Susan L. Hautala

Subtropical mode water in the Northeast Pacific Basin

A new type of mode water in the upper thermocline of the eastern subtropical North Pacific is identified and examined using data from World Ocean Circulation Experiment high-resolution repeat expendable bathythermograph (XBT) section PX37 and archives of historical XBT data. This water mass (labeled Eastern Subtropical Mode Water) is characterized by a subsurface potential vorticity minimum and is located east of Hawaii (Northeast Pacific Basin) in a density range of 24–25.4 σθ. It is a distinct water mass from the classical subtropical mode water (STMW) of the western Pacific. Eastern STMW is formed as a relatively deep late-winter mixed layer, associated with the subtropical/subpolar water mass boundary near 25°–30°N, 135°–140°W, and is capped and subducted into the permanent thermocline. Along a section between San Francisco and Honolulu, Eastern STMW production is seen in every year for which there is adequate data. In this section the volume of Eastern STMW formed each winter and the temperature of the potential vorticity minimum are similar during the periods 1970–1979 and 1991–1997.

Northward abyssal transport through the Samoan passage and adjacent regions

A conductivity-temperature-depth/hydrographic survey in January–February 1994 and a 17-month deployment of current meter moorings from September 1992 to March 1994 were carried out to determine the volume transport, water mass characteristics, and diathermal fluxes of northward flowing abyssal waters in the Samoan Passage and adjacent regions of the South Pacific Ocean. Geostrophic calculations relative to 1.2°C potential temperature indicated northward transport of 7.8 Sv in the Samoan Passage, 1.1 Sv through a gap in Robbie Ridge, and 2.8 Sv along the eastern flank of the Manihiki Plateau. All of the total of 11.7 Sv of northward geostrophic transport was in waters colder than 1.1°C. The northward transport distribution was bimodal in temperature, with a cold mode of 3.6 Sv in the range 0.65°–0.70°C occurring entirely in the Samoan Passage and a warm mode of 3.0 Sv in the range of 0.80°–0.85°C occurring mainly along the Manihiki Plateau. Within the Samoan Passage, 7.1 Sv of the northward transport was below 4000 m where the geostrophic calculation was confirmed by an equal estimate of transport from current meters during the simultaneous 3-day period. The 17-month mean transport from the moored array was 6.0 Sv ± 0.5. By using the observed temporally varying flow within the Samoan Passage together with the hydrographic snapshot across the region, an estimate of the total mean northward transport of 10.6 Sv ± 1.7 was obtained. Estimates of the flow across near-bottom potential temperature surfaces indicate extraordinarily high rates of mixing, with heating of the abyssal layer up to 20 W m−2, corresponding to diffusivities up to 10−1 m2 s−1.

The distribution and mixing of Pacific water masses in the Indonesian Seas

The salinity and dissolved oxygen of water masses in the Indonesian Seas, from historical hydrographic data, are examined on isopycnal surfaces. We focus primarily on the Banda Sea, from which the bulk of the throughflow transport flows into the Indian Ocean. Dissolved oxygen proves to be a problematic conservative tracer in this region due to biological consumption in the upwelling regime of the Arafura Sea and the subsequent spreading of relatively low oxygen water over a broad area. The remaining analysis is thus restricted to salinity. We first consider a hypothesis of simple isopycnal advection and mixing between North and South Pacific low-latitude western boundary current sources. Three regimes are apparent. The surface and upper thermocline layers, down to 25.8σθ, are too fresh to fit the hypothesis. Vertical mixing of surface precipitation and runoff excesses down into the water column must be invoked. Vertical mixing is also apparent on deeper isopycnals, below 27.0σθ, where a contribution from the deep Indian Ocean can be discerned. In between, in the lower thermocline, the θ-S data are consistent with the hypothesis of purely isopycnal spreading, given a simple variation in the ratio of sources toward increasing South Pacific contribution with depth. In this regime the juxtaposition of sources leads to relatively strong gradients in water mass properties across the Banda Sea. This gradient translates into a difference in outflow characteristics between Timor and Ombai Straits, which appear to draw their waters, respectively, from the eastern and western Banda Sea. We then consider how the presence of vertical mixing modifies our inferred water mass ratio under a variety of boundary conditions. Vertical mixing effects are particularly important in the upper thermocline. In the Banda Sea, on all isopycnals down to σθ = 26.5, an increase in the degree of vertical mixing tends to decrease the relative importance of the North Pacific source. The sensitivity of the water mass ratio to degree of vertical mixing decreases with depth, and the tendency is reversed below 26.5σθ. In the Banda Sea, for plausible values of throughflow residence time and vertical diffusivity (λ = (κt)1/2 < 100 m), the North Pacific contributes 80–90% of the water mass at 26σθ, 50–60% at 26.5σθ, and only 10–30% at 27σθ. This large South Pacific contribution in the lower thermocline is supported by other studies.

Dynamics of the South Java Current in the Indo‐Australian Basin

A year-long deployment of a mooring in the South Java Current (SJC) of Indonesia provides a fascinating insight into this poorly understood, semi-annually reversing boundary current. A striking three-week period of south- eastward flow begins in mid-May 1997. An analytical model directly account for changes in velocity at this time due to the passage of a westerly wind-forced, downwelling Kelvin wave from the equatorial western Indian Ocean. The entire water column is warmed, with a fresh cap overlying salty water, consistent with the Indian Ocean source. Following the wave passage, the SJC is north-westward, and the prevailing south- easterly monsoon winds lead to upwelling of cold, salty water. In early August, the SJC abruptly returns to south-eastward flow, and remains so until November 1997 in the face of steady south-easterly local winds. The anomalous flow direction and cooler water are related to an upwelling Kelvin wave, forced b y unseasonal prolonged easterly wind anomalies in the equatorial Indian Ocean. After a small reversal of flow in November 1997, the SJC is south-eastward, as expected during the north-east monsoon. A trend toward increasing salinities in the record is attributed to the increased input of salty Indian Ocean water, enhanced evaporation, and a lack of freshwater advection due to the regionally reduced precipitation during the 1997-98 E1 Nifo.

Large‐scale sea level, thermocline, and wind variations in the Indonesian throughflow region

The Indonesian throughflow is presumed to be driven by a sea level gradient from the Pacific to the Indian Ocean. Deep throughflow transport may also be driven by a steric gradient between the two basins. The sea level gradient, in turn, is thought to be maintained by the differing wind patterns in the two basins: monsoonal in the Indian Ocean and trades in the western equatorial Pacific. In the interaction between sea level, wind stress, and thermocline depth as identified from historical measurements, we find (1) over the Indian, Indonesian, and equatorial Pacific basins and specifically within the throughflow region, sea level, and thermocline seasonal variations are negatively correlated (sea level rise corresponding to thermocline deepening) and sea level and meridional wind stress are also correlated; (2) the expected strong seasonal gradients in sea level through the eastern throughflow region (near the island of Timor) are found, though without an accompanying thermocline depth gradient; (3) seasonal convergence in baroclinic, upper ocean throughflow transport previously identified [Meyers et al., 1995] in the Timor Sea is associated with changes in sea level as well as upper ocean dynamic height at annual period but not at semiannual; (4) interannual variability explains more of the sea level variance in the eastern throughflow region than is explained by seasonal harmonics; however, there does not appear to be a strong interannual signal in the sea level gradient to drive fluctuations in the upper ocean throughflow. We hypothesize that seasonal variability in the upper layer throughflow and interannual variability in the deep throughflow are the predominant results of the complex interaction of forcing mechanisms.

Is the North Pacific in Sverdrup balance along 24°N?

Hydrographic data from a zonal section along approximately 24°N is used to demonstrate that basin-scale baroclinic geostrophic transport agrees with the Sverdrup relation in the subtropical North Pacific. Moreover, profiles of vertical velocity, as derived from the linear vorticity equation, are consistent with constraints upon deeper flows. Sverdrup balance from the eastern boundary to about 137°E is in accord with most other elements of the general circulation to the extent that they are known. In the Philippine Sea, west of 137°E, the results suggest that a mean northward flow of 5–10 Sv occurs. In the northeast Pacific Basin, a significant subbasin-scale deviation of about 7 Sv from Sverdrup balance is revealed; this deviation is distinguished by excess shallow southward flow near the water mass boundary between the subtropical gyre and the subpolar waters adjacent to the American coast.

Characteristics of the Indo-Pacific Throughflow in the eastern Indian Ocean

Five hydrographic sections across the Indonesian throughflow from the World Ocean Circulation Experiment (WOCE) and the Java Australia Dynamic Experiment (JADE) are used to examine seasonal water mass and density distributions, including semiannual variation of the South Java Current. Altimetric sea surface height variations match dynamic height along each WOCE cruise track remarkably well. They also reveal energetic eddies and meanders of the current system south of Java with intraseasonal (60-day) changes in surface dynamic height comparable in magnitude to seasonal and interannual variations. Through the formation of discrete eddies and meanders of the large-scale zonal flows, intraseasonal fluctuations account for much of the variation in water mass structure between cruises.

Temperature and salinity variability in the exit passages of the Indonesian Throughflow

Abstract The Indonesian Throughflow was monitored from December 1995 until May 1999 in the five major exit passages of the Lesser Sunda Islands, as it flows from the Indonesian interior seas into the southeast Indian Ocean. The monitoring array included pairs of shallow pressure gauges at each side of the straits, equipped with temperature and salinity sensors. As in the inferred geostrophic velocity from the cross-strait pressure gauge data, the temperature and salinity data show strong variability over all time scales related to the local regional and remote forcing mechanisms of heat, freshwater and wind. The annual cycle dominates the temperature time series, with warmest temperatures occurring during the austral summer northwest monsoon, except in Lombok Strait where the semi-annual signal is dominant, and related to the Indian Ocean westerly wind-forced Kelvin waves during the monsoon transitions that supply Indian Ocean warmer surface water to the strait. In the salinity data, the annual signal again dominates the time series in all straits, with a distinct freshening occurring in March–May. This is partly related to the rainfall and resultant voluminous river runoff impacting the region, one month after the wetter northwest monsoon ends in March. The fresh, warm water from the monsoon-transition Indian Ocean Kelvin wave also contributes to the freshening observed in May. There is little cross-strait gradient in near-surface temperature and salinity through the outflow straits, except in Lombok Strait, where Lombok is warmer (except during the northwest monsoon) and fresher than the Bali site (especially during March through May). A fortnightly signal in temperature is found in Ombai and Sumba Straits, and is probably related to the proximity of these straits to the interior Banda Sea where the fortnightly tidal signal is strong. The fortnightly signal is also evident at the Bali site, although not at the Lombok site. Numerous ADCP surveys taken during the survey period suggest a western intensification of the flow through Lombok Strait, such that the Bali site also may be more influenced by the internal Indonesian seas. Finally, there is regional variability in temperature and salinity on interannual time scales. From mid-1997 through early 1998, the region is cooler and saltier than normal. These property changes are related to both the strong 1997–1998 El Nino event in the Pacific, and the strong 1997 Dipole Mode in the Indian Ocean, which together can result in lower regional precipitation; lower transport of the fresh, warm Throughflow water; and changes in the upwelling regime along the Lesser Sunda Island chain. From mid-1998 on, warmer conditions returned to the region probably related to the La Nina event.

Interaction between the Indonesian Seas and the Indian Ocean in Observations and Numerical Models

Recent measurements from six bottom-mounted gauges are used with numerical model results to study the exchange of water between the Indonesian seas and the Indian Ocean via the Lesser Sunda Islands known collectively as Nusa Tengarra. The observations are approximately three years in length, from late 1995 to early 1999, and include measurements of bottom pressure, temperature, and salinity. The locations of the gauges are at the boundaries of three straits connecting the southern Indonesian seas with the eastern Indian Ocean: the Lombok Strait, the Ombai Strait, and the Timor Passage. The magnitude of intraseasonal variations in the pressure data dominates over that of the seasonal cycle. Intraseasonal variability appears most frequently and largest in magnitude at the westernmost strait (Lombok) and decreases along the coastline to the Timor Passage. Comparison to wind data shows these intraseasonal variations to be due to Kelvin wave activity in the Indian Ocean, forced by two distinct wind variations: semiannual monsoon reversals and Madden‐Julian oscillation (MJO) activity. Sea level variations from both forcing mechanisms are then adjusted by local, alongshore winds. Longer-duration model results show the observation period (1996 through early 1999) to be a time of increased ENSO-related interannual variability and of suppressed annual cycle. MJO activity is also increased during this time. These factors explain the dominance of the higher frequency signals in the pressure data and the relative lack of a distinct annual cycle. An optimal fit of model sea level variations to model through-strait transport variations is used to estimate transport variability from the observed pressure records. At each strait the optimal fit is consistent with a cross-strait geostrophic balance for transport variations in the upper 250 m.

A Nonconservative β-Spiral Determination of the Deep Circulation in the Eastern South Pacific

Abstract The horizontal mean circulation and diapycnal flux divergence field in the South Pacific between Tahiti and the East Pacific Rise are obtained from a nonconservative β-spiral inverse method applied to high quality hydrographic data. The diapycnal flux divergence term is found to be an essential part of the deep vorticity balance and proper resolution of this term is critical to the success of the β-spiral calculation. The subthermocline circulation in this region of the South Pacific consists of three vertical regimes. Between the thermocline and approximately 1800–2000 m a cyclonic circulation pattern exists. A zone of minimum motion is found between 1800 and 2000 m. Below this zone, northward flow along the rise and westward flow in the north indicate an anticyclonic gyre, centered on 15°S, that extends approximately 1000–2000 km to the west of the East Pacific Rise. The deep vorticity balance associated with this flow system is primarily between meridional advection of planetary vorticity and ...