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Arnold L. Gordon

Abstract

The transfer of water from the Pacific to the Indian Oceans within the Indonesian Seas is comprised primarily of North Pacific water masses. To state that this water is in reality South Pacific water and that it only “appears” to be North Pacific water is misleading and does not properly reflect the large-scale climate role of the North Pacific Ocean.

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Arnold L. Gordon

Abstract

In February 1977 a column of water (14 km radius), within the central region of the Weddell Gyre west of Maud Rise. was observed in which the normal Antarctic stratification sequence of temperature-minimum to temperature-maximum was absent. The column appeared as a cold, low-salinity, high-oxygen, cyclonic flowing (surface velocity above 50 cm s−1) eddy extending to at lest 4000 m. It is hypothesized that similar eddies were common in this region (at least in Austral summer 1977) and represent winter structures which have survived into the summer period. Eddy formation is explained as a product of winter period static instability, similar to the MEDOC observations in the Mediterranean, but without the subsequent sinking and spreading phase. Winter period static instability in the Weddell Gyre is shown to be a likely condition and may be related to the frequent occurrence of a large polynya within the central region of the Weddell Gyre. Deep penetration of winter surface water within the eddy supplies the characteristics of a deep. low-salinity, high-oxygen intrusion near sigma-2 stratum 37.21 to 37.23 (between 1500 and 2000 m). This intrusion may represent a distinct water type formed within the Weddell Gyre. It would represent a variety of Antarctic Bottom Water [or Antarctic Deep Water (Wüst 1933)] with an open ocean origin. It may spread by isopycnal processes north of the Argentine Basin over the Rio Grande Ridge. The continental-margin-produced Antarctic Bottom Water may be partly topographically confined within the Weddell-Argentine-Crozet Basin trio.

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Debra Tillinger and Arnold L. Gordon

Abstract

Simple Ocean Data Assimilation (SODA) reanalysis data are used to produce a 50-yr record of flow through the Makassar Strait, the primary conduit for the Indonesian Throughflow (ITF). Two time series are constructed for comparison to the flow through the Makassar Strait as observed during 1997–98 and 2004–06: SODA along-channel speed within the Makassar Strait and Pacific to Indian Ocean interocean pressure difference calculated on isopycnal layers from SODA hydrology. These derived time series are compared to the total ITF as well as to the vertical distribution and frequency bands of ITF variability. The pressure difference method displays higher skill in replicating the observed Makassar ITF time series at periods longer than 9 months, particularly within the thermocline layer (50–200 m), the location of maximum flow. This is attributed to the connection between the thermocline layer and large-scale wind forcing, which affects the hydrology of the ITF inflow and outflow regions. In contrast, the surface layer (0–50 m) is more strongly correlated with local wind flow, and it is better predicted by SODA along-channel velocity. The pressure difference time series is extended over the 50-yr period of SODA and displays a strong correlation with ENSO as well as a correlation at the decadal scale with the island rule.

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Amy Ffield and Arnold L. Gordon

Abstract

Western Pacific central and tropical waters characterized by a subsurface salinity maximum spread into the Indonesian seas as part of the Indonesian throughflow. Within the Indonesian seas this salinity maximum is attenuated and, in some places, completely removed. A simple advection-diffusion model verifies the importance of vertical mixing in the transformation of Western Pacific waters to Indonesian thermocline water. The profiles indicate a predominant North Pacific presence in most of the seas, although some South Pacific water is present in the eastern ses of Halmahera, Seram, and Banda. The main interocean route is through the western seas of Sulawesi, Makassar, and Flores, while the flow pathway in the eastern seas is less certain. The Banda Sea can be renewed from either the northern passages (Halmahera and Maluku) or from the south via the Flores Sea. Using representative basin property profiles derived from the archieved data allows determination of a range of vertical diffusivities and residence times that best reproduce the transformation of Pacific waters into Indonesian water. In the Makassar thermocline a lower limit of 1 × 10−4 m2 s−1 for vertical diffusivity is inferred from the model results with reasonable throughflow and precipitation values. This estimate is roughly an order of magnitude greater than those deduced for the interior oceanic thermocline in an environment not conductive to salt fingers. In the Banda Sea a Kz of 1 × 10−4 m2 s−1 implies a predominant North Pacific source. If Kz is higher, then a larger South Pacific presence is possible.

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Amy Ffield and Arnold L. Gordon

Abstract

Expressions of low-frequency tidal periods are found throughout the Indonesian Seas' temperature field, supporting the hypothesis that vertical mixing is enhanced within the Indonesian Seas by the tides. The thermal signatures of tidal mixing vary mostly at the fortnightly and monthly tidal periods due to nonlinear dynamics redistributing tidal energy into these periods. Away from the coasts, the largest tidal mixing signatures are observed in sea surface temperature within the Scram and Banda Seas. Most of the Indonesian Throughflow passes through the Banda Sea where strong vertical mixing modifies the thermocline by transferring surface heat and freshwater to deeper layers before the upper water column is exported to the Indian Ocean. Modulation of vertical eddy fluxes within the Indonesian Seas by fortnightly and monthly tides may act to regulate ocean-atmosphere fluxes.

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Arnold L. Gordon and Julie L. McClean

Abstract

The Indonesian Throughflow, weaving through complex topography, drawing water from near the division of the North Pacific and South Pacific water mass fields, represents a severe challenge to modeling efforts. Thermohaline observations within the Indonesian seas in August 1993 (southeast monsoon) and February 1994 (northwest monsoon) offer an opportunity to compare observations to model output for these periods. The simulation used in these comparisons is the Los Alamos Parallel Ocean Program (POP) 1/6 deg (on average) global model, forced by ECMWF wind stresses for the period 1985 through 1995. The model temperature structure shows discrepancies from the observed profiles, such as between 200 and 1200 dbar where the model temperature is as much as 3°C warmer than the observed temperature. Within the 5°–28°C temperature interval, the model salinity is excessive, often by more than 0.2. The model density, dominated by the temperature profile, is lower than the observed density between 200 and 1200 dbar, and is denser at other depths. In the model Makassar Strait, North Pacific waters are found down to about 250 dbar, in agreement with observations. The model sill depth in the Makassar Strait of 200 m, rather than the observed 550-m sill depth, shields the model Flores Sea from Makassar Strait lower thermocline water, causing the Flores lower thermocline to be dominated by salty water from the Banda Sea. In the Maluku, Seram, and Banda Seas the model thermocline is far too salty, due to excessive amounts of South Pacific water. Observations show that the bulk of the Makassar throughflow turns eastward into the Flores and Banda Seas, before exiting the Indonesian seas near Timor. In the model, South Pacific thermocline water spreads uninhibited into the Banda, Flores, and Timor Seas and ultimately into the Indian Ocean. The model throughflow transport is about 7.0 Sv (Sv ≡ 106 m3 s−1) in August 1993 and 0.6 Sv in February 1994, which is low compared to observationally based estimates. However, during the prolonged El Niño of the early 1990s the throughflow is suspected to be lower than average and, indeed, the model transports for the non–El Niño months of August 1988 and February 1989 are larger. It is likely that aspects of the model bathymetry, particularly that of the Torres Strait, which is too open to the South Pacific, and the Makassar Strait, which is too restrictive, may be the cause of the discrepancies between observations and model.

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Alberto R. Piola and Arnold L. Gordon

Abstract

A simple box model based on mass, heat and salinity conservation combined with existing estimates of ocean–atmosphere heat and freshwater exchanges is used to calculate the oceanic mean meridional volume fluxes of three water masses at 30°S. Model results lead to relatively large volume fluxes and questionable flow directions in the South Pacific Ocean. It is shown that although solutions are sensitive to changes in the mean temperature and salinity of each water mass, changes of these properties within realistic limits cannot lead to large changes in the mass fluxes or flow reversals.

The effect of changes in the ocean–atmosphere fluxes north of 30°S on the oceanic mass transports is evaluated. In the South Atlantic Ocean, reduction of the excess evaporation would significantly reduce the required volume fluxes whereas in the South Pacific Ocean, reduction of the volume fluxes and reversing the flow direction in the Antarctic Intermediate Water requires reduction of both heat and freshwater fluxes into the ocean. The role of a possible flow of Pacific Ocean waters into the Indian Ocean at equatorial latitudes, through the Southeast Asian Seas, and its effects on the Pacific and Indian Oceans' mass, heat and freshwater budgets are evaluated. Such a flow would strongly relax the requirement of southward freshwater flux at 30°S in the South Pacific Ocean.

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Arnold L. Gordon and Alberto R. Piola

Abstract

Production of North Atlantic Deep Water (NADW) transfers upper-layer thermocline water into abyssal depths. Export of NADW across 35°S in the Atlantic Ocean into the Indian and Pacific Oceans by the Antarctic Circumpolar Current (ACC) requires a compensating flow of upper-layer water from the circumpolar zone of the Southern Ocean into the Atlantic. This water, enroute to the NADW production regions, becomes saltier because evaporation exceeds precipitation and continental runoff. This process is responsible for a relatively salty Atlantic Ocean. Using estimates of the net freshwater flux, the increase of upper-layer salinity versus latitude is calculated for two NADW production rates: 15 × 106 and 20 × 106 m3 s−1. The 20 × 106 m3 s−1 production rate provides the best relationship with the linear trend in salinity as determined from hydrographic data. It is suggested that a contributing factor to the establishment of a salty Atlantic Ocean, and possibly of NADW formation, is the removal of freshwater from the Atlantic Ocean by the ACC.

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Arnold L. Gordon and Worth D. Nowlin Jr.

Abstract

Hydrographic data were obtained within the Bransfield Strait and adjacent waters during February and March 1975 by R/V Conrad and R/V Melville as part of FDRAKE 75. Within the Strait the circumpolar Deep Water is either missing or its influence is weak. The salinity maximum, oxygen minimum and silicate maximum present in the upper layers of the Strait attenuate toward the east, demonstrating the eastward decrease of Bellingshausen Sea influence. The Strait contains three basins separated from one another by sills less than 1500 m deep and from adjacent ocean areas by depth near or less than 500 m, except for a channel to the northeast of slightly over 1100 m depth. The deep and bottom waters of these basins, with depths to nearly 2600 m, are significantly colder, less saline, higher in oxygen and lower in nutrient concentrations than the deep exterior water adjacent to the Strait. These characteristics confirm Clowes' (1934) contention that the waters of these basins are renewed by local convection. Supportive evidence for post-bomb renewal is provided by tritium measurements from the easternmost basin of the Strait. Bottom (2566 m) tritum values are essentially the same as surface values, which are greater than expected for subsurface water which has not recently been in contact with the surface waters. Comparison of T-S relations suggests that one mixing component of near-surface water in the convective renewal of Bransfield bottom water is the same as that involved in Weddell Sea bottom water formation. The FDRAKE data set shows that the character of the deep and bottom waters is different within each of the three major basins, suggesting significant spatial (or temporal) variability of the convective events occurring in the Strait. Water mass distributions of the southern Drake Passage and the Weddell Sea are apparently not influenced by outflow of Bransfield basin water. Likewise, there seems to be no direct outflow of deep or bottom waters from the Bransfield basins into the Weddell-Scotia confluence Zone.

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Qian Song, Arnold L. Gordon, and Martin Visbeck

Abstract

The Indonesian Throughflow (ITF) spreading pathways and time scales in the Indian Ocean are investigated using both observational data and two numerical tracer experiments, one being a three-dimensional Lagrangian trajectory experiment and the other a transit-time probability density function (PDF) tracer experiment, in an ocean general circulation model. The model climatology is in agreement with observations and other model results except that speeds of boundary currents are lower. Upon reaching the western boundary within the South Equatorial Current (SEC), the trajectories of the ITF tracers within the thermocline exhibit bifurcation. The Lagrangian trajectory experiment shows that at the western boundary about 38%±5% thermocline ITF water flows southward to join the Agulhas Current, consequently exiting the Indian Ocean, and the rest, about 62%±5%, flows northward to the north of SEC. In boreal summer, ITF water penetrates into the Northern Hemisphere within the Somali Current. The primary spreading pathway of the thermocline ITF water north of SEC is upwelling to the surface layer with subsequent advection southward within the surface Ekman layer toward the southern Indian Ocean subtropics. There it is subducted and advected northward in the upper thermocline to rejoin the SEC. Both the observations and the trajectory experiment suggest that the upwelling occurs mainly along the coast of Somalia during boreal summer and in the open ocean within a cyclonic gyre in the Tropics south of the equator throughout the year. All the ITF water eventually exits the Indian Ocean along the western boundary within the Mozambique Channel and the east coast of Madagascar and, farther south, the Agulhas Current region. The advective spreading time scales, represented by the elapsed time corresponding to the maximum of transit- time PDF, show that in the upper thermocline the ITF crosses the Indian Ocean, from the Makassar Strait to the east coast of the African continent, on a time scale of about 10 yr and reaches the Arabian Sea on a time scale of over 20 yr.

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