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  • Author or Editor: Henk A. Dijkstra x
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Ziqing Zu
,
Mu Mu
, and
Henk A. Dijkstra

Abstract

Within a three-dimensional ocean circulation model, the nonlinear optimal initial perturbations (NOIP) of sea surface salinity (SSS) and sea surface temperature (SST) to excite variability in the Atlantic meridional overturning circulation (AMOC) were obtained under prescribed heat and freshwater flux boundary conditions, using the conditional nonlinear optimal perturbation (CNOP) method. After 10 years, the optimal SSS and SST perturbations lead to reductions of the AMOC by 3.6 and 2.5 Sv (1 Sv = 106 m3 s−1), respectively, followed by multidecadal oscillations with a period of about 50 years. During the first 30 years, nonlinear processes have an important influence on the AMOC strength: convection strengthens the AMOC during years 0–2, zonal density advection promotes the slowdown of the AMOC during years 7–20, and meridional density advection inhibits the slowdown of meridional velocities in the upper ocean during years 5–18. The linear optimal initial perturbation (LOIP) was also computed using the first singular vector (FSV) method. For SSS perturbations with an amplitude of 0.5 psu, the LOIP will cause an underestimation of the amplitude of the multidecadal AMOC variability by about 1 Sv, compared to that induced by the NOIP. This underestimation will become more significant as the amplitudes of SSS perturbations increase.

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Lianke te Raa
,
Jeroen Gerrits
, and
Henk A. Dijkstra

Abstract

The aim of this paper is to identify the physical mechanism of interdecadal variability in simulations of the North Atlantic Ocean circulation with the Modular Ocean Model of the Geophysical Fluid Dynamics Laboratory. To that end, a hierarchy of increasingly complex model configurations is used. The variability in the simplest case, that of viscous, purely thermally driven flows in a flat-bottom ocean basin with a box-shaped geometry, is shown to be caused by an internal interdecadal mode. The westward propagation of temperature anomalies and the phase difference between the anomalous zonal and meridional overturning that characterize the interdecadal mode are used as “fingerprints” of the physical mechanism of the variability. In this way, the variability can be followed toward a less viscous regime in which the effects of continental geometry and bottom topography are also included. It is shown that, although quantitative aspects of the variability like period and spatial pattern are changing, the physical mechanism of the interdecadal variability in the more complex simulations can be attributed to the same processes as in the simplest model configuration.

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Stefano Pierini
,
Henk A. Dijkstra
, and
Angelo Riccio

Abstract

The Kuroshio Extension (KE) flow in the North Pacific Ocean displays a very distinctive decadal variability of bimodal character involving two completely different states (a large-meander “elongated” state and a small-meander “contracted” state) connected by very asymmetric temporal transitions. Although such a flow has been widely studied by means of a suite of mathematical models and by using several observational platforms, a satisfactory theoretical framework answering quite elementary questions is still lacking, the main question being whether such variability is induced by a time-varying wind forcing or, rather, by intrinsic oceanic mechanisms. In this context, the chaotic relaxation oscillation produced by a process-oriented model of the KE low-frequency variability, with steady climatological wind forcing, was recently recognized to be in substantial agreement with altimeter data. Here those model results are further compared with a comprehensive altimeter dataset. The positive result of such a comparison allows the conclusion that a minimal model for the KE bimodality has been identified and that, consequently, nonlinear intrinsic oceanic mechanisms are likely to be the main cause of the observed variability. By applying the methods of nonlinear dynamical systems theory, relevant dynamical features of the modeled flow are then explained, such as the origin of the relaxation oscillation as a consequence of a homoclinic bifurcation, the spatiotemporal character of the bimodal behavior, and the degree of predictability of the flow in the different stages of the oscillation (evaluated through a field of finite-time Lyapunov exponents and the corresponding Lagrangian time series).

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Henk A. Dijkstra
,
Juan A. Saenz
, and
Andrew McC. Hogg

Abstract

Oscillatory behavior of the Atlantic meridional overturning circulation (MOC) is thought to underlie Atlantic multidecadal climate variability. While the energy sources and sinks driving the mean MOC have received intense scrutiny over the last decade, the governing energetics of the modes of variability of the MOC have not been addressed to the same degree. This paper examines the energy conversion processes associated with this variability in an idealized North Atlantic Ocean model. In this model, the multidecadal variability arises through an instability associated with a so-called thermal Rossby mode, which involves westward propagation of temperature anomalies. Applying the available potential energy (APE) framework from stratified turbulence to the idealized ocean model simulations, the authors study the multidecadal variability from an energetics viewpoint. The analysis explains how the propagation of the temperature anomalies leads to changes in APE, which are subsequently converted into the kinetic energy changes associated with variations in the MOC. Thus, changes in the rate of generation of APE by surface buoyancy forcing provide the kinetic energy to sustain the multidecadal mode of variability.

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Caroline A. Katsman
,
Sybren S. Drijfhout
, and
Henk A. Dijkstra

Abstract

Recent modeling and observational studies have indicated that the interaction of the Gulf Stream and the deep western boundary current (DWBC) in the North Atlantic may induce low-frequency (decadal timescale) variability. To understand the origin of this low-frequency variability, a line of studies is continued here addressing the stability and variability of the wind-driven circulation using techniques of dynamical systems theory. In an idealized quasigeostrophic 2-layer model setup, stationary solutions of the coupled wind-driven gyres/DWBC system are computed, using the lateral friction as control parameter. Simultaneously, their stability is assessed. When a DWBC is absent, only oscillatory instabilities with intermonthly timescales are found. However, when the strength of the DWBC is increased, the coupled 2-layer flow becomes susceptible to instabilities with interannual timescales. By computing transient flows at relatively low friction, it is found that the existence of these interannual modes induces low-frequency variability in the coupled Gulf Stream/DWBC system with a preferred interannual timescale.

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Andrew McC. Hogg
,
Henk A. Dijkstra
, and
Juan A. Saenz

Abstract

A well-studied example of natural climate variability is the impact of large freshwater input to the polar oceans, simulating glacial melt release or an amplification of the hydrological cycle. Such forcing can reduce, or entirely eliminate, the formation of deep water in the polar latitudes and thereby weaken the Atlantic meridional overturning circulation (MOC). This study uses a series of idealized, eddy-permitting numerical simulations to analyze the energetic constraints on the Atlantic Ocean's response to anomalous freshwater forcing. In this model, the changes in MOC are not correlated with the global input of mechanical energy: both kinetic energy and available potential energy (APE) increase with northern freshwater forcing, while the MOC decreases. However, a regional analysis of APE density supports the notion that local maxima in APE density control the response of the MOC to freshwater forcing perturbations. A coupling between APE input and changes in local density anomalies accounts for the difference in time scales between the recovery and collapse of the MOC.

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Lisa Hahn-Woernle
,
Henk A. Dijkstra
, and
Hans J. van der Woerd

Abstract

Vertical mixing is thought to play an essential role in phytoplankton blooms, yet measurements of mixing properties are very sparse. This paper presents a methodology to estimate profiles of the upper-ocean vertical mixing from satellite color observations, using a coupled turbulence–phytoplankton model and data assimilation–based calibration techniques. The method is tested at a location in the eastern North Atlantic for which an integrated set of observations (vertical mixing, phytoplankton, nutrients) is available. Results of identical twin experiments show that the method is very robust and achieves accurate turbulence model parameter calibrations even with noisy or sparsely sampled observations. The application of surface chlorophyll-a (Chl a) concentration to MODIS Aqua satellite observations leads two independent cases (data for the years 2009 and 2011) to a calibration of the model parameterization that produces weaker winter mixing compared to the standard configuration. As a consequence of the weaker mixing, the timing and intensity of increased surface Chl a satellite observations in spring and summer was reproduced by the model. Moreover, the weaker mixing resembles the in situ observations of vertical mixing better than the stronger mixing based on the standard configuration. This shows that the new calibration indeed improves the performance of the turbulence model.

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Leela M. Frankcombe
,
Anna von der Heydt
, and
Henk A. Dijkstra

Abstract

The issue of multidecadal variability in the North Atlantic has been an important topic of late. It is clear that there are multidecadal variations in several climate variables in the North Atlantic, such as sea surface temperature and sea level height. The details of this variability, in particular the dominant patterns and time scales, are confusing from both an observational as well as a theoretical point of view. After analyzing results from observational datasets and a 500-yr simulation of an Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) climate model, two dominant time scales (20–30 and 50–70 yr) of multidecadal variability in the North Atlantic are proposed. The 20–30-yr variability is characterized by the westward propagation of subsurface temperature anomalies. The hypothesis is that the 20–30-yr variability is caused by internal variability of the Atlantic Meridional Overturning Circulation (MOC) while the 50–70-yr variability is related to atmospheric forcing over the Atlantic Ocean and exchange processes between the Atlantic and Arctic Oceans.

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Elian Vanderborght
,
Jonathan Demaeyer
,
Georgy Manucharyan
,
Woosok Moon
, and
Henk A. Dijkstra

Abstract

In recent theory trying to explain the origin of baroclinic low-frequency atmospheric variability, the concept of eddy memory has been proposed. In this theory, the effect of synoptic-scale heat fluxes on the planetary-scale mean flow depends on the history of the mean meridional temperature gradient. Mathematically, this involves the convolution of a memory kernel with the mean meridional temperature gradient over past times. However, the precise shape of the memory kernel and its connection to baroclinic wave dynamics remains to be explained. In this study we use linear and proxy response theory to determine the shape of the memory kernel of a truncated two-layer quasigeostrophic atmospheric model. We find a memory kernel that relates the eddy heat flux to the zonal mean meridional temperature gradient on time scales greater than 2 days. Although the shape of the memory kernel is complex, we show that it may be well approximated as an exponential, particularly when reproducing baroclinic low-frequency intraseasonal modes of variability. By computing the terms in the Lorenz energy cycle, we find that the shape of the memory kernel can be linked to the finite time that growing baroclinic instabilities require to adapt their growth properties to the local zonal mean atmospheric flow stability. Regarding the explanation for observed baroclinic annular modes in the Southern Hemisphere, our results suggest that it is physical for these modes to be derived directly from the thermodynamic equation by considering an exponentially decaying memory kernel, provided accurate estimates of the necessary parameters are incorporated.

Significance Statement

The goal of this study was to derive the memory of the zonal mean temperature field contained in eddy heat fluxes. To do this we used recent developments in a theory stemming from statistical mechanics, called proxy response theory. This theory facilitated direct numerical computations of the parameterization that links eddy heat fluxes to the zonal mean temperature field. Notably, this parameterization incorporates a crucial memory component, which we demonstrated to be essential in explaining the periodicity of low-frequency modes of variability, specifically the baroclinic annular mode (BAM). Understanding the role of memory as a driver of this variability holds great significance, as the BAM constitutes a dominant pattern of large annular variability within the Southern Hemisphere circulation. Enhanced comprehension of this driver, which is memory, can lead to improved understanding and predictive capabilities concerning observed annular weather patterns.

Open access
Henk A. Dijkstra
and
Wilhelmus P. M. de Ruijter

Abstract

From previous model studies, it has become clear that several physical mechanisms may be at work in the retroflection of the Agulhas Current. Here, a systematic study of steady barotropic flows connecting the Indian Ocean and South Atlantic Ocean in several idealized setups is performed. By solving directly for the steady circulation with continuation methods, the connection between different retroflection regimes can be monitored as external conditions, such as the wind forcing or bottom topography, as well as parameters, such as the lateral friction and layer depth, are changed. To distinguish the different steady retroflecting flows, an objective measure of the degree of retroflection, a retroflection index R , is introduced. By monitoring R along a branch of steady solutions, using the horizontal friction as control parameter, several steady retroflecting regimes are found. At large friction there exist stable steady states with viscously dominated retroflection. When friction is decreased, inertial effects become more dominant, and eventually unstable steady states with strong retroflection characteristics exist. Within this framework, different results from earlier studies can be reconciled.

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