind forcing and the Brazil Current (BC) to the upwelling of nutrient-rich slope water onto the shelf. The model results indicate that the water mass structure of the SBB is controlled by the synergy between wind-driven, inner-shelf upwelling and geostrophic, shelf-break upwelling. The later extends yearlong but the former peaks during the austral summer and decreases towards the winter. The interaction between the poleward flow of the BC and the bottom topography greatly influences the shelf circulation, particularly in the bottom boundary layer. Changes of the SBB coastline direction and shelf width modulate the along-shore pressure gradient and the magnitude of the shelf-break upwelling and downwelling. Thus, although the summer upwelling winds extend over large part of the SBB surface temperatures are warmer in the south because of the cooling effect of the shelf-break upwelling in the northern region. At difference with previous studies of shelf-break dynamics the shelf-break upwelling in our model is not controlled by the uplifting associated with the presence of instabilities of the boundary current or nonlinear accelerations under a variable shelf width. The proposed mechanism is relatively simple. As the boundary current flows along the continental slope, changes in the coastline orientation and along-shore bottom topography modify the along-shore pressure gradient which through geostrophy leads to inshore bottom flow and hence shelf-break upwelling. Such a mechanism can provide insight into upwellings on other western boundary current regions where similar topographic variations exist.

Sea Surface Height Signals as Indicators for Oceanic Meridional Mass Transports

Abstract Numerical models are used to test whether the sea surface height (SSH) can be used as an indicator for the variability of Atlantic meridional oceanic mass transports. The results suggest that if the transports over the western boundary current region and those in the eastern part of the basin are considered separately, significant correlations (0.3–0.9) are found between zonal SSH differences and the meridional transports in the top 1100 m. Much weaker correlations are found for the basinwide transport, which corresponds to the surface branch of the meridional overturning circulation (MOC). For the eastern and western branches of the meridional transport, combining the SSH signal with the baroclinic structure obtained from Rossby wave theory enables calculation of a quantitative estimate of the transport variability in the top 1100 m. The results of the method are less convincing for the variability of the MOC. The reason for this is that even small relative errors in the variability of the eastern and western branches can be large compared with the MOC variability. These errors project onto the sum of the eastern and western transports and therefore onto the surface branch of the MOC. Nevertheless, being able to infer transport anomalies from SSH signals in the eastern and western parts of the Atlantic might prove useful in interpreting MOC observations from the U.K. Natural Environment Research Council Rapid Climate Change (RAPID) mooring array at 26°N, which show a large subannual variability that is mainly due to changes at the western boundary. Transports inferred from the SSH could help to identify the origin of this variability and whether transport anomalies propagate into the western boundary region from the basin interior or from other latitudes.

Global Interannual Trends and Amplitude Modulations of the Sea Surface Height Anomaly from the TOPEX/Jason-1 Altimeters

Abstract This study uses the global Ocean Topography Experiment (TOPEX)/Jason-1 altimeters’ time series to estimate the 13-yr trend in sea surface height anomaly. These trends are estimated at each grid point by two methods: one fits a straight line to the time series and the other is based on the difference between the average height between the two halves of the time series. In both cases the trend shows large regional variability, mostly where the intense western boundary currents turn. The authors hypothesize that the regional variability of the sea surface height trends leads to changes in the local geostrophic transport. This in turn affects the instability-related processes that generate mesoscale eddies and enhances the Rossby wave signals. This hypothesis is verified by estimates of the trend of the amplitude of the filtered sea surface height anomaly that contains the spectral bands associated with Rossby waves and mesoscale eddies. The authors found predominantly positive tendency in the amplitude of Rossby waves and eddies, which suggests that, on average, these events are becoming more energetic. In some regions, the variation in amplitude over 13 yr is comparable to the standard deviation of the data and is statistically significant according to both methods employed in this study. It is plausible that in this case, the energy is transferred from the mean currents to the waves and eddies through barotropic and baroclinic instability processes that are more pronounced in the western boundary current extension regions. If these heat storage patterns and trends are confirmed on longer time series, then it will be justified to argue that the warming trend of the last century provides the energy that amplifies both Rossby waves and mesoscale eddies.

Contrasting spawning temperature optima: Why are anchovy and sardine regime shifts synchronous across the North Pacific?

Anchovy and sardine are both abundant small pelagic fish that have exhibited out-of-phase population oscillations in both the western boundary current region off Japan and the eastern boundary current upwelling region off California. These species alternations could indicate approximately synchronous patterns (with some variations in timing between ecosystems) despite the reversed temperature regimes for certain periods across the North Pacific, at least until recently. Here we suggest a contrasting pattern of temperature optima for anchovy and sardine spawnings between the opposite sides of the North Pacific as a possible mechanism of the synchronous phases of species alternations. Spawning temperature optima were examined for Japanese anchovy ( Engraulis japonicus) and Japanese sardine ( Sardinops melanostictus), based on occurrence of eggs and larvae in relation to sea surface temperature. The analyses were based on an updated long-term data set of egg and larval surveys from 1978 to 2004 (102,660 net tows) in the western North Pacific. The ratio of relative frequencies of plankton net samples that are positive for eggs or larvae versus all samples was calculated for temperature intervals of 0.1 °C for both species. This spawning temperature index exceeded 1.0 (baseline for optimum) from ca. 15 to 28 °C with a mid-point at 22 °C for anchovy and ca. 13–20 °C with a marked peak at 16–17 °C for sardine. Patterns of spawning temperature clearly show “warm” and “eurythermal” Japanese anchovy and “cool” and “stenothermal” Japanese sardine in the western North Pacific. This relationship between anchovy and sardine showed a marked contrast to earlier published reports on the spawning temperature optima of northern anchovy ( E. mordax) and California sardine ( S. sagax) in the California Current system. The spawning temperature optimum appears to be species-specific rather than genus-specific. The reversed species-specific temperature optima of anchovy and sardine under the reversed temperature regimes could provide a possible theoretical solution to the synchronous anchovy and sardine regime shifts across the North Pacific.

The shedding of mesoscale anticyclonic eddies from the Alaskan Stream and westward transport of warm water

Abstract The south-flowing waters of the Kamchatka and Oyashio Currents and west-flowing waters of the Alaskan Stream are key components of the western sub-Arctic Pacific circulation. We use CTD data, Argo buoys, WOCE surface drifters, and satellite-derived sea-level observations to investigate the structure and interannual changes in this system that arise from interactions among anticyclonic eddies and the mean flow. Variability in the temperature of the upstream Oyashio and Kamchatka Currents is evident by warming in mesothermal layer in 1994–2005 compared to 1990–1991. A major fraction of the water in these currents is derived directly from the Alaskan Stream. The stream also sheds large anticyclonic (Aleutian) eddies, averaging approximately 300 km in diameter with a volume transport significant in comparison with that of the Kamchatka Current itself. These eddies enclose pools of relatively warm and saline water whose temperature is typically 4 °C warmer and salinity is 0.4 greater than that of cold-core Kamchatka eddies in the same density range. Aleutian eddies drift at approximately 1.2 km d−1 and retain their distinctive warm and salty characteristics for at least 2 years. Selected westward pathways during 1990–2004 are identified. If the shorter northern route is followed, Aleutian eddies remain close to the stream and persist sufficiently long to carry warm and saline water directly to the Kamchatka Current. This was observed during 1994–1997 with substantial warming of the waters in the Kamchatka Current and upstream Oyashio. If the eddies take a more southern route they detach from the stream but can still contribute significant quantities of warm and saline water to the upstream Oyashio, as in 2004–2005. However, the eddies following this southern route may dissipate before reaching the western boundary current region.

An Assessment of the Southern Ocean Mixed Layer Heat Budget

Abstract The mixed layer heat balance in the Southern Ocean is examined by combining remotely sensed measurements and in situ observations from 1 June 2002 to 31 May 2006, coinciding with the period during which Advanced Microwave Scanning Radiometer-Earth Observing System (EOS) (AMSR-E) sea surface temperature measurements are available. Temperature/salinity profiles from Argo floats are used to derive the mixed layer depth. All terms in the heat budget are estimated directly from available data. The domain-averaged terms of oceanic heat advection, entrainment, diffusion, and air–sea flux are largely consistent with the evolution of the mixed layer temperature. The mixed layer temperature undergoes a strong seasonal cycle, which is largely attributed to the air–sea heat fluxes. Entrainment plays a secondary role. Oceanic advection also experiences a seasonal cycle, although it is relatively weak. Most of the seasonal variations in the advection term come from the Ekman advection, in contrast with western boundary current regions where geostrophic advection controls the total advection. Substantial imbalances exist in the regional heat budgets, especially near the northern boundary of the Antarctic Circumpolar Current. The biggest contributor to the surface heat budget error is thought to be the air–sea heat fluxes, because only limited Southern Hemisphere data are available for the reanalysis products, and hence these fluxes have large uncertainties. In particular, the lack of in situ measurements during winter is of fundamental concern. Sensitivity tests suggest that a proper representation of the mixed layer depth is important to close the budget. Salinity influences the stratification in the Southern Ocean; temperature alone provides an imperfect estimate of mixed layer depth and, because of this, also an imperfect estimate of the temperature of water entrained into the mixed layer from below.

The Different Nature of the Interdecadal Variability of the Thermohaline Circulation under Mixed and Flux Boundary Conditions

Abstract The differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse-resolution ocean model in an idealized flat-bottom single-hemisphere basin. Objective features are determined that allow one type of oscillation to be distinguished versus the other. First, by performing a linear stability analysis of the steady state obtained under restoring boundary conditions, it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises, respectively, from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state. Based on the budgets of density variance, it is shown next that the two types of oscillations have different energy sources: Under the constant-flux boundary condition (the thermal mode), the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability, whereas under mixed boundary conditions (the salinity mode), a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation. Furthermore, the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance. In addition, the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux, while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure. The authors propose that these differences will allow one to identify which type of oscillation, if any, is at play in the more exhaustive climate models.

Detecting unstable structures and controlling error growth by assimilation of standard and adaptive observations in a primitive equation ocean model

Abstract. Oceanic and atmospheric prediction is based on cyclic analysis-forecast systems that assimilate new observations as they become available. In such observationally forced systems, errors amplify depending on their components along the unstable directions; these can be estimated by Breeding on the Data Assimilation System (BDAS). Assimilation in the Unstable Subspace (AUS) uses the available observations to estimate the amplitude of the unstable structures (computed by BDAS), present in the forecast error field, in order to eliminate them and to control the error growth. For this purpose, it is crucial that the observational network can detect the unstable structures that are active in the system. These concepts are demonstrated here by twin experiments with a large state dimension, primitive equation ocean model and an observational network having a fixed and an adaptive component. The latter consists of observations taken each time at different locations, chosen to target the estimated instabilities, whose positions and features depend on the dynamical characteristics of the flow. The adaptive placement and the dynamically consistent assimilation of observations (both relying upon the estimate of the unstable directions of the data-forced system), allow to obtain a remarkable reduction of errors with respect to a non-adaptive setting. The space distribution of the positions chosen for the observations allows to characterize the evolution of instabilities, from deep layers in western boundary current regions, to near-surface layers in the eastward jet area.