On the history of meridional overturning circulation schematic diagrams

Recent global warming caused by humans and the prediction of a reduced Atlantic Ocean meridional overturning circulation in the future has increased interest in the role of the overturning circulation in climate change. A schematic diagram of the overturning circulation called the “Great Ocean Conveyor Belt,” published by Wallace Broecker in 1987, has become a popular image that emphasizes the inter-connected ocean circulation and the northward flux of heat in the Atlantic. This seems a good time to review the development of the conveyor belt concept and summarize the history of overturning circulation schematics.In the 19th century it was thought that symmetric overturning circulation cells were located on either side of the equator in the Atlantic. As new hydrographic measurements were obtained, circulation schematics in the early 20th century began to show the inter-hemispheric overturning circulation in the Atlantic. In the second half of the 20th century schematics showed the global ocean overturning circulation including connections between the Atlantic and the Pacific and Indian Oceans. Some recent schematics of the overturning circulation show its complexities, but as more information is included these schematics have also become complex and not as easy to understand as the simple Broecker 1987 version. However, these complex schematics, especially the quantitative ones, represent valuable syntheses of our developing knowledge of the overturning circulation.     

The alpha/beta ocean distinction: A perspective on freshwater fluxes, convection, nutrients and productivity in high-latitude seas

Stratification is perhaps the most important attribute of oceans with regards to climate and biology. Two simple aspects of the ocean's climate system appear to have a surprisingly important role in transforming waters that feed the global thermohaline circulation, dominating patterns of biogeochemical flux and establishing macroecological domains. First, largely because of meridional distillation (mainly due to the atmospheric transport of freshwater across the Isthmus of Panama) the North Pacific is fresher than the North Atlantic. Second, largely because of zonal distillation (e.g., warming and evaporation at low latitudes and poleward transport of latent heat and moisture by the atmosphere) the upper layers of subtropical seas are permanently stratified by temperature (N_T²=gαdT/dz>0; here called alpha oceans), while the upper layers of high-latitude seas are permanently stratified by salinity (N_S²=gβdS/dz>0; here called beta oceans). The physical basis for the boundary separating alpha and beta oceans is unclear, but may lie in the thermodynamical equations published by Fofonoff [1961. Energy transformations in the sea. Fisheries Research Board of Canada, Report Series 109, 82pp]. Nevertheless, it is clear that the resulting thermohaline distributions establish a ‘downhill journey’ of low-salinity (and nutrient-rich) waters from the North Pacific to the Arctic and then into the North Atlantic. The Arctic Ocean—itself—acts a double estuary, whereby waters entering from the North Atlantic become either denser through cooling (negative estuary) or lighter by freshening (positive estuary) as they circulate within the basin and then return to the North Atlantic as a variety of components of the ocean's conveyor. Intermediate and deep waters generally form within cyclonic beta oceans in close proximity to alphas systems. Similar patterns of stratification, nutrients and biogeographical boundaries persist in the Southern Hemisphere. It is thus argued that this simple distinction—alpha versus beta oceans—provides a broad, conceptual framework for simple interpretation of key physical and biological processes and rates, including the impacts of climate variability.     

Causes of a fresher, colder northern North Atlantic in late 20th century in a coupled model

Observational evidence indicates that in the northern North Atlantic, especially in the Labrador Sea, almost the whole column of the ocean water is fresher, and colder in late 20th century than in 1950–1960s. Here we analyze a four-member ensemble of the 20th century simulations from a coupled climate model to examine the possible causes for these observed changes. The model simulations resemble the observed changes in the northern North Atlantic. The simulated results show that a decreased meridional freshwater divergence and an increased meridional heat divergence associated with a weaker thermohaline circulation in the North Atlantic are the primary causes for the freshening and cooling in the northern North Atlantic. The increased precipitation less evaporation tends to enforce the freshening, but the reduced sea ice flux into this region tends to weaken it. On the other hand, the surface warming induced by a higher atmospheric CO₂ concentration tends to heat up the northern North Atlantic, but is overcome by the cooling from increased meridional heat divergence.     

Why are rings regularly shed in the western equatorial Atlantic but not in the western Pacific?

The western equatorial Atlantic is characterized by the formation and shedding of 3–4 large anticyclonic rings per year. These rings originate from the North Brazil Current which, in response to the vanishing wind stress curl (over the ocean interior), retroflects and turns eastward at around 4°N. After their formation and shedding the rings propagate toward the northwest along the South American coast carrying an annual average of about 4Sv. As such, the rings constitute an important part of the meridional heat flux in the Atlantic.The same cannot be said, however, of the western equatorial Pacific. Here, the situation is entirely different even though the South Equatorial Current retroflects at roughly the same latitude as its Atlantic counterpart, the North Brazil Current. Although the South Equatorial Current retroflection is flanked by two quasi-permanent eddies (the so-called Halmahera and the Mindanao eddies), these eddies are an integral part of the current itself and are not shed. Consequently, they are not associated with any meridional heat flux. An important question is, then, why the two oceans behave in such a fundamentally different way even though the source of the rings, the retroflected currents, are very similar in the two oceans.To answer this question, the two oceans are compared using recently developed analytical and numerical models for the western equatorial oceans. It is first pointed out that, according to recent developments in the modelling of the western equatorial Atlantic, the North Brazil Current retroflection rings are formed, shed and drift to the west because, in the Atlantic, this is the only way by which the momentum flux of the approaching and retroflecting current can be balanced. In this scenario, the northwestward flow force exerted by the approaching and retroflecting North Brazil Current (analogous to the force created by a rocket) is balanced by the southwestward force exerted by the rings as they are formed (analogous in some sense to the kickback associated with a firing gun).On the other hand, in the western equatorial Pacific, the formation and shedding of rings is unnecessary because the southward flowing Mindanao Current provides an alternative mechanism for balancing the northward momentum flux of the South Equatorial Current. This implies that it is the absence of a counter current (such as the Mindanao) in the western Atlantic that causes the formation and shedding of North Brazil Current rings. A remaining difficulty with the above scenario is that most colliding and retroflecting currents (i.e. the Mindanao and South Equatorial currents) are not “balanced” in the sense that they cannot be stationary but rather must drift along the coast. It is shown that, in the case of the western Pacific, the long-shore migration is arrested by the Indonesian Throughflow which allows the “unbalanced” fraction of the approaching currents to leak out into the Indian Ocean. This resolves the above difficulty and allows the retroflection to be approximately steady.     

Increase in total carbonate in the western North Pacific water and a hypothesis on the missing sink of anthropogenic carbon

Sea water samples were collected from various depths in the North Pacific (40–21°N) along 165°E in 1991. Their total carbonate (total dissolved carbonate species) contents were determined with random errors less than 0.2% by a coulometric method. The preformed carbonate contents defined by Chen (1982) were calculated from the obtained data and other observed data including potential temperature, salinity, dissolved oxygen and total alkalinity. The same calculation was done for the GEOSECS data obtained in nearly the same region in 1973. The difference between the two data sets reveals that the preformed carbonate has increased by 180±41 gC/m² during the last 18 years. This value is comparable or somewhat larger than 150 gC/m² obtained in the case that the ocean uptakes 3 GtC/yr for 18 years and distributes it equally among the world oceans. Based on the results, a hypothesis on the missing sink for the anthropogenic carbon dioxide is presented, in that the missing sink is the intermediate waters formed in the northern North Pacific and the Southern Ocean besides the deep waters formed in the North Atlantic and the Southern Ocean.     

The connection between Labrador Sea buoyancy loss, deep western boundary current strength, and Gulf Stream path in an ocean circulation model

The sensitivity of the North Atlantic gyre circulation to high latitude buoyancy forcing is explored in a global, non-eddy resolving ocean general circulation model. Increased buoyancy forcing strengthens the deep western boundary current, the northern recirculation gyre, and the North Atlantic Current, which leads to a more realistic Gulf Stream path. High latitude density fluxes and surface water mass transformation are strongly dependent on the choice of sea ice and salinity restoring boundary conditions. Coupling the ocean model to a prognostic sea ice model results in much greater buoyancy loss in the Labrador Sea compared to simulations in which the ocean is forced by prescribed sea ice boundary conditions. A comparison of bulk flux forced hindcast simulations which differ only in their sea ice and salinity restoring forcings reveals the effects of a mixed thermohaline boundary condition transport feedback whereby small, positive temperature and salinity anomalies in subpolar regions are amplified when the gyre spins up as a result of increased buoyancy loss and convection. The primary buoyancy flux effects of the sea ice which cause the simulations to diverge are ice melt, which is less physical in the diagnostic sea ice model, and insulation of the ocean, which is less physical with the prognostic sea ice model. Increased salinity restoring ensures a more realistic net winter buoyancy loss in the Labrador Sea, but it is found that improvements in the Gulf Stream simulation can only be achieved with the excessive buoyancy loss associated with weak salinity restoring.     

Modelling hydrographic changes in the Labrador sea over the past five decades

Inter-annual to inter-decadal changes of hydrographic structure and circulation in the subpolar North Atlantic are studied using a coarse resolution ocean circulation model. The study covers 1949 through 2001, inclusive. A “time-mean state nudging” method is applied to assimilate the observed hydrographic climatology into the model. The method significantly reduces model biases in the long-term mean distribution of temperature and salinity, which commonly exist in coarse-resolution ocean models. By reducing the time-mean biases we also significantly improve the model’s representation of inter-annual to inter-decadal variations. In the central Labrador Sea, the model broadly reproduces the heat and salt variations of the Labrador Sea Water (LSW) as revealed by hydrographic observations. Model sensitivity experiments confirm that the low-frequency hydrographic changes in the central Labrador Sea are closely related to changes in the intensity and depth of deep convection. Changes in surface heat flux associated with the winter North Atlantic Oscillation (NAO) index play a major role in driving the changes in T–S and sea surface height (SSH). Changes in wind stress play a secondary role in driving these changes but are important in driving the changes in the depth-integrated circulation. The total changes in both SSH and depth-integrated circulation are almost a linear combination of the separate influences of variable buoyancy and momentum fluxes.     

Winter-to-winter recurrence of sea surface temperature, salinity and mixed layer depth anomalies

The mean seasonal cycle of mixed layer depth (MLD) in the extratropical oceans has the potential to influence temperature, salinity and mixed layer depth anomalies from one winter to the next. Temperature and salinity anomalies that form at the surface and spread throughout the deep winter mixed layer are sequestered beneath the mixed layer when it shoals in spring, and are then re-entrained into the surface layer in the subsequent fall and winter. Here we document this ‘re-emergence mechanism’ in the North Pacific Ocean using observed SSTs, subsurface temperature fields from a data assimilation system, and coupled atmosphere–ocean model simulations. Observations indicate that the dominant large-scale SST anomaly pattern that forms in the North Pacific during winter recurs in the following winter. The model simulation with mixed layer ocean physics reproduced the winter-to-winter recurrence, while model simulations with observed SSTs specified in the tropical Pacific and a 50 m slab in the North Pacific did not. This difference between the model results indicates that the winter-to-winter SST correlations are the result of the re-emergence mechanism, and not of similar atmospheric forcing of the ocean in consecutive winters. The model experiments also indicate that SST anomalies in the tropical Pacific associated with El Niño are not essential for re-emergence to occur.The recurrence of observed SST and simulated SST and SSS anomalies are found in several regions in the central North Pacific, and are quite strong in the northern (>50°N) part of the basin. The winter-to-winter autocorrelation of SSS anomalies exceed those of SST, since only the latter are strongly damped by surface fluxes. The re-emergence mechanism also has a modest influence on MLD through changes in the vertical stratification in the seasonal thermocline.     

Freshwater transport estimates and the global overturning circulation: Shallow, deep and throughflow components

Meridional ocean freshwater transports and convergences are calculated from absolute geostrophic velocities and Ekman transports. The freshwater transports are analyzed in terms of mass-balanced contributions from the shallow, ventilated circulation of the subtropical gyres, intermediate and deep water overturns, and Indonesian Throughflow and Bering Strait components. The following are the major conclusions:

1.

Excess freshwater in high latitudes must be transported to the evaporative lower latitudes, as is well known. The calculations here show that the northern hemisphere transports most of its high latitude freshwater equatorward through North Atlantic Deep Water (NADW) formation (as in [Rahmstorf, S., 1996. On the freshwater forcing and transport of the Atlantic thermohaline circulation. Climate Dynamics 12, 799-811]), in which saline subtropical surface waters absorb the freshened Arctic and subpolar North Atlantic surface waters (0.45 ± 0.15 Sv for a 15 Sv overturn), plus a small contribution from the high latitude North Pacific through Bering Strait (0.06 ± 0.02 Sv). In the North Pacific, formation of 2.4 Sv of North Pacific Intermediate Water (NPIW) transports 0.07 ± 0.02 Sv of freshwater equatorward.In complete contrast, almost all of the 0.61 ± 0.13 Sv of freshwater gained in the Southern Ocean is transported equatorward in the upper ocean, in roughly equal magnitudes of about 0.2 Sv each in the three subtropical gyres, with a smaller contribution of <0.1 Sv from the Indonesian Throughflow loop through the Southern Ocean. The large Southern Ocean deep water formation (27 Sv) exports almost no freshwater (0.01 ± 0.03 Sv) or actually imports freshwater if deep overturns in each ocean are considered separately (−0.06 ± 0.04 Sv).This northern-southern hemisphere asymmetry is likely a consequence of the “Drake Passage” effect, which limits the southward transport of warm, saline surface waters into the Antarctic [Toggweiler, J.R., Samuels, B., 1995a. Effect of Drake Passage on the global thermohaline circulation. Deep-Sea Research I 42(4), 477-500]. The salinity contrast between the deep Atlantic, Pacific and Indian source waters and the denser new Antarctic waters is limited by their small temperature contrast, resulting in small freshwater transports. No such constraint applies to NADW formation, which draws on warm, saline subtropical surface waters .

2.

The Atlantic/Arctic and Indian Oceans are net evaporative basins, hence import freshwater via ocean circulation. For the Atlantic/Arctic north of 32°S, freshwater import (0.28 ± 0.04 Sv) comes from the Pacific through Bering Strait (0.06 ± 0.02 Sv), from the Southern Ocean via the shallow gyre circulation (0.20 ± 0.02 Sv), and from three nearly canceling conversions to the NADW layer (0.02 ± 0.02 Sv): from saline Benguela Current surface water (−0.05 ± 0.01 Sv), fresh AAIW (0.06 ± 0.01 Sv) and fresh AABW/LCDW (0.01 ± 0.01 Sv). Thus, the NADW freshwater balance is nearly closed within the Atlantic/Arctic Ocean and the freshwater transport associated with export of NADW to the Southern Ocean is only a small component of the Atlantic freshwater budget.For the Indian Ocean north of 32°S, import of the required 0.37 ± 0.10 Sv of freshwater comes from the Pacific through the Indonesian Throughflow (0.23 ± 0.05 Sv) and the Southern Ocean via the shallow gyre circulation (0.18 ± 0.02 Sv), with a small export southward due to freshening of bottom waters as they upwell into deep and intermediate waters (−0.04 ± 0.03 Sv).The Pacific north of 28°S is essentially neutral with respect to freshwater, −0.04 ± 0.09 Sv. This is the nearly balancing sum of export to the Atlantic through Bering Strait (−0.07 ± 0.02 Sv), export to the Indian through the Indonesian Throughflow (−0.17 ± 0.05 Sv), a negligible export due to freshening of upwelled bottom waters (−0.03 ± 0.03 Sv), and import of 0.23 ± 0.04 Sv from the Southern Ocean via the shallow gyre circulation.

3.

Bering Strait’ssmall freshwater transport of <0.1 Sv helps maintains the Atlantic-Pacific salinity difference. However, proportionally large variations in the small Bering Strait transport would only marginally impact NADW salinity, whose freshening relative to saline surface water is mainly due to air-sea/runoff fluxes in the subpolar North Atlantic and Arctic. In contrast, in the Pacific, because the total overturning rate is much smaller than in the Atlantic, Bering Strait freshwater export has proportionally much greater impact on North Pacific salinity balances, including NPIW salinity.

     

大西洋热盐环流年代际变化机制研究Ⅲ.北大西洋海气要素对热盐环流年代际振荡的影响

基于该系列文章前文研究中构建的海气耦合气候模式和所揭示的北大西洋热盐环流年代际振荡机制,针对海气要素对该振荡机制的影响问题进行了重点的探讨。为细致准确的研究北大西洋海洋要素同北大西洋热盐环流年代际振荡的关系,有针对性的定义了副极地海区表层密度指数和北大西洋暖流强度指数并对模式结果进行了全面分析。分析结果表明副极地海区表层密度变化领先大西洋径向翻转环流(MOC)变化7 a,北大西洋暖流的变化领先 MOC变化4 a,格陵兰－苏格兰海脊溢流水强度(包括丹麦海峡溢流水和法鲁海峡溢流水,是北大西洋深层水的重要来源)的变化领先 MOC的变化3 a;北大西洋大气要素变化对北大西洋热盐环流年代际振荡有非常重要的调制作用,当副极地流环和北大西洋暖流(NAC)达到最强的2 a之前,高纬度地区大气为气旋式环流异常,中纬度地区大气为反气旋式环流异常,海表热通量在大西洋副极地海区是负异常,这都有利于副极地流环和NAC的加强,更多高盐度的北大西洋水进入格陵兰-冰岛-挪威海(GIN)海域,由此可以导致GIN海域表层密度上升,使水体的层结稳定性减弱,有利于深层对流的发生,同时大气变化通过风应力旋度和海表热通量也直接影响GIN海域深层水的生成,进而导致格陵兰－苏格兰海脊溢流水的强度增加。     

大西洋热盐环流年代际变化机制研究Ⅲ.北大西洋海气要素对热盐环流年代际振荡的影响

基于该系列文章前文研究中构建的海气耦合气候模式和所揭示的北大西洋热盐环流年代际振荡机制,针对海气要素对该振荡机制的影响问题进行了重点的探讨.为细致准确的研究北大西洋海洋要素同北大西洋热盐环流年代际振荡的关系,有针对性的定义了副极地海区表层密度指数和北大西洋暖流强度指数并对模式结果进行了全面分析.分析结果表明副极地海区表...     

Roles of Physical Processes in the Carbon Cycle Using a Simplified Physical Model

A simplified physical model is proposed in this article to describe differences among basins in substance distributions which were not well described by previous simplified models. In the proposed model, the global ocean is divided into the Pacific/Indian Ocean (PI), the Atlantic Ocean (AT), the Southern Ocean and the Greenland/Iceland/Norwegian Sea. The model is consisted of five physical parameters, namely the air-sea gas exchange, the thermohaline circulation, the horizontal and vertical diffusions, and the deep convection in the high-latitude regions. Individual values of these parameters are chosen by optimizing model distribution of natural ¹⁴C as a physical tracer. The optimal value for a coefficient of vertical diffusion in the low-latitude region is 7.5 × 10^–5 [m²s^–1]. Vertical transports by the Antarctic Bottom Water and the North Atlantic Deep Water are estimated at 1.0 Sv and 9.0 Sv. Global-mean air-sea gas exchange time is calculated at 9.0 years. Using these optimal values, vertical profiles of dissolved inorganic carbon without biological production in PI and AT are estimated. Oceanic responses to anthropogenic fluctuations in substance concentrations in the atmosphere induced by the industrialization and nuclear bomb are also discribed, i.e., the effects appear significantly in AT while a signal is extremely weak in PI. A time-delay term is effective to make the PI water older near the bottom boundary.     

近惯性内波破碎混合方案对MOM4模式的影响

风生近惯性内波破碎引起的跨等密度面混合在海洋内部混合中起重要作用。然而其参数化对海洋模式的模拟影响仍有待进一步认识。本文给出的是在模块化海洋模式（MOM）中海洋表面边界层以下引入一个考虑风驱动近惯性内波破碎引起的跨等密度面混合参数化方案的研究工作。模拟结果显示,该方案有效改善MOM4模拟的上层1 000 m以上的温盐偏差,特别是在北太平洋和北大西洋的通风地区。数值试验表明,风生近惯性内波破碎有可能是维持海洋通风过程的重要机制之一,它使得海洋通风区的位温变冷,盐度变淡,整层等位密面加深。维持的通风过程使得北太平洋副极地大涡的影响延伸到副热带大涡。从而模拟的北太平洋中层水源头及其副热带大涡东侧的温盐更接近观测实际。同时,模拟的北大西洋经圈翻转环流强度也更为合理。     

Thallium concentrations in seawater

Measurements of the concentration of thallium in seawater collected from numerous ocean locations ranged from 12 to 16 ng kg⁻¹. Variations between the Atlantic and Pacific Oceans, between the northern and southern hemispheres of the Pacific Ocean, and between surface and deep waters of both the Pacific and Atlantic oceans were comparable with the precision of the analyses. This relatively constant distribution indicates that the element's cycle in seawater may be similar to those of the alkali metals which are its principal biogeochemical analogues.     

A Coupled Ice-Ocean Model in the Pan-Arctic and North Atlantic Ocean: Simulation of Seasonal Cycles

A coupled ice-ocean model is configured for the pan-Arctic and northern North Atlantic Ocean with a 27.5 km resolution. The model is driven by the daily atmospheric climatology averaged from the 40-year NCEP reanalysis (1958–1997). The ocean model is the Princeton Ocean Model (POM), while the sea ice model is based on a full thermodynamical and dynamical model with plastic-viscous rheology. A sea ice model with multiple categories of thickness is utilized. A systematic model-data comparison was conducted. This model reasonably reproduces seasonal cycles of both the sea ice and the ocean. Climatological sea ice areas derived from historical data are used to validate the ice model performance. The simulated sea ice cover reaches a maximum of 14 × 10⁶ km² in winter and a minimum of 6.7 × 10⁶ km² in summer. This is close to the 95-year climatology with a maximum of 13.3 × 10⁶ km² in winter and a minimum of 7 × 10⁶ km² in summer. The simulated general circulation in the Arctic Ocean, the GIN (Greenland, Iceland, and Norwegian) seas, and northern North Atlantic Ocean are qualitatively consistent with historical mapping. It is found that the low winter salinity or freshwater in the Canada Basin tends to converge due to the strong anticyclonic atmospheric circulation that drives the anticyclonic ocean surface current, while low summer salinity or freshwater tends to spread inside the Arctic and exports out of the Arctic due to the relaxing wind field. It is also found that the warm, saline Atlantic Water has little seasonal variation, based on both simulation and observations. Seasonal cycles of temperature and salinity at several representative locations reveals regional features that characterize different water mass properties.     

Variation of Indo-Pacific upper ocean heat content during 2001–2012 revealed by Argo

Understanding of the temporal variation of oceanic heat content(OHC) is of fundamental importance to the prediction of climate change and associated global meteorological phenomena. However, OHC characteristics in the Pacific and Indian oceans are not well understood. Based on in situ ocean temperature and salinity profiles mainly from the Argo program, we estimated the upper layer(0–750 m) OHC in the Indo-Pacific Ocean(40°S–40°N, 30°E–80°W). Spatial and temporal variability of OHC and its likely physical mechanisms are also analyzed. Climatic distributions of upper-layer OHC in the Indian and Pacific oceans have a similar saddle pattern in the subtropics, and the highest OHC value was in the northern Arabian Sea. However, OHC variabilities in the two oceans were different. OHC in the Pacific has an east-west see-saw pattern, which does not appear in the Indian Ocean. In the Indian Ocean, the largest change was around 10°S. The most interesting phenomenon is that, there was a long-term shift of OHC in the Indo-Pacific Ocean during 2001–2012. Such variation coincided with modulation of subsurface temperature/salinity. During 2001–2007, there was subsurface cooling(freshening)nearly the entire upper 400 m layer in the western Pacific and warming(salting) in the eastern Pacific. During2008–2012, the thermocline deepened in the western Pacific but shoaled in the east. In the Indian Ocean, there was only cooling(upper 150 m only) and freshening(almost the entire upper 400 m) during 2001–2007. The thermocline deepened during 2008–2012 in the Indian Ocean. Such change appeared from the equator to off the equator and even to the subtropics(about 20°N/S) in the two oceans. This long-term change of subsurface temperature/salinity may have been caused by change of the wind field over the two oceans during 2001–2012, in turn modifying OHC.     

An extended variable-grid global ocean circulation model and its preliminary results of the equatorial Pacific circulation

1IntroductionThetropicalPacificOceanplaysanimpor-tantroleintheclimatevariabilitiessuchasElNi-no-SouthernOscillation(ENSO)phenomenon(Chao,1993).ManystudieshavefoundthatthetropicalPacificvariabilitiescanhavesignifi-cantinfluenceontheoceancirculationintheseasadjacenttoChina(Yu,1985;Chaoetal.,1996;Wangetal.,2002).TheseaareaadjacenttoChinaischaracterizedbyitscomplextopog-raphyandnumerousnarrowstraits,andthusre-quiresafinegridtoresolve.Tostudytheinter-actionbetweenthetropicalPacificandChinas…     

Oxygen minimum zones in the eastern tropical Atlantic and Pacific oceans

Within the eastern tropical oceans of the Atlantic and Pacific basin vast oxygen minimum zones (OMZ) exist in the depth range between 100 and 900 m. Minimum oxygen values are reached at 300–500 m depth which in the eastern Pacific become suboxic (dissolved oxygen content <4.5 μmol kg⁻¹) with dissolved oxygen concentration of less than 1 μmol kg⁻¹. The OMZ of the eastern Atlantic is not suboxic and has relatively high oxygen minimum values of about 17 μmol kg⁻¹ in the South Atlantic and more than 40 μmol kg⁻¹ in the North Atlantic. About 20 (40%) of the North Pacific volume is occupied by an OMZ when using 45 μmol kg⁻¹ (or 90 μmol kg⁻¹, respectively) as an upper bound for OMZ oxygen concentration for ocean densities lighter than σ_θ < 27.2 kg m⁻³. The relative volumes reduce to less than half for the South Pacific (7% and 13%, respectively). The abundance of OMZs are considerably smaller (1% and 7%) for the South Atlantic and only 0% and 5% for the North Atlantic. Thermal domes characterized by upward displacements of isotherms located in the northeastern Pacific and Atlantic and in the southeastern Atlantic are co-located with the centres of the OMZs. They seem not to be directly involved in the generation of the OMZs.OMZs are a consequence of a combination of weak ocean ventilation, which supplies oxygen, and respiration, which consumes oxygen. Oxygen consumption can be approximated by the apparent oxygen utilization (AOU). However, AOU scaled with an appropriate consumption rate (aOUR) gives a time, the oxygen age. Here we derive oxygen ages using climatological AOU data and an empirical estimate of aOUR. Averaging oxygen ages for main thermocline isopycnals of the Atlantic and Pacific Ocean exhibit an exponential increase with density without an obvious signature of the OMZs. Oxygen supply originates from a surface outcrop area and can also be approximated by the turn-over time, the ratio of ocean volume to ventilating flux. The turn-over time corresponds well to the average oxygen ages for the well ventilated waters. However, in the density ranges of the suboxic OMZs the turn-over time substantially increases. This indicates that reduced ventilation in the outcrop is directly related to the existence of suboxic OMZs, but they are not obviously related to enhanced consumption indicated by the oxygen ages. The turn-over time suggests that the lower thermocline of the North Atlantic would be suboxic but at present this is compensated by the import of water from the well ventilated South Atlantic. The turn-over time approach itself is independent of details of ocean transport pathways. Instead the geographical location of the OMZ is to first order determined by: (i) the patterns of upwelling, either through Ekman or equatorial divergence, (ii) the regions of general sluggish horizontal transport at the eastern boundaries, and (iii) to a lesser extent to regions with high productivity as indicated through ocean colour data.     

North Pacific Tropical Water: its climatology and temporal changes associated with the climate regime shift in the 1970s

North Pacific Tropical Water (NPTW) is characterized as a subsurface salinity maximum flowing in the North Equatorial Current and is the main source of salt for the North Pacific. We briefly describe the climatological features of its formation and circulation, and then examine temporal changes in its properties associated with the climate regime shift in the 1970s. We use a variety of data, which include the repeat hydrographic sections along 130°E, 137°E, 144°E and 155°E meridians, the hydrographic data from the Hawaii Ocean Time-series, the World Ocean Atlas 1994, and available gridded data of wind stress and evaporation. The classical idea that NPTW originates from the zone of the highest sea surface salinity at 20°–30°N centered around the international date line and spreads along the isopycnal geostrophic flow patterns is confirmed. Further, it is shown that the meridional extent of NPTW along 137°E is from 10°N to 23°N on average and the highest salinity core lies at about 15°N and 24.0σ_θ, and that the portion of NPTW north (south) of about 15°N originates from the formation region west (east) of the date line. NPTW in the 137°E section changed remarkably associated with the mid-1970s regime shift. North of 15°N NPTW increased both in its salinity and thickness while to the south of 15°N only its salinity increased and its thickness remained unchanged. The westward geostrophic velocity is increased significantly in both the southern and northern parts of NPTW. The northern thickening and speedup and the southern speedup increased NPTW transport across 137°E. The changes in the thermohaline forcing such as evaporation and Ekman salt convergence in the NPTW formation region possibly contributed to the increases in salinity in the southern part of NPTW, but not to that of the northern part. On the other hand, the increased Ekman pumping accounts for the increase of the NPTW inventory and transport at 137°E. The increased salinity of NPTW at 137°E, especially its northern portion, was presumably caused by an increase in its formation rate rather than changes in the sea surface salinity in its formation region; the thicker the NPTW layer is, the saltier is the core that tends to survive the mixing processes.