Search Results

You are looking at 1 - 8 of 8 items for

  • Author or Editor: Will De Ruijter x
  • Refine by Access: All Content x
Clear All Modify Search
Will De Ruijter

Abstract

An asymptotic analysis is carried out of a linear and a nonlinear transport model of the large-scale wind-driven ocean circulation in the subtropical region of the Atlantic and Indian Oceans. The complicated geometry with a continent that terminates in mid-ocean is reflected in both models into the Atlantic Ocean through free boundary layers. It is shown that inertia must be incorporated to be able to produce a significant retroflection of the Agulhas Current south of Africa. In such a model the transport of the Return Agulhas Current becomes large when the wind stress curl decreases substantially over the north–south scale of an inertial boundary layer region around South Africa. The reason is that the overshooting Agulhas Current can then “unload” more of its transport eastward into the Sverdrup regime. The remainder of the transport bends westward into the free boundary layer. Wind stress curls calculated from the observed wind field over the Indian Ocean show such a rapid decrease. Thus the existence of a stagnation point near South Africa is shown to be a consequence of the large-scale conditions especially over the Indian Ocean part of the domain. The length scale that results from a formal asymptotic analysis for the distance of that retroflection point to the southern tip of the peninsula and the associated transport of the Return Agulhas Current agree with values from observations. Without the fast latitudinal variation of the wind stress curl the transport of the Brazil Current would he of the same order as that in the Agulhas. Almost the full Agulhas Current would then flow around South Africa and proceed as a westward jet into the Atlantic Ocean.

Full access
Will P. M. de Ruijter

Abstract

Through an analysis of a multi-dimensional extension of the bulk mixed layer model of Kraus and Turner (1967) it is shown how, even under spatially uniform atmospheric conditions, an initially smooth horizontal temperature gradient in the surface mixed layer can still develop into a front. For this to occur, it is essential that the net downward heat flux at the air-water interface be related to the difference between the mixed layer temperature T and an apparent atmospheric temperature TA (Haney, 1971), so that the initial horizontal gradient in T corresponds to a horizontal variation in the surface buoyancy flux. As a result the mixed layer depth also differs from place to place. Depending on the direction of the wind-driven transport, this produces either a steepening or a flattening of the initial temperature profile. A lower bound condition for the initial horizontal advective heat flux is derived in terms of the initial surface heat flux, the stirring by the wind, and the time rate of chance of the apparent atmospheric temperature. If that condition is satisfied, a front develops. If not, then frontogenesis is prevented by the damping effect of the local atmospheric heating. It is shown that the condition can be satisfied on the scales of lakes (or small seas) and in near-equatorial oceanic regions. Mathematically, the physical process can be described by a first-order quasilinear hyperbolic partial differential equation that is solved exactly by the method of characteristics.

Full access
Will P. M. de Ruijter

Abstract

A general multidimensional model of the upper mixed layer of oceans and lakes is presented. The density profile is approximated as uniform over the depth of the layer. Such an assumption is not made for the distribution of the horizontal velocity component, as there is no observational evidence for it. In fact, many observations indicate that important velocity shears exist in regions where advection can be expected to play an important role in the dynamics of the mixed layer (such as lakes, near upwelling and frontal areas, and in equatorial oceanic regions).

It is shown that vertical shear in the horizontal velocity field combined with a horizontal density gradient can result in the production or absorption of mechanical energy. Production results if the velocity shear tends to destabilize the density profile, consumption if the tendency is stabilizing so that work against gravity must be done to mix the stabler density profile to uniformity. In the mechanical energy equation, this effect of the velocity shear is represented by a well-defined term. Comparison with other terms shows that for many relevant examples the shear mechanism contributes substantially to the energy balance. As a consequence, the deepening characteristics of a mixed layer with shear dispersion are different from those in a slab layer. These considerations culminate in the formulation of a generalized entrainment law that is nonlinearly coupled to the vertically integrated heat transport equation. The system is solved exactly for several examples with an imposed velocity structure. One of the important results is that vertical shear in the horizontal velocity components can lead to a considerable enhancement or reduction of the speed with which density anomalies propagate horizontally. For example, if an Ekman spiral is embedded in the mixed layer and cooler water is upstream, that speed turns out to be approximately a factor 2 − 1/π larger than the average (“slab”) velocity.

Full access
Peter Jan van Leeuwen and Will P. M. de Ruijter

Abstract

The existence of inertial steady currents that separate from a coast and meander afterward is investigated. By integrating the zonal momentum equation over a suitable area, it is shown that retroflecting currents cannot be steady in a reduced gravity or in a barotropic model of the ocean. Even friction cannot negate this conclusion. Previous literature on this subject, notably the discrepancy between several articles by Nof and Pichevin on the unsteadiness of retroflecting currents and steady solutions presented in other papers, is critically discussed. For more general separating current systems, a local analysis of the zonal momentum balance shows that given a coastal current with a specific zonal momentum structure, an inertial, steady, separating current is unlikely, and the only analytical solution provided in the literature is shown to be inconsistent. In a basin-wide view of these separating current systems, a scaling analysis reveals that steady separation is impossible when the interior flow is nondissipative (e.g., linear Sverdrup-like). These findings point to the possibility that a large part of the variability in the world’s oceans is due to the separation process rather than to instability of a free jet.

Full access
Claudia E. Wieners, Henk A. Dijkstra, and Will P. M. de Ruijter

Abstract

The effect of long-term trends and interannual, ENSO-driven variability in the Indian Ocean (IO) on the stability and spatial pattern of ENSO is investigated with an intermediate-complexity two-basin model. The Pacific basin is modeled using a fully coupled (i.e., generating its own background state) Zebiak–Cane model. IO sea surface temperature (SST) is represented by a basinwide warming pattern whose strength is constant or varies at a prescribed lag to ENSO. Both basins are coupled through an atmosphere transferring information between them. For the covarying IO SST, a warm IO during the peak of El Niño (La Niña) dampens (destabilizes) ENSO, and a warm IO during the transition from El Niño to La Niña (La Niña to El Niño) shortens (lengthens) the period. The influence of the IO on the spatial pattern of ENSO is small. For constant IO warming, the ENSO cycle is destabilized because stronger easterlies induce more background upwelling, more thermocline steepening, and a stronger Bjerknes feedback. The SST signal at the east coast weakens or reverses sign with respect to the main ENSO signal [i.e., ENSO resembles central Pacific (CP) El Niños]. This is due to a reduced sensitivity of the SST to thermocline variations in case of a shallow background thermocline, as found near the east coast for a warm IO. With these results, the recent increase in CP El Niño can possibly be explained by the substantial IO (and west Pacific) warming over the last decades.

Full access
Claudia E. Wieners, Henk A. Dijkstra, and Will P. M. de Ruijter

Abstract

In recent years it has been proposed that a negative (positive) Indian Ocean dipole (IOD) in boreal autumn favors an El Niño (La Niña) at a lead time of 15 months. Observational analysis suggests that a negative IOD might be accompanied by easterly anomalies over the western Pacific. Such easterlies can enhance the western Pacific warm water volume, thus favoring El Niño development from the following boreal spring onward. However, a Gill-model response to a negative IOD forcing would lead to nearly zero winds over the western Pacific. The authors hypothesize that a negative IOD—or even a cool western Indian Ocean alone—leads to low-level air convergence and hence enhanced convectional heating over the Maritime Continent, which in turn amplifies the wind convergence so as to cause easterly winds over the western Pacific. This hypothesis is tested by coupling an idealized Indian Ocean model and a convective feedback model over the Maritime Continent to the Zebiak–Cane model. It is found that, for a sufficiently strong convection feedback, a negative (positive) IOD indeed forces easterlies (westerlies) over the western Pacific. The contribution from the eastern IOD pole dominates. IOD variability is found to destabilize the El Niño–Southern Oscillation (ENSO) mode, whereas Indian Ocean basinwide warming (IOB) variability dampens ENSO, even in the presence of convection. The influence of the Indian Ocean on the spectral properties of ENSO is dominated by the IOB, while the IOD is a better predictor for individual ENSO events.

Open access
Weiqing Han, Jérôme Vialard, Michael J. McPhaden, Tong Lee, Yukio Masumoto, Ming Feng, and Will P.M. de Ruijter

The international scientific community has highlighted decadal and multidecadal climate variability as a priority area for climate research. The Indian Ocean rim region is home to one-third of the world's population, mostly living in developing countries that are vulnerable to climate variability and to the increasing pressure of anthropogenic climate change. Yet, while prominent decadal and multidecadal variations occur in the Indian Ocean, they have been less studied than those in the Pacific and Atlantic Oceans. This paper reviews existing literature on these Indian Ocean variations, including observational evidence, physical mechanisms, and climatic impacts. This paper also identifies major issues and challenges for future Indian Ocean research on decadal and multidecadal variability.

Full access
Claudia E. Wieners, Will P. M. de Ruijter, Wim Ridderinkhof, Anna S. von der Heydt, and Henk A. Dijkstra

Abstract

A multichannel singular spectrum analysis (MSSA) applied simultaneously to tropical sea surface temperature (SST), zonal wind, and burstiness (zonal wind variability) reveals three significant oscillatory modes. They all show a strong ENSO signal in the eastern Pacific Ocean (PO) but also a substantial SST signal in the western Indian Ocean (IO). A correlation-based analysis shows that the western IO signal contains linearly independent information on ENSO. Of the three Indo-Pacific ENSO modes of the MSSA, one resembles a central Pacific (CP) El Niño, while the others represent eastern Pacific (EP) El Niños, which either start in the central Pacific and grow eastward (EPe) or start near Peru and grow westward (EPw). A composite analysis shows that EPw El Niños are preceded by cooling in the western IO about 15 months earlier. Two mechanisms are discussed by which the western IO might influence ENSO. In the atmospheric bridge mechanism, subsidence over the cool western IO in autumn (year 0) leads to enhanced convection above Indonesia, strengthening easterlies over the western PO, and the creation of a large warm water volume. This is essential for the creation of (EP) El Niños in the following spring–summer. In the state-dependent noise mechanism, a cool western IO favors a strong intraseasonal zonal wind variability over the western PO in early spring (year 1), which can partly be attributed to the Madden–Julian oscillation. This intraseasonal variability induces Kelvin waves, which in early spring lead to a strong warming of the eastern PO and can initiate EPw El Niños.

Full access