Claes Rooth

Salinity-driven Thermocline Transients in a Wind- and Thermohaline-forced Isopycnic Coordinate Model of the North Atlantic

Abstract An isopycnic-coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented. Special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation. The model performance is illustrated by a 30-year spinup run with coarse horizontal resolution (2° mesh) in a domain with North Atlantic topography extending from 10° to 60°N latitude. The vertical structure encompasses 10 isopycnic layers in steps of 0.2 σ units, capped by a thermodynamically active mixed layer. From an initially isohaline state with isopycnals prescribed by zonally averaged climatology, the model is forced by seasonally varying wind stress, radiative and freshwater fluxes, and by a thermal relaxation process at the surface. After a mechanical spinup time of about 15 years, a quasi-stationary pattern of mean circulation and annual variability ensues, characterized by pronounced subtropical mode-water formation...

Influence of Florida Current, gyres and wind-driven circulation on transport of larvae and recruitment in the Florida Keys coral reefs

Abstract Physical processes with high potential influence on the transport and recruitment of fish, lobster and other larvae in the Florida Keys are discussed using current measurements from standard moored instrumentation, plus bottom mounted Acoustic Doppler Current Profile observations, interdisciplinary surveys of water mass properties, nutrients and planktonic distributions, and satellite derived surface thermal patterns. A cold, cyclonic gyre forms over the Pourtales Terrace seaward of the middle and lower Keys where the Florida Current shifts from eastward to northward flow. Formation of the gyre appears to be related to offshore meander motion of the Florida Current and its cyclonic curvature. Prevailing easterly winds over the gyre circulation cause a convergence of Ekman transports into the coastal zone. The gyre circulation combined with the shoreward convergence of Ekman flow facilitates the transport of pelagic larvae from the Current to the fringing reefs. Duration of the gyre is approximately 1 month, which matches the planktonic stage of fish and slipper lobster larvae, and thus provides a mechanism for larvae retention and local recruitment. Abundant microzooplankton food supply for the larvae is available in the gyre interior due in part to concentration mechanisms and ecosystem response to gyre-induced upwelling of deeper, nutrient enriched waters. Gyre retention times are too short to be a factor in recruitment of locally spawned Florida spiny lobster larvae, which have a planktonic stage that may last up to 12 months. This indicates that Florida spiny lobster should recruit from remote upstream sources in the Caribbean or spend most of their long planktonic stage in Florida Bay of the southwest Florida shelf.

Geosecs North Atlantic radiocarbon and tritium results

Radiocarbon data for 11 stations and tritium data for 16 stations in the North Atlantic Ocean from 74°N to 3°N are presented. For radiocarbon, normal errors inΔ14C are± 4‰, and in tritium,± 0.09TU or± 3%, whichever is larger. There is a remarkable, but not simply linear, correlation between oceanic bomb transients in14C and3H. The deep convective mixing in the Greenland Sea is reflected in substantial bomb tracer penetration to all depths, with residence time for the deep, cold core water that seems to be 20 to 30 years. The outflow in the bottom layer southward over the sills of the Denmark Strait and Faroe Passage carries significant tritium concentration, at least to 40°N. Complicated, but coherent, profile structures in the subtropical Atlantic suggest effects of large-scale lateral advection. In particular, a pronounced minimum in both14C and3H might be associated with the Antarctic intermediate water.

On the Connection between the Mediterranean Outflow and the Azores Current

As the salty and dense Mediteranean overflow exits the Strait of Gibraltar and descends rapidly in the Gulf of Cadiz, it entrains the fresher overlying subtropical Atlantic Water. A minimal model is put forth in this study to show that the entrainment process associated with the Mediterranean outflow in the Gulf of Cadiz can impact the upper-ocean circulation in the subtropical North Atlantic Ocean and can be a fundamental factor in the establishment of the Azores Current. Two key simplifications are applied in the interest of producing an economical model that captures the dominant effects. The first is to recognize that in a vertically asymmetric twolayer system, a relatively shallow upper layer can be dynamically approximated as a single-layer reduced-gravity controlled barotropic system, and the second is to apply quasigeostrophic dynamics such that the volume flux divergence effect associated with the entrainment is represented as a source of potential vorticity. Two sets of computations are presented within the 1‰-layer framework. A primitive-equation-based computation, which includes the divergent flow effects, is first compared with the equivalent quasigeostrophic formulation. The upper-ocean cyclonic eddy generated by the loss of mass over a localized area elongates westward under the influence of the b effect until the flow encounters the western boundary. In the steady state, the circulation pattern consists of bidirectional zonal flows with a limited meridional extent: eastward to the south of the sink and westward to the north of the sink. The localized sink drives a horizontal circulation in the interior ocean whose strength is approximately an order of magnitude greater than the sink’s strength. It is demonstrated that the induced circulation in the far field from a localized sink is insensitive to the neglect of the divergent flow component. A set of parameter sensitivity experiments is then undertaken with the quasigeostrophic model for an idealized midlatitude circulation, driven both by wind forcing and ‘‘thermohaline’’ flow through the open southern and northern boundaries. When a sink near the eastern boundary is superimposed on the idealized midlatitude circulation, it is shown to alter significantly the upper-ocean flow and induce an eastward zonal current, which resembles the Azores Current in location and transport. This mechanism also generates a westward current to the north of the sink location, which could be associated with the Azores Countercurrent. An extensive series of sensitivity experiments is conducted to determine the response of this current system to changes in the boundary layer processes, sink strength, sink distribution, model resolution, and wind forcing. The magnitude of the current transports is found to be sensitive to the sink intensity and to its distance from the coastline.

Formation and exchange of deep water in the Greenland and Norwegian seas

Abstract The formation and exchange of deep water in the Greenland and Norwegian seas are modeled by a time-dependent box model. Using tritium and radiocarbon data, primarily from the Geochemical Sections (GEOSECS) 1972 operation, the time scale for deep convective mixing in the Greenland Sea is estimated to be about 30 years, which implies that about 100 m of surface water are mixed down annually. The time scale for exchange between the deep Greenland Sea and the deep Norwegian Sea is estimated to be at least 100 years. It appears, on the basis of tritium data, that the formation and exchange of deep water in the Greenland and Norwegian seas are essentially isolated from the formation of the dense overflow waters of the Denmark Strait and the Iceland-Faroe Passage.

Inclusion of Thermobaricity in Isopycnic-Coordinate Ocean Models

Buoyancy anomalies caused by thermobaricity, that is, the modulation of seawater compressibility by potential temperature anomalies, underlie a long-standing argument against the use of potential-density-framed numerical models for realistic circulation studies. The authors show that this problem can be overcome by relaxing the strict correspondence between buoyancy and potential density in isopycnic-coordinate models. A parametric representation of the difference between the two variables is introduced in the form of a ‘‘virtual potential density,’’ which can be viewed as the potential density that would be computed from the in situ conditions using the compressibility coefficient for seawater of a fixed (but representative) salinity and potential temperature. This variable is used as a basis for effective dynamic height computations in the dynamic equations, while the traditionally defined potential density may be retained as model coordinate. The conservation properties of the latter assure that adiabatic transport processes in a compressibility-compliant model can still be represented as exactly two-dimensional. Consistent with its dynamic significance, the distribution of virtual potential density is found to determine both the local static stability and, to a lesser degree, the orientation of neutrally buoyant mixing surfaces. The paper closes with a brief discussion of the pros and cons of replacing potential density by virtual potential density as vertical model coordinate.

The late Wisconsin flood into the Gulf of Mexico

Abstract 18 O/ 16 O and 14 C analysis of deep-sea cores from the Gulf of Mexico revealed a 2.4% isotopic anomaly between 12,000 and 11,000 years ago. This value, together with estimates of the oxygen isotopic composition of ice meltwater and rates of mixing within the Gulf of Mexico indicates that the average yearly discharge of the Mississippi River was between 100,000 and 230,000 m 3 /s (as compared to 57,000 m 3 /s for the peak flood of February 17, 1937), with probable yearly peak floods twice as large. Corresponding sea-level rise was between 1 and 2 m per century.

Effects of low-frequency current variability on near-inertial submesoscale vortices

From September 30 to October 14, 1993, and from May 18 to June 13, 1994, ocean surface currents along the inshore boundary of the Florida Current were measured using a land-based ocean surface current radar (OSCR) deployed along the Florida Keys. A dual-station OSCR mapped the current fields at 20-min intervals with a horizontal resolution of 1.2 km over a 30×44 km domain using the high-frequency (HF, 25.4 MHz) mode. An upward looking, narrowband acoustic Doppler current profiler (ADCP) was moored in 150 m of water seaward of the shelf break concurrently sampling the three-dimensional current vector between 15 and 130 m at 30-min intervals. Regression analyses between surface and subsurface currents at 15 m depth from the ADCP indicated biases of 3–12 cm s−1 and slopes of O(0.8–1). Root-mean-square differences were about 18 cm s−1 because of a 15-m-depth separation between the two measurements, and bulk current shears were O(10−3 s−1) in the Florida Current where maximum velocities exceeded 150 cm s−1 between 40 and 70 m. Surface tidal currents were 2–10 cm s−1 in the diurnal and semidiurnal bands and revealed fine-scale variability in the tidal ellipses due to topographical variations. Differences in the tidal current amplitudes between the surface and 15 m ranged from 1 to 3 cm s−1 in these constituents and explained more of the current variance with depth. During the second experiment a submesoscale surface feature was detected along the shelf break and progressed eastward at a rate of about 30 cm s−1. This feature was embedded in the near-inertial wave band where the local inertial period was ≈29 hours that may have been forced by an abrupt change in the wind direction. This transient feature was trapped and advected along the inshore edge of the Florida Current where the anticyclonic vorticity of the subinertial flow shifted the frequency of the nearinertial motions by a few percent of f. These energetic near-inertial currents of 15–20 cm s−1 had horizontal and vertical wavelengths of 40 km and 50–100 m, respectively.

Ventilation of the Cariaco Trench, a case of multiple source competition?

The joint evolution of potential temperature, salinity and tritium concentrations in the deep basin in the Cariaco Trench, over a 13 year period from late 1972 to early 1986, suggests a complex pattern of ventilation of the basin. Scrantonet al. (1987, Deep-Sea Research, 34, 945–963) found that a purely diffusive model could accommodate the observed evolution from 1973 to 1982 of concentrations of a range of hydrographic and chemical trace species, but the tritium data demand inclusion of two distinct convective ventilation sources with differently distributed depth penetration. Exploratory computations with an extension of Scrantons model, including penetrative convection effects, suggest that a single ventilation mode cannot explain the total observed evolution. The observed trends in temperature, salinity and tritium concentration can be individually matched by fitting simpler models, but jointly they demand injection of warm hypersalline shelf waters which reach the basin bottom, as well as input of Caribbean thermocline waters at the sill. While the overflow events at the sill appear not to have penetrated into the deepest basin layers in recent times, the observed bottom temperature demands at least episodic ventilation with Caribbean thermocline waters colder than 16.5°C. The apparent absence of such activity, for several decades through the mid-1980s, implies climatically persistent differences in the circulation dynamics of the southern Caribbean Sea. A theoretical discussion suggests that this may be but one example of extreme sensitivity of bottom ventilation processes in enclosed deep basins to forcing fluctuations, and that even weak geothermal heating may exert a significant influence on the stratification and ventilation statistics in such systems.

Climate response to astronomical forcing

Abstract Power spectrum analysis of the oxygen isotopic records of deep-sea cores cannot be directly used for correlation studies with the astronomical forcing functions because of the rapidity of the major deglaciations. The ensuing step change in the isotopic curves, repeated at somewhat irregular intervals, produces, in fact, substantial spectral energy in a wide frequency band that includes the frequencies of the precessional parameters. The isotopic fluctuations within the ramp sections of a composite core curve have been found to be highly correlated with the precessional parameters, indicating a linear response of climate to these parameters and verifying, at the same time, the essential correctness of the time scale adopted. A direct correlation of eccentricity with the major glacial/interglacial cycle is doubtful because some of the eccentricity minima have absolute values of the same order as some of the maxima, while the isotopic amplitude remains essentially constant. In order to test if the major cycle could be related to a terrestrial parameter exhibiting an asymmetric relaxation fluctuation, we have added an asymmetric saw-tooth function to an artificial curve constructed from the precessional parameters on the basis of optimal response amplitude. The curve thus generated reproduces the original isotopic curve with a similarity sufficient to warrant a close search and analysis of the relaxation process.