Dean Roemmich

Climatic Warming and the Decline of Zooplankton in the California Current

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�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.

Two transatlantic sections: meridional circulation and heat flux in the subtropical North Atlantic Ocean

Abstract 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 σ 2 = 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 × 10 15 W across 24°N and 0.8 × 10 15 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.

Decadal Spinup of the South Pacific Subtropical Gyre

Abstract An increase in the circulation of the South Pacific Ocean subtropical gyre, extending from the sea surface to middepth, is observed over 12 years. Datasets used to quantify the decadal gyre spinup include satellite altimetric height, the World Ocean Circulation Experiment (WOCE) hydrographic and float survey of the South Pacific, a repeated hydrographic transect along 170°W, and profiling float data from the global Argo array. The signal in sea surface height is a 12-cm increase between 1993 and 2004, on large spatial scale centered at about 40°S, 170°W. The subsurface datasets show that this signal is predominantly due to density variations in the water column, that is, to deepening of isopycnal surfaces, extending to depths of at least 1800 m. The maximum increase in dynamic height is collocated with the deep center of the subtropical gyre, and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre, by at least 20% at 1000 m. A comparison of WOCE and...

Ocean heat transport across 24°N in the Pacific

Ocean heat transport across 24°N in the North Pacific is estimated to be 0.76 × 1015 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 × 1015 W northward and 1.17 × 1015 southward. The error in this new direct estimate of Pacific heat transport is approximately 0.3 × 1015 W. In addition, it is suggested that the annual variation in poleward heat transport across 24°N in the Pacific is of order 0.2 × 1015 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°N so that the total ocean heat transport across 24°N is estimated to be 2.0 × 1015 W northward. This ocean heat transport is larger than the northward atmospheric energy transport across 24°N of 1.7 × 1015 W. The ocean and atmospheric together transport 3.7 × 1015 W of heat across 24°N, which is in reasonable agreement with classic values of 4.0 × 1015 W derived from consideration of the Earths radiation budget but which is markedly less than the 5.3 × 1015 W required by recent satellite radiation budget determinations.

Argo profiling floats bring new era of in situ ocean observations

The Argo profiling float project will enable, for the first time, continuous global observations of the temperature, salinity, and velocity of the upper ocean in near-real time.This new capability will improve our understanding of the oceans role in climate, as well as spawn an enormous range of valuable ocean applications. Because over 90% of the observed increase in heat content of the air/land/sea climate system over the past 50 years occurred in the ocean [Leuitus et al., 2001], Argo will effectively monitor the pulse of the global heat balance.The end of 2003 was marked by two significant events for Argo. In mid-November 2003, over 200 scientists from 22 countries met at Argos first science workshop to discuss early results from the floats. Two weeks later, Argo had 1000 profiling floats—one-third of the target total—delivering data. As of 7 May that total was 1171.

Eddy Transport of Heat and Thermocline Waters in the North Pacific: A Key to Interannual/Decadal Climate Variability?

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. ...

Seasonal evolution of upper ocean thermal structure between Tasmania and Antarctica

We describe the upper ocean thermal structure between Tasmania and Antarctica based on thirteen repeat temeprature 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°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° and 5°C isotherms. The northern part of the SAF is located between 50°S and 51°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°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°S and 53°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°S and 60°S and is constant through the 1993–1994 austral summer to within the accuracy of the method (rms error ≈ 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°C between December and March south of 54°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.

Optimal Estimation of Hydrographic Station Data and Derived Fields

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.

The mean and variability of ocean circulation past northern New Zealand: Determining the representativeness of hydrographic climatologies

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 Cape. 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.

Large scale circulation of the North Pacific Ocean

Abstract 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 × 10 14 W and modest heat gain elsewhere. Heat transport across 24°N is estimated to be 7.5 × 10 14 W. The subpolar ocean has a large excess of precipitation and runoff over evaporation, about 5.6 × 10 5 m 3 s −3 north of 35°N, while in the subtropics there is excess evaporation, about 2.7 × 10 5 m 3 s −1 between 24°N and 35°N.