Herman Ridderinkhof

On the role of the Agulhas system in ocean circulation and climate

The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments—backed by palaeoceanographic and sustained modern observations—are required to establish firmly the role of the Agulhas system in a warming climate.

Eddies and variability in the Mozambique Channel

Abstract Between 1995 and 2000, on average 4 eddies per year are observed from satellite altimetry to propagate southward through the Mozambique Channel, into the upstream Agulhas region. Further south, these eddies have been found to control the timing and frequency of Agulhas ring shedding. Within the Mozambique Channel, anomalous SSH amplitudes rise to 30 cm , in agreement with in situ measured velocities. Comparison of an observed velocity section with GCM model results shows that the Mozambique Channel eddies in these models are too surface intensified. Also, the number of eddies formed in the models is in disagreement with our observational analysis. Moored current meter measurements observing the passage of three eddies in 2000 are extended to a 5-year time series by referencing the anomalous surface currents estimated from altimeter data to a synoptic LADCP velocity measurement. The results show intermittent eddy passage at the mooring location. A statistical analysis of SSH observations in different parts of the Mozambique Channel shows a southward decrease of the dominant frequency of the variability, going from 7 per year in the extension of the South Equatorial Current north of Madagascar to 4 per year south of Madagascar. The observations suggest that frequency reduction is related to the Rossby waves coming in from the east.

Strong inertial currents and marginal internal wave stability in the central North Sea

Solar insolation stabilizes the water column and suppresses vertical exchange. Observations from the central North Sea clearly show that increased heating in summer is accompanied by enhanced de-stabilizing vertical current differences (shear), surprisingly to such extent that the equilibrium state is marginally stable. Under calm weather conditions, the shear is predominantly generated by near-inertial motions while the internal wave spectrum primarily results from nonlinear interaction between the dominating tidal and near-inertial motions. In terms of the associated enhanced vertical mixing across the largest vertical temperature gradients, shelf seas are not different from the abyssal ocean, despite the proximity to energy sources near boundaries in the former. By the lack of sufficiently strong wind- and tidal-mixing this internal mixing is considered to be responsible for the diapycnal transport of nutrients leading to the observed increase in near-surface values and triggering the late-summer phytoplankton bloom.

Impact of interannual variability in hydrodynamic circulation on egg and larval transport of plaice Pleuronectes platessa L. in the southern North Sea

Abstract A realistic 2D circulation model of the southern North Sea was applied to simulate the interannual variability in dispersal (advection and diffusion) of plaice Pleuronectes platessa L. eggs and larvae from the spawning area in the Southern Bight of the North Sea towards the Dutch coastal nursery areas. The model is driven with daily varying current fields, which are the result of driving a general hydrodynamical model with realistic tidal and meteorological (wind) forcing for the period 1974–1981. Model simulations of plaice egg and larval transport are compared with observed larval concentrations near coastal nursery areas (Delta area and Wadden Sea). The model simulations show that: (1) the interannual variability in transport is large and of the same order as the year-to-year variations observed in larval concentrations near the nursery areas; (2) the interannual variability increases with increasing distance from the spawning area; (3) for the years in which larval estimates were available, the model predictions were positively correlated with observed estimates for the Marsdiep tidal inlet. This suggests that the variability in circulation patterns during the early pelagic stages in the open sea might be a key factor in determining year-class strength of plaice.

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.

The impact of thermal stratification on phytoplankton and nutrient dynamics in shelf seas: a model study

Abstract Local heating rate within the water column depends on energy input by solar radiation and on heat exchange across the surface as controlled by wind and convection. The heating results in thermal stratification of the water column, which in turn affects the vertical transport of, for example, nutrients. In this paper the implications of the stratification on the biota by focusing on the time of its onset and its variability in time and (vertical) space are evaluated. Especially the consequences of the stratification on the phytoplankton dynamics and the trophic interactions are shown. For this purpose, an integrated ecosystem model is developed which includes a physical submodel and an ecological model. The model is calibrated with data from a mooring project, during which a large number of physical and biological parameters have been measured at a site on the Oyster Grounds in the North Sea over a 15-month period. The physical model is a one-dimensional entrainment/detrainment model. The ecological model consists of submodels which are part of the ERSEM ecological model and which describe the biological and chemical processes in the water column and in the benthos. Results show that stratification has a major impact on the biota. Especially the timing of the onset of the stratification has major consequences for the production and succession of phytoplankton and the structure of the food web during the entire growing season.

Variability of the southwest Indian Ocean.

The variability in the southwest Indian Ocean is connected to the basin–scale and global–scale ocean circulation. Two bands of enhanced variability stretch across the Southern Indian Ocean east of Madagascar around 12○ S and 25○ S, respectively. They mark the preferred routes along which anomalies, generated by varying forcing over the central basin, near the eastern boundary or in the equatorial region, propagate westward as baroclinic Rossby waves. Sea–surface height anomalies pass along the northern tip of Madagascar and are observed by satellite altimetry to propagate into the central Mozambique Channel. There, eddies are subsequently formed that propagate southward into the Agulhas retroflection region. The anomalies along the southern band trigger the formation of large dipolar vortex pairs in the separation region of the East Madagascar Current at the southern tip of the island. South of Africa these eddies and dipoles trigger the shedding of Agulhas Rings that feed the Atlantic meridional overturning circulation with warm, salty, Indian Ocean water. Interannual variability of the forcing over the Indian Ocean, such as that associated with the Indian Ocean Dipole/El Niño climate modes, propagates along these pathways and leads to associated modulations of the eddy transports into the South Atlantic.

Ferry observations on temperature, salinity and currents in the marsdiep tidal inlet between the North Sea and Wadden Sea

Publisher Summary This chapter discusses the observation made by a ferry on temperature, salinity, and currents in the Marsdiep tidal inlet between the North Sea and Wadden Sea. In a cooperation between Netherlands Institute for Sea Research and the ferry company, Texels Eigen Stoomboot Onderneming (TESO), continuous observations from the ferry—Schulpengat—are carried out in the Marsdiep tidal inlet between the North Sea and Dutch Wadden Sea. The measurements include surface temperature, salinity, and fluorescence as obtained from a through-flow system, as well as vertical profiles of velocity and echo-intensity, obtained every 2 s with an acoustic Doppler current profiler mounted below the hull of the ferry. A dedicated computer onboard the ferry combines and stores the data from an acoustic doppler current profiler (ADP), the through-flow system, and the DGPS and GYRO system. Part of the data is presented directly on a screen in the passengers room of the ferry. The chapter presents a figure showing daily averages of salinity and temperature observations obtained in 1998. The temperature observations clearly show the seasonal variability in surface heating. It has been found that on this time scale, the temporal variations in salinity mainly reflect the temporal variability of the fresh water discharge to the Wadden Sea and, more indirectly, the fresh water discharge to the coastal zone by rivers south of the inlet.

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.