W. P. M. de Ruijter

Observations of a young Agulhas ring, Astrid, during MARE in March 2000

The MARE project studies the effects of inter-ocean exchange between the Indian and Atlantic Ocean, via Agulhas rings, on the Atlantic meridional overturning circulation. The field programme of MARE concentrates on the study of the decay and modification of a single Agulhas ring named Astrid, formed in January 2000. The ring was clearly visible in the analysis of satellite altimetry data, and surface drifters confirmed the anti-cyclonic rotation. During a detailed survey of this 2-month-old ring in March 2000, it appeared that the water mass properties of this ring only differed from the surrounding water above the 12°C isotherm. The observed fine-structure near its boundary suggested that exchange of water with its surroundings already had started. Observations with a lowered acoustic Doppler current profiler showed that the ring had a significant barotropic component, additional to the baroclinic flow around its warm centre. Meteorological observations indicated that during the summer survey the ring was losing heat to the atmosphere. This heat loss maintained convective mixing in the surface mixed layer. Compared to other reported rings, Astrid had a very large kinetic energy, a property probably characteristic for very young Agulhas rings. In other aspects Astrid did not differ strongly from the other rings, although Astrid was slightly larger than their typical ‘average’ size.

Seasonal and interannual variability in the Mozambique Channel from moored current observations

Direct observations from an array of current meter moorings across the Mozambique Channel in the south-west Indian Ocean are presented covering a period of more than 4 years. This allows an analysis of the volume transport through the channel, including the variability on interannual and seasonal time scales. The mean volume transport over the entire observational period is 16.7 Sv poleward. Seasonal variations have a magnitude of 4.1 Sv and can be explained from the variability in the wind field over the western part of the Indian Ocean. Interannual variability has a magnitude of 8.9 Sv and is large compared to the mean. This time scale of variability could be related to variability in the Indian Ocean Dipole (IOD), showing that it forms part of the variability in the ocean-climate system of the entire Indian Ocean. By modulating the strength of the South Equatorial Current, the weakening (strengthening) tropical gyre circulation during a period of positive (negative) IOD index leads to a weakened (strengthened) southward transport through the channel, with a time lag of about a year. The relatively strong interannual variability stresses the importance of long-term direct observations.

Long-term observations of transport, eddies, and Rossby waves in the Mozambique Channel

Data from an array of current meter moorings covering a period of two and a half years are used to estimate the varying transport through the Mozambique Channel. The total transport during this period is small. Below 1200 m the transport is weak but a prominent deep western boundary undercurrent with cores at 1700 and 2200 m is found that transports 1.5 Sv to the north. The transport shows a large temporal variability, and neither a continuous upper layer western boundary current nor a continuous deep undercurrent is found. The variability in the upper layer is dominated by a period of 68 days and results mainly from eddies that migrate southward through the Mozambique Channel. In addition to this southward propagation, a westward-propagating signal is evident from a space-time diagram of the throughflow. The signal is interpreted as a Mozambique Channel Rossby normal mode. This interpretation is consistent with results from a Principal Oscillation Pattern Analysis (that estimates normal modes from the data) and a quasi-geostrophic channel model. A detailed inspection of a single ‘‘eddy event’’ shows that a precursor of an anticyclone is a strong southward current along the Madagascar coast that propagates westward to the center of the Channel. During the westward propagation, the current becomes unstable inducing an anticyclone. This scenario connects the westward-propagating mode with the eddy growth and explains the coincidence of the eddy and Rossby mode frequency.

Three-dimensional current structure in the Dutch coastal zone

Abstract As part of a more comprehensive programme on the hydrography in the Netherlands coastal zone its complex current structure is investigated. One of the aims is to identify the various dominant factors that control the patterns of the coastal flow. To that end field observations covering a one year period were taken at strategic locations in the Dutch coastal area. One of the main results reported here concerns the long term ( i.e. one year) average residual currents. Near the bottom these appear to have a significant onshore directed component, with values of the order 2.5 to 3.5 cm/s. In the upper layer typical average values are between 7 and 11 cm/s. The near surface residual flow is mostly in longshore direction with an onshore component increasing with distance from shore, due to the prevailing southwesterly winds ( i.e. close to longshore) over the area. The observed large variability of the three-dimensional current structure on different time-scales is largely induced by variations in wind conditions. If strong winds are accompanied by a high river discharge, a distinct two layer system results with very high residual current speeds in the upper-layer, up to 40 cm/s. The observations also produce evidence for the outflow of Rhine river water to be at least partly responsible for the significant onshore component of residual currents near the bottom, as it determines the density structure in this area. Tidal current ellipses show a marked vertical structure both with respect to shape and sense of rotation. For the region considered the spatial distribution of these parameters clearly illustrates the large influence of bottom friction and coastal boundaries, in combination with earths rotation. Furthermore the data show that variations due to the spring-neap tidal cycle are substantial.

A weaker Agulhas Current leads to more Agulhas leakage

Time series of transports in the Agulhas region have been constructed by simulating Lagrangian drifter trajectories in a 1/10 degree two-way nested ocean model. Using these 34 year long time series it is shown that smaller (larger) Agulhas Current transport leads to larger (smaller) Indian-Atlantic inter-ocean exchange. When transport is low, the Agulhas Current detaches farther downstream from the African continental slope. Moreover, the lower inertia suppresses generation of anti-cyclonic vorticity. These two effects cause the Agulhas retroflection to move westward and enhance Agulhas leakage. In the model a 1 Sv decrease in Agulhas Current transport at 32 degrees S results in a 0.7 +/- 0.2 Sv increase in Agulhas leakage

A link between low-frequency mesoscale eddy variability around Madagascar and the large-scale Indian Ocean variability

[1] A connection is shown to exist between the mesoscale eddy activity around Madagascar and the large-scale interannual variability in the Indian Ocean. We use the combined TOPEX/Poseidon-ERS sea surface height (SSH) data for the period 1993– 2003. The SSH-fields in the Mozambique Channel and east of Madagascar exhibit a significant interannual oscillation. This is related to the arrival of large-scale anomalies that propagate westward along 10–15S in response to the Indian Ocean dipole (IOD) events. Positive (negative) SSH anomalies associated to a positive (negative) IOD phase induce ashift intheintensity andpositionofthetropicaland subtropical gyres.Aweakening (strengthening) results in the intensity of the South Equatorial Current and its branches along east Madagascar. In addition, the flow through the narrows of the Mozambique Channel around 17S increases (decreases) during periods of a stronger and northward (southward) extension of the subtropical (tropical) gyre. Interaction between the currents in the narrows and southward propagating eddies from the northern Channel leads to interannual variability in the eddy kinetic energy of the central Channel in phase with the one in the SSH-field.

The influence of the Agulhas Current on the adjacent coastal ocean: possible impacts of climate change

The dynamics of the coastal ocean along the southeastern coast of Africa is dominated by a strong and intense western boundary current, the Agulhas Current. With a near-uniform, narrow continental shelf and a steep shelf slope that stabilizes this current, the trajectory of the Agulhas exhibits great stability. The only substantial perturbation occurs with the irregular passage of a Natal Pulse, a soliton meander. The initiation of this meander at the Natal Bight is due to a barotropic instability when the intensity of the landward border of the current exceeds a certain threshold value. This may come about with natural fluctuations in the current or with the adsorption of deep-sea eddies onto the current. Under a climate change scenario of altered wind stress curl over the South Indian Ocean it is conceivable that the threshold for the triggering of a Natal Pulse will occur more frequently. This will lead to a situation where the current axis on average lies further offshore. The possible consequences of such a situation on the rainfall of the coast, on the ecology of estuaries and the coastal ocean, and on the socio-economics of the region is discussed.

The distribution of suspended matter in the Dutch coastal zone

Abstract Results are presented of suspended matter observations taken at bi-weekly intervals during the period 1975–1983, in a strip 70 km wide along the Dutch coast. The average distribution showed a (weak) minimum zone located north of Noordwijk at a distance of about 30 km from and parallel to the Dutch coast, between the salinity maximum and the coast. Variability was large between different years, seasons and individual cruises. A clear seasonal cycle emerged from the observed suspended-matter patterns: in winter a pronounced minimum existed, while in summer generally a monotonically decreasing concentration was measured, to open-sea values much lower than in winter. This seasonal cycle may be explained by a combination of variation in wind, river discharge and the activity of suspended-matter sources. The Flemish Banks and Channel waters are the main sources supplying suspended matter to the Dutch coastal area. Especially the varying transport through the Strait of Dover, large in winter, small in summer, determines the seasonal variation in the total amount of suspended matter. The existence of a localized turbidity minimum is tied to relatively large discharges from the Rhine and associated steep salinity gradients, also occurring mainly in winter and early spring. A simple model of cross-shore density-driven circulation shows the possible influence of the strong salinity gradients on the sedimentation of suspended matter. Particles with a settling rate comparable to the vertical velocity component of the circulation are forced to move offshore until they reach an area they sink out of the surface layer. The strength of this circulation is determined by the cross-shore density gradient due to the inflow of fresh water from the Rhine-Meuse estuary.

Seasonal variability in the Agulhas Retroflection Region

The objective of this article is to present evidence for the existence of seasonal variability in sea surface height (SSH) anomaly in the Agulhas Retroflection region. TOPEX/POSEIDON altimeter data are used to estimate seasonal changes in the mesoscale SSH variability. There is a seasonal oscillation of SSH variability characterized by a maximum during the austral summer and a minimum during the austral winter. The amplitude of this seasonal change is approximately 30% of its mean value. During the winter season the spatial distribution of SSH variability resembles that of the annual mean variability, with relative maxima centered at approximately 18°E, 27°E and 38°E. During the summer there is an additional maximum which extends from approximately 20°E to 25°E and from 40° to 42°S. Analysis of longitude-time diagrams reveals that at low latitudes planetary waves propagate freely throughout the basin. Along the latitude of the Agulhas Retroflection region, the East Madagascar Ridge hampers the westward propagation of planetary waves. It is speculated that the difference between summer and winter patterns is caused by an inertially driven bifurcation of the Agulhas Current.

Comparison between observations and models of the Mozambique Channel transport: Seasonal cycle and eddy frequencies

[1] A time series of the observed transport through an array of moorings across the Mozambique Channel is compared with that of six model runs with ocean general circulation models. In the observations, the seasonal cycle cannot be distinguished from red noise, while this cycle is dominant in the transport of the numerical models. It is found, however, that the seasonal cycles of the observations and numerical models are similar in strength and phase. These cycles have an amplitude of 5 Sv and a maximum in September, and can be explained by the yearly variation of the wind forcing. The seasonal cycle in the models is dominant because the spectral density at other frequencies is underrepresented. Main deviations from the observations are found at depths shallower than 1500 m and in the 5/y–6/y frequency range. Nevertheless, the structure of eddies in the models is close to the observed eddy structure. The discrepancy is found to be related to the formation mechanism and the formation position of the eddies. In the observations, eddies are frequently formed from an overshooting current near the mooring section, as proposed by Ridderinkhof and de Ruijter (2003) and Harlander et al. (2009). This causes an alternation of events at the mooring section, varying between a strong southward current, and the formation and passing of an eddy. This results in a large variation of transport in the frequencyrangeof5/y–6/y.Inthemodels,theeddiesareformedfurthernorthandpropagate through the section. No alternation similar to the observations is observed, resulting in a more constant transport.