Deep Winter Mixed Layer Research Articles

Decadal Variability in the Formation of the North Pacific Subtropical Mode Water: Oceanic versus Atmospheric Control

Abstract In situ temperature and altimetrically derived sea surface height data are used to investigate the low-frequency variations in the formation of the North Pacific Ocean Subtropical Mode Water (STMW) over the past 12 yr. Inside the Kuroshio Extension (KE) recirculation gyre where STMW forms, the dominant signal is characterized by a gradual thinning in the late winter mixed layer depth and in the 16°–18°C thermostad layer from 1993 to 1999 and a subsequent steady thickening of these features after 2000. This same decadal signal is also seen in the low-potential-vorticity (PV) STMW layer in the interior subtropical gyre south of the recirculation gyre. By analyzing the air–sea flux data from the NCEP–NCAR reanalysis project, little correlation is found between the decadal STMW signal and the year-to-year changes in the cumulative wintertime surface cooling. In contrast, the decadal signal is found to be closely related to variability in the dynamic state of the KE system. Specifically, STMW formation is reduced when the KE path is in a variable state, during which time high regional eddy variability infuses high-PV KE water into the recirculation gyre, increasing the upper-ocean stratification and hindering the development of a deep winter mixed layer. A stable KE path, on the other hand, favors the maintenance of a weak stratification, leading to a deep winter mixed layer and formation of a thick STMW layer. The relative importance of the surface air–sea flux forcing versus the preconditioning stratification in controlling the variations in the late winter mixed layer depth is quantified using both a simple upper-ocean heat conservation model and a bulk mixed layer model. The majority of the variance (∼80%) is found to be due to the stratification changes controlled by the dynamic state of the KE system.

Formation and pathways of intermediate water in the Parallel Ocean Circulation Model's Southern Ocean

The formation mechanisms and pathways of intermediate water in the Southern Ocean are analyzed from output of a high‐resolution ocean general circulation model. Deep winter mixed layer formation in the Southern Ocean is diagnosed from the model results and is found to be mostly consistent with observations. Diapycnal water mass transformations by air‐sea fluxes and internal mixing are quantified and split into mean and eddy components. The diapycnal formation of the water masses that constitute the Antarctic intermediate water layer in the southeast Pacific is found to occur mainly in the western Pacific Ocean in this model. In winter, convection up to 900 m is found to set the potential vorticity characteristics of this layer. Eddy fluxes of heat and buoyancy play an important role in the formation of the intermediate waters by transferring water from the southern parts of the subtropical gyres into the Antarctic Circumpolar Current (ACC) and vice versa. The effects of eddy fluxes are found to vary significantly along the path of the ACC. They are strongly concentrated in the regions near the Agulhas Return Current in the Indian Ocean and the Brazil‐Malvinas Confluence in the Atlantic.

Formation and Circulation of Mode Waters of the North Pacific in a High-Resolution GCM

Abstract Formation and circulation of mode waters in the subtropical North Pacific are investigated by performing a numerical simulation with a high-resolution general circulation model. In the model, important features for forming mode waters, such as separation of western boundary currents, frontal systems, mixed layer processes, and eddy activities, are well reproduced. Annual subduction rates, calculated by following the mixed layer base, have major peaks in three density ranges and are related to the deep winter mixed layer at the south of the fronts in the western North Pacific, that is, the Kuroshio Extension with a winter surface density of 25.5 σθ, the Kuroshio bifurcation front (26.2 σθ), and the subarctic front (26.5 σθ). The lightest peak corresponds to the Subtropical Mode Water (STMW). It is formed at the south of the Kuroshio Extension and enters the main pycnocline as it crosses the mixed layer depth (MLD) front to the south. The remaining two peaks correspond to the Central Mode Water (CM...

A role of eddies in formation and transport of North Pacific Subtropical Mode Water

Hydrographic data acquired during 2001 by Argo profiling floats deployed in the Kuroshio recirculation region are used to verify the inference that mesoscale eddies prevailing in the recirculation region play an important role in the formation and transport of North Pacific Subtropical Mode Water (NPSTMW). That is, the deeper winter mixed layer is formed preferentially where the thermocline is deeper in association with anticyclonic eddies. In the succeeding seasons, mesoscale eddies retain NPSTMW during their southwestward movement from the NPSTMW formation region, so that anticylonic eddies contribute substantially to the NPSTMW transportation. The spatial distributions of the mixed layer depth in winter and the NPSTMW thickness in the succeeding seasons, based on the float data, suggest that they are affected by mesoscale eddies. The float data also provide the statistical relations of these quantities against the thermocline depth, which are consistent with our inference.

Understanding the Persistence of Sea Surface Temperature Anomalies in Midlatitudes

An extension of the simple stochastic climate model of Frankignoul and Hasselman that includes the effects of seasonal variations in upper-ocean mixed layer depth upon the persistence of winter sea surface temperature (SST) anomalies is proposed. Seasonal variations in mixed layer depth allow for the “reemergence mechanism,” whereby thermal anomalies stored in the deep winter mixed layer persist at depth through summer and become partially reentrained into the mixed layer during the following winter. In this way, SST anomalies can recur from winter to winter without persisting through the intervening summer. Reformulating the simple stochastic climate model in terms of an effective ocean thermal capacity given by the depth of the winter mixed layer, thereby implicitly taking into account reemergence, is shown to provide a favorable fit to the observed winter-to-winter SST autocorrelations in the North Atlantic and Pacific, and represents a considerable improvement over the original model. The extended model also compares favorably with results from an entraining bulk ocean mixed layer model coupled to an atmospheric general circulation model. The authors propose that the extended model be adopted as the new “null hypothesis” for interannual SST variability in middle and high latitudes.

On the Reemergence of North Atlantic SST Anomalies

The reemergence mechanism, whereby temperature anomalies extending over the deep winter mixed layer are stored beneath the surface in summer and are reentrained into the mixed layer when it deepens again in the following autumn and winter, is studied in the North Atlantic using approximately 40 years of surface and subsurface data. Reemergence is found to be robust in the Sargasso Sea and the northeast Atlantic, regions where (i) the mixed layer is much deeper in winter than in summer, (ii) currents are relatively weak, and (iii) temperature anomalies are coherent over broad areas. The two leading empirical orthogonal functions of North Atlantic SST anomalies also exhibit strong reemergence signatures. A novel application of empirical orthogonal function analysis to temperature anomalies in the time–depth plane, which also incorporates information from all grid points, is shown to be an efficient and useful approach for detecting reemergence without dependence on specific spatial patterns or prior selection of regions.

Mechanisms of North Atlantic Wintertime Sea Surface Temperature Anomalies

The authors address the question: What are the oceanic mechanisms that control North Atlantic sea surface temperature (SST) anomalies? The approach is to examine the sensitivity dynamics of a non-eddy-resolving North Atlantic Ocean general circulation model (GCM) using its adjoint. The adjoint GCM yields the sensitivity of end-of-winter SSTs to the prior ocean state and prior air‐sea forcing over a seasonal cycle. Diagnosis of the sensitivity results identifies the oceanic mechanisms involved in controlling SST anomalies. The most effective way to alter SST is to change the local, contemporaneous air‐sea heat flux. Wind stress and freshwater perturbations are ineffective over one year. Upstream, wintertime heat flux anomalies can cause SST fluctuations in the following winter but heat flux anomalies during summer weakly affect subsequent end-of-winter SSTs. The dominant mechanism is the end-of-winter detrainment of warmer or colder water and its subsequent entrainment downstream into the mixed layer the next winter. This process is more effective in the midlatitude and subpolar North Atlantic where deep winter mixed layers occur, than in the tropical‐subtropical regions, which are characterized by a shallow mixed layer and a weak seasonal cycle. Mean-flow advection in the seasonal thermocline of the North Atlantic Current is moderately important in the subpolar gyre. Dynamical mechanisms, such as planetary waves and anomalous currents, are much less important over one year. The GCM results indicate that internal ocean anomalies forced by remote heat fluxes do affect SST variability. But, overall, contemporaneous winter heat flux anomalies are 3‐30 times more effective at causing SST anomalies than heat flux anomalies from the previous winter. The loss of sensitivity to prior air‐sea fluxes suggests that North Atlantic SST fluctuations are thus primarily a response to local, recent forcing.

Eastern North Pacific Subtropical Mode Water in a general circulation model: Formation mechanism and salinity effects

The Eastern North Pacific Subtropical Mode Water (ESTMW) is a water mass of low potential vorticity (PV) and appears as a weak pycnostad or thermostad. Distinct from other subtropical mode waters, it forms in the absence of a deep winter mixed layer. The formation mechanism of this ESTMW is investigated using an ocean general circulation model that is forced by monthly climatological temperature, salinity, and wind stress at the sea surface. An equation based on the ventilated thermocline theory is used to diagnose the initial PV of a water mass right after its subduction. In this equation, three factors affect the initial PV: the spacing of density outcrop lines, the mixed layer depth gradient, and the vertical velocity at the bottom of mixed layer. Among them the wide spacing between outcrop lines is the most important for ESTMW's low PV instead of the deep mixed layer, which is most important for classical mode waters. It is found that weak gradients in both sea surface temperature and salinity in the direction of mixed layer flow are important for the low PV formation. A low‐salinity tongue that extends southeastward off North America is responsible for the small surface density gradient in the eastern North Pacific and contributes to the formation of the ESTMW. An additional experiment forced with observed freshwater flux demonstrates that the southward advection of fresher water from the high latitude along the eastern boundary is the cause of this low‐salinity tongue.

Impact of the Midlatitude Storm Track on the Upper Pacific Ocean

Transient eddies in the atmosphere induce a poleward transport of heat and moisture. A moist static energy budget of the surface layer is determined from the NCEP reanalysis data to evaluate the impact of the storm track. It is found that the transient eddies induce a cooling and drying of the surface layer with a monthly mean maximum of 60 W m22. The cooling in the midlatitudes extends zonally over the entire basin. The impact of this cooling and drying on surface heat fluxes, sea surface temperature (SST), water mass transformation, and vertical structure of the Pacific is investigated using an ocean model coupled to an atmospheric mixed layer model. The cooling by atmospheric storms is represented by adding an eddy-induced transfer velocity to the mean velocity in an atmospheric mixed layer model. This is based on a parameterization of tracer transport by eddies in the ocean. When the atmospheric mixed layer model is coupled to an ocean model, realistic SSTs are simulated. The SST is up to 3 K lower due to the cooling by storms. The additional cooling leads to enhanced transformation rates of water masses in the midlatitudes. The enhanced shallow overturning cells affect even tropical regions. Together with realistic SST and deep winter mixed layer depths, this leads to formation of homogeneous water masses in the upper North Pacific, in accordance to observations.

Supply and demand of nutrients and dissolved organic matter at and across the NW European shelf break in relation to hydrography and biogeochemical activity

As part of the OMEX I project, nutrient determinations were made on 17 cruises in the region of the Goban Spur and La Chapelle Bank between 46 and 51°N, in all seasons of the year, between 1993–1995. Over this period no change was detectable in the structure of the water masses below the deep winter mixed layer. The N : P (dissolved nitrate-to-phosphate) ratio changed from 16 at 100-m depth to less than 15 at 3300-m depth. At intermediate depths nutrient and oxygen data indicate the presence of Mediterranean Outflow water overlying Labrador Sea Water at its most eastern extension. Estimated maximum levels of production in the spring bloom are the total N-limited new primary production equivalent between 24 and 41 gC m−2, the equivalent maximum diatom production is 11 gC m−2. Measurements during the spring bloom suggest a conversion factor of 1 μM nitrate to 1 μg l−1 chlorophyll, at the shelf break, which is consistent with other recent measurements in European shelf seawaters. Sediment trap data suggest that 80% (5.4 g m−2) of the opal produced in the spring bloom dissolved before reaching the sediment trap at 600 m. A comparison of the winter and summer profiles for dissolved silicon suggests a similar dissolution of 9±3 g opal m−2 above 300-m depth. Measurements of dissolved organic carbon (DOC) in September 1994 show an enrichment of 7 μM-C above the seasonal thermocline relative to the winter values (52±4 μM). In winter dissolved organic nitrogen represents 40% of the pool of total dissolved nitrogen. There is no consistent evidence of an increase in the concentration of DON during summer. Measurements of nitrate in surface waters in January 1994 show that concentrations off-shelf vary with the temperature of the water and are related to the depth of winter mixing. Mixing in surface waters is discontinuous at the shelf break, demonstrating the degree to which exchange across the shelf break is limited even in winter. OMEX winter measurements of nitrate concentrations can be used to estimate the flow of water across the shelf break that would be required to maintain the nitrogen balance in the North Sea at a steady state. The estimate is 0.6 Sv (1 Sv=106 m3 s−1), which is similar to an earlier estimate of a total flow of 1.7 Sv based on salt budgets (cf. Huthnance, Deutsche Hydrographische Zeitschrift, 49 (1997) 153).

Processes that influence sea surface temperature and ocean mixed layer depth variability in a coupled model

A 50‐year coupled atmosphere‐ocean model integration is used to study sea surface temperature (SST) and mixed layer depth (h), and the processes which influence them. The model consists of an atmospheric general circulation model coupled to an ocean mixed layer model in ice‐free regions. The midlatitude SST variability is simulated fairly well, although the maximum variance is underestimated and located farther south than observed. The model is clearly deficient in the vicinity of the Gulf Stream and in the eastern tropical Pacific where advective processes are important. The model generally reproduces the observed structure of the mean h in both March and September but underestimates it in the North Atlantic during winter. The net surface heat flux strongly regulates both the mean (¯) and the anomalous (') SSTs throughout the year. The entrainment heat flux, which is proportional to the product of the entrainment rate (We) and the temperature jump at the base of the mixed layer (ΔT), influences SSTs in summer and fall, especially north of ∼35°N (45°N) in the Pacific (Atlantic). is more important for the development of SST' in fall compared to , which is larger in summer. The entrainment rate is dominated by wind‐induced mixing in summer and surface buoyancy forcing in winter; the density jump at the base of the mixed layer is of secondary importance. In addition, anomalies in h have a significant impact on the heat balance of the mixed layer during spring and summer. Deep winter mixed layers and the storage of thermal anomalies beneath the shallow mixed layer in summer leads to large winter‐to‐winter persistence of SST anomalies in the far North Atlantic, in accord with observations and stochastic climate theory.

Comparison of methods to determine the anthropogenic CO2 invasion into the Atlantic Ocean

A comparison of different methods for estimating the anthropogenic CO 2 burden in the Atlantic Ocean is performed using referenced, high quality total dissolved inorganic carbon (DIC) data. The dataset is from two cruises through the center of the basin between 62°N and 43°S in 1991 and 1993. The specific anthropogenic input is determined utilizing empirical procedures as described in Gruber et al. (1996) and Chen and Millero (1979) to correct for remineralization and to estimate preanthropogenic endmembers. These estimates are compared with output of the Princeton ocean biogeochemical model and the NCAR ocean model. The results show that the specific inventories of anthropogenic carbon agree to within 20% but with different storage and uptake patterns. The empirical estimates differ because of assumptions about mixing and winter outcrop endmembers. The same remineralization quotients (Redfield ratios) were used for each method. Varying these constants within the range of literature values causes changes in specific inventories of similar magnitude as the differences observed with different methodologies. Comparison of anthropogenic CO 2 uptake and chlorofluorocarbon ages suggests that the anthropogenic CO 2 penetration in the North Atlantic is too shallow following the procedure according to Gruber et al. (1996), and too deep using those of Chen and Millero (1979). The results support these previous observations in that the uptake of CO 2 in the North Atlantic is disproportionate to its surface area. This is caused by a combination of deep water formation and deep winter mixed layers. DOI: 10.1034/j.1600-0889.1999.00027.x

Comparison of methods to determine the anthropogenic CO2 invasion into the Atlantic Ocean

A comparison of different methods for estimating the anthropogenic CO2 burden in the Atlantic Ocean is performed using referenced, high quality total dissolved inorganic carbon (DIC) data. The dataset is from two cruises through the center of the basin between 62°N and 43°S in 1991 and 1993. The specific anthropogenic input is determined utilizing empirical procedures as described in Gruber et al. (1996) and Chen and Millero (1979) to correct for remineralization and to estimate preanthropogenic endmembers. These estimates are compared with output of the Princeton ocean biogeochemical model and the NCAR ocean model. The results show that the specific inventories of anthropogenic carbon agree to within 20% but with different storage and uptake patterns. The empirical estimates differ because of assumptions about mixing and winter outcrop endmembers. The same remineralization quotients (Redfield ratios) were used for each method. Varying these constants within the range of literature values causes changes in specific inventories of similar magnitude as the differences observed with different methodologies. Comparison of anthropogenic CO2 uptake and chlorofluorocarbon ages suggests that the anthropogenic CO2 penetration in the North Atlantic is too shallow following the procedure according to Gruber et al. (1996), and too deep using those of Chen and Millero (1979). The results support these previous observations in that the uptake of CO2 in the North Atlantic is disproportionate to its surface area. This is caused by a combination of deep water formation and deep winter mixed layers.

A pycnostad on the bottom of the ventilated portion in the central subtropical North Pacific: Its distribution and formation

A water mass characterized by the pycnostad on the bottom of the ventilated portion in the central subtropical North Pacific is described through the comparison with the Subtropical Mode Wate (STMW). In this paper, this water mass is called the North Pacific Central Mode Water (CMW), because of its vertical homogeneity. The distribution of CMW is examined based on the climatological maps of annual mean potential vorticity. On the other hand, its formation area is examined based on the climatological winter temperature data set and the STD sections across the Kuroshio Extension in early spring of individual years. The main results are summarized as follows: 1) STMW is formed in the deep winter mixed layer south of the main path of the Kuroshio Extension (termed 12°C Front in this paper). On the other hand, CMW is formed in the deep winter mixed layer in the east-west band surrounded by a branch of the Kuroshio Extension (termed 9°C Front in this paper) and the boundary of two water masses representing the subtropical and subpolar gyres. 2) The winter mixed layer between the 12°C Front and the 9°C Front is shallower than that in the CMW and STMW formation areas. 3) These geographical features of the winter mixed layer depths near the subarcticsubtropical transition zone result in two pycnostads (STMW and CMW) in the main thermocline of the subtropical North Pacific through the advection caused by the subtropical gyre.

A seasonal model of the Mediterranean Sea general circulation

This paper describes the seasonal characteristics of the Mediterranean Sea general circulation as simulated by a primitive equation general circulation model. The forcing is composed of climatological monthly mean atmospheric parameters, which are used to compute the heat and momentum budgets at the air‐sea interface of the model. This allows heat fluxes to be determined by a realistic air‐sea interaction physics. The Strait of Gibraltar is open, and the model resolution is in the horizontal and 19 levels in the vertical. The results show the large seasonal cycle of the circulation and its transient characteristics. The heat budget at the surface is characterized by lateral boundary intensifications occurring in downwelling and up welling areas of the basin. The general circulation is composed of subbasin gyres, and cyclonic motion dominates the northern and anticyclonic motion the southern part of the basin. The Atlantic stream which enters from Gibraltar and assumes the form of different boundary current subsystems is a coherent structure at the surface. At depth it appears as current segments and jets around a vigorous gyre system. The seasonal variability is manifested not only by a change in amplitude and location of the gyres but also by the appearance of seasonally recurrent gyres in different parts of the basin. Distinct westward propagation of these gyres occurs, together with amplitude changes. For the first time a Mersa‐Matruh Gyre is successfully simulated due to the introduction of our heat fluxes at the air‐sea interface. The seasonal thermocline is formed each summer, and a deep winter mixed layer is produced in the region of Levantine intermediate water formation. Deep water renewal does not occur, probably due to the climatological forcing used.

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.

Sea ice, winter convection, and the temperature minimum layer in the Southern Ocean

The structure of the near surface waters in the Southern Ocean, poleward of the Antarctic Polar Front but away from continental margins, is investigated with a three‐dimensional time‐dependent numerical model which resolves the annual sea ice cycle. The growth and decay of the ice field is predicted, using one of Semtner's (1976) thermodynamic ice models, in terms of specified atmospheric data and computed thermohaline characteristics of the ocean layers. The ice field is found to be sensitive to the lateral advection of heat by the oceanic circulation as well as vertical heat transports due to deep winter convection. The model treats the temperature minimum layer in the Southern Ocean as the remnant of a deep winter mixed layer which becomes capped by surface heating and precipitation in summer. The predicted thermohaline characteristics of the temperature minimum layer and the surface mixed layer are in good agreement with observations. Finally, the annual air‐sea heat exchange predicted by the model is discussed. The model's predicted area averaged heat loss experienced by the ocean south of the Atlantic Polar Front is much smaller than was previously estimated. This is attributed to errors in the southern region of the model domain. It is suggested that models, which include the interactions between sea ice and surface, deep and bottom waters along the continental margins, are needed to investigate this region.