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2001
Nowlin, WD, Briscoe M, Smith N, McPhaden MJ, Roemmich D, Chapman P, Grassle JF.  2001.  Evolution of a sustained ocean observing system. Bulletin of the American Meteorological Society. 82:1369-1376.   10.1175/1520-0477(2001)082<1369:eoasoo>2.3.co;2   AbstractWebsite

The Global Ocean Observing System (GOOS) was initiated in the early 1990s with sponsorship by the Intergovemmental Oceanographic Commission, the International Council for Science, the United Nations Environment Programme, and the World Meteorological Organization. Its objective is to design and assist with the implementation of a sustained, integrated, multidisciplinary ocean observing system focused on the production and delivery of data and products to a wide variety of users. The initial design for the GOOS is nearing completion, and implementation has begun. The initial task in developing a sustained observing system is to identify the requirements of users for sustained data and products. Once such needs are known, the next task is to examine observing system elements that already exist; many necessary elements will be found to exist. The next tasks are to identify and integrate the useful elements into an efficient and effective system, while removing the unneeded elements, and to develop and implement effective data management activities. Moreover, the system must be augmented with new elements because some requirements cannot be met with existing elements and because of technological advances. Our key objective is to discuss the mechanism whereby new candidate observing system elements are transformed from development status into elements of the sustained system. Candidate systems normally will pass through many different phases on the path from idea and concept to a mature, robust technique. These stages are discussed and examples are given: 1. Development of an observational/analysis technique within the ocean community. 2. Community acceptance of the methodology gained through experience within pilot projects to demonstrate the utility of the methods and data. 3. Pre-operational use of the methods and data by researchers, application groups, and other end users, to ensure proper integration within the global system and to ensure that the intended augmentation (and perhaps phased withdrawal of an old technique) does not have any negative impact on the integrity of the GOOS data set and its dependent products. 4. Incorporation of the methods and data into an operational framework with sustained support and sustained use to meet societal objectives.

Roemmich, D, Gilson J.  2001.  Eddy transport of heat and thermocline waters in the North Pacific: A key to interannual/decadal climate variability? Journal of Physical Oceanography. 31:675-687.   10.1175/1520-0485(2001)031<0675:etohat>2.0.co;2   AbstractWebsite

High-resolution XBT transects in the North Pacific Ocean, at an average latitude of 22 degreesN, 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 degreesC in the center of the thermocline. The features slant westward with decreasing depth, by 0.8 degrees 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 equivalent to 10(6) m(3) 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.

Stanton, B, Roemmich D, Kosro M.  2001.  A shallow zonal jet south of Fiji. Journal of Physical Oceanography. 31:3127-3130.   10.1175/1520-0485(2001)031<3127:aszjso>2.0.co;2   AbstractWebsite
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2000
McCarthy, MC, Talley LD, Roemmich D.  2000.  Seasonal to interannual variability from expendable bathythermograph and TOPEX/Poseidon altimeter data in the South Pacific subtropical gyre. Journal of Geophysical Research-Oceans. 105:19535-19550.   10.1029/2000jc900056   AbstractWebsite

Estimates of dynamic height anomalies from expendable bathythermograph (XBT) and TOPEX/Poseidon (T/P) sea surface height (SSH) measurements were compared along a, transect at similar to 30 degrees S in the South Pacific. T/P SSH anomalies were calculated relative to a 5 year time mean. XBT dynamic height was calculated relative to 750 m using measured temperature and an objectively mapped climatological temperature-salinity relationship. The anomaly was obtained by subtracting out an objectively-mapped climatological dynamic height relative to 750 m. XBT temperature sections show evidence of a double-gyre structure, related to changes in shallow isopycnals near the gyre's center. XBT dynamic height and T/P SSH anomalies compare well with an RMS difference of 3.8 cm and a coherence above 0.7 for scales larger than 300 km. The differences between the two measures of dynamic height yield systematic patterns. Time-varying spatial averages of the differences are found to be related to changes in Sverdrup transport, zonal surface slope differences, and the 6 degrees C isotherm depth. Higher zonally averaged altimetry SSH than zonally averaged XBT height and larger northward transport from altimetry SSH than from XBT height correspond to gyre spinup determined from Sverdrup transport changes. This implies mass storage during gyre spinup due to the phase lag between the Ekman pumping and the full baroclinic Sverdrup response. Increases in the spatially averaged differences and zonal slope differences, associated with gyre spinup, correspond to shoaling in the 6 degrees C isotherm depth, requiring deep baroclinic changes out of phase with the 6 degrees C isotherm depth changes.

Roemmich, D, Owens WB.  2000.  The Argo Project: Global ocean observations for understanding and prediction of climate variability. Oceanography. 13:45-50.   10.5670/oceanog.2000.33   Abstract

Oceanography is now engaged in the ambitious enterprise of designing and installing a global ocean observing system to provide unprecedented observation of seasonal to decadal variability (OCEANOBS99, 1999). This will enable major advances in understanding and prediction of climate along with other practical applications. The in situ backbone of the global system, indeed the only element that will produce a global subsurface dataset, is the Argo array of profiling floats (Argo Science Team, 1998, 1999a, 1999b).

1999
Sprintall, J, Roemmich D.  1999.  Characterizing the structure of the surface layer in the Pacific Ocean. Journal of Geophysical Research-Oceans. 104:23297-23311.   10.1029/1999jc900179   AbstractWebsite

The structure and statistics of the oceanic surface layer are characterized using quarterly surveyed, eddy-resolving expendable bathythermograph (XBT) and expendable conductivity-temperature-depth (XCTD) data collected along a number of routes spanning the Pacific Ocean. This data set consists of more than 18,000 temperature casts to 800 mi with station spacing of 10 to 40 km along transects between Auckland and Seattle (beginning in 1986), San Francisco and Taiwan (1991), Auckland and Valparaiso (1993), and Honolulu and Valdez (1993). The surface layer can assume many different shapes. It can include strongly stratified layers, actively mixed layers. salinity barrier layers, fossil mixed layers, and inversions. The spatial and temporal distribution of these features within the XBT and XCTD profiles is examined. Fossil layers are predominantly a springtime feature and are associated with regions of Subtropical Mode Water formation in the southwest Pacific (near New Zealand) and the northeast Pacific (near San Francisco). Inversions are less seasonally dependent and most commonly related to interleaving of different water masses in the high shear regions of the California Transition Zone and its counterpart eastern boundary systems in the far northeast and southeast Pacific and in the tropical zonal current system. The XCTD casts show a rich and varied surface layer structure in the equatorial and subpolar regions of the pacific Ocean chat is strongly influenced by the salinity stratification. This highlights the need for complementary salinity and density information in these areas to accurately categorize the true nature of the active mixed layer.

Roemmich, DH, The ARGO Science Team.  1999.  Argo, the global array of profiling floats. OCEANOBS99 : International Conference on the Ocean Observing System for Climate, 18 - 22 October 1999, Saint-Raphael, France. ( Centre national d'Ètudes spatiales (Ocean Observations Panel for Climate) , Ed.). Abstract
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1998
Chiswell, SM, Roemmich D.  1998.  The East Cape Current and two eddies: a mechanism for larval retention? New Zealand Journal of Marine and Freshwater Research. 32:385-397. AbstractWebsite

Current meters deployed near East Cape, New Zealand, for 1 year show large differences between the offshore flow and that inshore of the 1000 m isobath. Offshore, mean flows are to the south-east, and are the East Cape Current. Inshore, mean flows are to the north-east, indicating that there may be a persistent inshore counter current. Comparison between the offshore current meter measurements and geostrophic currents inferred from TOPEX/Poseidon altimetry shows good agreement. We use this agreement to argue that passive drifter trajectories can be modelled using the altimeter data. By simulating drifter releases into the region, and modelling their trajectories, we develop a statistical picture of likely retention times for passive drifters. Drifters can get retained in one or other of two permanent eddies: the East Cape and Wairarapa Eddies, and retention within the system can be as high as 2-3 years, but depends on release time and location. If weak-swimming larvae such as rock lobster larvae behave as passive drifters, retention and recirculation within the eddies may provide a mechanism allowing them to survive within the system long enough to recruit as juvenile lobsters.

Gilson, J, Roemmich D, Cornuelle B, Fu LL.  1998.  Relationship of TOPEX/Poseidon altimetric height to steric height and circulation in the North Pacific. Journal of Geophysical Research-Oceans. 103:27947-27965.   10.1029/98jc01680   AbstractWebsite

TOPEX/Poseidon altimetric height is compared with 20 transpacific eddy-resolving realizations of steric height. The latter are calculated from temperature (expendable bathythermograph (XBT)) and salinity (expendable conductivity and temperature profiler (XCTD)) profiles along a precisely repeating ship track over a period of 5 years. The overall difference between steric height and altimetric height is 5.2 cm RMS. On long wavelengths (lambda < 500 km), the 3.5 cm RMS difference is due mainly to altimetric measurement errors but also has a component from steric variability deeper than the 800 m limit of the XBT. The data sets are very coherent in the long wavelength band, with coherence amplitude of 0.89. This band contains 65% of the total variance in steric height. On short wavelengths (lambda > 500 km), containing 17% of the steric height variance, the 3.0 cm RMS difference and lowered coherence are due to the sparse distribution of altimeter ground tracks along the XBT section. The 2.4 cm RMS difference in the basin-wide spatial mean appears to be due to fluctuations in bottom pressure. Differences between steric height and altimetric height increase near the western boundary, but data variance increases even more, and so the signal-to-noise ratio is highest in the western quarter of the transect. Basin-wide integrals of surface geostrophic transport from steric height and altimetric height are in reasonable agreement. The 1.9 x 10(4) m(2) s(-1) RMS difference is mainly because the interpolated altimetric height lacks spatial resolution across the narrow western boundary current. A linear regression is used to demonstrate the estimation of subsurface temperature from altimetric data. Errors diminish from 0.8 degrees C at 200 m to 0.3 degrees C at 400 m. Geostrophic volume transport, 0-800 m, shows agreement that is similar to surface transport, with 4.8 Sverdrup (Sv) (10(6) m(3) s(-1)) RMS difference. The combination of altimetric height with subsurface temperature and salinity profiling is a powerful tool for observing variability in circulation and transport of the upper ocean. The continuing need for appropriate subsurface data for verification and for statistical estimation is emphasized. This includes salinity measurements, which significantly reduce errors in specific volume and steric height.

Roemmich, D, Sutton P.  1998.  The mean and variability of ocean circulation past northern New Zealand: Determining the representativeness of hydrographic climatologies. Journal of Geophysical Research-Oceans. 103:13041-13054.   10.1029/98jc00583   AbstractWebsite

Eastward flow in the Tasman Sea, from the separated East Australia Current, reattaches to the shelf break near North Cape, New Zealand, and then continues alongshore to the southeast as the East Auckland Current. A series of three permanent warm core eddies occurs along the offshore side. The mean transport of the East Auckland Current is about 9 Sv, with an additional 10 Sv or more of circulation in the eddies. An extensive hydrographic data set, archived broad scale expendable bathythermograph (XBT) data, two repeating high-resolution XBT transects, neutrally buoyant float trajectories, and TOPEX altimetric data are used to estimate the temperature and absolute flow fields and to characterize variability. The aim is to examine the usefulness of time series information and absolute velocity measurements in the interpretation of hydrographic Snapshots and climatologies, as well as to describe a region having intrinsic oceanographic interest and complexity. Issues of representativeness of the hydrographic data, of the magnitude and scales of the underlying variability, of the existence of permanent fine-scale features, and of the appropriateness of deep reference levels are addressed directly. The relatively well sampled hydrographic climatology is shown to contain the equivalent of as many as 10 independent realizations. Temperature errors, relative to the true mean, are typically a few tenths of a degree. Significant seasonal bias is identified in the surface layer, and interannual bias is seen in the position of an eddy near North Caps. The dynamic height field at 1000 dbar relative to 2000 dbar is similar to estimates based on float trajectories and the assumption of geostrophic dynamics. This study underlines the value of time series data in the interpretation of a hydrographic climatology, in quantifying the errors in the estimated mean field as well as determining the magnitude and nature of variability. It also highlights the fact that the mean circulation of the oceans contains significant mesoscale structure, unnoticed in coarsely smoothed climatologies.

Hautala, SL, Roemmich DH.  1998.  Subtropical mode water in the Northeast Pacific Basin. Journal of Geophysical Research-Oceans. 103:13055-13066.   10.1029/98jc01015   AbstractWebsite

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 sigma(theta). 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 degrees-30 degrees N, 135 degrees-140 degrees 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.

Morris, MY, Roemmich DH, Meyers G, Weisberg R.  1998.  Upper ocean heat and freshwater advection in the western Pacific Ocean. Journal of Geophysical Research-Oceans. 103:13023-13039.   10.1029/98jc00195   AbstractWebsite

Mean net surface heating of the Pacific Ocean shows areas of high heat gain in the eastern tropics and heat loss in the western midlatitudes. For steady state conditions, westward and poleward transport of heat is implied. Mean geostrophic and Ekman fluxes of heat and freshwater through the sides of a western tropical Pacific box bounded by World Ocean Circulation Experiment (WOCE) high-resolution expendable bathythermograph (XBT) transects are presented. Water mass properties of currents transporting heat and freshwater through the enclosed region are seen to be modified in transit. Net transport convergences within the volume of water extending to 800 m in the vertical show a shallow ocean circulation cell (sigma(theta) < 26) with relatively dense subtropical gyre water entering the volume and lighter water leaving in western boundary currents, eastward flowing tropical currents, Ekman flow, and outflow to the Indian ocean. Surface buoyancy fluxes are dominated by freshwater rather than heat fluxes. Freshwater gain through the surface allows warm, salty water to enter in the same density range as cool, fresh water leaving the volume. For steady state conditions the XBT-derived oceanic heat convergence of 0 +/- 0.3 PW implies negligible heat loss to the atmosphere. Traditional surface heat flux climatologies predict too much heat gain. Part of the discrepancy may be due to anomolous meteorological conditions during the XBT experiment (1987-1995). Oceanic freshwater divergence of 0.70 +/- 0.08 Sv falls in the range of climatological predictions from surface freshwater gain.

1997
Rintoul, SR, Donguy JR, Roemmich DH.  1997.  Seasonal evolution of upper ocean thermal structure between Tasmania and Antarctica. Deep-Sea Research Part I-Oceanographic Research Papers. 44:1185-1202.   10.1016/s0967-0637(96)00125-2   AbstractWebsite

We describe the upper ocean thermal structure between Tasmania and Antarctica based on thirteen repeat temperature sections occupied between 1991 and 1994. The sections cross three main fronts. The subtropical front is found between Tasmania and the South Tasman Rise in each of the sections. The subantarctic front (SAF) is composed of two parts, which have distinct thermohaline signatures and behave somewhat independently: the northern part, associated with the 6-8 degrees C isotherms, is characterised by large meridional gradients of both temperature and salinity; the southern part is associated with a weaker meridional temperature gradient and negligible salinity gradient between the 3 degrees and 5 degrees C isotherms. The northern part of the SAF is located between 50 degrees S and 51 degrees S in each of the sections, but the position of the southern part of the SAF is more variable with time. A cold core eddy or meander is found north of the SAF throughout the 1993-1994 austral summer. The polar front (PF) is found near 53 degrees S in all sections. Dynamic height is estimated for each of the XBT sections by exploiting the tight correlation in this region between vertically-integrated temperature and dynamic height. Dynamic height decreases relatively smoothly with latitude between 50 degrees S and 53 degrees S, so that the SAF, PF and the water between the two fronts forms a broad belt of eastward flow relative to a deeper level. The difference in dynamic height at the sea surface relative to 2000 m is 1.03 dyn m between 47 degrees S and 60 degrees S and is constant through the 1993-1994 austral summer to within the accuracy of the method (rms error approximate to 0.07 dyn m). The dynamic height expression of the cold core eddy reaches a maximum of 0.23 dyn m in February 1994. The upper 100 m of the water column warms by about 1.6 degrees C between December and March south of 54 degrees S, corresponding to an average warming rate of 95 W m(-2). Changes in heat content at other latitudes are dominated by meridional shifts of the fronts, and no clear seasonal trend can be identified. (C) 1997 Elsevier Science Ltd.

1996
Morris, M, Roemmich D, Cornuelle B.  1996.  Observations of variability in the South Pacific subtropical gyre. Journal of Physical Oceanography. 26:2359-2380.   10.1175/1520-0485(1996)026<2359:oovits>2.0.co;2   AbstractWebsite

Variability of the subtropical gyre in the South Pacific Ocean was investigated using high-resolution expendable bathythermograph sections along a repeated track between New Zealand and Hawaii. The southern part of the section sampled most of the zonal flow in the subtropical gyre with the eastward flowing branch between New Zealand and Fiji and the westward branch extending north of Fiji to approximately 10 degrees S. The time series began in September 1987 and extended through 1994, averaging four cruises every year. The geostrophic shear field was calculated, relative to 800 m, with the aid of a mean T-S relationship. Variability was present at a broad range of spatial and temporal scales but annual fluctuations were particularly prominent. The authors conclude that 30 snapshots of temperature, measured over a period of seven years, are sufficient to resolve the annual cycle of the gyre scale circulation along the transect. The shape and intensity of the gyre varied seasonally throughout the water column (0-800 m). Geostrophic transport was most intense (15 Sv, where Sv=10(6)m(3)s(-1)) in November. At this time, the northern edges of eastward dow at the surface and in the thermocline were closest together and the ratio of thermocline to surface transport was highest. Most intense flow occurred approximately two to three months after the basinwide seasonal peak in Ekman pumping. Transport was weakest(ll Sv) in May and was associated with an increase in the poleward slant of the gyre center with depth and a decrease in the ratio of thermocline to surface transport. Seasonal wind forcing was considered as a possible mechanism for the observed annual intensification of the gyre-scale circulation. A simple linear model of thermocline response to local changes in wind stress curl explained a significant fraction of the observed annual variability. Conservation of potential vorticity q yielded an estimate for the absolute mean how (-1 cm s(-1) at 800 m), consistent with direct measurements in the region. Interannual variability, possibly related to the El Nino-Southern Oscillation cycle, was observed. The cold event of 1988/89 appeared to be associated with relatively weak gyre-scale transport. After 1991, gyre-scale transport was more intense and a prominent change in the small-scale circulation occurred, with a shift in the alongtrack wavenumber spectral energy to higher wavenumbers.

Roemmich, D, Hautala S, Rudnick D.  1996.  Northward abyssal transport through the Samoan passage and adjacent regions. Journal of Geophysical Research-Oceans. 101:14039-14055.   10.1029/96jc00797   AbstractWebsite

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 degrees 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 degrees C. The northward transport distribution was bimodal in temperature, with a cold mode of 3.6 Sv in the range 0.65 degrees-0.70 degrees C occurring entirely in the Samoan Passage and a warm mode of 3.0 Sv in the range of 0.8 degrees-0.85 degrees 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) m(2) s(-1).

1995
Roemmich, D, McGowan J.  1995.  Climatic warming and the decline of zooplankton in the California Current. Science. 267:1324-1326.   10.1126/science.267.5202.1324   AbstractWebsite

Since 1951, the biomass of macrozooplankton in waters off southern California has decreased by 80 percent. During the same period, the surface layer warmed-by more than 1.5 degrees C in some places-and the temperature difference across the thermocline increased. Increased stratification resulted in less lifting of the thermocline by wind-driven upwelling. A shallower source of upwelled waters provided less inorganic nutrient for new biological production and hence supported a smaller zooplankton population. Continued warming could lead to further decline of zooplankton.

Sprintall, J, Roemmich D, Stanton B, Bailey R.  1995.  Regional climate variability and ocean heat transport in the southwest Pacific Ocean. Journal of Geophysical Research-Oceans. 100:15865-15871.   10.1029/95jc01664   AbstractWebsite

The winter of 1992 was the coldest on record in New Zealand since the 1940s. Temperatures in New Zealand were as much as 3 degrees C below average, with heavy rain and unusual snow. The oceanic surface layer in the southwest Pacific was also anomalously cold over the same period. A World Ocean Circulation Experiment (WOCE) hydrographic section (P14C) between Auckland and Fiji during September 1992 found cold sea surface temperatures and deep mixed layers near New Zealand when compared to 8 years of high-resolution expendable bathy thermograph (XBT) temperature measurements collected along the same transect. High nutrient and low dissolved oxygen concentrations in the surface layer indicated recent entrainment of thermocline waters. The Auckland to Fiji XBT section is ope of three WOCE high-resolution XBT survey lines in the ''Tasman Box'' region, whose boundaries are Auckland-Fiji, Fiji-Brisbane and Wellington-Sydney transects plus the Australian coast. Geostrophic shear and transport were estimated from 10 realizations of the Tasman Box during the period 1991-1993. The time series of geostrophic transport shows that following a convergence in late 1991, early in 1992 there was a substantial divergence of mass in the upper waters, equivalent to a thinning of the warm water layer. The phase of this anomalous divergence is matched to an observed amplification of the seasonal oceanic heat storage cycle in 1991-1992. The top 200-m average temperature was warmer in the 1992 summer than in 1991 or 1993, but the winter of 1992 was the coldest of the 8-year record along the Auckland-Fiji line. The divergence (thinning) of the warm water layer appears to have preconditioned the region for the exceptionally cold 1992 winter. The alternative, cool conditions from anomalous air-sea heat exchange caused by variability in the wind field, is considered unlikely as large fluctuations in heat loss are not observed in the air-sea flux data during this period. The severe weather conditions and anomalous ocean heat transport are most likely related to the prolonged El Nino-Southern Oscillation episode that began in early 1991.

1994
Roemmich, D, Morris M, Young WR, Donguy JR.  1994.  Fresh equatorial jets. Journal of Physical Oceanography. 24:540-558.   10.1175/1520-0485(1994)024<0540:fej>2.0.co;2   AbstractWebsite

A vertically sheared eastward jet in the equatorial Pacific in late 1991 and early 1992 carried relatively fresh water from the western Pacific overriding the saltier surface layer of the central region. Salinity anomalies of about - 1.0 psu were observed over a period of several months in a surface layer 50 m thick near the equator. Below this fresh layer there was a steep halocline having very little temperature stratification, so that the density changes were dominated by salinity. In December 199 1, eastward surface velocities in the fresh jet at 170-degrees-W were 100 cm s^-1 with a shear of about 40 cm s^-1 in the top 100 m; the core of the jet was about 200 km in width, centered at 1.5-degrees-S. The jet decayed and vanished over the next few months, though the surface halocline remained. A simple extension of the familiar 1 1/2-layer model can account for the initial development of the sheared eastward jet. The surface pressure gradient in this initial value problem, tending to accelerate the fluid eastward, diminishes with depth because there is a zonal salinity gradient in the initially mixed layer. The depth dependence of the pressure gradient causes the accelerating flow to be vertically sheared, resulting in a tilting over of the isohalines. The shear progressively unmixes the mixed layer. The vertically integrated part of this solution is the Yoshida jet. The depth-dependent part of the solution results from a local conversion of potential to kinetic energy as the tilting isohalines lower the center of gravity of the surface layer. For added realism, generalizations of the model include wind forcing and a meridional salinity gradient. While not discounting the conventional explanation of westerly wind stress in driving the eastward jet, it is shown that the tilting/shearing mechanism can be comparable to wind stress and is important in the production of salinity barrier layers. Fresh equatorial jets may provide a key to a better understanding of the physics of tropical ocean circulation and air-sea interaction during El Nino.

Hautala, SL, Roemmich DH, Schmitz WJ.  1994.  Is the North Pacific in Sverdrup balance along 24°N? Journal of Geophysical Research-Oceans. 99:16041-16052.   10.1029/94jc01084   AbstractWebsite

Hydrographic data from a zonal section along approximately 24-degrees-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-degrees-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-degrees-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.

1993
Cornuelle, BD, Morris MY, Roemmich DH.  1993.  An objective mapping method for estimating geostrophic velocity from hydrographic sections including the equator. Journal of Geophysical Research-Oceans. 98:18109-18118.   10.1029/93jc01729   AbstractWebsite

Objective mapping can remove the equatorial singularity from the problem of estimating geostrophic shear from noisy density measurements. The method uses the complete thermal wind relation, so it is valid uniformly on and off the equator. Errors in the thermal wind balance are due to neglected terms in the momentum balance, which are treated as noise in the inverse problem. The question of whether the geostrophic balance holds near the equator is restated as a need to estimate the size of the ageostrophic noise in the thermal wind equation. Objective mapping formalizes the assumptions about the magnitudes and scales of the geostrophic currents and about the magnitudes and scales of the ageostrophic terms and measurement errors. The uncertainty of the velocity estimates is calculated as part of the mapping and depends on the signal to noise ratio (geostrophic density signal to ageostrophic ''noise'') in the data, as well as the station spacing and the scales assumed for the geostrophic velocities. The method is used to map zonal velocity from a mean Hawaii-Tahiti Shuttle density section. These are compared with previous velocity estimates for the same dataset calculated using other techniques. By choosing appropriate scales, the objective map can duplicate previous results. New temperature data are presented from a repeating, high-resolution expendable bathythermograph section crossing the equator at about 170-degrees-W with four cruises a year between 1987-1991. There appear to be significant differences between this mean temperature and the shuttle mean temperature. Temperature is converted to density with the aid of a mean T-S relation and geostrophic velocity maps are calculated for the 4-year mean. The mean geostrophic undercurrent obtained from our sections is weaker than in the shuttle estimate and is centered slightly north of the equator. Enforcing symmetry about the equator removes the offset of the current, giving a stronger, but narrow undercurrent. The density field apparently includes significant (O(0.5 kg M-3)) large-scale ageostrophic variability which makes velocity estimates from single cruises poorly determined near the equator.

1992
Roemmich, D, Cornuelle B.  1992.  The subtropical mode waters of the South Pacific Ocean. Journal of Physical Oceanography. 22:1178-1187.   10.1175/1520-0485(1992)022<1178:tsmwot>2.0.co;2   AbstractWebsite

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-degrees-19-degrees-C and vertical temperature gradient less than about 2-degrees-C per 100 m. Typical salinity is 35.5 psu at 16.5-degrees-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-degrees-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.

Roemmich, D.  1992.  Ocean warming and sea level rise along the Southwest U.S. coast. Science. 257:373-375.   10.1126/science.257.5068.373   AbstractWebsite

Hydrographic time-series data recorded during the past 42 years in the upper 500 meters off the coast of southern California indicate that temperatures have increased by 0.8-degrees-C uniformly in the upper 1 00 meters and that temperatures have risen significantly to depths of about 300 meters. The effect of warming the surface layer of the ocean and thereby expanding the water column has been to raise sea level by 0.9 +/- 0.2 millimeter per year. Tide gauge records along the coast are coherent with steric height and show upward trends in sea level that vary from about 1 to 3 millimeters per year.

1991
Bryden, HL, Roemmich DH, Church JA.  1991.  Ocean heat transport across 24°N in the Pacific. Deep-Sea Research Part a-Oceanographic Research Papers. 38:297-324.   10.1016/0198-0149(91)90070-v   AbstractWebsite

Ocean heat transport across 24-degrees-N in the North Pacific is estimated to be 0.76 X 10(15) W northward from the 1985 transpacific hydrographic section. This northward heat transport is due half to a zonally averaged vertical meridional circulation cell and half to a horizontal circulation cell. The vertical meridional cell is a shallow one, in which the northward Ekman transport of warm surface waters returns southward only slightly deeper and colder, all within the upper 700 m of the water column. In terms of its meridional heat transport, the horizontal circulation cell is also shallow with effectively all of its northward heat transport in the upper 700 m of the water column. Previous estimates of North Pacific heat transport at subtropical latitudes had ranged between 1.14 X 10(15) W northward and 1.17 X 10(15) W southward. The error in this new direct estimate of Pacific heat transport is approximately 0.3 X 10(15) W. In addition, it is suggested that the annual variation in poleward heat transport across 24-degrees-N in the Pacific is of order 0.2 X 10(15) W, as long as the deep circulation below 1000 m exhibits little variation in water mass transport. Together, the Pacific and Atlantic transoceanic sections essentially close off the global ocean north of 24-degrees-N so that the total ocean heat transport across 24-degrees-N is estimated to be 2.0 X 10(15) W northward. This ocean heat transport is larger than the northward atmospheric energy transport across 24-degrees-N of 1.7 X 10(15) W. The ocean and atmosphere together transport 3.7 X 10(15) W of heat across 24-degrees-N, which is in reasonable agreement with classic values of 4.0 X 10(15) W derived from consideration of the Earth's radiation budget but which is markedly less than the 5.3 X 10(15) W required by recent satellite radiation budget determinations.

Chereskin, TK, Roemmich D.  1991.  A comparison of measured and wind-derived Ekman transport at 11°N in the Atlantic-Ocean. Journal of Physical Oceanography. 21:869-878.   10.1175/1520-0485(1991)021<0869:acomaw>2.0.co;2   AbstractWebsite

A comparison of measured and wind-derived ageostrophic transport is presented from a zonal transect spanning the Atlantic Ocean along 11-degrees-N. The transport per unit depth shows a striking surface maximum that decays to nearly zero at a depth of approximately 100 m. We identify this flow in the upper 100 m as the Ekman transport. The sustained values of wind stress and the penetration depth of the Ekman transport reported here are considerably greater than in previous observations, which were made in conditions of light winds. The transport of 12.0 +/- 5.5 x 10(6) m3 s-1, calculated from the difference of geostrophic shear and shear measured by an acoustic Doppler current profiler, is in good agreement with that estimated from the shipboard winds, 8.8 +/- 1.9 x 10(6) m3 s-1, and from climatology, 13.5 +/- 0.3 x 10(6) m3 s-1. Qualitatively, the horizontal distribution of the wind-driven flow was best predicted by the shipboard winds. The cumulative transport increased linearly over the western three-fourths of the basin, where the winds were large and spatially uniform, and remained constant over the eastern fourth where the easterly stress was uncharacteristically low. The mean depth of the Ekman transport extended below the mixed layer depth, which varied from 25 to 90 m. The profile of ageostrophic transport does not appear consonant with slablike behavior in the mixed layer, even when spatial variations in mixed layer depth are taken into account.

Roemmich, D, McCallister T, Swift J.  1991.  A transpacific hydrographic section along latitude 24°N: the distribution of properties in the subtropical gyre. Deep-Sea Research Part a-Oceanographic Research Papers. 38:S1-S20.   10.1016/S0198-0149(12)80002-1   AbstractWebsite

An intensively sampled transpacific hydrographic section along 24-degrees-N was completed in the spring of 1985. The data are described here in terms of the spatial distribution of properties, the distribution along isopycnal surfaces, and, where possible, the relationship of these distributions to the large-scale circulation of the Pacific Ocean. Near-surface waters of subtropical origin display a salinity maximum in mid-ocean, with lower salinity to the west due to greater rainfall and lower salinity in the east due to advection of water from the north. In the next layers down, containing waters of subpolar origin, the low salinity and high dissolved oxygen concentrations of those waters are most pronounced in the eastern ocean where the subpolar water is swept clockwise into the subtropical gyre. Differences between patterns of dissolved oxygen concentration and salinity indicate that both horizontal advection and upwelling contribute to observed distributions near the eastern boundary and that the two tracers contain independent information. In the upper kilometer, the eastern Pacific is richer in tracer signals and has steeper property gradients than the west. The deep Pacific has long been recognized to be the most uniform of the oceans. Although property gradients are small, they are significant, and it is found that on all isopycnal surfaces below the upper kilometer salinity increases and dissolved oxygen concentration decreases towards the east on basin-wide scales. These zonal gradients are weakest in the abyss, where there is a substantial net input of southern water, and strongest at mid-depth. Vertical diffusion is the likely cause of the uniformity in this pattern over so much of the deep North Pacific, with oxygen consumption in waters of greater age in the east also being a plausible contributor. With a highly sampled data set such as the 24-degrees-N transpacific section it is appropriate to ask how many stations are required to define property distributions and to estimate large-scale circulation and transport. Estimation of geostrophic transport requires high spatial resolution to detect flow near sloping topography at all depths. A 50% decimation of the 24-degrees-N station pattern yields a severe degradation in the estimation of transport.