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Roemmich, D, Cornuelle B.  1990.  Observing the fluctuations of gyre-scale ocean circulation: a study of the Subtropical South Pacific. Journal of Physical Oceanography. 20:1919-1934.   10.1175/1520-0485(1990)020<1919:otfogs>;2   AbstractWebsite

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-degrees-S, 175-degrees-E) and Fiji (18-degrees-S, 178-degrees-E). The experiment spans much of the subtropical gyre and is a protype 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-degrees-S which is likely an extension of the Tasman Front, and a weaker feature, the Tropical Convergence at about 22-degrees-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-degrees-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.

Roemmich, DH.  1990.  Sea level and the thermal variability of the ocean. Sea level change. ( National Research Council Geophysics Study Committee , Ed.).:208-217., Washington, D.C.: National Academy Press Abstract
Roemmich, D.  1989.  Mean transport of mass, heat, salt and nutrients in southern California coastal waters: implications for primary production and nutrient cycling. Deep-Sea Research Part a-Oceanographic Research Papers. 36:1359-1378.   10.1016/0198-0149(89)90088-5   AbstractWebsite

A close balance is demonstrated between geostrophic convergence, relative to 500 m, and Ekman divergence, in a control volume defined by the modern (1984–1987) CalCOFI surveys of southern California coastal waters. Both the geostrophic and Ekman calculations yield an area mean vertical velocity of about 3.5 × 10−4 cm s−1 upward at the base of the Ekman layer. Cruise-to-cruise and year-to-year variations in geostrophic transport are large and a long time series is required for a good estimate of the mean field. With the mass field balanced by geostrophic and Ekman transport, the resulting oceanic heat flux is equivalent to 68 W m−2 of heat gain by the ocean and is in good agreement with calculations of air-sea heat transfer. Agreement in the heat budget sets a limit on possible errors due to ambiguity in the reference level of the geostrophic velocity field. Mean annual and seasonal fluxes of nutrients, oxygen, chlorophyll a, and salinity are calculated, together with error bars, and are used to estimate primary production as well as rates of air-sea oxygen exchange and net water vapor loss to the atmosphere. The nitrate import, converted to equivalent carbon by means of a Redfield ratio, and including local estimates of nitrogen recycling, yields an estimate of total primary production of roughly 600 mg C m−2 d−1, consistent with direct measurements of carbon uptake. The geostrophic convergence and subsequent upwelling of subsurface nitrate-rich waters is the dominant physical mechanism for nutrient importation, with a much smaller contribution due to alongshore advection of nutrients in the surface layer. It is suggested that some additional measurements would permit the flux calculations to be made with a substantially greater degree of accuracy and that the seasonal and interannual variations of the mass transport field could be determined together with rates of biologically and physically important processes.

Roemmich, D, McCallister T.  1989.  Large scale circulation of the North Pacific Ocean. Progress in Oceanography. 22:171-204.   10.1016/0079-6611(89)90005-0   AbstractWebsite

A least squares inversion procedure is used to estimate the large scale cirulation and transport of the subtropical and subpolar North Pacific Ocean from a modern data set of long hydrographic transects. Initially a deep surface of known motion is specified using information derived from abyssal property distributions, moored current meter observations, and basin scale topographic constraints. A geostrophic solution is obtained which conserves mass while devaiting as little as possible in a least squares sense from the initial field. The sensitivity of the solution is tested with regard to changes in the initial field and to the addition of conservation constraints in layers.It is found that about 10 Sv of abyssal water flows northward across 24°N, principally between the dateline and 160°E, in the deepest part of the Northwest Pacific Basin. The flow turns westward across 152°E and then mostly northward again near the Izu-Ogasawara Ridge and the coast of Japan. It then feeds a strong deep anti-cyclonic recirculation beneath the cyclonic subpolar gyre in the Northwest Pacific Basin. The abyssal waters near the western boundary region are found to have a strong component of flow that is upward and across isopycnal surfaces. Here, the abyssal waters complete an important loop in the global thermohaline circulation, entering as bottom water from the South Pacific and returning southward in a less dense and shallower layer. Deep flow into the Northeast Pacific Basin, and circulation within that basin, appear to be weak, making it remote from the main pathway of deep water renewal.The circulation of the subtropical and subpolar gyres dominates transport in the upper layers. The subtropical gyre appears to penetrate to about 1500–2000 m on both sides of the Izu-Ogasawara Ridge, which blocks deeper flow between the Philippine Basin and the Northwest Pacific Basin. The Kuroshio is estimated to carry about 32 Sv northward in the East China Sea. Farther east, as the thermocline slopes upward toward the eastern boundary, the eastward flow is even shallower. In terms of eddy activity, three regimes are observed at 24°N. Peak-to-rough eddy fluctuations in geostrophically balanced sea level diminish from about 40 cm in the west to about 5 cm in the east. Overall, the western boudary of the ocean is about 25 cm higher than the eastern boundary in the 24°N section.Patterns of heat and freshwater flux determined in the North Pacific are in accord with those from air-sea heat flux estimates and hydrological data although the magnitudes are in some cases different. There is large heat loss in the western ocean amounting to about 9.6 × 1014 W and modest heat gain elsewhere. Heat transport across 24°N is estimated to be 7.5 × 1014 W. The subpolar ocean has a large excess of precipitation and runoff over evaporation, about 5.6 × 105 m3s−3 north of 35°N, while in the subtropics there is excess evaporation, about 2.7 × 105 m3s−1 between 24°N and 35°N.

Roemmich, D, Cornuelle B.  1987.  Digitization and calibration of the expendable bathythermograph. Deep-Sea Research Part a-Oceanographic Research Papers. 34:299-307.   10.1016/0198-0149(87)90088-4   AbstractWebsite

A study was undertaken of signal digitization and temperature calibration in expendable bathythermographs (XBT's) to learn how to minimize temperature errors in that system. An XBT digitizer was built into a PC-type microcomputer and used to calibrate 24 XBT probes at 5 temperature points, and later, to calibrate 72 probes at a single temperature. Twenty of the first set of probes were fastened rigidly in pairs and dropped in the ocean as a field test of the calibrations. Calibration of individual probes reduced the standard deviation of temperature calibration errors from around 0.05°C to <0.01°C. The calibration procedure is simple and nondestructive, so the probes can be used normally after calibration. Errors in the temperature digitizer can be held to about 0.01°C by periodic adjustment. An advantage of the PC-based digitizer is the ease with which the calibrations are accomplished and applied to the ocean tracers. Two substantial sources of transient systematic error in XBT temperatures are mentioned: an electronic transient lasting about 0.1 s occurs on entry of the probe into seawater, and a longer transient is due to the thermal mass of the XBT nose.

Roemmich, DH.  1987.  Estimates of net transport, upwelling, and heat flux in the tropical oceans from inverse methods and related geostrophic models. Further progress in equatorial oceanography : a report of the U.S. Toga Workshop on the Dynamics of the Equatorial Oceans, Honolulu HI, August 11-15, 1986. ( Katz EJ, Witte JM, Eds.)., Fort Lauderdale, Fla.: Nova University Press Abstract
Wunsch, C, Roemmich D.  1985.  Is the North Atlantic in Sverdrup balance. Journal of Physical Oceanography. 15:1876-1880.   10.1175/1520-0485(1985)015<1876:itnais>;2   AbstractWebsite

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.

Roemmich, D, Wunsch C.  1985.  Two transatlantic sections: meridional circulation and heat flux in the subtropical North Atlantic Ocean. Deep-Sea Research Part a-Oceanographic Research Papers. 32:619-664.   10.1016/0198-0149(85)90070-6   AbstractWebsite

Transatlantic hydrographic sections were obtained in mid-1981 along latitudes 24.5° and 36.25°N. The tracks nearly duplicated sections made 23 years earlier as part of the International Geophysical Year. A total of 215 stations were occupied: data from a conductivity-temperature-depth (CTD) probe and water samples for analyses of oxygen, nutrient, and other tracer concentrations were collected from the ocean surface to near bottom. The 1981 sections are described and displayed, and the circulation is compared to that of the earlier survey.Large-scale meridional velocity and basin-integrated transport are compared in the 1981 and IGY sections, using a hierarchy of geostrophic models. In the simplest model, a reference level is based on the gross thermohaline flow and consideration of water mass characteristics. The only transport constraint is that the geostrophic plus Ekman flows sum to zero. Subsequent models impose mass conservation in a set of layers and then conservation of potential vorticity in a single layer. Distributions of salinity and potential vorticity on density surfaces are examined in order to identify layers in which an advective balance of tracer distribution is plausible and where gradients are strong enough to be of practical use.It was found that the 1981 and IGY sections have similar features in their large-scale velocity fields and similar zonally averaged meridional transport. In each case, net northward transports of approximately 17 Sv of surface and intermediate water (above σ2 = 36.82) were balanced by equal southward flow in the deep water. A significant shift toward greater depth occurred in the depth distribution of the deep southward flow in the 1981 sections. The wind-driven subtropical gyre is superimposed on this thermohaline overturning, and transport calculations in the gyre interior show the Sverdrup relation to be of questionable direct applicability.Ocean heat transport in 1981 was found to be about 1.2 × 1015 W across 24°N and 0.8 × 1015 W across 36°N. These values are indistinguishable from those obtained from the IGY data and from computations of air-sea heat exchange. The steadiness of the heat transport is attributed to the invariance of the zonally averaged meridional circulation.

Roemmich, D, Wunsch C.  1984.  Apparent changes in the climatic state of the deep North Atlantic Ocean. Nature. 307:447-450.   10.1038/307447a0   AbstractWebsite

Determination of any long-term changes in the large-scale characteristics of the deep ocean circulation would be an important clue in understanding the climatic interactions of the ocean and atmosphere. In the summer of 1981, the RV Atlantis II reoccupied two transatlantic sections at nominal latitudes of 24°30′ N and 36°16′ N with a conductivity–temperature-depth instrument (CTD). One purpose of the work was to make a comparison with previous surveys conducted during the International Geophysical Year (IGY)1, when sections were obtained in October 1957 and April–May 1959. We report here that significant warming occurred in an ocean-wide band from 700 m to 3,000 m with a maximum temperature difference of 0.2 °C. These changes are sufficient to expand the water column by several centimetres. The historical temperature–salinity curve was apparently unchanged. Interannual changes in local water mass characteristics have been proposed previously2,3. Perhaps it would be most surprising if no changes were seen to occur. What remains obscure is the significance of these changes and the extent to which they represent long-term climate trends, or merely the minor and random fluctuations to be expected in any complex fluid system.

Roemmich, D.  1984.  Indirect sensing of equatorial currents by means of island pressure measurements. Journal of Physical Oceanography. 14:1458-1469.   10.1175/1520-0485(1984)014<1458:isoecb>;2   AbstractWebsite

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.

Roemmich, D.  1983.  The balance of geostrophic and Ekman transports in the tropical Atlantic Ocean. Journal of Physical Oceanography. 13:1534-1539.   10.1175/1520-0485(1983)013<1534:tbogae>;2   AbstractWebsite

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.

Roemmich, D.  1983.  Optimal estimation of hydrographic station data and derived fields. Journal of Physical Oceanography. 13:1544-1549.   10.1175/1520-0485(1983)013<1544:oeohsd>;2   AbstractWebsite

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.

Luyten, JR, Roemmich DH.  1982.  Equatorial currents at semi-annual period in the Indian Ocean. Journal of Physical Oceanography. 12:406-413.   10.1175/1520-0485(1982)012<0406:ecasap>;2   AbstractWebsite

Moored records of zonal velocity in the western equatorial Indian Ocean are dominated by motion of semi-annual period. The motion is coherent over an array spanning more than 1000 km along the equator, 160 km across the equator, and 500 m in the vertical The signal has an amplitude of 0.15 m s−1. It has zonal, vertical and meridional length scales which are long compared to the dimensions of the array, and it shows upward propagation of phase. Behavior is characteristic of an eastward propagating equatorial Kelvin wave and a westward long equatorial Rossby wave. The vertical wavelength is an estimated 5000 m, implying an equatorial trapping scale of 400 km. Zonal wavelengths are 8000 km for the equatorial Rossby wave and 24000 km for the equatorial Kelvin wave. A downward energy flux, estimated to be 3 × 1016 erg s−1, most likely represents propagation away from surface forcing by the zonal wind.

Roemmich, D, Wunsch C.  1982.  On combining satellite altimetry with hydrographic data. Journal of Marine Research. 40:605-619. AbstractWebsite

It is shown, by random construction of many sea surfaces, each consistent with geostrophy and mass conservation in the underlying ocean, that absolute sea surface topography relative to a geopotential surface can be estimated to 10 cm accuracy from appropriate in situ measurements of density. If one were to use hydrography to remove the oceanographic signal from an altimetric geoid, then 10 cm accuracy is the limit of our present capability. If, on the other hand, altimetry is to be used with a gravimetric geoid in order to improve estimates of large-scale ocean circulation, then 10 cm is the upper bound on acceptable errors for the combined altimeter and geoid system, over the spatial scales of interest. A hypothetical smoothed altimetric sea surface is used to demonstrate improvement in sea surface estimation in the combined hydrography-altimetry-geoid problem. Linear inverse theory forms the computational framework.

Roemmich, D.  1981.  Circulation of the Caribbean Sea: A well-resolved inverse problem. Journal of Geophysical Research-Oceans and Atmospheres. 86:7993-8005.   10.1029/JC086iC09p07993   AbstractWebsite

The Caribbean Sea is selected as a region where the large-scale circulation is well determined by historical hydrographic measurements through application of the inverse method. A simple example is used to illustrate the technique and to demonstrate how some physically relevant quantities may be well determined in the formally underdetermined inverse problem. The geostrophic flow field in the Caribbean is found by imposing mass and salt conservation constraints in seven layers separated by surfaces of constant potential density. An unsmoothed solution is displayed that has weak dependence on an initial choice of reference level. In addition, a unique smoothed solution is shown. Above σθ = 27.4, the total flow leaving the western Caribbean is estimated to be 29 × 106 m3 s−1, in agreement with direct measurements. This flow is made up of 22 × 106 m3 s−1 entering the Caribbean from the east and flowing across the southern half of the basin as the Caribbean Current, and 7 × 106 m3 s−1 entering from the north through Windward Passage. Both of these currents show small-scale variability that diminishes with distance from the respective passages. The deep flow has no net transport, as required by the shallow exit, but a well organized clockwise recirculation is found in the deep eastern Caribbean.

Roemmich, D.  1980.  Estimation of meridional heat flux in the North Atlantic by inverse methods. Journal of Physical Oceanography. 10:1972-1983.   10.1175/1520-0485(1980)010<1972:eomhfi>;2   AbstractWebsite

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.