Subpolar Waters Research Articles

Coherent high- and low-latitude climate variability during the holocene warm period

A faunal record of sea-surface temperature (SST) variations off West Africa documents a series of abrupt, millennial-scale cooling events, which punctuated the Holocene warm period. These events evidently resulted from increased southward advection of cooler temperate or subpolar waters to this subtropical location or from enhanced regional upwelling. The most recent of these events was the Little Ice Age, which occurred between 1300 to 1850 A.D., when subtropical SSTs were reduced by 3 degrees to 4 degrees C. These events were synchronous with Holocene changes in subpolar North Atlantic SSTs, documenting a strong, in-phase link between millennial-scale variations in high- and low-latitude climate during the Holocene.

Climate Variability Inferred from a Continuously Stratified Model of the Ideal-Fluid Thermocline*

Climate variability in the subtropical gyre interior induced by anomalous surface thermal forcing, Ekman pumping, mixed layer depth variability, and anomalous subpolar water formation is examined, using a continuously stratified model of the ideal-fluid thermocline. Cooling (heating) induces a negative (positive) potential vorticity perturbation in the ventilated thermocline, and the associated density perturbations propagate downstream in the form of second and higher baroclinic modes. The second baroclinic mode resembles the traditional second baroclinic mode because it has a thermal structure with cooling (warming) in the upper thermocline and warming (cooling) in the lower thermocline. Anomalous Ekman pumping can also induce density perturbations that propagate westward in the form of the first baroclinic mode. In addition, if the outcrop lines are nonzonal, there are density perturbations that propagate downstream in the form of the second or third baroclinic modes. Perturbations in the sea surface elevation are mostly confined to the region of anomalous forcing. On the other hand, when the low potential vorticity anomaly in the subpolar mode water spreads into the subtropical basin, both the unventilated and ventilated thermocline move downward. Consequently, temperature at a given depth seems to increase.

Microbial consumption and production of dimethyl sulfide (DMS) in the Labrador Sea

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 18:197-205 (1999) - doi:10.3354/ame018197 Microbial consumption and production of dimethyl sulfide (DMS) in the Labrador Sea Gordon V. Wolfe*, Maurice Levasseur**, Guy Cantin, Sonia Michaud Institut Maurice-Lamontagne, Ministère des Pêches et des Océans, CP 1000, Mont-Joli, Québec G5L 3A1, Canada *Present address: College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean. Admin. Bldg., Corvallis, Oregon 97331-5503, USA. **Addressee for correspondence. E-mail: levasseurm@dfo-mpo.gc.ca ABSTRACT: We examined microbial production and consumption of dimethyl sulfide (DMS) in Labrador Sea surface waters ranging in temperature from -0.1 to 6.9°C. 200 nM dimethyl disulfide (DMDS) was used to inhibit DMS consumption. We also studied DMS consumption kinetics by additions of 5 to 50 nM DMS, DMS production from added dimethylsulfoniopropionate (DMSP), and DMS production and consumption during zooplankton grazing. During the cruise, DMS concentrations were low, ranging from 1 to 7 nM throughout the study area, which included a bloom of the colonial haptophyte alga Phaeocystis pouchetii. DMDS additions often revealed rapid DMS production and consumption (up to 5 nM d-1) and very rapid turnover (<1 to 3 d), similar to rates found in coastal waters at much higher temperatures. There was no clear effect of temperature on DMS consumption; rather, DMS consumption appeared to be tightly coupled with production. Turnover was most rapid at low DMS concentrations, and DMS consumption was stimulated by additions of DMS, or by increased DMS production from additions of dissolved DMSP. DMDS additions to zooplankton grazing incubations revealed rapid gross DMS production and consumption which were nearly balanced, resulting in net steady-state DMS patterns. DMDS did not affect production or grazing of algal pigments or DMSP. DMS consumption saturated at 18 to 32 nM [DMS] and saturation kinetics were similar within the photic zone, but consumption was near-zero at greater depths. We suggest that DMS consumption likely saturates more easily than microbial DMS production from DMSP, and this, combined with temperature limitation on the growth of prokaryotic DMS consumers, may lead to the periodic buildup of high DMS concentrations previously observed in polar and subpolar waters. KEY WORDS: Dimethyl sulfide · Dimethylsulfoniopropionate · DMSP · DMS · Inhibitor technique · Consumption kinetics · Zooplankton grazing Full text in pdf format PreviousExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 18, No. 2. Publication date: August 09, 1999 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1999 Inter-Research.

Modification of the intermediate waters in the northeastern subpolar Pacific

The modification of the intermediate layers in and around the Alaskan Gyre are investigated in relation to general circulation features of the region, using tracer data from the World Ocean Circulation Experiment P17N line. The Alaskan Gyre (AG) core lies between the North Pacific Current and the Alaskan Stream (AS) in the 52–54.5°N range and to the west of 144°W. The North Pacific Current feeds the Transition Zone (TZ) and the Western Subpolar Waters (WSW), which consequently constitute the inflow to the region from the northwestern Pacific. The Alaska Current (AC) is observed to the west of 144°W, between the North American coast and the AG. The AC halocline (σθ &lt; 26.7) has distinct property distributions (low chlorofluorocarbon and oxygen saturations and high nutrient concentrations), which are correlated with eddies that are characterized by higher salt content and warmer temperatures (most significantly at 26.0 σθ). The AG is found to act as a reservoir where water mass modification takes place and the AS transports a mixture of waters from the AG and the AC west‐ward out of the region. The residence time estimate for the intermediate and upper layer water in the AG (σθ ≤ 27.2) is about 2 years, which corresponds to a volume exchange between the gyre and the surrounding circulation of 4.7 Sv in the 0–1000 m range. The layers above 26.9 σθ (∼0–200 m) are modified by vertical mixing mostly of WSW in the AG; the flux of modified waters out of the AG is approximately 0.9 Sv. Comparison of the present data with the 1984–1989 period suggests that mixing of water from different regimes has not changed dramatically.

Characteristics of the South Atlantic subtropical frontal zone between 15°W and 5°E

In this paper we present data from a number of crossings of the boundary between subtropical and subpolar water in the 15°W–5°E region of the South Atlantic and discuss the implications. The previous paucity of synoptic data sets near 40°S between 25°W and the Greenwich Meridian meant that up to now it has not been possible to fix the position of the boundary in the mid-South Atlantic or to deduce the effects of the midocean ridge on the strength of the South Atlantic Current (SAC). Using hydrographic and chemical tracer data we confirm that the transition from subtropical to subpolar waters occurs within a Subtropical Frontal Zone (STFZ), which constrains the South Atlantic Current flow and is bounded on each side by a distinct front. The northern one, the Northern Subtropical Front (NSTF), varies by only 1.5° of latitude, whereas the southern one, the South Subtropical Front (SSTF), the traditional Subtropical Convergence (STC, defined by Deacon, 1937), migrates over 2.5° of latitude and remains south of the island of Tristan da Cunha. This finding goes some way in resolving the disparity in the literature to the position and seasonal migration of the STC. The data confirm the existence of a subsurface salinity maximum lobe, which appears to be formed at the NSTF rather than at the SSTF. By closely investigating the mesoscale structure and comparing it with historical data from a number of meridional cruises, we have shown strong seasonality in the frontal structure between 30 and 45°S in the South Atlantic. Having resolved the fine structure, we have made an estimate of the geostropic flow of the SAC and suggest that there are east to west differences, which may be related to recirculation in the Argentine Basin rather than to a slowing down by the midocean ridge.

Response of nannoplankton to major changes in sea-surface temperature and movements of hydrological fronts over Site DSDP 594 (south Chatham Rise, southeastern New Zealand), during the last 130 kyr

A high-resolution history of paleoceanographic changes in the subpolar waters of the southern margin of the Subtropical Convergence Zone during the last 130 kyr, is present in foraminiferal assemblages of DSDP Site 594. The foraminifera indicate that sea-surface temperatures during the Last Interglacial Climax were warmer than today, and that between substage 5d through to the end of isotope stage 2, temperatures were mostly cooler than Holocene temperatures. The paleotemperatures suggest that (1) the Subtropical Convergence was located over the site during substage 5e, later moving further north, then moving southwards to near the site during the Holocene, and (2) the Polar Front was positioned over the Site during glacial stages 6, 4, 2 and possibly parts of stage 3. Several major events are indicated by the nannofloral assemblages during these large changes in sea-surface temperature and associated reorganization of ocean circulation. First, the time-progressive trends between E. huxleyi and medium to large Gephyrocapsa are unique to this site, with E. huxleyi dominating over medium Gephyrocapsa during stages 5c-a, middle part of stage 4 and after the middle point of stage 3. This unusual trend may (at least partly) be caused by the shift of the Polar Front across the site. Second, upwelling flora ( E. huxleyi and small placoliths) increase in abundance during stages 1, 3 and 5, suggesting that upwelling or disturbance of water stratification took place during the interglacials. Thirdly, there are no significant differences between the distribution patterns of the various morphotypes of medium to large Gephyrocapsa, and the combined value of all medium Gephyrocapsa increases in abundance during glacials (stages 2 and 4 and the end of stage 6), similar to the abundance trends in benthic foraminifera. Finally, subordinate nannofossil taxa also show distinctive. climatic trends during the last glacial cycle: (1) Syracosphaera spp. are present in increased abundance during warmer extremes in climate (substages 5e, 5a, and stage 1); (2) Coccolithus pelagicus and Calcidiscus leptoporus dominate the subordinate nannofossil taxa, and their relative proportions seem to provide a useful paleoceanographic index, with C. pelagicus dominating when the Polar Front Zone is over the site (stages 6, 4 and 2), whilst C. leptoporus is relatively more abundant when the STC is positioned over the site (stages 1 and 5e). Increased abundance of C. pelagicus also can indicate intensified coastal upwelling.

North Pacific Intermediate Water Transports in the Mixed Water Region

Abstract Initial mixing between the subtropical and subpolar waters of Kuroshio and Oyashio origin occurs in the mixed water region (interfrontal zone) between the Kuroshio and Oyashio. The relatively fresh water that enters the Kuroshio Extension from the Mixed Water Region is this already mixed subtropical transition water. Subtropical transition water in the density range 26.64–27.4 σθ can be considered to be the newest North Pacific Intermediate Water (NPIW) in the subtropical gyre; this density range is approximately that which is ventilated in the subpolar gyre with significant influence from the Okhotsk Sea. Freshening of the Kuroshio Extension core occurs between 140° and 165°E in the upper part of the NPIW (26.64–27.0 σθ), with the greatest freshening associated with the eastern side of the first and second Kuroshio meanders. Kuroshio Extension freshening in the lower part of the NPIW (27.0–27.4 σθ) occurs more gradually and farther to the east. There is nearly no distinction in water properties ...

Alkenones and alkenes in surface waters and sediments of the Southern Ocean: Implications for paleotemperature estimation in polar regions

The concentration of C37-C39 long-chain alkenones and alkenes were determined in surface water and surface sediment samples from the subpolar waters of the Southern Ocean. Distributions of these compounds were similar in both sample sets indicating little differential degradation between or within compound classes. The relative amounts of the tri- to tetra-unsaturated C 37 alkenones increased with increasing temperature for temperatures below 6°C similar to the di- and tri-unsaturated C 37 alkenones. The C 37 di-, tri-, and tetra-unsaturated methyl alkenones are used in paleotemperature calculations via the U 37 K and the U 37 K ratios. In these datasets, the relative abundances of the C 37:2 and the C 37.3 alkenones as a proportion of the total C 37 alkenones were opposite and strongly related to temperature (the latter with more scatter), but the abundance of the C37:4 alkenone showed no relationship with temperature. The original definition of U 37 K includes the abundance of 37:4 in both the numerator and denominator, and thus it is perhaps not surprising that there is considerable scatter in the values obtained for U 37 K at low temperatures. Of the two, we suggest that U 37 K′ is the better parameter for use in paleotemperature estimations, even in cold locations. U 37 K′ values in the sediments fall on virtually the same regression line obtained for the water column samples of Sikes and Volkman (1993), indicating that their calibration is suitable for use in Southern Ocean sediments. The comparison of water column data with sedimentary temperature estimates suggests that the alkenone distributions are dominated by contributions from the summer when the biomass of Emiliania huxleyi and presumably flux to the sediment, is expected to be high.

Distribution of radiolaria in surface sediments and its relation to the oceanography of the Iceland and Greenland seas

Abstract In order to depict the distribution of radiolarians in the sea-floor of the Greenland and Iceland Seas, 31 surface sediment samples were analyzed. Three radiolarian assemblages were defined through Q-mode factor analysis and the importance of each species in each biofacies, was determined by means of a graphic multivariate analysis. The first assemblage (factor 1) is called ‘Arctic Water’ because it dwells in the Arctic Surface Water (ASW). Assemblage 2 inhabits Subarctic Water along the Arctic Front, while assemblage 3 is associated with the Subpolar Water along the Polar Front. Amphimelissa setosa (Cleve) is the most prominent species in sediment samples below Arctic Surface Water, whereas Botryocyrtis platycephala (Petrushevskaya) and Cycladophora davisiana (Ehrenberg) are more prolific below Subarctic Water. Lithelius spiralis (Haeckel) and Artobotrys borealis (Cleve) predominate below Subpolar Water and represent the coldest radiolarian province in the study area.

Reevaluation of the open ocean source of carbonyl sulfide to the atmosphere

Carbonyl sulfide (COS) concentrations were measured in surface seawater samples and the overlying marine boundary layer of the Pacific Ocean by using gas chromatography (GC) and electron capture sulfur detection (ECD‐S). A wide latitudinal range was covered (55°N–70°S) on two cruises 9 months apart. COS saturation ratios (SRs) in seawater were found to be less than 1 (undersaturated) across wide regions of the open ocean, especially in the subtropical gyres and wintertime subpolar waters. SRs were highest in coastal/shelf regions and in spring/summertime temperate waters. Extensive undersaturation is attributed to a low COS photoproduction potential of the water, limited sunlight, and/or a rapid hydrolysis rate constant. Decreasing COS concentrations during diurnal cycles in tropical waters were fitted to first‐order exponentials, with resulting decay times agreeing with calculated hydrolysis lifetimes to within 15%. Air‐sea fluxes of COS from the open ocean were calculated by using two different expressions for the transfer velocity and averaged into six latitude bands and three seasons. On the basis of these data we report a global open ocean sea‐air flux of −0.032 (−0.010 to −0.054) Tg COS/yr, which is much lower than and of different sign from the current global estimate (0.14–0.58 Tg COS/yr). Atmospheric COS mixing ratios averaged 470 pptv on the first cruise and 442 pptv on the second cruise, with much of the difference possibly a result of a seasonal decrease in the northern hemisphere COS mixing ratio of up to 10%.

Ventilation of the North Atlantic and North Pacific: Subduction Versus Obduction

Ventilation in the North Atlantic and North Pacific is examined by analyzing the Levitus climatological data and the Hellerman and Rosenstein wind stress data. Ventilation between the permanent pycnocline and the overlying seasonal pycnocline and mixed layer consists of two physical processes: subduction and obduction. Subduction takes place mainly in the subtropical basin where surface water is irreversibly transferred into the permanent pycnocline below. Obduction takes place in the subpolar basin where water from the permanent pycnocline is irreversibly transferred into the mixed layer above. Veatilation in the North Atlantic and North Pacific can be clarified into four physically different regions: the subductive region, the obductive region, the ambiductive region where both subduction and obduction take place, and the insulated region where neither subduction nor obduction occurs. Although the total subduction rates in these two oceans are comparable, the total obduction rates are considerably different. In the North Atlantic, obduction is strong (23.5 Sv), consistent with the notion of the fast thermohaline circulation and the relatively short renewal time of the subpolar water masses in the Atlantic basin. Obduction is weak in the North Pacific (7.8 Sv), this is consistent with the sluggish thermohaline circulation and the slower renewal process of the subpolar water masses there. Accordingly, the water mass renewal time based on the subduction/obduction rate is calculated and compared with previous estimations.

Regional models for phytoplankton absorption as a function of chlorophyll a concentration

Empirical relationships for predicting phytoplanktonic absorption at 676 and 436 nm from water column chlorophyll a concentration are presented for distinct geographic regions defined by latitude. The forms of the predictive equations are controlled by underlying biological mechanisms and lend insight into these mechanisms. These region‐specific models allow prediction of phytoplanktonic absorption from more easily measured parameters such as chlorophyll a concentration or in situ fluorescence and increase accuracy in modeling optical properties or primary production rates from specific absorption coefficients. Phytoplanktonic absorption can be predicted from chlorophyll a concentrations estimated from satellite‐based ocean color measurements. Temperate and tropical regions exhibited statistically indistinguishable relationships at low chlorophyll so these regions were combined and treated as one. Nonlinear relationships between phytoplanktonic absorption at both 436 and 676 nm and chlorophyll a concentration for the combined temperate/tropical region suggested that pigment packaging effects were important and variable. Higher slopes between absorption and chlorophyll a at low chlorophyll supported the concept of low pigment packaging effects (thus higher specific absorption) in oligotrophic, low chlorophyll a waters. Subpolar waters displayed a distinct pattern and were defined as a separate region. Near‐linear and linear relationships between phytoplanktonic absorption at 436 and 676 nm and chlorophyll a concentration indicated that influences of pigment packaging on phytoplankton specific absorption coefficients were relatively constant and uncoupled from water column chlorophyll a concentration in the subpolar region. Optical depth correlated inversely with specific absorption at 436 nm in the subpolar region, illustrating the role of photoadaptation in determining specific absorption and predictive relationships. Differences between predictive quadratic equations for particulate and phytoplanktonic absorption indicated that detritus made only a small contribution to the nonlinear nature of the relationships. Absorption by detritus at 436 nm ranged from 25 to 90% of the total particulate absorption. The proportion of absorption by detritus did not exhibit any strong patterns as a function of chlorophyll a concentration, arguing against the previously invoked explanation that proportional increases in absorption by detritus as environments become more oligotrophic cause the observed nonlinearity between absorption and chlorophyll a concentration.

Recent climatic trend and thermal response of permafrost in Salluit, Northern Quebec, Canada

AbstractNot all places may in fact warm under global change scenarios. This paper presents an example of climatic cooling from northern Quebec, Canada. Ground temperature measurements along the southern shore of Hudson Strait, northern Quebec indicate a cooling trend over the last seven years (1987‐93). Long‐term air temperature records show that this area has actually experienced continuous cooling for more than 40 years. Related studies suggest that the cooling is likely to continue due to freshening of subpolar water in the North Atlantic and Labrador Sea area. A one‐dimensional geothermal model was used to simulate the effect of continued cooling on permafrost thermal regime in Salluit, northern Quebec. The results show that, if the climatic trend continues for the next 50 years, the thickness of the active layer would decrease by 30cm (from 2.3 to 2.0 m) in gneiss and by 20cm (from 1.3 to 1.1 m) in till. Permafrost temperature at 20 m depth would decrease by 0.65 °C. Under the cooling scenario, rate of permafrost creep and slope activities would be reduced. Ice‐wedge regrowth would continue, and the buried ice wedges may even become reactivated. The results also indicate that regional snow precipitation data cannot be used directly in predicting ground thermal regimes.

Is the North Pacific in Sverdrup balance along 24°N?

Hydrographic data from a zonal section along approximately 24°N is used to demonstrate that basin‐scale baroclinic geostrophic transport agrees with the Sverdrup relation in the subtropical North Pacific. Moreover, profiles of vertical velocity, as derived from the linear vorticity equation, are consistent with constraints upon deeper flows. Sverdrup balance from the eastern boundary to about 137°E is in accord with most other elements of the general circulation to the extent that they are known. In the Philippine Sea, west of 137°E, the results suggest that a mean northward flow of 5–10 Sv occurs. In the northeast Pacific Basin, a significant subbasin‐scale deviation of about 7 Sv from Sverdrup balance is revealed; this deviation is distinguished by excess shallow southward flow near the water mass boundary between the subtropical gyre and the subpolar waters adjacent to the American coast.

Seasonal variation of CO2 and nutrients in the high‐latitude surface oceans: A comparative study

Seasonal data for pCO2 and the concentrations of CO2 and nutrients in high‐latitude surface oceans obtained by the Lamont‐Doherty CO2 group and Marine Research Institute, Reykjavik, are presented and analyzed. The seasonal progression and relationships between these properties are described, and their inter‐ocean variation is compared. Spring phytoplankton blooms in the surface water of the North Atlantic Ocean and Iceland Sea caused a precipitous reduction of surface water pCO2 and the concentrations of CO2 and nutrients within two weeks, and proceeded until the nutrient salts were exhausted. This type of seasonal behavior is limited to the high‐latitude (north of approximately 40°N) North Atlantic Ocean and adjoining seas. In contrast, seasonal changes in CO2 and nutrients were more gradual in the North Pacific and the nutrients were only partially consumed in the surface waters of the subarctic North Pacific Ocean and Southern Ocean. The magnitude of seasonal changes in nutrient concentrations in the North Pacific and Southern Oceans was similar to that observed in the North Atlantic and adjoining seas. In the subpolar and polar waters of the North and South Atlantic and North Pacific Oceans, pCO2 and the concentrations Of CO2 and nutrients were much higher during winter than summer. During winter, the high latitude areas of the North Atlantic, North Pacific, and Weddell Sea were sources for atmospheric CO2; during summer, they became CO2 sinks. This is attributed to the upwelling of deep waters rich in CO2 and nutrients during winter, and the intense photosynthesis occurring in strongly stratified upper layers during summer. On the other hand, subtropical waters were a CO2 source in summer and a sink in winter. Since these waters were depleted of nutrients and could only sustain low levels of primary production, the seasonal variation of pCO2 in subtropical waters and the CO2 sink/source condition were governed primarily by temperature. An intense CO2 sink zone was found along the confluence of the subtropical and subpolar waters (or the subtropical convergence). Its formation is attributed to the combined effects of cooling in subtropical waters and photosynthetic drawdown of CO2 in subpolar waters.

Ventilation and transport of thermocline and intermediate waters in the northeast Pacific during recent El Niños

We present time series of tritium (3H) concentrations in seawater from stations in the eastern subpolar (50°N, 145°W) and subtropical (28°N, 122°W) North Pacific. In the eastern subpolar North Pacific the tritium gradient between surface water and North Pacific Intermediate Water is smallest during years that coincide with El Niño events. In the eastern subtropical North Pacific between 200 and 400 m the tritium signature suggests that the water is of subpolar origin during non‐El Niño years. During El Niño years, the water at this location is devoid of tritium. We hypothesize that the El Niño phenomenon alters both the ventilation of thermocline and intermediate waters in the eastern subpolar North Pacific, as well as the transport of this water to the eastern subtropical gyre. Combined with satellite altimeter data, these results offer both a mechanism and a time frame by which subpolar water ventilates the vast reservoir of the subtropical North Pacific.

Annual cycles of phytoplankton chlorophyll concentrations in the global ocean: A satellite view

Conceptual and mathematical models show that annual cycles of phytoplankton biomass are different within different regions of the ocean. The purpose of this manuscript is to use coastal zone color scanner chlorophyll imagery (CZCS‐Ch1) to determine annual cycles in phytoplankton chlorophyll (biomass) averaged over very large areas of the global ocean. A possible result is that large‐scale averaging of CZCS‐Ch1 will yield no interpretable signals because of spatial variability in annual cycles at scales much smaller than our averaging scale. Alternatively, if our analyses show regular and persistent global patterns, then our results will jprovide a basin‐scale overview of phytoplankton biomass seasonally for comparison with model results or with other large‐scale oceanographic measurements. Our results show that monthly mean CZCS‐Ch1 imagery (and using in situ concentrations for winter at latitudes poleward of 40 deg) resolves important differences in annual phytoplankton chlorophyll cycles for different ocean basins and latitude belts. As predicted by simple models of plankton dynamics, our results show: (1) global subtropical waters have circa 2X higher CZCS‐Ch1 concentrations in winter than in summer and (2) subpolar waters in the northern hemisphere (NH) have mean monthly CZCS‐Ch1 concentrations during May and June that are manyfold higher than in winter, particularly in the North Atlantic. Our results also show: (1) Northern Indian Ocean is the major subtropical anomaly, (2) subpolar waters in the SH do not show differences between spring maxima and winter minima as large as those in the subpolar NH and (3) larger ocean area in the SH is compensated by higher mean annual CZCS‐Ch1 concentrations in the NH, so that annual hemispherical integrals (mean annual concentrations multiplied by ocean areas) are very similar. The simple patterns we report imply that mean annual cycles in phytoplankton biomass averaged over very large areas of the global ocean are largely explainable by very simple mathematical models such as those presented several decades ago by Cushing, Riley, Steele, and others.

A chlorofluorocarbon section in the eastern North Atlantic

We present the distributions of two chlorofluorocarbons (CFCs) CFC-11 and CFC-12, measured as part of a hydrographic section between Iceland and the equator during July and August 1988. CFC-tagged water has filled the entire subpolar water column and subtropical thermocline in the eastern North Atlantic. Measurable CFC concentrations are observed at the ocean bottom as far south as 35°N, and the CFC penetration depth shoals to ≈750 m in the tropics. Specific features in the CFC distributions include a clear signal of Labrador Sea mid-depth ventilation, a CFC-enriched overflow water boundary current along the Iceland slope, and a mid-depth, equatorial plume of upper North Atlantic Water. The CFC data are used, in conjuction with the hydrographic data from the cruise, to illustrate the ventilation time-scales and pathways for the water masses in the eastern basin. A subsurface CFC maximum at about 100–200 m depth in the subtropics is shown to be a by-product of the heating and degassing of the seasonal thermocline and of the temperature sensitivity of CFC solubility. The CFC concentrations in the subpolar mode water are undersaturated by 15–18% relative to the atmosphere, reflecting the age of the mode waters and the very deep winter mixed layers in the eastern subpolar gyre. The CFC concentrations in the oxygen minimum off tropical Africa are much lower than the concentrations in the subtropical gyre, supporting previous work that suggests that isolation and enhanced productivity both contribute to the formation of the tropical oxygen minimum. In addition, the CFC inventories at the tropical stations have increased between 1982 and 1983 (TTO/TAS) and the summer of 1988 at a slower rate relative to the subtropical inventories over the same period. Thermocline oxygen utilization rates calculated from the CFC concentration data range from 5 to 10 μmol kg −1y −1 and are in line with previous estimates for the eastern subtropical thermocline. The low CFC concentrations in Mediterranean Water, about one-quarter those in the Labrador Sea Water, are shown to result from entrainment near the Straits of Gibraltar of a large component of low CFC, lower Atlantic thermocline water. Based on the CFC and other transient tracer distributions, the deep eastern basin can be divided into two regions: the Iceland Basin and surrounding area influenced by Iceland-Scotland Overflow Water that is ventilated on a decadal time-scale, and the area south of ≈50°N that has little or no CFC and is ventilated from a southern source on a much longer time-scale. A northward flowing boundary current of low CFC, modified Eastern Basin Bottom Water is also found along the Rockall Plateau and in the Iceland Basin.

April and August Northeast Atlantic surface water masses reflected in planktic foraminifera

Northeast Atlantic surface water masses and corresponding frontal zones were alike in April and August, although April frontal zones were enlarged compared to August. The observed water masses and fronts are the Subpolar Water (SW), the Subpolar Front (SF), the North Atlantic Current (NAC), North Atlantic Transitional Water (NATW), the Azores Frontal Zone (AFZ) and the Azores Current (AC). During August, clusters of relative and absolute frequencies of planktic foraminifera corresponded with water masses as conventionally distinguished on oceanic conditions. However, the distinct faunal assemblages deviated from the observed oceanic regime in April, when the transition between the immutable winter and the highly variable spring periods resulted in broad frontal zones. Under those conditions absolute frequencies and information indices prove to be better suited to characterize oceanic conditions than assemblage composition. The highly mixed and migrating fronts strongly influenced the planktic community and affected the horizontal as well as vertical species distribution.

A transpacific hydrographic section along latitude 24°N: the distribution of properties in the subtropical gyre

An intensively sampled transpacific hydrographic section along 24°N was completed in the spring of 1985. The data are described here in terms of the spatial distribution of properties, the distribution along isopycnal surfaces, and, where possible, the relationship of these distributions to the large-scale circulation of the Pacific Ocean. Near-surface waters of subtropical origin display a salinity maximum in mid-ocean, with lower salinity to the west due to greater rainfall and lower salinity in the east due to advection of water from the north. In the next layers down, containing waters of subpolar origin, the low salinity and high dissolved oxygen concentrations of those waters are most pronounced in the eastern ocean where the subpolar water is swept clockwise into the subtropical gyre. Differences between patterns of dissolved oxygen concentration and salinity indicate that both horizontal advection and upwelling contribute to observed distributions near the eastern boundary and that the two tracers contain independent information. In the upper kilometer, the eastern Pacific is richer in tracer signals and has steeper property gradients than the west.The deep Pacific has long been recognized to be the most uniform of the oceans. Although property gradients are small, they are significant, and it is found that on all isopycnal surfaces below the upper kilometer salinity increases and dissolved oxygen concentration decreases towards the east on basin-wide scales. These zonal gradients are weakest in the abyss, where there is a substantial net input of southern water, and strongest at mid-depth. Vertical diffusion is the likely cause of the uniformity in this pattern over so much of the deep North Pacific, with oxygen consumption in waters of greater age in the east also being a plausible contributor.With a highly sampled data set such as the 24°N transpacific section it is appropriate to ask how many stations are required to define property distributions and to estimate large-scale circulation and transport. Estimation of geostrophic transport requires high spatial resolution to detect flow near sloping topography at all depths. A 50% decimation of the 24°N station pattern yields a severe degradation in the estimation of transport.