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Paul C. F. van der Vaart
,
Henk A. Dijkstra
, and
Fei Fei Jin

Abstract

The equatorial tropical Pacific climate system is a delicate coupled system in which winds driven by gradients of sea surface temperature (SST) within the basin interact with the ocean circulation to maintain SST gradients. This results in a time mean state having a strong zonal temperature contrast along the equator with an eastern cold tongue and a western warm pool. By the same coupled processes, interannual variability, known as the El Niño–Southern Oscillation (ENSO), is present in the Pacific. This variability can be attributed to an oscillatory coupled mode, the ENSO mode, in the equatorial ocean–atmosphere system. Using a Zebiak–Cane-type intermediate coupled model, the coexistence of an eastern cold tongue in the annual mean state and ENSO in the Pacific climate system is investigated. The ENSO mode arises as a robust oscillatory mode on a coupled mean state and becomes unstable if the cold tongue of the mean state is sufficiently strong. The origin of this mode, its propagation mechanism, its sensitivity to parameters, and its relation to the spatial structure of the annual mean state are considered.

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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.

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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.

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Wilbert Weijer
,
Wilhelmus P. M. de Ruijter
,
Henk A. Dijkstra
, and
Peter Jan van Leeuwen

Abstract

The thermohaline exchange between the Atlantic and the Southern Ocean is analyzed, using a dataset based on WOCE hydrographic data. It is shown that the salt and heat transports brought about by the South Atlantic subtropical gyre play an essential role in the Atlantic heat and salt budgets. It is found that on average the exported North Atlantic Deep Water (NADW) is fresher than the return flows (basically composed of thermocline and intermediate water), indicating that the overturning circulation (OC) exports freshwater from the Atlantic.

The sensitivity of the OC to interbasin fluxes of heat and salt is studied in a 2D model, representing the Atlantic between 60°N and 30°S. The model is forced by mixed boundary conditions at the surface, and by realistic fluxes of heat and salt at its 30°S boundary. The model circulation turns out to be very sensitive to net buoyancy fluxes through the surface. Both net surface cooling and net surface saltening are sources of potential energy and impact positively on the circulation strength. The vertical distributions of the lateral fluxes tend to stabilize the stratification, and, as they extract potential energy from the system, tend to weaken the flow. These results imply that a change in the composition of the NADW return transports, whether by a change in the ratio thermocline/intermediate water, or by a change in their thermohaline characteristics, might influence the Atlantic OC considerably.

It is also shown that the circulation is much more sensitive to changes in the shape of the lateral buoyancy flux than to changes in the shape of the surface buoyancy flux, as the latter does not explicitly impact on the potential energy of the system. It is concluded that interocean fluxes of heat and salt are important for the strength and operation of the Atlantic thermohaline circulation, and should be correctly represented in models that are used for climate sensitivity studies.

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Caroline A. Katsman
,
Paul C. F. Van der Vaart
,
Henk A. Dijkstra
, and
Wilhelmus P. M. de Ruijter

Abstract

Ocean rings, when isolated from major ocean currents, can have life spans on the order of years. This study focuses on the stability of such isolated ocean rings. Assuming axisymmetric basic-state profiles, the linear stability of a wide variety of rings is analyzed by examining the properties of the modes to which they become unstable and the associated energy conversions. Earlier studies have indicated that corotating rings, with a large barotropic component, are far less unstable than counterrotating ones. This sharp contrast between co- and counterrotating rings appears to be a consequence of the choice for a radial profile of the azimuthal velocity that decays only gradually on the ring's outer flank. For more realistic velocity profiles, co- and counterrotating rings have similar growth rates. Nearly compensated rings, that is, those with a weak flow in the deepest layer, are found to be the least unstable ones. In this paper, the problem for warm-core rings with a Gaussian profile is first revisited in a two-layer setup. A systematic survey of the sensitivity of the results for this standard case with respect to various ring parameters, such as the stratification, ring width, and, in particular, the radial profile of the azimuthal velocity, is presented. Besides exponential profiles, as used in earlier studies, the stability of rings with a core in solid-body rotation is also examined. Subsequently, more realistic cases are considered by discussing the stability of ocean rings designed as fits to an observed cold-core Gulf Stream ring and a warm-core Agulhas ring. Minimal growth rates for the latter rings are very large: the calculated e-folding timescales are about one week.

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Selma E. Huisman
,
Henk A. Dijkstra
,
A. S. von der Heydt
, and
W. P. M. de Ruijter

Abstract

The present-day global meridional overturning circulation (MOC) with formation of North Atlantic Deep Water (NADW) and the absence of a deep-water formation in the North Pacific is often considered to be caused by the fact that the North Pacific basin is a net precipitative, while the North Atlantic is a net evaporative basin. In this paper, the authors study the effect of asymmetries in continent geometry and freshwater fluxes on the MOC both in an idealized two-dimensional model and in a global ocean model. This study approaches the problem from a multiple equilibria perspective, where asymmetries in external factors constrain the existence of steady MOC patterns. Both this multiple equilibria perspective and the fact that a realistic global geometry is used add new aspects to the problem. In the global model, it is shown that the Atlantic forced by net precipitation can have a meridional overturning circulation with northern sinking and a sea surface salinity that resembles the present-day salinity field. The model results are suggestive of the importance of factors other than the freshwater flux asymmetries, in particular continental asymmetries, in producing the meridional overturning asymmetry.

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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.

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Matthijs den Toom
,
Henk A. Dijkstra
,
Wilbert Weijer
,
Matthew W. Hecht
,
Mathew E. Maltrud
, and
Erik van Sebille

Abstract

The strongly eddying version of the Parallel Ocean Program (POP) is used in two 45-yr simulations to investigate the response of the Atlantic meridional overturning circulation (AMOC) to strongly enhanced freshwater input due to Greenland melting, with an integrated flux of 0.5 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1). For comparison, a similar set of experiments is performed using a noneddying version of POP. The aim is to identify the signature of the salt advection feedback in the two configurations. For this reason, surface salinity is not restored in these experiments. The freshwater input leads to a quantitatively comparable reduction of the overturning strength in the two models. To examine the importance of transient effects in the relation between AMOC strength and density distribution, the results of the eddy-resolving model are related to water mass transformation theory. The freshwater forcing leads to a reduction of the rate of light to dense water conversion in the North Atlantic, but there is no change in dense to light transformation elsewhere, implying that high density layers are continuously deflating. The main focus of the paper is on the effect of the AMOC reduction on the basinwide advection of freshwater. The low-resolution model results show a change of the net freshwater advection that is consistent with the salt advection feedback. However, for the eddy-resolving model, the net freshwater advection into the Atlantic basin appears to be unaffected, despite the significant change in the large-scale velocity structure.

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