North Pacific Central Mode Water Research Articles

Variations in the Central Mode Water in the North Pacific as a manifestation of the Pacific Decadal Oscillation

The North Pacific Central Mode Water (CMW) is examined from the viewpoint of its volume variations. The volume of the CMW layers thicker than 150m\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$150 m$$\\end{document} is used as an index of CMW variations, which successfully represents the year-to-year and decadal variations in the CMW volume. The CMW index shows the variation close to that of the Pacific Decadal Oscillation (PDO). The CMW variation is strongly tied with large-scale, dominant variations in sea surface temperature (SST), surface dynamic height (SDH), and sea surface height (SSH) anomalies in the North Pacific, with a significant correlation with the PDO. Year-to-year and decadal variations of CMW volume in May to July are significantly correlated with the wintertime Aleutian Low and 500hPa\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$hPa$$\\end{document} geopotential height variations, which indicates that the Aleutian Low induces the eastward extension/retreat of the strong winter westerlies and resultant net surface heat flux anomaly over the CMW distribution region. Thus, the CMW volume variation can be regarded as a significant manifestation of the PDO. The SST, SDH, and SSH anomalies are associated with the surface cooling in the northern sector of the CMW distribution region. On the other hand, the SDH and SSH anomalies throughout a year and the SST anomaly in the cold season in the southeastern sector of the CMW region are formed due to the heaving of isopycnal surfaces in the subsurface layer above the CMW in response to its volume variations.

Inter-annual zonal shift of the formation region of the lighter variety of the North Pacific Central Mode Water

Inter-annual variability in the formation and subduction of the lighter variety of the Central Mode Water (L-CMW) in the North Pacific is investigated using Argo profile data from 2003 to 2013. The deep mixed layer (ML) in winter (February–April) with the L-CMW property (potential temperature of 10–16 °C and salinity of 34.2–34.6) shows a zonal shift inter-annually. In winters of 2003–2005 and 2010, deep MLs are more frequently generated in the eastern part of the formation region (30°–40°N, 180°–160°W), which is mainly caused by large oceanic heat loss associated with strengthening of the westerlies. Relatively large parts of the L-CMW that formed in this eastern region survive to subduct into the subsurface layer and, in the following summer, spread widely over the North Pacific subtropical gyre. On the other hand, during the winters of 2006–2009 and 2011–2013, deep MLs were more frequently formed in the western part of the formation region (36°–40°N, 140°–155°E). These were induced by weakening of surface ocean stratification attributable to anti-cyclonic meso-scale eddies pinched off northward from the Kuroshio Extension. In contrast to the L-CMW formed in the eastern part of the formation region, the L-CMW formed in the western part is hardly detected in the subsurface layer during the following summer.

Mesoscale eddy effects on the subduction of North Pacific mode waters

AbstractMesoscale eddy effects on the subduction of North Pacific mode waters are investigated by comparing observations and ocean general circulation models where eddies are either parameterized or resolved. The eddy‐resolving models produce results closer to observations than the noneddy‐resolving model. There are large discrepancies in subduction patterns between eddy‐resolving and noneddy‐resolving models. In the noneddy‐resolving model, subduction on a given isopycnal is limited to the cross point between the mixed layer depth (MLD) front and the outcrop line whereas in eddy‐resolving models and observations, subduction takes place in a broader, zonally elongated band within the deep mixed layer region. Mesoscale eddies significantly enhance the total subduction rate, helping create remarkable peaks in the volume histogram that correspond to North Pacific subtropical mode water (STMW) and central mode water (CMW). Eddy‐enhanced subduction preferentially occurs south of the winter mean outcrop. With an anticyclonic eddy to the west and a cyclonic eddy to the east, the outcrop line meanders south, and the thermocline/MLD shoals eastward. As eddies propagate westward, the MLD shoals, shielding the water of low potential vorticity from the atmosphere. The southward eddy flow then carries the subducted water mass into the thermocline. The eddy subduction processes revealed here have important implications for designing field observations and improving models.

Mixed layer saturations of CFC-11, CFC-12, and SF6 in a global isopycnal model

[1] The use of CFC-11, CFC-12, and SF6 to quantify oceanic ventilation rates, interior water age, and formation rates requires knowledge of the saturation levels at the sea surface. While their atmospheric histories are relatively well known, physical processes in the mixed layer in conjunction with limited air-sea gas exchange can cause surface concentrations to be in disequilibrium with the atmosphere. We use an offline tracer advection-diffusion code that evolves tracers using along-isopycnal and cross-isopycnal mass fluxes from a global, climatological run of the Hallberg Isopycnal Model to reconstruct the saturation level of all three tracers over the entirety of their atmospheric histories. Disequilibria on a global scale occur in regions associated with deep winter mixed layers and are found throughout the time period of the release of these chemicals into the atmosphere. Sensitivity studies using targeted model simulations, focusing on the North Pacific, show that seasonal cycles in temperature and salinity that affect gas solubility as well as entrainment of water containing low concentration of tracers during mixed layer deepening are the dominant causes of undersaturation. When using the transit time distribution method, our results show that these undersaturations introduce a significant bias toward older ages for North Pacific Central Mode Water but do not significantly affect estimates of anthropogenic carbon inventory.

Origin and Pathway of Equatorial 13°C Water in the Pacific Identified by a Simulated Passive Tracer and Its Adjoint*

Abstract The origin and pathway of the thermostad water in the eastern equatorial Pacific Ocean, often referred to as the equatorial 13°C Water, are investigated using a simulated passive tracer and its adjoint, based on circulation estimates of a global general circulation model. Results demonstrate that the source region of the 13°C Water lies well outside the tropics. In the South Pacific, some 13°C Water is formed northeast of New Zealand, confirming an earlier hypothesis on the water’s origin. The South Pacific origin of the 13°C Water is also related to the formation of the Eastern Subtropical Mode Water (ESTMW) and the Sub-Antarctic Mode Water (SAMW). The portion of the ESTMW and SAMW that eventually enters the density range of the 13°C Water (25.8 < σθ < 26.6 kg m−3) does so largely by mixing. Water formed in the subtropics enters the equatorial region predominantly through the western boundary, while its interior transport is relatively small. The fresher North Pacific ESTMW and Central Mode Water (CMW) are also important sources of the 13°C Water. The ratio of the southern versus the northern origins of the water mass is about 2 to 1 and tends to increase with time elapsed from its origin. Of the total volume of initially tracer-tagged water in the eastern equatorial Pacific, approximately 47.5% originates from depths above σθ = 25.8 kg m−3 and 34.6% from depths below σθ = 26.6 kg m−3, indicative of a dramatic impact of mixing on the route of subtropical water to becoming the 13°C Water. Still only a small portion of the water formed in the subtropics reaches the equatorial region, because most of the water is trapped and recirculates in the subtropical gyre.

Subduction of North Pacific central mode water associated with subsurface mesoscale eddy

During a shipboard high‐density hydrographic survey carried out in the western North Pacific in fall 2008, we observed an anticyclonic eddy with a thickness of 150 dbar and a diameter of 40 km near 500 dbar depth at 27.5°N, 145°E. This subsurface mesoscale eddy contains the North Pacific central mode water (CMW), which has anomalously low potential vorticity and high dissolved oxygen compared to the climatological CMW properties in the same region. Profiling float measurements detect similar CMW patches near and south of the Kuroshio Extension as well as southward CMW migration within the CMW formation region north of the Kuroshio Extension. These observed facts suggest that CMW is subducted into the permanent pycnocline not only through large‐scale eastward advection near the northern edge of the subtropical gyre but also through southward cross‐frontal advection associated with the formation and migration of subsurface mesoscale eddies.

Formation Mechanism of the “Stability Gap” and the North Pacific Central Mode Water

AbstractLocal feature of the formation region (165°E~160°W, 38°N~42°N) of the North Pacific Central Mode Water (NPCMW) is first put forward from data analysis, and for which, the external atmospheric forcing (solar shortwave radiation, net heat flux and wind stress curl) could not solely give acceptable explanation. Further analysis on the seasonal variability of the upper ocean stratification shows that a special weak zone of the ocean stratification in the upper ocean (<75 m)‐the “stability gap” is detected in the zone of (165°E~160°W, 38°N~42°N) in autumn (September‐October). As “Precondition Mechanism”, the “stability gap” provides a reliable answer for the “local feature” of the formation of the NPCMW. Based on a heat balance equation of the upper ocean mixed layer, diagnostic analysis suggests that the formation of the “stability gap” is the cooperative product of the surface heat flux forcing, vertical entrainment, Ekman advection and geostrophic advection. Among which, the latitudinal differences of the surface heat flux forcing, the cold Ekman advection and the warm geostrophic advection play the crucial roles on determining the critical eastern and western bound of the “stability gap”.

Further evidence for non-reemergence of winter SST anomalies in the North Pacific eastern subtropical mode water area

Following our previous study (Sugimoto and Hanawa, 2005b), we further investigate the reason why reemergence of winter sea surface temperature anomalies does not occur in the North Pacific eastern subtropical mode water (NPESTMW) area, despite its occurrence in the North Pacific subtropical mode water and North Pacific central mode water areas. We use vertical temperature and salinity profiles of the World Ocean Circulation Experiment Hydrographic Program and Argo floats with high vertical and temporal resolution, together with heat flux data through the sea surface. We point out first that one of the causes for non-occurrence of reemergence is that the thickness of NPESTMW is very thin. In addition to this basic cause, two major reasons are found: a vigorous mixing in the lower portion of NPESTMW and less heat input from the atmosphere in the warming season. Since, in the lower portion of NPESTMW and deeper, the stratification is favorable for salt-finger type convection to occur compared with the other mode water areas, vigorous mixing takes place. This is confirmed by both a large Turner Angle there and the existence of staircase structures in vertical temperature and salinity profiles. From the viewpoint of heat input, the NPESTMW area gradually gains heat in the warming season compared with other mode water areas. As a result, NPESTMW cannot be capped so quickly by the shallow summer mixed layer, and water properties of NPESTMW are to be gradually modified, even in the upper portion.

New type of pycnostad in the western subtropical-subarctic transition region of the North Pacific: Transition Region Mode Water

A new type of pycnostad has been identified in the western subtropical-subarctic transition region of the North Pacific, based on the intensive hydrographic survey carried out in July, 2002. The potential density, temperature and salinity of the pycnostad were found to be 26.5–26.7 σθ, 5°–7°C and 33.5–33.9 psu respectively. The pycnostad is denser, colder and fresher than those of the North Pacific Central Mode Water and different from those of other known mode waters in the North Pacific. The thickness of the pycnostad is comparable to that of other mode waters, spreading over an area of at least 650 × 500 km around 43°N and 160°E in the western transition region. Hence, we refer to the pycnostad as Transition Region Mode Water (TRMW). Oxygen data, geostrophic current speed and climatology of mixed layer depth in the winter suggest that the TRMW is formed regularly in the deep winter mixed layer near the region where it was observed. Analysis of surface heat flux also supports the idea and suggests that there is significant interannual variability in the property of the TRMW. The TRMW is consistently distributed between the Subarctic Boundary and the Subarctic Front. It is also characterized by a wide T-S range with similar density, which is the characteristic of such a transition region between subtropical and subarctic water masses, which forms a density-compensating temperature and salinity front. The frontal nature also tends to cause isopycnal intrusions within the pycnostad of the TRMW.

Sensitivity of the Ventilation Process in the North Pacific to Eddy-Induced Tracer Transport*

Abstract A coarse-resolution isopycnal model coupled with a bulk mixed layer model is used to examine the effect of isopycnal thickness diffusion, which parameterizes the subgrid-scale eddy-induced tracer transport, on ventilation of the North Pacific Ocean. Three numerical experiments with thickness diffusivities of 0 m2 s−1 and around 500 and 2000 m2 s−1 are carried out. The model successfully reproduces a deep winter mixed layer in the subarctic North Pacific, leading to well-formed mode waters and the subtropical countercurrent in the experiment with thickness diffusivity around 500 m2 s−1. The annual-mean subduction rate has peaks at densities of 25.0–25.4 and 26.4 σθ. The former peak spans the densities of North Pacific Subtropical Mode Water and North Pacific Eastern Subtropical Mode Water, whereas the latter peak is centered near the density of North Pacific Central Mode Water. The annual mean obduction rate also has the former peak and a slight enhancement corresponding to the latter peak. The Kuroshio plays a crucial role in obduction of North Pacific Subtropical Mode Water by transferring it northward from the permanent pycnocline to the seasonal pycnocline around the Kuroshio Extension, the importance of which has been overlooked in previous studies. In contrast to the simple expectation that the eddy-induced tracer transport enhances the ventilation process, stronger circulation with lower thickness diffusion increases the annual-mean subduction rate by carrying the subducted water quickly away from the seasonal pycnocline into the permanent pycnocline, as well as the annual-mean obduction rate by transferring much water from the permanent pycnocline to the seasonal pycnocline. As thickness diffusivity increases, the former peaks in the subduction and obduction rates occur at lighter densities, whereas the latter peak in the subduction rate is shifted toward higher densities.

Differential Formation and Circulation of North Pacific Central Mode Water

Abstract A repeat hydrographic section along 165°E was analyzed to verify a westward extension of the formation region of the North Pacific Ocean Central Mode Water (CMW) suggested by previous synoptic observations, and to investigate the relation between the formation region and thermohaline fronts. The CMW formation region extends at least as far west as 155°E, much farther than recognized in a previous study based on climatology. It is located in two interfrontal regions between the Kuroshio Extension front and the Kuroshio Bifurcation front (KBF), and between the KBF and the subarctic front, where two types of CMW—namely, the lighter variety with potential density of 25.8–26.2 kg m−3 and the denser one of 26.3–26.4 kg m−3—are formed. How this differential formation of CMW is reflected in its gyrewide distribution was examined using one-time sections of the World Ocean Circulation Experiment (WOCE) Hydrographic Program in the North Pacific. The main circulation paths of the two types of CMW diverge east of the date line; the lighter variety is located in the inner part of the eastern subtropical gyre, and the denser variety is located in the outer part. These results demonstrate that the frontal structure around the northern boundary of the subtropical gyre, particularly the existence of KBF, is essential in determining the properties and the gyrewide distribution of CMW.

Remote reemergence areas of winter sea surface temperature anomalies in the North Pacific

Taking account of water movement, we detect pairs of ‘formation’ and ‘reemergence’ areas of winter sea surface temperature (SST) anomalies in the North Pacific, by analyzing various kinds of data sets. Two pairs of formation and reemergence areas are detected. One pair is located in the northern subtropical gyre, and water properties in both the formation and reemergence areas well correspond to those of North Pacific central mode water (NPCMW). Another pair is found in the western to central part of the subtropical gyre. The formation area corresponds well to that of North Pacific subtropical water (NPSTMW), but the reemergence area is remote from the formation area. Examinations on upper ocean current field, trace of temperature anomalies in the subsurface, and change of water properties from the formation area to the reemergence area, clearly indicate that waters with winter SST anomalies move from the formation area to the reemergence area in a year.

Interdecadal Temperature Variations in the North Pacific Central Mode Water Simulated by an OGCM

The North Pacific Central Mode Water (CMW) is a water mass that forms in the Kuroshio-Oyashio Extension (KOE) region with characteristic low potential vorticity. Recent studies have suggested that the CMW, as low potential vorticity water, plays an important role in the adjustment of the subtropical gyre and subsurface variability on decadal to interdecadal timescales. We have forced a realistic ocean general circulation model (OGCM) with observed wind stress and sea surface temperature (SST) forcing to investigate the decadal variations of the CMW. Associated with the large atmospheric changes after the mid-1970s climate regime shift, the upper thermocline experiences a cooling as negative SST anomalies in the central North Pacific are subducted and advected southward. In addition to this thermodynamic response, the CMW's path shifts anomalously eastward in response to anomalous Ekman pumping. This eastward shift of the core of the CMW produces a lowering of the isotherms, and a consequent warming, on the path of the CMW core. This warming partially counteracts the cooling associated with subducted surface anomalies, and it may be responsible for the reduced temperature variations at the climatological position of the CMW when both anomalous wind and heat fluxes are given. Lateral induction across the sloping bottom of the winter mixed layer in the KOE is critical to the formation of the low potential vorticity CMW. Coarse resolution models, which are widely used in climate modeling, underestimate the horizontal gradient of the mixed layer depth and form only a weak CMW or none at all. We have conducted a coarse resolution experiment with the same OGCM, showing that the subsurface response is much reduced. In particular, there is no dynamic warming in the CMW and the thermodynamic response to the SST cooling dominates. The resultant total response differs substantially from that in the finer resolution run where a strong CMW forms. This sensitivity to the model resolution corroborates the important dynamical role that the CMW may play with its distinctive low potential vorticity character and calls for its improved simulation.

Interdecadal temperature variations in the North Pacific Central Mode Water simulated by an OGCM

The North Pacific Central Mode Water (CMW) is a water mass that forms in the Kuroshio-Oyashio Extension (KOE) region with characteristic low potential vorticity. Recent studies have suggested that the CMW, as low potential vorticity water, plays an important role in the adjustment of the subtropical gyre and subsurface variability on decadal to interdecadal timescales. We have forced a realistic ocean general circulation model (OGCM) with observed wind stress and sea surface temperature (SST) forcing to investigate the decadal variations of the CMW. Associated with the large atmospheric changes after the mid-1970s climate regime shift, the upper thermocline experiences a cooling as negative SST anomalies in the central North Pacific are subducted and advected southward. In addition to this thermodynamic response, the CMW’s path shifts anomalously eastward in response to anomalous Ekman pumping. This eastward shift of the core of the CMW produces a lowering of the isotherms, and a consequent warming, on the path of the CMW core. This warming partially counteracts the cooling associated with subducted surface anomalies, and it may be responsible for the reduced temperature variations at the climatological position of the CMW when both anomalous wind and heat fluxes are given. Lateral induction across the sloping bottom of the winter mixed layer in the KOE is critical to the formation of the low potential vorticity CMW. Coarse resolution models, which are widely used in climate modeling, underestimate the horizontal gradient of the mixed layer depth and form only a weak CMW or none at all. We have conducted a coarse resolution experiment with the same OGCM, showing that the subsurface response is much reduced. In particular, there is no dynamic warming in the CMW and the thermodynamic response to the SST cooling dominates. The resultant total response differs substantially from that in the finer resolution run where a strong CMW forms. This sensitivity to the model resolution corroborates the important dynamical role that the CMW may play with its distinctive low potential vorticity character and calls for its improved simulation.

Chlorofluorocarbons (CFCs) in the North Pacific Central Mode Water: Possibility of under-saturation of CFCs in the wintertime mixed layer

Analysis of chlorofluorocarbons (CFCs) data in seawater in the Kuroshio-Oyashio Transition Area obtained during the WOCE Campaign P-13 along 165°E in August to October 1992 and P-14 along 179°E in July to September 1993 revealed that CFC-12 and CFC-11 in the North Pacific Central Mode Water (NPCMW) were significantly under-saturated with respect to the atmospheric CFC-12 (75.5 ± 12.3% for P-13 and 85.9 ± 6.3% for P-14) and CFC-11 (72.9 ± 10.6% for P-13 and 82.6 ± 5.3% for P-14). Since the mode water is derived from the vertical convection in the surface layer in winter and is considered not greatly influenced by mixing with ambient water during the subsequent advection, the observed under-saturation of CFC-12 and CFC-11 in the mode water suggests that CFCs in the mixed layer in winter has already been under-saturated with respect to the atmospheric CFC-12 and CFC-11. A result from a simple one-dimensional model for the mixed layer also supports the entrainment of the water with lower degree of CFC-12 and CFC-11 saturation and the under-saturation in the wintertime mixed layer.

Subsurface water masses in the Central North Pacific transition region: The repeat section along the 180° meridian

A repeat hydrographic section has been maintained over two decades along the 180° meridian across the subarctic-subtropical transition region. The section is naturally divided into at least three distinct zones. In the Subarctic Zone north of 46°N, the permanent halocline dominates the density stratification, supporting a subsurface temperature minimum (STM). The Subarctic Frontal Zone (SFZ) between 42°–46°N is the region where the subarctic halocline outcrops. To the south is the Subtropical Zone, where the permanent thermocline dominates the density stratification, containing a pycnostad of North Pacific Central Mode Water (CMW). The STM water colder than 4°C in the Subarctic Zone is originated in the winter mixed layer of the Bering Sea. The temporal variation of its core temperature lags 12–16 months behind the variations of both the winter sea surface temperature (SST) and the summer STM temperature in the Bering Sea, suggesting that the thermal anomalies imposed on the STM water by wintertime air-sea interaction in the Bering Sea spread over the western subarctic gyre, reaching the 180° meridian within a year or so. The CMW in this section originates in the winter mixed layer near the northern edge of the Subtropical Zone between 160°E and 180°. The CMW properties changed abruptly from 1988 to 1989; its temperature and salinity increased and its potential density decreased. It is argued that these changes were caused by the climate regime shift in 1988/1989 characterized by weakening of the Aleutian Low and the westerlies and increase in the SST in the subarctic-subtropical transition region.

Subsurface Subtropical Fronts of the North Pacific as Inherent Boundaries in the Ventilated Thermocline

The two subsurface temperature–density fronts and the accompanying eastward currents in the central and southern part of the North Pacific subtropical gyre are identified using the repeat hydrographic sections in the western part of the gyre and the sections over the whole gyre from the World Ocean Circulation Experiment (WOCE) Hydrographic Program (WHP). The northern eastward current corresponds to what has been known as the subtropical countercurrent (STCC). The countercurrent/front is located typically near 24°N and extends from the western boundary approximately to the international date line. The previous suggestion that the STCC is located at the southern edge of North Pacific Subtropical Mode Water (STMW) is confirmed; the front appears as the southern boundary of the lower potential vorticity (PV)/apparent oxygen utilization (AOU) waters on the isopycnals within the STMW layer. The southern eastward current corresponds to what was recognized earlier but has not been documented in detail. The southern countercurrent/front is located near 18°N west of the date line, but shifts to the north east of the date line. Its eastern limit is around 165°W at 25°N. The southern front corresponds to the southern boundary of the lower PV/AOU waters on the isopycnal surfaces of the wide σθ range including those of North Pacific Central Mode Water (CMW). While CMW seems to contribute largely to this low PV/AOU layer, the contribution from other waters is still significant. Judging from the associated PV/AOU features in the thermocline, both fronts can be regarded as inherent boundaries in the ventilated thermocline, dividing the regions ventilated to distinctive degrees. Correspondence between the observational features of the countercurrents/fronts and some of the previously proposed theories concerning the STCC are discussed.

Water Mass Formation in an Isopycnal Model of the North Pacific

Abstract An isopycnal model coupled with a mixed layer model is used to study transformation and formation rates in the North Pacific. When annual formation rates are averaged over the entire North Pacific, a large peak in water mass formation is found at a density of approximately σθ = 26 kg m−3. This peak in formation rate corresponds to the formation of North Pacific Central Mode Water (CMW) in the model. No corresponding peaks in formation rate are found at the densities of Subtropical Mode Water (STMW; σθ ∼25.4 kg m−3) or Eastern Subtropical Mode Water (ESMW; σθ ∼24–25.4 kg m−3) when averaged over the entire model basin. However, when calculated locally, enhanced formation rates are found at the densities of these mode water masses. The formation of each of the three types of North Pacific mode water in the model occurs because of different circumstances. As expected, STMW formation is dependent on the strong cooling and resultant deep mixed layers over the Kuroshio Current region. However, formation...

North Pacific Subtropical Mode Waters Observed in Long XBT Cross Sections along 32.5°N Line

The S/V Shoyo, of the Hydrographic Department, Japan Coast Guard, has conducted high-density expendable bathythermograph (XBT) measurements along the 32.5°N line in the North Pacific every year from 1990 to 1993 as a part of the Japanese-World Ocean Circulation Experiment (WOCE). These XBT data are analyzed here, focusing on year-to-year variations of the inventory and core layer temperature (CLT) of the North Pacific subtropical mode water (NPSTMW). Large year-to-year changes are found in the NPSTMW CLTs estimated in longitudes between 140°E and 160°E. CLT values were found of 17.4°C in 1990, 17.1°C in 1991, 17.3°C in 1992 and 17.6°C in 1993. Inspection of the wintertime westerlies over the formation area and sea surface temperature distribution revealed that this change in CLT can be qualitatively attributed to the strength of atmospheric cooling in the formation area in the previous winter. Although a large year-to-year variation of NPSTMW inventory was also found, it is hard to state any relationship between CLT and atmospheric forcing. There is a possibility that different observational seasons may affect the inventory. It has also been found that the thermocline depth in 1991 was shallower in the sea area east of 180° than in 1992 and 1993. Associated with this change, the North Pacific central mode water (NPCMW), characterized by thermostad with temperatures ranging from 14°C to 11°C, appears in the sea area east of 180° in the 1992 and 1993 cross sections. The 1993 cross section, which ranged from the Japanese coast to the west coast of North America, possessed another thermostad in the surface layer, with a temperature of about 17°C in the eastern part of the cross section, off California.

Decadal Changes in the Mode Waters in the Midlatitude North Pacific

Abstract Temporal changes in the properties of the North Pacific subtropical mode water (NPSTMW) and the North Pacific central mode water (NPCMW), which occurred around the mid-1970s, are investigated using temperature data composited for the two decades bounded by the mid-1970s: 1966–75 and 1976–85. Properties of these mode waters changed greatly after the mid-1970s. The colder NPCMW was formed and widely distributed during 1976–85. In the NPSTMW formation area, warmer water occupied the southwestern part, and colder water occupied the northeastern part during 1976–85. The cause of this change is discussed with regard to the heat flux and wind stress data. The cooling can be explained as a result of changes in surface heat flux and heat divergence in the Ekman layer, that is, a larger amount of heat released from the ocean surface and an increased southward Ekman transport of cold water due to intensification of the westerlies. In particular, the latter plays a dominant role in the observed cooling. On t...