Boundary Current Regions Research Articles

Influence of upwelling on distribution of chaetognath (zooplankton) in the oxygen deficient zone of the eastern Arabian Sea

The study addresses the coupled spatial scales in the physico-chemical variables and chaetognaths in the upwelling system of the eastern Arabian Sea during the 2005 summer monsoon. We studied the taxon between 0 and 1000m depth along seven zonal sections between 8°N and 19°N and especially observed the vertical stratification of the animals. In the upwelling regions, higher chaetognath abundance was observed resulted by the local population growth and advection of the upwelling preferred epipelagic species. Abundance weight (Aw) value further helped to understand the mode of distribution of the chaetognath community around the upwelling locations. The variation in the depth-weighted average values of different species between the upwelling and non-upwelling sites helped to identify the dissimilarity in their coupling with the abiotic components. As this eastern boundary current region further draws research interest as one of the major natural oxygen deficient system in the global ocean, the role of this hypoxia was shelved separately from the influence of upwelling in the heterogeneity of distribution of chaetognaths. In our study, two mesopelagic species Eukrohnia fowleri and Eukrohnia minuta were identified as the indicator species of this upwelling process. Our observation suggests that the temporal physical event (upwelling) plays a decisive role in the heterogeneity of the spatial abundance, community composition and diversity of chaetognaths in this least studied eastern boundary current system.

An examination of a multi-scale three-dimensional variational data assimilation scheme in the Kuroshio Extension using the naval coastal ocean model

The implementation of a multi-scale three dimensional variational (MS3DVAR) data assimilation scheme for use with the Navy Coastal Ocean Model (NCOM) in the Kuroshio Extension western boundary current region is presented here. This work leverages on Li et al. (2013), who initially developed this method. MS3DVAR data assimilation allows for the effective assimilation of both spatially coarse and dense collections of observations. Traditional 3DVAR produces an inherent filtering of dynamical features smaller than the decorrelation length. The MS3DVAR allows for a scale selective background error covariance capable of handling a wider range of ocean scales. Here the MS3DVAR is examined in an energetic coastal regime using simulated and real observations. The results show that the MS3DVAR reduces analysis errors when compared to a traditional 3DVAR scheme. Forecast errors appear to be similar for both systems and are most likely due to the coarse resolution of the surface forceing being applied.

Operational ocean prediction experiments for smart coastal fishing

This paper describes a new combination of in situ, high-density observations gathered by fishermen, and a real-time, high-resolution (approx. 1.5km) prediction model developed toward more efficient fishing. Flow field data can be successfully collected by observations from acoustic Doppler current profilers installed on commercial fishing boats, which uncover sub-mesoscale structures such as small (approx. 10km) eddies in the eastern boundary current region of the Japan/East Sea. Frequent vertical temperature profiles observed by sensors attached to casting trawl nets indicate fine feature of summertime upwelling area associated with fishing grounds. These observational assets back up routine observations conducted by using stationary buoys, research vessels, commercial passenger lines, and tide gauges. These assets enable evaluation of system predictability and improvement through calibration of physical model parameters in addition to data assimilation using low-resolution remote-sensing satellites. Our prediction system is automated with high-end computers and enables better understanding of sub-mesoscale phenomena for more accurate determination of fishing conditions. High-resolution forecasts of hazardous rapid currents can be delivered via mobile phone to prevent damage to nets.

Coastal upwelling off Cape São Tomé (22°S, Brazil): The supporting role of deep ocean processes

The regional ocean off Cape São Tomé (CST, 22°S, Brazil) is known to feature transient coastal upwelling and intense mesoscale activity associated with the Brazil Current (BC). Satellite and in situ observations are used to characterize the coastal upwelling and the oceanic pycnocline water intrusions onto the continental shelf. Coastal upwelling events around CST are found to be less intense than the ones around Cape Frio (23°S), confirming previously reported findings. It is shown that the quasi-standing growth of a BC cyclonic meander is an effective supporting mechanism to this primarily wind-driven coastal upwelling system. A typical propagating cyclonic meander event is described and compared with its quasi-standing counterpart. The propagating cyclones also appear to promote oceanic pycnocline water intrusions, but at a lesser extent than the quasi-standing features. The supporting effect of the BC cyclones was quantified via simplified numerical experiments carried out with a 2D, primitive-equation numerical model. It is shown that meanders enhance intrusions as they grow, and may decrease by ≈50% the momentum input needed from the wind to cause coastal upwelling. Also, the role of the sloping of the isolines linked to the mean baroclinic structure of the Brazil Current is examined in idealized numerical experiments. This structure is shown to be sufficient to explain the observed time scales of coastal upwelling. The kind of meander-driven intrusion investigated here appears to be a regional singularity of the CST region, and may provide insight into the cross-shelf dynamics of other Western Boundary Current regions where similar quasi-standing instabilities exist.

Seasonal mixed-layer dynamics in an eddy-resolving ocean circulation model

[1] Mean and seasonal mixed-layer depths (MLDs) derived from an eddy-resolving ocean general circulation model with a horizontal resolution of (1/10)° are validated with climatological observations. Associated heat budgets on seasonal timescales are analyzed for six boundary current regions with high eddy kinetic energy (Somali Current, Agulhas Current region, Kuroshio, East Australian Current, Gulf Stream, and Brazil-Malvinas/Falkland Confluence). In all of these regions and on seasonal timescales, (a) horizontal advection significantly contributes to the mixed-layer heat budget (MLHB) on eddy scales and locally exceeds ±5°C/month; (b) lateral mixing (calculated as a residual term) is similar in size to surface net heat flux, horizontal advection, and vertical entrainment in defining the mixed-layer temperature; (c) seasonal vertical entrainment has a cooling effect on mixed-layer temperature throughout the year in the regions investigated; and (d) a phase lag between MLD and changes in mixed-layer heat content exists such that local cooling (warming) in the mixed layer precedes maxima (minima) in MLD by 1–3 months. A rather complex picture emerges where the MLHB in ocean boundary currents on larger spatial scales is determined by net surface heat fluxes and entrainment, whereas local, eddy-related advection and stirring modulate the large-scale signals.

Climate change projection of the Tasman Sea from an Eddy‐resolving Ocean Model

The ocean's western boundary current regions display the greatest rate of twentieth century warming and global climate models project that the accelerated rate of warming will continue with climate change. All existing global climate change projections come from simulations that do not fully resolve either these boundary currents or their eddies. Using an Ocean Eddy‐resolving Model (OEM) that captures the dynamics of the East Australian Current (EAC) and its eddies we show the response of the Tasman Sea to climate change differs from what is projected with a coarse resolution Global Climate Model (GCM). With climate change, the OEM projects increased EAC transport with increased eddy activity and an approximately 1° southward latitudinal shift in the point where the EAC separates from the shelf and flows eastward. The OEM increased eddy activity in the Tasman Sea with climate change increases the nutrient supply to the upper ocean and causes an increase in the phytoplankton concentrations and primary productivity by 10% in the oligotrophic waters of the Tasman Sea. The increase in primary productivity is absent in the GCM climate change projection, which projects the region will have a decrease in primary productivity with climate change. Applying the OEM climate change projection for the Tasman Sea to other western boundary current regions suggests the projected intensification of all western boundary currents with climate change should increase eddy activity and provide an important nutrient supply mechanism to counter the increased stratification projected with global warming.

Global surface eddy diffusivities derived from satellite altimetry

Velocities derived from AVISO sea‐surface height observations, adjusted to be nondivergent, are used to simulate the evolution of passive tracers at the ocean surface. Eddy mixing rates are derived from the tracer fields in two ways. First, the method of Nakamura is applied to a sector in the East Pacific. Second, the Osborn‐Cox diffusivity is calculated globally to yield estimates of diffusivity in two dimensions. The results from the East Pacific show weak meridional mixing at the surface in the Southern Ocean (<1000 m2 s−1, consistent with previous results) but higher mixing rates (~3000–5000 m2 s−1) in the tropical ocean. The Osborn‐Cox diagnostic provides a global picture of mixing rates and agrees reasonably well with the results from the East Pacific. It also shows extremely high mixing rates (~104 m2 s−1) in western boundary current regions. The Osborn‐Cox diffusivity is sensitive to the tracer initialization, which we attribute to the presence of anisotropic mixing processes. The mixing rates are strongly influenced by the presence of a mean flow nearly everywhere, as shown by comparison with an eddy‐only calculation, with the mean flow absent. Finally, results are compared with other recent estimates of mixing rates using Lagrangian and inverse methods.

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.

The California Current System: A multiscale overview and the development of a feature-oriented regional modeling system (FORMS)

Over the past decade, the feature-oriented regional modeling methodology has been developed and applied in several ocean domains, including the western North Atlantic and tropical North Atlantic. This methodology is model-independent and can be utilized with or without satellite and/or in situ observations. Here we develop new feature-oriented models for the eastern North Pacific from 36° to 48°N – essentially, most of the regional eastern boundary current. This is the first time feature-modeling has been applied to a complex eastern boundary current system. As a prerequisite to feature modeling, prevalent features that comprise the multiscale and complex circulation in the California Current system (CCS) are first overviewed. This description is based on contemporary understanding of the features and their dominant space and time scales of variability. A synergistic configuration of circulation features interacting with one another on multiple and sometimes overlapping space and time scales as a meander-eddy-upwelling system is presented. The second step is to define the feature-oriented regional modeling system (FORMS). The major multiscale circulation features include the mean flow and southeastward meandering jet(s) of the California Current (CC), the poleward flowing California Undercurrent (CUC), and six upwelling regions along the coastline. Next, the typical synoptic width, location, vertical extent, and core characteristics of these features and their dominant scales of variability are identified from past observational, theoretical and modeling studies. The parameterized features are then melded with the climatology, in situ and remotely sensed data, as available. The methodology is exemplified here for initialization of primitive-equation models. Dynamical simulations are run as nowcasts and short-term (4–6 weeks) forecasts using these feature models (FM) as initial fields and the Princeton Ocean Model (POM) for dynamics. The set of simulations over a 40-day period illustrate the applicability of FORMS to a transient eastern boundary current region such as the CCS. Comparisons are made with simulations initialized from climatology only. The FORMS approach increases skill in several factors, including the: (i) maintenance of the low-salinity pool in the core of the CC; (ii) representation of eddy activity inshore of the coastal transition zone; (iii) realistic eddy kinetic energy evolution; (iv) subsurface (intermediate depth) mesoscale feature evolution; and (v) deep poleward flow evolution.

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.

Deep South China Sea circulation

[1] The analysis of an updated monthly climatology of observed temperature and salinity from the U.S. Navy Generalized Digital Environment Model reveals a basin-scale cyclonic circulation over the deep South China Sea (SCS). The cyclonic circulation lies from about 2400 m to the bottom. The boundary current transport of the cyclonic circulation is around 3.0 Sv. Our results suggest that the cyclonic circulation is mainly forced by the Luzon overflow, with bottom topography playing an important role. The structures of potential temperature, salinity, and potential density in the deep SCS are consistent with the existence of the cyclonic circulation. Specifically, low salinity water is found in the interior region west of Luzon Island, and surrounded by saline Pacific water in boundary current regions to the north, west and southwest. Our results show the potential density distribution and the corresponding cyclonic circulation in deep SCS are primarily controlled by salinity variations in the deep basin.

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.