Subtropical Frontal Zone Research Articles

Observed Influence of North Pacific SST Anomalies on the Atmospheric Circulation

Abstract A lagged maximum covariance analysis (MCA) of monthly anomaly data from the NCEP–NCAR reanalysis shows significant relations between the large-scale atmospheric circulation in two seasons and prior North Pacific sea surface temperature (SST) anomalies, independent from the teleconnections associated with the ENSO phenomenon. Regression analysis based on the SST anomaly centers of action confirms these findings. In late summer, a hemispheric atmospheric signal that is primarily equivalent barotropic, except over the western subtropical Pacific, is significantly correlated with an SST anomaly mode up to at least 5 months earlier. Although the relation is most significant in the upper troposphere, significant temperature anomalies are found in the lower troposphere over North America, the North Atlantic, Europe, and Asia. The SST anomaly is largest in the Kuroshio Extension region and along the subtropical frontal zone, resembling the main mode of North Pacific SST anomaly variability in late winter and spring, and it is itself driven by the atmosphere. The predictability of the atmospheric signal, as estimated from cross-validated correlation, is highest when SST leads by 4 months because the SST anomaly pattern is more dominant in the spring than in the summer. In late fall and early winter, a signal resembling the Pacific–North American (PNA) pattern is found to be correlated with a quadripolar SST anomaly during summer, up to 4 months earlier, with comparable statistical significance throughout the troposphere. The SST anomaly changes shape and propagates eastward, and by early winter it resembles the SST anomaly that is generated by the PNA pattern. It is argued that this results via heat flux forcing and meridional Ekman advection from an active coupling between the SST and the PNA pattern that takes place throughout the fall. Correspondingly, the predictability of the PNA-like signal is highest when SST leads by 2 months. In late summer, the maximum atmospheric perturbation at 250 mb reaches 35 m K−1 in the MCA and 20 m K−1 in the regressions. In early winter, the maximum atmospheric perturbation at 250 mb ranges between 70 m K−1 in the MCA and about 35 m K−1 in the regressions. This suggests that North Pacific SST anomalies have a substantial impact on the Northern Hemisphere climate. The back interaction of the atmospheric response onto the ocean is also discussed.

On the characteristics of subtropical fronts in the North Atlantic

The characteristics of surface temperature (SST) fronts in the subtropical frontal zone (STCZ) in the North Atlantic were investigated using a time series of front data produced by an edge‐detection algorithm applied to 18 years of 9.3 km resolution NOAA/NASA AVHRR Pathfinder SST imagery. Front probability, averaged over subregions of the STCZ, varies seasonally with highest probability in early spring in the western part of the region. In the eastern part of the region, maximum probability occurs in late spring to early summer, and the annual cycle in front probability is far less robust, being nearly absent prior to 1991. The location and strength of the fronts across the entire STCZ varies strongly with season, moving southward (northward) and strengthening (weakening) in winter/spring (summer). From estimates of the cross‐frontal SST step and gradient, the cross‐frontal scale of SST variability at fronts is estimated to be approximately 30 km. The monthly mean total meridional SST step across the STCZ is partitioned into a component due to frontal pixels and a component due to the background SST gradient at nonfrontal pixels, with approximately 30% of the total step attributable to the SST step across fronts. While the total meridional step is largest in winter due to the meridional gradient in surface heat flux, the frontal contribution lags the total step by 2 months or more in the western part of the STCZ. This suggests a frontogenesis mechanism requiring the presence of a background meridional SST gradient rather than one arising from surface heat flux gradients alone.

Structure of the Subtropical Front in the Tasman Sea

The structure of the Subtropical Front (STF) in the southwest Pacific is established via a practical separation observed between the thermohaline properties of the subtropical and subantarctic waters (SAWs) converging at the STF. Previous examinations have been unable to separate waters in and about the STF zone, and could not identify the actual frontal interface of the STF, leaving its structure and properties largely unresolved. Consequently, other fronts in the area have been confused with the STF or misidentified. Away from the land boundaries of Australia and New Zealand, the STF appears as a well-defined zonal trending band, in which subtropical and SAWs over-ride, intrude, and interleave. The zone is bounded by quasi-parallel fronts, each associated with near definitive surface salinity values. The shallower northern front marks the equatorwards limits of surface SAW in the water column, and the deeper southern front represents the polewards limits of subsurface subtropical water (Tasman Sea Central Water). Empirical prescriptions used to approximately locate the STF over the last 70 years are explained and reconciled on comparison with the double frontal structure. Antarctic Intermediate Water (AAIW) at the STF also exhibits a double frontal structure, inside of which is maintained a pool of AAIW with distinctive properties. The AAIW frontal system forms a leaky barrier to northward movement of Southern Ocean AAIW in the eastern Tasman Sea.

Temporal and spatial characteristics of sea surface height variability in the North Atlantic Ocean

Abstract. We investigate the spatial and temporal variability of sea surface height (SSH) in the North Atlantic basin using satellite altimeter data from October 1992–January 2004. Our primary aim is to provide a detailed description of such variability, including that associated with propagating signals. We also investigate possible correlations between SSH variability and atmospheric pressure changes as represented by climate indices. We first investigate interannual SSH variations by deriving the complex empirical orthogonal functions (CEOFs) of altimeter data lowpass-filtered at 18 months. We determine the spatial structure of the leading four modes (both in amplitude and phase) and also the associated principal component (PC) time series. Using wavelet analysis we derive the time-varying spectral density of the PCs, revealing when particular modes were strongest between 1992–2004. The spatial pattern of the leading CEOF, comprising 30% of the total variability, displays a 5-year periodicity in phase; signal propagation is particularly marked in the Labrador Sea. The second mode, with a dominant 3-year signal, has strong variability in the eastern basin. Secondly, we focus on the Azores subtropical frontal zone. The leading mode (35%) is strong in the south and east of this region with strong variations at 3- and 5-year periods. The second mode (21%) has a near-zonal band of low variance between 22°–27° N, sandwiched between two regions of high variance. Thirdly, we lowpass filter the altimeter data at a cutoff of 30 days, instead of 18 months, in order to retain signals associated with propagating baroclinic Rossby waves and/or eddies. The leading mode is the annual steric signal, around 46% of the SSH variability. The third and fourth CEOFs, 11% of the remaining variability, are associated with westward propagation which is particularly dominant in a "waveband" between 32°–36° N. For all three cases considered above, no significant cross-correlation is found between the North Atlantic Oscillation index and the amplitude of the leading four PCs of interannual SSH variability. The only exception is an anti-correlation found over the North Atlantic basin between the NAO and the 4th PC. In the subtropical front, the East Atlantic Pattern index is anti-correlated with the leading PC for SSH variations lowpass filtered at 30 days. Further investigation of forcing mechanisms is suggested using hindcasts from ocean general circulation models.

Thorium isotopes as tracers of particles dynamics and deep water circulation in the Indian sector of the Southern Ocean (ANTARES IV)

Dissolved and particulate samples were collected to study the distribution of thorium isotopes ( 234Th, 232Th and 230Th) in the water column of the Indian sector of the Southern Ocean (from 42°S to 47°S and from 60°E to 66°E, north of the Polar Front) during Austral summer 1999. Vertical profiles of excess 230Th ( 230Th xs) increases linearly with depth in surface water (0–100 m) and a model was applied to estimate a residence time relative to the thorium scavenging ( τ scav). Low τ scav in the Polar Front Zone (PFZ) are found, compared to those estimated in the Subtropical Front Zone (STZ). Changes in particle composition between the PFZ and STZ could influence the 230Th xs scavenging efficiency and explain this difference. An innovative coupling between 234Th and 230Th xs was then used to simultaneously constrain the settling velocities of small (0.6–60 μm) and large (above 60 μm) particles. Although the different hydrological and biogeochemical regimes visited during the ANTARES IV cruise did not explain the spatial variation of sinking velocity estimates, our results indicate that less particles may reach the seafloor north (60 ± 2 m d − 1 , station 8) than south of the Agulhas Return Current (119 ± 23 and 130 ± 5 m d − 1 at stations 3 and 7, respectively). This information is essential for understanding particle transport and by extension, carbon export. In the deep water column, the 230Th xs concentrations did not increase linearly with depth, probably due to lateral transport of North Atlantic Deep Water (NADW) from the Atlantic to the Indian sector, which renews the deep waters and decreases the 230Th xs concentrations. A specific 230Th xs transport model is applied in the deep water column and allows us to assess a “travel time” of NADW ranging from 2 to 15 years.

Amino acid uptake of Prochlorococcus spp. in surface waters across the South Atlantic Subtropical Front

To test the hypothesis that surface-living Prochlorococcus spp. (Pro) cyanobacteria metabolism declines towards the boundaries of its natural habitat, a latitudinal transect of surface waters was sampled from the centre of the Southern Atlantic Gyre (SAG, 20 to 35° S) to the South Subtropical Frontal Zone (SSFZ, 35 to 45° S). Along this transect, amino acid uptake rates of Pro, Synechococcus spp. (Syn) and an average bacterioplankton cell were determined using 35S-methionine precursor and flow-cytometry sorting, with methionine uptake rate as an index of cellular metabolic activity. Methionine and possibly other amino acids were a very minor nutrient source for Syn, while their contribution to Pro production was significant. Contrary to expectations, the mean methionine uptake rate per Pro cell in the SSFZ was about 3 times higher than in the SAG. The uptake rates per unit Pro biomass were equal to or higher than that of an average bacterioplankton cell in both the SAG and SSFZ. About 20 and 5% of total bacterioplankton consumption of amino acids could be assigned to Pro in the SAG and SSFZ, respectively. Methionine and leucine turnover rates were 3.5 and 3 times higher in the SSFZ than in the SAG, respectively. These results suggest that Pro remained highly metabolically active and acquired more methionine at its habitat boundaries, despite higher rates of bacterioplankton activity and therefore greater competition, as well as exposure to deep water mixing, low light and low temperature conditions.

Differing body size between the autumn and the winter–spring cohorts of neon flying squid (Ommastrephes bartramii) related to the oceanographic regime in the North Pacific: a hypothesis

AbstractThe neon flying squid (Ommastrephes bartramii), which is the target of an important North Pacific fishery, is comprised of an autumn and winter–spring cohort. During summer, there is a clear separation of mantle length (ML) between the autumn (ML range: 38–46 cm) and the winter–spring cohorts (ML range: 16–28 cm) despite their apparently contiguous hatching periods. We examined oceanic conditions associated with spawning/nursery and northward migration habitats of the two different‐sized cohorts. The seasonal meridional movement of the sea surface temperature (SST) range at which spawning is thought to occur (21–25°C) indicates that the spawning ground occurs farther north during autumn (28–34°N) than winter–spring (20–28°N). The autumn spawning ground coincides with the Subtropical Frontal Zone (STFZ), characterized by enhanced productivity in winter because of its close proximity to the Transition Zone Chlorophyll Front (TZCF), which move south to the STFZ from the Subarctic Boundary. Hence this area is thought to become a food‐rich nursery ground in winter. The winter–spring spawning ground, on the other hand, coincides with the Subtropical Domain, which is less productive throughout the year. Furthermore, as the TZCF and SST front migrate northward in spring and summer, the autumn cohort has the advantage of being in the SST front and productive area north of the chlorophyll front, whereas the winter–spring cohort remains to the south in a less productive area. Thus, the autumn cohort can utilize a food‐rich habitat from winter through summer, which, we hypothesize, causes its members to grow larger than those in the winter–spring cohort in summer.

Pliocene sea surface temperature changes in ODP Site 1125, Chatham Rise, east of New Zealand

Planktonic foraminiferal census counts were converted to sea surface temperature (SST) estimates using the modern analogue technique (MAT) for the middle–late Pliocene (4.0–2.37 Ma) in ODP Site 1125, north side of Chatham Rise, SW Pacific Ocean. MAT SST warm records range between 8°C and 20.5°C, and MAT SST cold records parallel that pattern but with a temperature range of 5–15°C. The modern position of Site 1125 is just north of the Subtropical Front and has an annual temperature range of ∼14–18°C. Pliocene warmest temperatures are 1–2° warmer than modern summers, whereas cold season SST records are up to 6–10°C cooler than modern winters. Overall average temperatures at the site are 2–3°C cooler than modern temperatures during a time of sustained global warmth. Three major cold excursions centred on 3.35, 3.0, and 2.8 Ma showed warm season temperatures over 5°C colder than the last glacial maximum, experiencing temperatures typical of modern subantarctic waters. Two minor cold excursions at 2.7 Ma and 2.4 Ma experienced temperatures cooler than modern winters but not as cold as last glacial conditions. Cold season SSTs show a shift to warmer climate upward through the study interval, whereas warm season estimates remain essentially unchanged. We interpret the strong regional cooling of subtropical Southwest Pacific water through the middle–late Pliocene as having been caused by increased upwelling. It is also possible that the subtropical frontal zone moved north over the site in the Pliocene, however, this is considered the least likely interpretation. Our record of cool conditions in the Southwest Pacific corroborate evidence of cooler than modern conditions in other regions of the western Pacific through the mid-Pliocene despite overall global warming.

Subtropical frontal zone with an inverse horizontal gradient of presipitable water over the midsummer western North Pacific

Subtropical frontal zone with an inverse horizontal gradient of presipitable water over the midsummer western North Pacific

Major nutrients and dissolved oxygen as indicators of the frontal zones in the Atlantic sector of the Southern Ocean

The chemical tracers of the main frontal zones of the Atlantic sector of the Southern Ocean are considered. Before the beginning of the spring bloom, frontal zones are distinguished by lateral gradients of dissolved oxygen, phosphate, nitrate, and silicate. During the spring bloom, the smoothing of nutrient concentrations on both sides of the fronts weakens lateral gradients of chemical properties. The position of surface gradients of nutrients within the Subtropical Frontal Zone (STFZ) does not coincide with the location of temperature and salinity gradients. As a result, fronts in this region have a stepped character. The best chemical indicator of the Northern STFZ front is the dissolved oxygen gradient, which coincides with the temperature and salinity gradients. The southern boundary of the STFZ is distinguished by the gradient of nitrate. The chemical criterion for identifying the Subantarctic Front is the gradient of oxygen, which ranges from 0.5 to 4.0 μmol kg−1 per km; the Polar Front is identified by the gradient of silicate (0.56 to 2.78 μM per km). At the surface, the Weddell‐Scotia Confluence (WSC) is distinguished not by the temperature and salinity, but by chemical parameters: The best year‐round criterion is the lateral gradient of silicate‐to‐phosphate atomic ratio, which ranges from 25 to 35. Other markers of the WSC are the gradients of silicate at the surface, oxygen at the upper boundary of the Circumpolar Deep Water, and the depths of its location.

The Greenland‐Norwegian Seaway: A key area for understanding Late Jurassic to Early Cretaceous paleoenvironments

The paleoclimatology and paleoceanology of the Late Jurassic and Early Cretaceous are of special interest because this was a time when large amounts of marine organic matter were deposited in sediments that have subsequently become petroleum source rocks. However, because of the lack of outcrops, most studies have concentrated on low latitudes, in particular the Tethys and the “Boreal Realm,” where information has been based largely on material from northwest Germany, the North Sea, and England. These areas were all south of 40°N latitude during the Late Jurassic and Early Cretaceous. We have studied sediment samples of Kimmeridgian (∼154 Ma) to Barremian (∼121 Ma) age from cores taken at sites offshore mid‐Norway and in the Barents Sea that lay in a narrow seaway connecting the Tethys with the northern polar ocean. During the Late Jurassic‐Early Cretaceous these sites had paleolatitudes of 42–67°N. The Late Jurassic‐Early Cretaceous sequences at these sites reflect the global sea‐level rise during the Volgian‐Hauterivian and a climatic shift from warm humid conditions in Volgian times to arid cold climates in the early Hauterivian. The sediments indicate orbital control of climate, reflected in fluctuations in the clastic influx and variations in carbonate and organic matter production. Trace element concentrations in the Volgian‐Berriasian sediments suggest that the central part of the Greenland‐Norwegian Seaway might have had suboxic bottom water beneath an oxic water column. Both marine and terrigenous organic matter are present in the seaway sediments. The Volgian‐Berriasian strata have unusually high contents of organic carbon and are the source rocks for petroleum and gas fields in the region. The accumulation of organic carbon is attributed to restricted conditions in the seaway during this time of low sea level. It might be that the Greenland‐Norwegian segment was the deepest part of the transcontinental seaway, bounded at both ends by relatively shallow swells. The decline in organic matter content of the sediments in the Valanginian‐Hauterivian indicates greater ventilation and more active flow through the seaway as the sea level rose. The same benthic foraminifera assemblages are encountered throughout the seaway. Endemic assemblages of arenaceous foraminifera in the Volgian‐Berriasian give way to more diverse and cosmopolitan Valanginian‐Hauterivian benthic communities that include calcareous species. The foraminiferal assemblages also suggest low oxygen content bottom waters during the earlier Cretaceous, changing to more fully oxygenated conditions later. The calcareous nannoplankton, particularlyCrucibiscutum salebrosum, which is rare at low latitudes and abundant in high latitudes, reflect the meridional thermal gradient. They indicate that the Greenland‐Norwegian segment of the seaway was north of a subtropical frontal zone that acted as a barrier between the Tethyan and Boreal Realms. This implies the existence of stable climatic belts during the early Valanginian and Hauterivian, significant meridional temperature gradients, and moderate “ice house” conditions.

An oceanographic characterization of swordfish (Xiphias gladius) longline fishing grounds in the springtime subtropical North Pacific

AbstractDuring January–May, surface manifestation of multiple, individual basin‐scale fronts accentuate the central North Pacific Subtropical Frontal Zone (STFZ) system. The most prominent of these fronts are climatologically located at 32°−34°N and at 28°−30°N latitudes [herein nominally referred to as the ‘Subtropical Front’ (STF) and the ‘South Subtropical Front’ (SSTF), respectively], although considerable interannual variability in both position and intensity is observed. This seasonally dynamic system is also the region typically targeted by the Hawaii‐based swordfish (Xiphias gladius) longline fishing fleet, where the presence, position, and strength of the convergent fronts are believed to play a key role with regard to the catch and catch rates of swordfish. Information furnished by a recent series of meridional hydrographic surveys and concurrent satellite remote sensing data elucidate structural patterns and coupling of the physics and biology associated with these fronts. This enables a re‐characterization of the spring North Pacific STFZ and offers new insight into the seasonal variability of the phytoplankton dynamics in the subtropical North Pacific.On synoptic time scales, geographical positioning of the fronts may be systematically identified through surface outcropping of diagnostic thermohaline isopleths and therefore readily discerned from both shipboard surveys and by spaceborne sensors. The STF during spring can be characterized by the surface expression of the 34.8 isohaline and the 17°C isotherm within the frontal gradient. Biologically, the STF marks the transition from low chloropigment (chlorophyll + phaeopigments), nutrient‐depleted surface waters to the south to a more productive regime to the north. To the south, the 20°C and 35.0 surface isotherm and isohaline, respectively, are characteristically embedded in the thermohaline gradients associated with the SSTF. A sharp increase in depth‐integrated chloropigment is also observed at the SSTF and is ascribed to an increase in the concentration and thickness of the subsurface chloropigment maximum (SCM) prompted by the shoaling of the nutricline with the thermocline structure into the euphotic zone.

Omnivory by copepods in the New Zealand Subtropical Frontal Zone

The importance of omnivorous feeding by epipelagic copepods for copepod nutrition, particle removal and generation of vertical export was investigated in Subantarctic (SA) and Subtropical (ST) water masses either side of the Subtropical Frontal Zone (SFZ) east of New Zealand, using biomass surveys and shipboard bottle incubations. Methodological biases in incubations are described. Ciliates were cleared from the food stock much more rapidly than phytoplankton by all copepod sizes, and heterotrophic nanoflagellates (HNAN) were shown to be a significant potential food source. Microheterotrophs (ciliates + HNAN) contributed at least 50% of the ration in the SA water mass, across copepod size classes. The summed auto- and heterotrophic rations were sufficient to exceed and nearly meet the metabolic and growth requirements for the small and medium-size copepods, respectively, but were well below the requirements for large copepods (including Neocalanus tonsus). Carbon flow calculations showed that this deficit for the larger copepods would be overcome if they were predators of the highly productive small copepods and their egg production. In spring, the copepod community consumed only ∼ 3-8% of phytoplankton and HNAN daily production, whereas larger proportions of ciliate production were predated (∼ 21-38%). The estimated egestion faecal production) rate by the spring SA copepod community was equal to half to one-third the sedimentation rate determined at 300 m, in spring SA sediment traps. Copepod omnivory, including predation on microheterotrophs and possibly cannibalism by large on small copepods, may increase vertical carbon flux because it increases connections in the food web to faecal production.

Detailed structure of the Subtropical Front over Chatham Rise, east of New Zealand

A detailed description of the Subtropical Front (STF) over Chatham Rise is presented. Information is provided by conductivity‐temperature‐depth (CTD) profiler temperature and salinity transects, high‐resolution (3–4 km spacing), near‐synoptic expendable bathythermograph surveys, shipboard acoustic Doppler current profiler measurements, a 160 km constant depth undulator tow, and satellite SST measurements in austral autumn and spring (April and October) near 178°30′E. Detailed sections across the front reveal that the STF can be divided into northern and southern fronts (NSTF and SSTF) separated by a frontal zone, consistent with observations of the STF elsewhere. Comparison with a western CTD section along 176°E reveals that this structure breaks down close to the New Zealand landmass. The dominant impact of seasonally is the formation and decay of a seasonal thermocline of fairly constant depth over the entire region. Velocity measurements indicate eastward zonal flows on the northern and southern flanks of Chatham Rise. The flow over Chatham Rise is complicated by stirring processes but has a mean southward trend consistent with the less dense, warm, salty subtropical water (STW) overriding the more dense, cold, fresh subantarctic water (SAW) to the south. Horizontal‐length‐scale analyses of the temperature field for a constant depth tow at 21 m indicate a k−2.4 spectral falloff with no proof of preferential length scales. The temperature salinity gradients are largely density compensated in this mixed layer tow. The horizontal temperature standard deviation shows subsurface maxima concentrated at the locations of the NSTF and SSTF. Vertical length scales vary across the front, with large scales (80–100 m) in the STW north of the STF and smaller scales (20–40 m) in the subtropical frontal zone and SAW to the south. The surface vertical length scales are dominated by the presence of a seasonal thermocline.

Global patterns of decadal‐scale variability observed in sea surface temperature and lower‐tropospheric circulation fields

The global patterns associated with decadal‐scale variability (DSV) are examined by a lag correlation technique based on local anomaly indices, using the fields of measured sea surface temperature (SST) and 850 hPa geopotential height for the last 50 years. The three dominant patterns are identified, and the variability is examined; the first spreads over the entire Pacific, which is concurrent with the decadal‐scale modulation of the El Niño/Southern Oscillation (DES variability), the second is confined to the local midlatitude North Pacific (LNP variability), and the third extends over the North Atlantic with the decadal North Atlantic Oscillation (NAO) (DNA variability). The global SST pattern of DES variability exhibits large‐scale equatorial symmetry in the Pacific, which is similar to that of the Pacific Decadal Oscillation but is distinguished by the signals in the subtropical frontal zones. These SST anomalies are accompanied by anomalous subtropical highs that appear prior to the anomalous depression around Australia. The LNP variability, which is related with the Arctic Oscillation, is characterized by the SST anomalies along the North Pacific subarctic frontal zone moving eastward accompanied by the anomalous Aleutian Low. This variability develops (decays) without (with) coherent variability in the tropics. It shows the 6‐year quadrature phase relationship with the DES variability, indicative of an interdecadal variability with a period of 24 years. The DNA variability is featured by the atmospheric NAO and by the SST anomalies in four zonal bands that spread in the North Atlantic from the tropics to high latitudes. This variability is independent of either the DES or LNP variability.

On the structure, paths, and fluxes associated with Agulhas rings

In January 1993, two Agulhas rings were sampled in the eastern South Atlantic during the World Ocean Circulation Experiment one‐time hydrographic section A11. The first of these, Ring 1, was sampled at 36°S, 4°E in the Subtropical Frontal Zone and was smaller and less energetic than other, more typical Agulhas rings, owing partly to its interactions with the North Subtropical Front. The other, Ring 2, was sampled at 33°S, 10°E in the subtropics. This ring was average in size and reasonably representative of the rings observed in the southeast Atlantic during the early 1990s. The path of the rings is found to be dominated by advection in the mean flow rather than self‐induced velocities of eddies under the β effect. We observe significant differences in reduced gravity and available heat and salt anomalies in the rings; from these we infer temporal variability in the stratification of rings upon their formation at the Agulhas Retroflection. This temporal variability in the structure of rings will affect the fluxes associated with them. Data collected within Ring 2 show that neglecting consideration of the water beneath the 10°C isotherm underestimates the volume flux by half and the potential temperature and salinity flux by in excess of one third and one half, respectively. Using a base of the 3.5°C isotherm and including the high‐salinity intermediate water from the Indian Ocean is found to be more appropriate. With these conditions, six Agulhas rings per year entering the subtropical South Atlantic would equate to a volume flux of 9 Sv and an absolute potential temperature and salinity flux of 84 Sv°C and 322 Sv psu (1 Sv psu ≈ 106 kg s−1). The significant flux of intermediate water from the Indian Ocean may be as important in the return path of the thermohaline circulation as the intermediate water that enters the South Atlantic through Drake Passage. Further, the water associated with Agulhas rings and cooler than 10°C potentially contributes a larger volume flux to the warm water return path than the corresponding surface and upper thermocline water.

Frontogenesis in the North Pacific Oceanic Frontal Zones—A Numerical Simulation

Abstract A primitive equation model [Geophysical Fluid Dynamics Laboratory’s (GFDL’s) MOM 2] with one degree horizontal resolution is used to simulate the seasonal cycle of frontogenesis in the subarctic frontal zone (SAFZ) and the subtropical frontal zone (STFZ) of the North Pacific Ocean. The SAFZ in the model contains deep (greater than 500 m in some places) regions with seasonally varying high gradients in temperature and salinity. The gradients generally weaken toward the east. The STFZ consists of a relatively shallow (less than 200 m in most places) region of high gradient in temperature that disappears in the summer/fall. The high gradient in salinity in the STFZ maintains its strength year round and extends across almost the entire basin. The model simulates the location and intensity of the frontal zones in good agreement with climatological observations: generally to within two degrees of latitude and usually at the same or slightly stronger intensity. The seasonal cycle of the frontal zones al...

Autotrophic carbon assimilation and biomass from size-fractionated phytoplankton in the surface waters across the subtropical frontal zone (Indian Ocean)

The composition of the phytoplanktonic communities in the surface waters of the La Reunion-Kerguelen transect (from 38°36S to 46°33S) has been investigated under spring conditions (Antares 3 cruise, France-JGOFS, 28 September–8 November, 1995). The study, conducted at six stations in the subtropical frontal zone, involved size fractionations (threshold: 2 μm). The large variations in the overall biomass and autotrophic carbon fixation, calculated via Rubisco activity measurements and expressed respectively in terms of μg chlorophyll (a + b + c) per liter and nmol fixed carbon dioxide per liter and per hour, were attributable only to phytoplanktonic cells of >2 μm, with a peak observed in the frontal zone. The picophytoplankton (<2 μm) biomass remained constant throughout the transect, but the evolution of the species composition of the picophytoplanktonic population, as calculated from flow cytometry measurements through this frontal zone, changed. This study provides evidence, for the first time in this area, of the disappearance of prochlorophytes from the south of the frontal zone (42–47°S). Picoeukaryotes (<2 μm) and cyanobacteria populations, resolved by flow cytometry, were present all along the transect. However, their abundance decreased southward up to the quasi-disappearance of cyanobacteria at the southernmost station (52°S) that is characteristic of antarctic waters. The presence of prochlorophytes that is exclusive to the subtropical surface waters, and the low carbon fixation activity associated with these waters, may be linked to the specific hydrological features encountered. In contrast, the marked reduction in the cyanobacteria and the abundance of picoeukaryotes along the north-south transect is more likely to be a result of the reduction in temperature through the frontal zone.

初夏の東アジアおよび西部北太平洋域にみられる対流活動と循環場の10-25日季節内変動

The space-time evolution of intraseasonal convection and circulation anomalies in the 10-25-day period range over East Asia and western North-Pacific Monsoon regions are explored for four early summer seasons from 1991 to 1994. The paper primarily focuses on the linkage between tropical convection over the South China Sea, and lower-tropospheric circulation over the East Asian subtropics on this time scale. The composite results show that quasi-periodic fluctuations of convection on the 10-25-day time scale over the South China Sea, are associated with large-scale circulation in the Asian-Pacific region. The development of subtropical circulation and convection takes place through changes in large-scale circulation on this time scale. 10-25-day enhanced (suppressed) convection occurs in conjunction with well-organized cyclonic (anti-cyclonic) circulation anomalies over the South China Sea. A downstream wave train extending from the South China Sea into the North Pacific, is built up in the lower troposphere associated with this convective activity. This wave train is probably interpreted as a Rossby mode response to the anomalous heating (cooling). Following the peak of enhanced convection, the East Asian subtropical anomalous anti-cyclone maintains its strength as part of this wave train, and couples with enhanced subtropical convection along the northern flank of this anti-cyclonic cell. This development of convection in the subtropical frontal zone is due to an increase in the low-level westerlies at the northern flank of this anti-cyclonic circulation. In addition, this subtropical anomalous anti-cyclone moves southwestward from the subtropics into the South China Sea region, and appears to initiate subsequent suppressed convection. In contrast, the southwestward migration of subtropical cyclonic anomalies also occurs following the inactive convection over the South China Sea, and this behavior appears to trigger the subsequent active convection over this region. These features suggest that mutual interactions between the tropics and the subtropics on this time scale play important roles on the variability of monsoon convection and circulation over East Asia, and western North Pacific. In the upper level strongest divergent outflow (convergent inflow) occurs in the convective (non-convective) region and the significant wave train is maximally intensified in the mid-latitude at the convective peak. Upper-level circulation also appears to be forced by heating (cooling) located over the South China Sea region. These circulation features confirm that 10-25-day variations of convection over the South China Sea are one of the effective forcings for the large-scale circulation during the early summer season.

Oceans and Climate Shifts

An article “As the oceans switch, climate shifts” by Richard A. Kerr (News Focus, 10 July, p. [157][1]) presents a timely summary of latest studies on large-scale oscillations in the climate system with (multi-) decadal time scales. Emphasized in the article are near-global or basin-wide patterns like PDO (Pacific Decadal Oscillation). Presumably, an underlying assumption is that decadal sea surface temperature (SST) anomalies must be distributed near globally or, at least, as broadly as El Nino-related anomalies whose time scales are shorter than a decade. Influence of El Nino spreads widely over the extratropics through atmospheric “teleconnection” and forces SST underneath to change. A pattern like PDO would emerge if a similar “teleconnection” occurs also on the decadal scale. Yet, if midlatitude oceanic processes play an active role in the decadal variability independently of tropical processes, then significant SST anomalies associated with the midlatitude mode may be confined within a local oceanic gyre or even along a narrow oceanic front. Several recent studies suggest that is likely the case. Over the North Pacific basin, decadal SST fluctuations correlated with the tropical variability were confined in the subtropical frontal zone. Stronger decadal fluctuations were observed in the subarctic frontal zone, but unlike in PDO they exhibited no simultaneous correlation with the tropical variability ([1][2]). This is suggestive of the presence of decadal variability inherent to the North Pacific atmosphere-ocean system, although the associated atmospheric anomalies extends over the North America. An empirical orthogonal function analysis applied globally tends to preferentially extract a near-global pattern like PDO. The PDO time series presented apparently includes decadal and shorter time scales, implying that part of decadal anomalies generated in midlatitudes may be mixed up in PDO with near-global anomalies associated with El Nino. 1. [↵][3]1. H. Nakamura, 2. G. Lin, 3. T. Yamagata , Bull. Am. Meteorol. Soc. 78, 2215 (1997). [OpenUrl][4] [1]: /lookup/doi/10.1126/science.281.5374.157 [2]: #ref-1 [3]: #xref-ref-1-1 View reference 1 in text [4]: {openurl}?query=rft.jtitle%253DBull.%2BAm.%2BMeteorol.%2BSoc.%26rft.volume%253D78%26rft.spage%253D2215%26rft.atitle%253DBULL%2BAM%2BMETEOROL%2BSOC%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx