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Dean Roemmich

Abstract

From November 1982 until March 1983, an experiment was conducted at Jarvis Island (0.4°S, 160°W) in order to study the energetics of swift currents encountering a small equatorial island, and to determine the relationship between the free stream zonal velocity and the pressure drop from the upstream stagnation point to the island wake. Vertical profiles of velocity and temperature show a conversion of kinetic to potential energy as water in the eastward flowing Equatorial Undercurrent approaches the upstream stagnation point. An energy deficit is observed in the wake region, and the energy drop from the upstream stagnation point to the downstream end of the island amounts to about 1.3 times the free stream kinetic energy. The measurements are consistent with laboratory studies of high Reynolds number flow and with a previous density survey at Jarvis Island.

Temperature–pressure recorders were set on the east and west sides of the island at nominal depths of 10 and 150 m. West-to-east pressure differences were converted to time series of free stream velocity using calibration data from the velocity-temperature profiles. These indirectly measured velocities agree reasonably well with a set of direct measurements made by E. Firing at 159°W on the equator. They show the relaxation of the anomalous oceanographic conditions of the 1982–83 El Niño event, including the reappearance of the South Equatorial Current and the Equatorial Undercurrent.

Pairs of sea level stations at small equatorial islands are suggested as a practical means of obtaining long time series of zonal velocity and also to provide improved estimates of open ocean sea level.

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Dean Roemmich

Abstract

Optimal estimation is applied to contouring and analysis of hydrographic sections. Measured fields, such as temperature and salinity, and derived fields, such as geostrophic velocity, are decomposed into large-scale and small-scale components. First, a heavily sampled basin-scale field is estimated and subtracted from the data. A small-scale field, barely resolved by the hydrography, is then found from the residuals. The power and utility of the technique are illustrated by means of examples, using a short section from the Straits of Florida and a transatlantic section along 24°N.

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Dean Roemmich

Abstract

Hydrographic sections spanning the Atlantic Ocean at 24, 36 and 48°N are used to make an estimate of meridional heat flux in the ocean. An inverse method provides reference level velocities for geostrophic calculations, consistent with assumptions of conservation of mass and salt in a multilayered ocean. The heat-flux calculation is made on the total geostrophic velocity together with observed temperature.

It is found that the dominant mechanism for heat transport in the North Atlantic is a meridional cell of northward flowing surface water balanced by deep southward flow. The strength of the meridional cell is determined best by the data at 24°N. This is attributed to higher information content and lower noise, from topographic roughness, in the southern transect. An ageostrophic correction to the heat flux is estimated, and the resulting total northward heat flux is 120×1013 W at 24°N and about 80×1013 W at 36°N. The heat flux was poorly determined at 48°N. It is concluded that the technique could be used to combine hydrographic data with other relevant measurements, such as air-sea heat exchange, to construct a heat budget for the world oceans.

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Dean Roemmich

Abstract

Hydrographic station data from 24°N, 8°N, 8°S and 24°S in the Atlantic Ocean are used to calculate geostrophic transport in eight layers separated by isopycnal surfaces. In the upper ocean, the geostrophic transport is strongly northward across 8°S and strongly southward across 8°N resulting in a geostrophic convergence of ∼25 × 106 m2 s−1 in water of density less than ρ = 26.8. This is equal to the magnitude of the Ekman divergence calculated from observed wind. Similarly, geostrophic divergences of surface layers between 8°N and 24°N and between 8°S and 24°S are balanced by estimated Ekman convergences in those areas. The net upper-ocean transport across each latitude, given by the sum of Ekman transport plus upper ocean geostrophic transport, is ∼10 × 106 m3 s−1 northward. This transport is a component of the large-scale meridional cell which carries surface water and heat northward in both hemispheres of the Atlantic, with a return flow of cold water at depth.

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Dean Roemmich and John Gilson

Abstract

High-resolution XBT transects in the North Pacific Ocean, at an average latitude of 22°N, are analyzed together with TOPEX/Poseidon altimetric data to determine the structure and transport characteristics of the mesoscale eddy field. Based on anomalies in dynamic height, 410 eddies are identified in 30 transects from 1991 to 1999, including eddies seen in multiple transects over a year or longer. Their wavelength is typically 500 km, with peak-to-trough temperature difference of 2.2°C in the center of the thermocline. The features slant westward with decreasing depth, by 0.8° of longitude on average from 400 m up to the sea surface. This tilt produces a depth-varying velocity/temperature correlation and hence a vertical meridional overturning circulation. In the mean, 3.9 Sv (Sv ≡ 106 m3 s−1) of thermocline waters are carried southward by the eddy field over the width of the basin, balanced mainly by northward flow in the surface layer. Corresponding northward heat transport is 0.086 ± 0.012 pW. The eddy field has considerable variability on seasonal to interannual timescales. For the 8-yr period studied here, eddy variability was the dominant mechanism for interannual change in the equatorward transport of thermocline waters, suggesting a potentially important forcing mechanism in the coupled air–sea climate system.

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Dean Roemmich and Bruce Cornuelle

Abstract

Seasonal and interannual variability of the subtropical gyre in the South Pacific Ocean are investigated by means of a time series of expendable bathythermograph (XBT) sections between New Zealand (36°S, 175°E) and Fiji (18°S, 178°E). The experiment spans much of the subtropical gyre and is a prototype for future basin-scale observations. Eddy-resolving transects along the precisely repeating ship track, spanning four years, are used to estimate the mean field and fluctuations of temperature and geostrophic velocity. The mean field dominates on very large spatial scales while the fluctuations dominate on small scales. Mean and fluctuations have equal energy at a horizontal wavelength of about 2000 km. The study region contains three recurring small-scale features. These are the East Auckland Current, flowing eastward along the New Zealand continental slope, a front at about 29°S which is likely an extension of the Tasman Front, and a weaker feature, the Tropical Convergence at about 22°S.

At lower latitudes in the study region, the entire thermocline migrates vertically at annual period. This annual oscillation ends near the front at 29°S. Farther poleward, the only substantial subsurface annual variation is in the strength of the East Auckland Current. Interannual variability of circulation during 1986–90 consisted of rapid transitions between two rather steady states. In one state, which persisted through 1987–88 and from mid-1989 to the present (mid-1990), the eastward flowing limb of the gyre was relatively strong and narrow, with a reversal in velocity at the ocean surface south of Fiji.

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Carl Wunsch and Dean Roemmich

Abstract

Evidence for the widespread assumption that Sverdrup balance describes the dynamics of the North Atlantic subtropical gyre is reviewed critically. If the balance were to hold up to the edge of the Gulf Stream system, then there is a serious conflict with existing estimates of the net meridional flux of heat. If a recirculation region near the Stream is excluded, then one loses numerical agreement between a geostrophic calculation of the interior mass flux and the opposite Gulf Stream transport. Our conclusion is that while the linear vorticity balance may well apply over much of the North Atlantic subtropical gyre, there is little evidence supporting the special version called Sverdrup balance even in the relatively quiescent region east of the Mid-Atlantic Ridge. To the contrary, simple order of magnitude estimates suggest that it is just as likely that the flows are dominated by bottom-induced vertical velocities as they are directly driven by the wind stress curl.

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Dean Roemmich and Bruce Cornuelle

Abstract

The subtropical mode waters (STMW) of the southwestern Pacific Ocean are described, including their physical characteristics, spatial distribution, and temporal variability. STMW is a thermostad, or minimum in stratification, having temperatures of about 15°–19°C and vertical temperature gradient less than about 2°C per 100 m. Typical salinity is 35.5 psu at 16.5°C. The STMW layer is formed by deep mixing and cooling in the eastward-flowing waters of the separated East Australia Current. Surface mixed layers are observed as deep as 300 m north of New Zealand in winter, in the center of a recurring anticyclonic eddy. The STMW thermostad in the South Pacific is considerably weaker than its counterparts in the North Atlantic and North Pacific, a contrast that may help to discriminate between physical processes contributing to its formation.

A quarterly time series of expendable bathythermograph transects between New Zealand and Fiji is used to study the temporal variability of STMW. Large fluctuations are observed at both annual and subannual periods. Based on the quarterly census of STMW volume, the lifetime of the thermostad is estimated to be of order 1 year. During the years 1986–91 wintertime sea surface and air temperature minima warmed by about 1.5°C. The volume of STMW decreased dramatically during that period, with the 1989–91 census showing only a small fraction of the 1986–87 STMW volume. The observed fluctuations may be due either to long-period change in air–sea heat exchange or to fluctuations in heat transport by ocean currents.

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Basil Stanton, Dean Roemmich, and Michael Kosro
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Xuebin Zhang, Bruce Cornuelle, and Dean Roemmich

Abstract

The bifurcation of the North Equatorial Current (NEC) plays an important role in the heat and water mass exchanges between the tropical and subtropical gyres in the Pacific Ocean. The variability of western boundary transport (WBT) east of the Philippine coast at the mean NEC bifurcation latitude (12°N) is examined here. A tropical Pacific regional model is set up based on the Massachusetts Institute of Technology general circulation model and its adjoint, which calculates the sensitivities of a defined meridional transport to atmospheric forcing fields and ocean state going backward in time. The adjoint-derived sensitivity of the WBT at the mean NEC bifurcation latitude to surface wind stress is dominated by curl-like patterns that are located farther eastward and southward with increasing time lag. The temporal evolution of the adjoint sensitivity of the WBT to wind stress resembles wind-forced Rossby wave dynamics but propagating with speeds determined by the background stratification and current, suggesting that wind-forced Rossby waves are the underlying mechanism. Interannual-to-decadal variations of the WBT can be hindcast well by multiplying the adjoint sensitivity and the time-lagged wind stress over the whole model domain and summing over time lags. The analysis agrees with previous findings that surface wind stress (especially zonal wind stress in the western subtropical Pacific) largely determines the WBT east of the Philippines, and with a time lag based on Rossby wave propagation. This adjoint sensitivity study quantifies the contribution of wind stress at all latitudes and longitudes and provides a novel perspective to understand the relationship between the WBT and wind forcing over the Pacific Ocean.

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