Subtropical Mode Water Formation Research Articles

Closing the Time-Varying Mass and Heat Budgets for Large Ocean Areas: The Tasman Box

Abstract The role of oceanic advection in seasonal-to-interannual balances of mass and heat is studied using a 12-yr time series of quarterly eddy-resolving expendable bathythermograph (XBT) surveys around the perimeter of a region the authors call the Tasman Box in the southwestern Pacific. The region contains the South Pacific’s subtropical western boundary current system and associated strong mesoscale variability. Mean geostrophic transport in the warm upper ocean (temperature greater than 12°C) is about 3.8 Sv (1 Sv ≡ 106 m3 s−1) southward into the box across the Brisbane, Australia–Fiji northern edge. Net outflows are 3.3 Sv eastward across the Auckland, New Zealand–Fiji edge, and 2.7 Sv southward across Sydney, Australia–Wellington, New Zealand. Mean Ekman convergence of 2.2 Sv closes the mass budget. Net water mass conversions in the upper ocean consist of inflow of waters averaging about 26°C and 35.4 psu balanced by outflow at about 18°C and 35.7 psu, and reflect the net evaporation and heat loss in the formation of South Pacific Subtropical Mode Water. The mean heat balance shows good agreement between ocean heat flux convergence (42.3 W m−2), heat loss to the atmosphere from the NCEP–NCAR reanalysis (39.2 W m−2), and heat storage calculated from data in the box interior (1.3 W m−2). On interannual time scales, volume transport through the box ranges from about 1 to 9 Sv, with heat flux convergence ranging from about 20 to 60 W m−2. An interannual balance in the heat budget of the warm layer is achieved to within about 10 W m−2 (or 6 W m−2 for the upper 100 m alone). Maxima in the advective heat flux convergence occurred in 1993, 1997, and 1999–2000, and corresponded to maxima in air–sea heat loss. The evolution of surface-layer temperature in the region is the residual of nearly equal and opposing effects of ocean heat flux convergence and air–sea exchange. Hence, ocean circulation is a key element in the interannual heat budget of the air–sea climate system in the western boundary current region.

Variations and trends of CO2 in the surface seawater in the Southern Ocean south of Australia between 1969 and 2002

Measurements of the partial pressure of CO2 in surface seawater (pCOsw2) were made in the Southern Ocean south of Australia during four cruises in January to February 1969, December 1983 to January 1984, December 1994 to January 1995 and January 2002. The spatial distribution of pCOsw2 for the four cruises showed the same pattern north of the Sub-Antarctic Front (SAF), while year-to-year changes were noted south of the SAF. We evaluated the long-term trend of the pCOsw2 representative of the zone between oceanographic fronts by taking into account changes in the seasonal variation in pCOsw2 and the long-term increase of the sea-surface temperature (SST) of the Southern Hemisphere. The observed growth rate of pCOsw2 was 0.7 ± 0.1 µatm yr−1 at its minimum, which was observed at the SST of 15 °C north of the Subtropical Front (STF), 1.0 ± 0.5 µatm yr−1 in the Sub-Antarctic Zone (SAZ) between STF and SAF, 1.5 ± 0.4 µatm yr−1 in the Polar Frontal Zone (PFZ) between SAF and the Polar Front (PF) and 1.8 ± 0.2 µatm yr−1 in the Polar Zone (PZ) between PF and 62°S, determined as the northern edge of the Seasonal Sea Ice Zone (SSIZ) on the basis of surface salinity and satellite images. These increases were caused by the uptake of anthropogenic CO2 as well as variations in the thermodynamic temperature effect, ocean transport and biological activity. In the SSIZ between 62 and 66.5°S, we could not clearly evaluate the long-term trend of pCOsw2 due to the remarkable CO2 drawdown due to biological activity in January 2002. The relatively low growth rates of pCOsw2 close to the STF and in the SAZ are probably associated with the formation of Subtropical Mode Water and Sub-Antarctic Mode Water in their respective zones. Between the north of the STF and the PZ, the growth rate of total dissolved inorganic carbon was calculated to be about 0.5–0.8 µmol kg−1 yr−1 via the buffer factor.

Preconditioning of winter mixed layer in the formation of North Pacific eastern subtropical mode water

The preconditioning of winter mixed layer in the formation of the eastern subtropical mode water (ESTMW) in the North Pacific Ocean is investigated using an eddy‐permitting ocean general circulation model. The result shows that convergence in the northward Ekman transport of saltier water and weak summertime heat fluxes due to the presence of stratus cloud in the ESTMW formation region are both central to the initiation of significant mixed layer deepening that subsequently evolves into the ESTMW pycnostad in the main thermocline. These two factors originate from air‐sea interactions inherent in the region of Northeast Pacific Basin. The approach described here has significant potential for determining the water‐mass distribution and may lead to an explanation of the Sverdrup's Eastern Gyral in the subtropical North Pacific.

Formation region of North Pacific subtropical mode water in the late winter of 2003

The mixed layer structure and properties in the formation region of the North Pacific Subtropical Mode Water (STMW) east of Japan in the late winter of 2003 were examined using temperature and salinity data primarily from Argo profiling floats. The formation region extends south of the Kuroshio Extension, between 30°N and 35°N and as far east as 175°E. It is characterized by a mixed layer with depths greater than 200db and meridionally uniform temperature between 16.5°C and 18.2°C, associated with a mixed layer front at its southern edge. The mixed layer in the formation region becomes colder, fresher, and denser toward the east, leading to the formation of warmer, saltier, and lighter STMW in the western part of the region. The spatial variation of the mixed layer depth in the formation region corresponds well with the underlying permanent thermocline depth.

Temporal variability of excess nitrate in the subtropical mode water of the North Atlantic Ocean

A dichotomy exists between rates of nitrogen fixation directly measured by biological techniques, and rates inferred from the geochemical distributions of excess nitrate within the thermocline of the North Atlantic Ocean. Part of the dichotomy relates to the temporal and spatial uncoupling between the event (i.e., nitrogen fixation by diazotrophs) and signal (i.e., excess nitrate (DIN xs) in the thermocline), as well as the interannual variability of both. Here, temporal variability of excess nitrate in the subtropical mode water (STMW) of the North Atlantic Ocean is evaluated for the 1988–2001 period. The excess nitrate signal has a maximum in this water mass, and it is by far the largest volumetric component of thermocline waters in the subtropical gyre. DIN xs variability and excess nitrate production rates in the STMW layer were well correlated with the North Atlantic Oscillation (NAO). For example, DIN xs values (∼1.5–2.8 μmol kg −1) and excess nitrate production rates (∼3.7 Tg N year −1) were generally high during positive phases of the NAO (e.g., 1989–1994; 1997–2000) and coincident with periods of higher atmospheric mineral dust input to the ocean. When the NAO was in its negative phase and dust inputs lower (e.g., 1995–1996; 2001), DIN xs values (∼0–1.0 μmol kg −1) and excess nitrate production rates were generally low (up to ∼0.6 Tg N year −1). The NAO potentially influences DIN xs variability by modulating the extent and magnitude of STMW formation, thereby changing the fate of accumulated DIN xs during circulation of STMW in the subtropical gyre, and the variability of nitrogen fixers through changes in dust inputs to the subtropical gyre.

South Pacific Eastern Subtropical Mode Water*

Abstract The structure, formation, and destruction of South Pacific Eastern Subtropical Mode Water (SPESTMW) are analyzed. Geographic extent and water properties are discussed by using high-quality CTD sections collected between 1991 and 1996. Defined as having a planetary potential vorticity magnitude of less than 3 × 10−10 m−1 s−1, SPESMTW has a volume of about 1.1 × 1015 m3, estimated from CTD data. The ventilation of this mode water is described by using data from a high-resolution XBT section in concert with 30-month time series from profiling CTD floats, some of the first Argo deployments. Published subduction rates allow a mode-water formation rate estimate of 8.7 × 106 m3 s−1. Combining this estimate with the volume yields a residence time of about 4 years. The density-compensating covarying patterns of late winter surface temperature and salinity in the ventilation region of SPESTMW are shown to contribute to the strength of the mode water. However, while the destabilizing salinity gradient in SP...

A short-term sink for atmospheric CO2 in subtropical mode water of the North Atlantic Ocean.

Large-scale features of ocean circulation, such as deep water formation in the northern North Atlantic Ocean, are known to regulate the long-term physical uptake of CO2 from the atmosphere by moving CO2-laden surface waters into the deep ocean. But the importance of CO2 uptake into water masses that ventilate shallower ocean depths, such as subtropical mode waters of the subtropical gyres, are poorly quantified. Here we report that, between 1988 and 2001, dissolved CO2 concentrations in subtropical mode waters of the North Atlantic have increased at a rate twice that expected from these waters keeping in equilibrium with increasing atmospheric CO2. This accounts for an extra 0.4-2.8 Pg C (1 Pg = 10(15) g) over this period (that is, about 0.03-0.24 Pg C yr(-1)), equivalent to 3-10% of the current net annual ocean uptake of CO2 (ref. 3). We suggest that the lack of strong winter mixing events, to greater than 300 m in depth, in recent decades is responsible for this accumulation, which would otherwise disturb the mode water layer and liberate accumulated CO2 back to the atmosphere. However, future climate variability (which influences subtropical mode water formation) and changes in the North Atlantic Oscillation (leading to a return of deep winter mixing events) may reduce CO2 accumulation in subtropical mode waters. We therefore conclude that, although CO2 uptake by subtropical mode waters in the North Atlantic--and possibly elsewhere--does not always represent a long-term CO2 sink, the phenomenon is likely to contribute substantially to interannual variability in oceanic CO2 uptake.

Inverse Modeling of Intra-annual Variability in the Subtropical North Pacific*

Abstract Climatological data on the oceanic and atmospheric variability are inverted to study seasonal variation of the Kuroshio Extension (KE) and the recirculation gyre to the south. The processed datasets include climatological fluxes of heat, salt, and momentum at the ocean surface; Levitus hydrography; TOPEX/Poseidon altimetry; and surface drifter data. A variational data assimilation technique is used to retrieve variability in the open ocean region (18°–42°N, 142°E–160°W) bounded at 1000 m from below. By optimizing the open boundary values of oceanic fields in combination with initial conditions and atmospheric forcing, model solutions that are consistent with various climatological datasets within limits of observational errors were found. Using this technique, the mean geographical positions of the Subtropical Mode Water (STMW) and Central Mode Water (CMW) formation sites were estimated, the structures were analyzed, and estimates of the mode water production rates were obtained. Computations ind...

A Lagrangian numerical investigation of the origins and fates of the salinity maximum water in the Atlantic

The origins and fates of the Atlantic salinity maximum water (SMW), formed through excess evaporation in the tropics and subtropics of both hemispheres, are studied using monthly mean outputs of a numerical simulation of the world ocean climatological circulation. After defining formation domains from the surface salinity field and the vertical stratification, a Lagrangian technique is used to estimate the formation rates and main pathways in each hemisphere and the role of this water in the framework of the warm water return flow of the meridional overturning cell. Formation rates around 9 and 11 Sv are found in the Southern and Northern Hemispheres, respectively. While the export of the southern SMW from its formation area is realized by the western boundary currents, that of the northern SMW mainly results from interior subduction. Equatorward of the formation regions, a fraction of each SMW variety is entrained in the subtropical cells that connect the subtropics to the equatorial region. Poleward of them, both varieties are seen to feed the regions of subtropical mode water (STMW) formation around 35° of latitude in both hemispheres. The bulk of the transport associated with each variety eventually turns northward: This amounts to ∼6 Sv of southern SMW gathered in the North Brazil Undercurrent (NBUC), and ∼10 Sv of northern SMW found in the Gulf Stream at 35°N, of which 8 Sv have gone through the Caribbean Sea. Of the 13.4‐Sv northward transport of the meridional overturning cell estimated by the model at 47°N, more than 50% (6.9 Sv) is found to have transited through at least one of the SMW regions. This gives an indication of the likely important role of SMW formation in the observed northward salinity increase of the upper Atlantic Ocean.

A model study on internally generated variability in subtropical mode water formation

Internally generated variability in the subtropical gyre is studied as a possible mechanism for the observed interannual to decadal variability in subtropical mode water formation. An isopycnic ocean model with idealized geometry and forcing which mimics the North Atlantic subtropical gyre is used for this purpose. The horizontal resolution is sufficiently high and the friction and diffusion sufficiently low for the flow to become barotropically and baroclinically unstable. Two modes of low‐frequency variability are found. Both modes consist of westward propagating thickness anomalies. The anomalies have a first baroclinic modal structure with a maximum amplitude at the thermocline. One mode has a timescale of 8 years and a basin wide spatial scale; the other has a timescale of 4.5 years and a smaller spatial scale. The modes appear to be damped when the diffusion is high. In that case, the 8‐year mode can be excited by a spatially coherent stochastic wind stress. The evolution of the modes is determined by an interaction between the mean flow and the low‐frequency variability itself. The modes are instabilities of the mean flow determined by the basic stratification. It appears that coupling to the atmosphere and a parameterization of surface mixing are necessary for the low‐frequency variability to appear in the mixed layer. The coupling and surface mixing do not play a role in generating the modes. It is concluded that these internally generated modes may play a role in the observed variability in mode water formation.

Formation Mechanisms for North Pacific Central and Eastern Subtropical Mode Waters

The effects of one-dimensional processes on the formation of deep mixed layers in the central mode water (CMW) and eastern subtropical mode water (ESMW) formation regions of the North Pacific have been analyzed using a mixed layer model. By running the model with various combinations of initial (August) background stratification and forcing fields (heat flux, E − P, and wind stress), and comparing the resultant March mixed layer depths, the relative importance of these effects on creating deep mixed layers was diagnosed. Model results suggest that the contributions of evaporation minus precipitation and wind mixing to mixed layer depth in both the CMW and the ESMW formation regions are negligible. In the ESMW formation region (centered at approximately 30°N, 140°W), the initial stratification is very important in determining where deep mixed layers form. Summer heating is quite weak in this region, resulting in a weak (or even nonexistent) seasonal pycnocline at the end of the summer at about 30°N. It is this lack of shallow seasonal stratification that allows a local maximum of winter mixed layer depth even though the wintertime cooling is much weaker than other regions of locally deep mixed layers. In the CMW formation region (approximately 40°N between 170°E and 160°W), in contrast to the ESMW formation region, wintertime cooling is strong enough to erode through the shallow seasonal pycnocline. In the region of deepest mixed layers in the CMW region, the deeper stratification (150–400 m) is quite weak. Once the seasonal pycnocline has been eroded away, the lack of deeper stratification becomes important in allowing the mixing to penetrate further.

Characterizing the structure of the surface layer in the Pacific Ocean

The structure and statistics of the oceanic surface layer are characterized using quarterly surveyed, eddy‐resolving expendable bathythermograph (XBT) and expendable conductivity‐temperature‐depth (XCTD) data collected along a number of routes spanning the Pacific Ocean. This data set consists of more than 18,000 temperature casts to 800 m, with station spacing of 10 to 40 km along transects between Auckland and Seattle (beginning in 1986), San Francisco and Taiwan (1991), Auckland and Valparaiso (1993), and Honolulu and Valdez (1993). The surface layer can assume many different shapes. It can include strongly stratified layers, actively mixed layers, salinity barrier layers, fossil mixed layers, and inversions. The spatial and temporal distribution of these features within the XBT and XCTD profiles is examined. Fossil layers are predominantly a springtime feature and are associated with regions of Subtropical Mode Water formation in the southwest Pacific (near New Zealand) and the northeast Pacific (near San Francisco). Inversions are less seasonally dependent and most commonly related to interleaving of different water masses in the high shear regions of the California Transition Zone and its counterpart eastern boundary systems in the far northeast and southeast Pacific and in the tropical zonal current system. The XCTD casts show a rich and varied surface layer structure in the equatorial and subpolar regions of the Pacific Ocean that is strongly influenced by the salinity stratification. This highlights the need for complementary salinity and density information in these areas to accurately categorize the true nature of the active mixed layer.

Origin and ages of mode waters in the Brazil‐Malvinas Confluence region during austral winter 1994

The ventilation of the main thermocline associated with the formation of Subtropical Mode Water (STMW) and Subantarctic Mode Water (SAMW) within the Brazil/Malvinas Confluence region is investigated using satellite data, hydrographic data, and dissolved CFCs as transient tracers. During the winter of 1994, two types of STMW were observed. Both types, 26.24 σθ and 26.46 σθ were formed within the warm pool of the South Atlantic Central Water (SACW). The SAMW is found on the 27.10 σθ density surface. This surface lies in the minimum salinity layer of the Antarctic Intermediate Water (AAIW). The mode waters are dated using CFC ratios. To our knowledge, this is the first estimate of the age of the mode waters in the region using the CFC‐113:CFC‐11 ratio method. The lighter STMW was being formed during the cruise. The heavier STMW was found to be 6±3 years old and is thought to have circulated in the subtropical gyre without losing its primary properties. The SAMW, or the lightest AAIW has an age of 3±2 years. This water is very young compared with the oldest (10±5 years) and heaviest component of the AAIW, which circulates within the subtropical gyre. The formation of the STMW in the confluence region is dependent on the southward extension of the Brazil Current.

Interannual Variations of North Pacific Subtropical Mode Water in the 137°E Section

Abstract Examination of interannual variations of North Pacific subtropical mode water (STMW) in the 137°E repeat hydrographic section by Suga et al. is extended to seek a relation between changes in STMW renewal and the wintertime cooling. Time series of isopycnal potential vorticity (PV) and apparent oxygen utilization (AOU) demonstrate substantial interannual variations of STMW. It is suggested that the interannual variations are closely related to the changes both in the Kuroshio recirculation system and in the formation rate. For the non-large-meander period of the Kuroshio, during which renewed STMW is advected vigorously from east of the section, the monsoon index as an indicator of cooling in a given winter, and PV and AOU at an STMW isopycnal surface in the following summer show significant correlation, indicating a strong relation between the wintertime cooling and the STMW formation.

Long-term trend of the partial pressure of carbon dioxide (pCO<sub>2</sub>) in surface waters of the western North Pacific, 1984-1993

The ocean is an important sink for anthropogenic CO 2 emissions, but there are only a few measurements which confirm the oceanic CO 2 uptake. Since 1981, partial pressure of CO 2 (pCO 2 ) in the western North Pacific (35°N−3°N, 128°E−155°E) and the overlying air have been measured periodically to clarify the seasonal and long-term trends of the oceanic carbonate system. The partial pressure of CO 2 in surface seawater (pCO 2 sea ) observed every boreal winter during the period from 1984 to 1993 give a growth rate of 1.8 ± 0.6 µ atm yr -1 ( n = 27) north of 15°N and 0.5 ± 0.7 µ atm yr -1 ( n = 23) south of 14°N with an average of 1.2 ± 0.9 µ atm yr -1 ( n = 50). The rate of pCO 2 sea a increase north of 15°N is equal to that of atmospheric CO 2 (1.8 µ atm yr -1 ) during the same period but that south of 14°N is lower. The difference in rate of pCO 2 sea a increase is suggestive of temporal variations in ΔpCO 2 distribution. After removing the long-term trend from the pCO 2 sea data, the seasonal variation of pCO 2 sea in the western North Pacific (132°E−142°E) was evaluated with a linear regression between the pCO 2 sea and sea surface temperature (SST). Generally, a thermodynamic process (temperature effect) plays a predominant role in determining the seasonal variations of pCO 2 sea . South of 14°N, however, a clear interannual variability is significant relative to the seasonal changes if an El Nino event is accompanied by enhanced vertical mixing. The annual air-sea CO 2 flux showed a large influx of CO 2 into the ocean north of 27°N (Kuroshio Counter Current) because of a large negative ΔpCO 2 (− 60 µ atm) and strong wind during the winter season. Toward the south, the annual average air-sea CO 2 flux increased by 9 mmol m -2 day -1 from – 8 mmol m -2 day -1 at 31°N to 1 mmol M -2 day -1 at 5°N. South of 10°N, the ocean acts as a source for atmospheric CO 2 (0.2-0.7 mmol m -2 day -1 ), but this is a considerably weaker source as compared with those of the central and eastern equatorial Pacific. The observed increase of pCO 2 sea and the estimated air/sea CO 2 flux suggest the importance of carbon transport from the mixed layer to the intermediate/deep water in the area of Subtropical Mode Water formation, south of the Kuroshio and east of Japan. DOI: 10.1034/j.1600-0889.47.issue4.2.x

Formation and spreading of subtropical mode water in the North Pacific

A time‐averaged description of the formation and spreading of North Pacific subtropical mode water (NPSTMW) is presented. The formation of NPSTMW is studied using TRANSPAC expendable bathythermograph data. The data are transformed into a stream coordinate system and averaged. The coordinate system uses the position of the Kuroshio front as its origin. Maps are presented of the temperature structure and mixed layer distribution in winter relative to the Kuroshio front. The formation of NPSTMW occurs in two separate stages, cooling and thickening. As water flows at the surface in the Kuroshio in winter, the water is cooled by the intense ocean‐atmosphere heat flux taking place in the area. When water leaves the Kuroshio to the south, the thermocline underneath becomes deeper, convection is able to penetrate deep into the water column, and the heat flux acts more to thicken the surface mixed layer rather than reduce the temperature. The area south of the Kuroshio, where relatively homogeneous waters are found, is the source region for NPSTMW. At the end of the winter the thick mixed layers are assumed to be covered over by a seasonal thermocline and become NPSTMW. The Levitus (1982) data are used to study the way in which NPSTMW is spread away from its source region by the subtropical circulation. The distribution of potential vorticity (PV) on four NPSTMW isopycnals is studied, with low PV used as a tracer for NPSTMW. The PV reveals a pattern of “differential spreading,” whereby NPSTMW formed in the western part of the source region is advected to the west after its formation. NPSTMW formed farther east is advected more to the south or east after formation. Differential spreading allows NPSTMW to be found in a large part of the northwestern subtropical gyre, well away from its source region. The distribution of NPSTMW described by Masuzawa (1969), with NPSTMW getting colder from west to east, is explained as a combination of the distribution of NPSTMW at its source and the way it is spread after it is formed.

Two decades of mixing tritium into the North Pacific Ocean

Tritium data collected in the North Pacific Ocean mostly from Long Lines cruises (1983–1985) are compared with GEOSECS data (1973–1974) to characterize the dominant circulation processes in the second decade after the major bomb injection. The total decaycorrected inventory for the North Pacific Ocean remained nearly constant during the decade between GEOSECS and Long Lines, although the spatial distribution changed. Tritium was lost from the subpolar region and was gained by the tropics. Exchange between the subpolar and subtropical regions appears to be occurring at both the extreme eastern and western boundaries. On isopycnals that are not thought to outcrop ≥26.80 σθ), the tritium inventory is about 30% higher in the Long Lines data.The GEOSECS tritium data show that the ventilation time scale of the thermocline is rapid; less than 10 years. The GEOSECS tritium distribution is characterized by north-south gradients, which are strongest in the east. The Long Lines tritium data show that the eastern portion of the subtropical gyre is well mixed and has similar values to the subpolar region. We attribute this difference in the data sets to the sharp decrease in the tritium source function after the mid-1960s. Without the addition of tritium at northern outcrops, gradients cannot be maintained in the presence of mixing. Higher tritium values in the eastern, as compared with the western, portion of the basin are consistent with Subtropical Mode Water and shallow salinity minimum water formation. Furthermore, differences between the eastern and western portions of the basin suggest that mixing and/or recirculation may be more important in the west.On non-outcropping isopycnals there is poleward and westward contraction of the subtropical gyre in both data sets. Cross i~opycnal mixing in the subpolar region ventilates isopycnals (as evidenced by the presence of bomb tritium) which do not outcrop. The Long Lines data suggest that after formation in the northwest, intermediate waters may be further ventilated within the Alaskan Gyre. The residence time in the subpolar region is estimated to be less than 14 years for North Pacific Intermediate Water and greater than 20 years for deeper waters. In the Long Lines data tritium penetration reaches at least to the oxygen minimum (27.40 σθ) in the subpolar and most of the SUbtropical region. The average increase in vertical tritium penetration in the Long Lines data is approximately 100 m. This is consistent with the small increase (from 12 to 14%) in the volume of the North Pacific Ocean exposed to bomb tritium. Fourteen per cent is a lower bound estimate of the ventilated volume of the North Pacific Ocean