Eric Lindstrom

The mixed layer of the western equatorial Pacific Ocean

The mixed layer of the western equatorial Pacific and its thermodynamics are poorly known because of a general lack of data. Conductivity-temperature-depth (CTD) profiles from the recent Western Equatorial Pacific Ocean Circulation Study (WEPOCS) cruises have been analyzed for various measures of the upper layer and mixed layer thickness, using criteria which depend on vertical gradients of temperature, salinity, and density. From 434 profiles, the average mixed layer depth in the western equatorial Pacific during the two WEPOCS cruises was 29 m, which is about a factor of 3 shallower than had previously been thought. The mean depth of the top of the thermocline was found to be 64 m, so there is a nearly isothermal layer that is deeper than the mixed layer. This discrepancy is attributable to salinity stratification. It is hypothesized that the waters in this “barrier” layer between the bottom of the mixed layer and the top of the thermocline are formed to the east of the WEPOCS region, and subducted below the shallow and lighter mixed layer waters found in the west. Under light wind conditions, there was a tendency for warm and thin layers to form at the sea surface as a result of diurnal heating; however, there did not appear to be any nighttime maximum to the mixed layer depth associated with convective overturn due to cooling. This contrast with the central Pacific may be caused by the influence of salinity on the thermodynamics of the mixed layer. A strong westerly wind burst was observed during WEPOCS II, and apparently the mixed layer nearly doubled in depth while cooling by more than 1°C. Evidence of downwelling near the equator, and upwelling off the equator, was seen in the distribution of temperature, salinity, and density in the meridional section along 143°E, which was occupied immediately following the wind event. This event was apparently strong enough to erode through the salinity-stratified layer and into the thermocline, resulting in the observed cooling. The results of this study suggest that except during strong wind events, entrainment cooling may not be an important component of the heat budget of the western Pacific warm pool. This has potentially important implications for the El Nino/Southern Oscillation (ENSO) phenomenon.

Source waters of the Pacific Equatorial Undercurrent

Abstract Hydrographic and direct current measurements were made north and east Papua New Guinea in June–August 1985 and January–February 1986 as part of the Western Equatorial Pacific Ocean Circulation Study (WEPOCS). Analyses of the data indicate that the major portion of the water in the Equatorial Undercurrent at its beginning north of Papua New Guinea is supplied from the south by a narrow western boundary undercurrent (New Guinea Coastal Undercurrent) transporting high-salinity, low-tritium, high-oxygen, low-nutrient water from the Solomon Sea northwestward along the north coast of Papua New Guinea through the Vitiaz Strait. The New Guinea Coastal Undercurrent has a maximum speed of 40–70 cm s −1 at a depth of about 200 m. It is a permanent feature despite the reversals of the wind and the surface current during the period of the northwest monsoon in austral summer. Its transport through the Vitiaz Strait is as high as 8 × 10 6 m 3 s −1 , which is of the same magnitude as the Equatorial Undercurrent transport at 143°E. The New Guinea Coastal Undercurrent revealed by the WEPOCS data is consistent with the low-latitude equatorward western boundary current implied in a calculation of the Sverdrup transport based on the observed wind-stress distribution for the tropical Pacific Ocean. High-salinity, low-tritium, low -oxygen, high -nutrient water which flows westward into the Bismarck Sea passing north of the Solomon Islands is entrained into the Equatorial Undercurrent north of New Ireland and returns to the east, resulting in a down-stream increae in the Undercurrent transport. Low-salinity, high-tritium, high-nutrient water of eastern North Pacific origin also contributes to the Equatorial Undercurrent in its source area west of the WEPOCS region. However, there is no evidence that northern waters are being continuously entrained into the Undercurrent in the WEPOCS region.

A Climatological Interpretation of the Circulation in the Western South Pacific

Abstract Time-averaged circulation is examined using historical hydrographic data near the Australia and Papua New Guinea coast in the Pacific. By averaging the data along isopycnal surfaces in a 0.5° × 0.5° grid, the authors are able to show many detailed phenomena associated with the narrow western boundary currents, including the vertical structure of the bifurcation latitude of the South Equatorial Current (SEC) and the connection between the Solomon and Coral Seas. The bifurcation latitude of the SEC is found to move southward from about 15°S near the surface to south of 22°S in the intermediate layers. The origin of the Great Barrier Reef Undercurrent (GBRUC) is identified to be at about 22°S. Farther to the north, the GBRUC intensifies underlying the surface East Australian Current, and merges with the North Queensland Current (NQC) at about 15°S. The NQC turns eastward to flow along the Papua New Guinea coast and feeds into the New Guinea Coastal Undercurrent (NGCUC) through the Louisiade Archipel...

Northward Intrusion of Antarctic Intermediate Water in the Western Pacific

Abstract The northward intrusion of Antarctic Intermediate Water (AAIW) is examined using historical data combined with synoptic observations from a repeated hydrographic section in the western Pacific Ocean. The results of this analysis suggest that AAIW is traced as a salinity minimum to only about 15°N via the New Guinea Coastal Undercurrent and the Mindanao Undercurrent. There is no northward extension of AAIW farther to the north along the western boundary. Although relatively high oxygen water does exist in the Okinawa Trough, it is connected with high-oxygen water in the South China Sea (SCS) through the Luzon Strait but not from the south as an extension of AAIW. Local circulation seems to play an essential role in localizing the oxygen maximum in the SCS. Evidence exists to suggest that high-oxygen water enters the SCS as part of the Pacific deep water around the still depth (∼2000 m) of the Luzon Strait; from there, part of it upwells and is entrained into shallower isopycnal surfaces by vertica...

Currents off the east coast of New Ireland, Papua New Guinea, and their relevance to regional undercurrents in the western equatorial Pacific Ocean

In the Western Equatorial Pacific Ocean a coastal undercurrent lying within 40 km of the east coast of New Ireland was in May 1988 found to be transporting high-salinity, low dissolved oxygen, high-nutrient thermocline waters northwestwards towards the equator. Acoustic doppler current profiler measurements showed the peak alongshore current speeds of approximately 60 cm s−1 occurred at 235 m depth and the volume transport between the 24.5 and 26.5 kg m3 isopycnals (approximately 176 to 320 m) was 2.0±0.1×106 m3 s−1. We suggest that this current be called the New Ireland Coastal Undercurrent (NICU). Just to the east of 149°E, the NICU was diverted to the north and forced to interact with the Equatorial Undercurrent (EUC). Transport budgets in the region bounded by 149°E, 153°E, 2°S and the equator, between the 24.5 and 26.5 kg m−3 isopycnals show that approximately 5×106 m3 s−1 of water originating from east of New Ireland, including the NICU was added to the EUC system. This accounted for approximately half of the southern hemisphere thermocline waters in the EUC system at 153°E.

The OSTM/Jason-2 Mission

The Ocean Surface Topography Mission/Jason-2 (OSTM/Jason-2) satellite altimetry mission was successfully launched on June 20, 2008, as a cooperative mission between CNES, EUMETSAT, NASA, and NOAA. OSTM/Jason-2 will continue to precisely measure the surface topography of the oceans and continental surface waters, following on the same orbit as its predecessors, TOPEX/Poseidon and Jason-1. To maintain the high-accuracy measurements, the mission carries a dual-frequency altimeter, a three-frequency microwave radiometer, and three precise positioning systems. The objectives of the mission are both operational and scientific. The mission will provide near-real time high-precision altimetric measurements for integration into ocean forecasting models and other products. The mission will also extend the precise surface topography time series started by TOPEX/Poseidon in 1992 over two decades in order to study long-term ocean variations such as mean sea level variations and interannual and decadal oscillations. The measurement system has been adapted to provide quality data nearer to the coasts, and over lakes and rivers. This paper provides an overview of the OSTM/Jason-2 mission in terms of the system design and a brief introduction to the science objectives.

The Flow through Vitiaz Strait and St. George’s Channel, Papua New Guinea

Hydrographic and current meter measurements collected in 1985 and 1986 in Vitiaz Strait and St. George’s Channel, Papua New Guinea are described. The subsurface, equatorward flowing current in Vitiaz Strait is remarkably strong (>1 m/s) and persistent throughout major seasonal wind shifts. Mass transport estimates through Vitiaz Strait of 8–14 Sverdrup are discussed. St. George’s Channel mass transport is smaller (perhaps half) of that in Vitiaz Strait.

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 Wate...

Structure and Origin of 18°C Water Observed during the POLYMODE Local Dynamics Experiment

Abstract Two distinct types of 18°C water (Subtropical Mode Water) were observed during the POLYMODE Local Dynamics Experiment (LDE; May–July 1978; 31.0°N, 69.5°W). These were revealed on isopyncals by salinity histograms which were bimodal. Salinity was highly correlated with oxygen, vortex stretching, and 17.5°–18.5°C thickness. The correlations are positive between salinity and both oxygen and thickness and negative between salinity and vortex stretching. The origins of the two water types are deduced using a variety of measurements in the Sargasso Sea including apparent oxygen utilization, vortex stretching and salinity. It is found that the modes were formed approximately 16 months prior to the LDE during the severe winter of 1976/77. Sharp horizontal salinity gradients between the two LDE water types are comparable to those observed more than a year earlier, and the spatial scale (∼100 km) of the regions of saline mode water is smaller in the LDE than immediately after the 1976/77 winter (∼200 km). ...

A Simple Model of Lake Ontario's Coastal Boundary Layer

Abstract An empirical forced wave model of currents and thermocline displacements in the coastal zone of Lake Ontario is derived from data from the International Field Year for the Great Lakes (1972). The model consists of three linear wave equations for predicting the depth of the thermocline, its slope and the longshore volume transport from the wind. The empirical phase speeds are consistent with internal Kelvin wave and topographic wave theory and the response to a unit longshore wind stress is consistent with cross-section models of long lakes.