Rosemary Morrow

Divergent pathways of cyclonic and anti-cyclonic ocean eddies

Satellite altimetry is used to study the propagation pathways of warm and cold ocean eddies in different ocean basins. We consider eddies that have a life span longer than 3 months, and we present three regional studies: in the southeast Indian, the southeast Atlantic, and the northeast Pacific Oceans. The case studies show that simple theories for vortex propagation on a β-plane work in regions where energetic eddies propagate though a weak background flow. Under these conditions, anticyclonic/cyclonic eddies propagate westward and equatorward/poleward. This divergence in the eddy pathways implies a net equatorward eddy heat flux, and has implications for the meridional transport of freshwater, carbon, nutrients, etc.

Surface Eddy Momentum Flux and Velocity Variances in the Southern Ocean from Geosat Altimetry

Abstract Satellite altimetry has previously been used to map the magnitude of the surface eddy variability of the global oceans, but the direction of the time-variable velocities have been more difficult to determine. Here, a technique is presented for resolving both magnitude and direction of residual surface geostrophic velocities at Geosat altimeter crossover points; providing a two-year time series with a temporal resolution of 17 days and horizontal resolution of around 100 km. The time series of residual velocity components are then used to determine surface eddy statistics in the Southern Ocean and to investigate the role of transient eddies in the Southern Ocean momentum balance. The surface eddy statistics from Geosat crossover points show a complex spatial distribution in the surface Reynolds stresses (u′2, v′2, u′v′). In contrast to the assumptions of isotropic variability in previous analyses of altimeter data, velocity variance ellipses are found to be distinctly anisotropic in many regions. ...

Source of the baroclinic waves in the southeast Indian Ocean

The dynamics of the seasonal and interannual sea level variability in the southeast Indian Ocean are investigated using a simple model of low-frequency quasi-geostrophic thermocline variability in order to determine whether the observed variability responds primarily to local or remote forcing. This region is important for climate studies, in relation to the interannual variations of the tropical oceans and atmosphere. The eastern Indian Ocean is directly forced by the strong seasonal monsoons as well as by a remote ocean forcing from the tropical Indian Ocean and by the western Pacific via the Indonesian Throughflow. As a result, the dynamics of the southeast Indian Ocean are unique, with unusually large variability and bands of energetic Rossby waves. The annual wave signal around 10°S is clearly marked and a band of propagating mesoscale variability between 20° and 35°S extends across the entire Indian Ocean, with characteristic timescales between 100 and 200 days. There is also strong interannual variability. To investigate the origin of the observed long baroclinic waves, we use a simple reduced gravity model allowing the radiation of long waves due to Ekman pumping and the radiation of long waves from the eastern boundary. Eastern boundary conditions are given by expendable bathythermograph data. In the band 10°–15°S the thermocline depth oscillation corresponds mainly to waves radiating from wind forcing in the east. Their amplitude is strongly damped west of 90°E. In the southeastern tropical Indian Ocean (STIO) the influence of free waves emanating from the eastern boundary is small but significant. In addition, a strong interannual signal appears to originate from Lombok Strait to the north and propagates southwestward into the STIO. In the band 20°–35°S the observed waves appear to be free waves generated by eastern boundary processes.

Variability in the southeast Indian Ocean from altimetry: Forcing mechanisms for the Leeuwin Current

Seasonal and interannual variability in the southeastern Indian Ocean is investigated with the aid of Topex/Poseidon (T/P) altimeter data for the 3-year period 1993–1995. The annual Rossby wave signal around 10°S is clearly marked, and consistent with modelling and Geosat results from earlier periods. A band of higher mesoscale variability between 20° and 35°S extends across the entire Indian Ocean, with characteristic timescales between 120 and 180 days and length scales of order 500 km. Sea level anomalies are shown to propagate at around 1.5 to 2 times the theoretical linear Rossby wave speed, with an associated signal in sea surface temperature (SST) anomalies, and the propagation suggests that the variability is not locally forced but originates near the eastern boundary. Altimeter data is also used to examine variations in the alongshore pressure gradient, thought to be the principal mechanism forcing the Leeuwin Current poleward against the prevailing equatorward winds. The T/P data confirms that the alongshore pressure gradient is maximum in May when the Leeuwin Current is strongest, but we find a consistent secondary peak in November which is not evident in the climatological data. There is also significant interannual variation, related to large interannual variations on the Australian Northwest Shelf. The seasonal and interannual variations also influence the thermal structure of the instabilities associated with the Leeuwin Current, which may be the source of the westward propagating anomalies between 20°S and 35°S. The results compare well with satellite SST and in situ XBT data at the eastern boundary and suggest that satellite data can be used to monitor the variability of the Leeuwin Current.

Eddy kinetic energy and momentum flux in the Southern Ocean: Comparison of a global eddy‐resolving model with altimeter, drifter, and current‐meter data

The ability of a seasonally forced high-resolution global ocean general circulation model to simulate eddy variability and associated energy and momentum transfer processes in the Southern Ocean is assessed by comparing model statistics with observations. The observations include Geosat altimeter data analyzed for surface velocity variance at satellite ground track crossover points, current-meter data from the Agulhas and Campbell plateaus, and surface drifter data in the Tasman Sea. In western boundary currents and energetic regions of the Antarctic Circumpolar Current model eddy kinetic energy is lower than observed by typically a factor of 4, and in less energetic regions by a factor of 10. Differences in the location and extent of energetic regions are related to smoothness of the model bathymetry and other features of the model configuration. Eddy momentum flux divergence and eddy to mean kinetic energy conversion at the surface are diagnosed from the model. These show regions where eddy activity accelerates the mean flow through instability processes. Observational estimates of these terms are computed using mean flow gradients from hydrography climatology and altimeter eddy statistics. Several features of the spatial distribution of the observational estimates are consistent with the model and suggest that future calculations of mean currents from altimeter data will allow direct computation of eddy to mean current momentum and energy conversion terms.

Adjoint assimilation of altimetric, surface drifter, and hydrographic data in a quasi-geostrophic model of the Azores Current

The flow characteristics in the region of the Azores Current are investigated by assimilating TOPEX/POSEIDON and ERS 1 altimeter data into the multilevel Harvard quasigeostrophic (QG) model with open boundaries (Miller et al., 1983) using an adjoint variational scheme (Moore, 1991). The study site lies in the path of the Azores Current, where a branch retroflects to the south in the vicinity of the Madeira Rise. The region was the site of an intensive field program in 1993, SEMAPHORE. We had two main aims in this adjoint assimilation project. The first was to see whether the adjoint method could be applied locally to optimize an initial guess field, derived from the continous assimilation of altimetry data using optimal interpolation (OI). The second aim was to assimilate a variety of different data sets and evaluate their importance in constraining our QG model. The adjoint assimilation of surface data was effective in optimizing the initial conditions from OI. After 20 iterations the cost function was generally reduced by 50–80%, depending on the chosen data constraints. The primary adjustment process was via the barotropic mode. Altimetry proved to be a good constraint on the variable flow field, in particular, for constraining the barotropic field. The excellent data quality of the TOPEX/POSEIDON (T/P) altimeter data provided smooth and reliable forcing; but for our mesoscale study in a region of long decorrelation times O(30 days), the spatial coverage from the combined T/P and ERS 1 data sets was more important for constraining the solution and providing stable flow at all levels. Surface drifters provided an excellent constraint on both the barotropic and baroclinic model fields. More importantly, the drifters provided a reliable measure of the mean field. Hydrographic data were also applied as a constraint; in general, hydrography provided a weak but effective constraint on the vertical Rossby modes in the model. Finally, forecasts run over a 2-month period indicate that the initial conditions optimized by the 20-day adjoint assimilation provide more stable, longer-term forecasts.

Separation of quasi-semiannual Rossby waves from the eastern boundary of the Indian Ocean

Westward propagating features, identified as semiannual Rossby waves, have been observed in the southern subtropical Indian Ocean between 20S and 35S. A previous study based on ERS satellite scatterometer wind stress data in this region has shown that the Rossby waves were not forced by local Ekman pumping in the ocean interior nor by local variations in wind stress near the Australian coast. In the present study, we investigate a third possible source, in the form of remotely forced semiannual coastal waves propagating along the Australian coast. Altimetric and tide gauge data show consistent semiannual anomalies along the northwestern and western coast, with a phase lag between the northernmost station and about 25S, producing a poleward phase speed of ∼0.4 m s -1 . South of the critical latitude for semiannual Rossby wave radiation at 27S, we still observe a significant sea level signal along the coast at the semiannual frequency that appears nearly in phase between the stations. Semiannual sea level anomalies lead the semiannual wind forcing by about one month, confirming that the signal is not locally forced. These semiannual coastal variations are in phase with the offshore radiation of Rossby waves observed between 20S and 35S. We suggest that, south of the critical latitude of 27S, the observed coastal wave signal could interact with the poleward coastal Leeuwin Current and that this wave-current interaction could stimulate nonlinear instabilities of this density-driven current and may be a key mechanism for the Rossby wave radiation in the subtropical Indian Ocean.

Seasonal and interannual variations of the upper ocean energetics between Tasmania and Antarctica

Nine years of Topex/Poseidon and ERS satellite altimetry and XBT data from the SURVOSTRAL program were used to analyze the seasonal and interannual variations of the eddy energetics in terms of its spatial distribution and relation with the upper ocean heat content. Eddy kinetic energy is calculated in two frequency bands one associated with transient and the other with low-frequency variability. The two eddy components have distinct geographical distribution. At the SURVOSTRAL line, the transient eddy energy is twice the low-frequency energy, with maximum transient energy occurring during the austral summer period and maximum low-frequency energy in winter. The site is one of growing eddy energy. Eddy momentum flux is northward over the SURVOSTRAL line, and the summertime eddy heat flux is poleward across the Subantarctic and Subtropical Fronts, and equatorward either side of the fronts. Eddy fluxes are strongly influenced by their position relative to the bathymetry and the mean current.

An Atlantic meridional transect of surface water dimethyl sulfide concentrations with 10-15 km horizontal resolution and close examination of ocean circulation

Underway measurements of dimethyl sulfide (DMS) in the Atlantic surface waters have been made during the ALBATROSS campaign from 65°N to 45°S along about 30°W. The main patterns of DMS variability in subtropical waters of both hemispheres were the existence of (1) a poleward negative gradient of DMS (0.04 nM/°latitude) paralleling the temperature and salinity meridional trends and opposite to that of chlorophyll a (chl a) and particulate DMSP (pDMSP), and (2) sharp DMS enhancements, up to twenty fold the background levels, coinciding almost systematically with thermohaline frontal zones. We observed that DMS concentrations and TOPEX/Poseidon sea level anomalies (SLAs) were clearly in opposition of phase in the subtropical and tropical waters of the Atlantic. Neither meridional changes in pDMSP nor in chl a concentrations account for these large-scale (15°–20° latitude) DMS variations. It is suggested that the spatial distribution of DMS is highly sensitive to the upper ocean dynamics. The tropical Atlantic is a zone of contrasted DMS levels with two broad maxima associated (1) with the cyclonic circulations generated by the North Equatorial currents and (2) with the South Equatorial Current, a situation very much resembling the autumnal meridional distribution of surface pC02. A close examination of the South Atlantic subtropical front (38°–43°S) show that DMS and in situ validated satellite chl a have a distinct spatial distribution suggesting important spatial segregation of biogeochemical processes in the frontal zones. These observations at different spatial scales provide indications for the existence of a DMS-climate link through frontogenesis and surface ocean circulation in the Atlantic.

Analysis of collinear passes of satellite altimeter data

The analysis of altimetry measurements along repeated satellite ground tracks, as in the Geosat Exact Repeat Mission, provides an excellent way of determining temporal variability in the sea surface height. This collinear analysis is usually carried out in two steps: first, by determining a mean sea level profile and then by removing a radial orbit error. We propose a new unified analysis method for the simultaneous estimation of orbit error, mean sea surface height, and residual sea surface heights (variability) with respect to each pass. All passes are treated in exactly the same way; the consequent rank or datum defect in the model is overcome by appending a minimal set of conditions, chosen either to give no overall distortion of the orbits or otherwise a best fit to a reference profile, such as the geoid. Gaps in the altimeter data are easily accommodated. Numerical results of the method are shown using data from the first year of the Geosat Exact Repeat Mission. Some consideration is given to the choice of radial orbit error model. The results indicate that for orbit error removal, a sinusoid model is the most appropriate over a one-revolution cycle. For passes less than one revolution in length, a component of the longer wavelength ocean signal is lost if using the sinusoid model. If passes longer than about two revolutions (some 200 mins) are analyzed, additional terms are needed to eliminate nongravitational perturbations.