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G. Lenderink and R. J. Haarsma

Abstract

In this study the variability of the thermohaline circulation on decadal and centennial time scales that is related to the process of deep-water formation is investigated. This is done within the context of a simple geostrophic three-layer ocean model with a rectangular closed basin. When slowly varying the atmospheric forcing the model response shows sudden transitions, characterized by local changes in convective activity. Many different equilibria were found within the thermally driven regime (in Stommel's sense, i.e., downwelling occurs near the poles). Next, the deep-water formation process was analyzed with a one-dimensional box model. In this box model four different regimes can be identified: convective, nonconvective, periodic, and a regime where both convection and no convection are possible with the same mixed boundary conditions. These regimes were identified in the circulation of the ocean model. A region was traced where convection is possible according to the authors’ analysis but not occurring. In this potentially convective region convection can be triggered easily. Similarly. there exists a region where the deep-water formation is easily suppressed. These sensitive areas generate multiple equilibria in the ocean model and contain a simple mechanism for variability on decadal and centennial time scales in the climate system.

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G. Lenderink and R. J. Haarsma

Abstract

A substantial part of the variability in the thermohaline circulation on decadal and longer timescales as found in ocean models is strongly related to the deep-water formation (DWF) process. Many of these studies, however, neglected the role of sea ice. Nevertheless, it is known that sea ice strongly influences the heat and salt budget of the polar ocean. In this study, DWF is studied in the presence of sea ice. This is done within the context of a simple geostrophic 6-level ocean model with a rectangular basin coupled to a thermodynamical sea ice model. The model is forced by mixed boundary conditions (MBCs). In general, the area of DWF changes rather discontinuously due to the self-sustaining character of convection under MBCS. For long times the DWF area remains basically unchanged, then, suddenly. the extension and/or location of the convective area dramatically changes in a few years time. Also, multiple equilibria are observed under MBCS. In this study, the main emphasis will be put on a case with rapid growth of the convective area. The authors start from a stationary state with a relatively small convective area and a large ice covered area. Then a rapid growth of the convective area is triggered by introducing a local salinity anomaly. The rapid growth is accompanied by a rapid increase in heat loss to the atmosphere. At the same time the ice cover retreats. The release of freshwater by melting is a negative feedback, which tends to stabilize the stratification and hence interrupt convection and prevent further ablation of the ice cover. This negative feedback is locally very effective; however, it can (but not necessarily) be compensated by the horizontal advection and diffusion of saline water beneath the ice. In the experiments the source of saline water is provided by convection south of the ice cover. The growth of the convective area, the retreat of the ice cover and, hence, the atmospheric heat flux strongly depend on the strength of this salt source. Also, a method is -presented to estimate the importance of convection for the simulated changes in the ocean temperature and salinity fields and atmospheric heat flux.

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R. J. Haarsma and J. D. Opsteegh

Abstract

We have investigated the nonlinear steady-state response of a barotropic model to an estimate of the observed anomalous tropical divergence forcing for the El Niño winter of 1982/83. The 400 mb climatological flow was made a forced solution of the model by adding a relaxation forcing. The Rayleigh friction coefficient (ε−1 = 20 days) was chosen such that this solution is marginally stable. The steady states were computed as a function of a dimensionless parameter α, that governs the strength of the anomalous forcing. The computed steady-state curve deviates markedly from a straight line, displaying a fold and an isolated branch. The linear steady state (α ≪ 1) compares well with the observed seasonal mean anomaly pattern. After the fold at α = 0.65, the agreement is smaller. A further increase in α after the fold results in saturation of the response. The streamfunction patterns of the isolated branch display unrealistically large amplitudes.

Time integrations show that the steady states govern the time-dependent behavior despite their unstable nature. The resulting time-mean patterns are very similar to the steady states. Periodic, quasi-periodic, and complete chaotic behavior are observed.

Increasing the Rayleigh friction coefficient to ε−1 = 10 days results in a disappearance of the fold as well as the isolated branch. As for ε−1 = 20 days, the agreement between the steady-state response and the observed pattern decreases when α is increased.

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R. J. Haarsma and J. D. Opsteegh

Abstract

The relevance of barotropic instability for the observed low-frequency variability in the atmosphere is investigated. The stability properties of the shallow-water equations on a sphere are computed for small values of Lamb's parameter (F = α2Ω2/gHe) where a is the earth's radius, Ω its angular velocity, g gravity and He the equivalent depth. For small values of F these equations describe the horizontal structure of external and deep internal modes that are basically barotropic in the troposphere.

The stability of simple zonal flows, as well as free and forced planetary Rossby waves, has been computed as a function of F. This is done numerically using a hemispheric spectral model with a T13 truncation. For F = 0 we have tried to interpret the numerical results by analytically computing the stability properties of the flow when only one triad is considered. The results show that for increasing F the critical amplitudes for instability decrease slightly, but in the area of instability both growth rate and frequency of the perturbations decrease with increasing F. The horizontal structure of the perturbations changes only slightly. In most cases the instability process occurs within one triad which is the triad closest to resonance. An analysis in terms of unstable triads stems equally relevant for zonal and for nonzonal flows. The stability properties of the observed 400 mb Northern Hemisphere winter climatological flow show the same dependence on F as found for simple flow patterns: both growth rate and frequency of the perturbations decrease for increasing F.

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F. M. Selten, R. J. Haarsma, and J. D. Opsteegh

Abstract

North Atlantic decadal climate variability is studied with a coupled atmosphere–ocean–sea ice model (ECBILT). After having reached an approximate statistical equilibrium in coupled mode without applying flux corrections, a subsequent 1000-yr integration is performed and analyzed. Compared to the current climate, the surface temperatures are 2°C warmer in the Tropics to almost 8°C warmer in the polar regions.

The covariability between the atmosphere and ocean is explored by performing a singular value decomposition (SVD) of boreal winter SST anomalies and 800-hPa geopotential height anomalies. The first SVD pair shows a red variance spectrum in SST and a white spectrum in 800-hPa height. The second mode shows a peak in both spectra at a timescale of about 16–18 yr. The geopotential height pattern is the model’s equivalent of the North Atlantic oscillation (NAO) pattern; the SST anomaly pattern is a north–south oriented dipole.

Additional experiments have revealed that the decadal oscillation in ECBILT is basically an oscillation in the subsurface of the ocean. The oscillation is excited by anomalies in the atmospheric NAO pattern, both through anomalous surface heat fluxes and anomalous Ekman transports. The atmospheric response to the SST anomaly enhances the oscillation and slightly modifies it, but is not essential. The atmospheric response consists primarily of a local surface air temperature adjustment to the SST anomaly. An important element in the physical mechanism of the oscillation is the geostrophic response of the ocean circulation to the forced temperature anomalies creating surface salinity anomalies through anomalous horizontal advection. These salinity anomalies influence the convective activity in the area of the temperature anomaly such as to break down the subsurface temperature anomaly. Both temperature and salinity anomalies slowly propagate eastward at a rate consistent with the mean current.

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R. J. Haarsma, F. M. Selten, and J. D. Opsteegh

Abstract

The variability in the subpolar Southern Hemisphere is studied with a coupled atmosphere–ocean–sea-ice model (the ECBilt). After having reached an approximate statistical equilibrium in coupled mode without flux corrections, a subsequent 1000-yr integration is performed and analyzed. A singular value decomposition of austral winter SST anomalies and 800-hPa geopotential height in the Antarctic Circumpolar Current region reveals a mode of covariability that resembles the observed Antarctic circumpolar wave. Subsequent analysis of this mode shows that it is basically an oscillation in the subsurface of the ocean. Additional experiments suggest that it is generated by the advective resonance mechanism: the oscillation is excited by the dominant modes of variability in the atmosphere, whereas the timescale is set by the ratio of the horizontal scale of these atmospheric modes and the advection velocity of the mean oceanic currents. The atmospheric response mainly consists of a local temperature adjustment to the SST anomaly, which reduces the damping of the SST anomalies. Salinity, wind stress, and sea-ice anomalies do modify the structure and intensity of the mode without playing an essential role.

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Regina R. Rodrigues, Edmo J. D. Campos, and Reindert Haarsma

Abstract

The impact of El Niño–Southern Oscillation (ENSO) on the South Atlantic subtropical dipole mode (SASD) is investigated using both observations and model simulations. The SASD is the dominant mode of coupled ocean–atmosphere variability in the South Atlantic. This study focuses on austral summer, when both ENSO and SASD peak. It is shown that negative SASD events are associated with central Pacific El Niño events by triggering the Pacific–South American wave train (PSA). The latter resembles the third leading mode of atmospheric variability in the Southern Hemisphere (PSA2) and causes a weakening and meridional shift of the South Atlantic subtropical high, which then generates the negative SASD events. On the other hand, a strengthening of the South Atlantic subtropical high related to central La Niña teleconnections causes positive SASD events. The results herein show that the PSA2, triggered by central Pacific ENSO events, connects the tropical Pacific to the Atlantic. This connection is absent from eastern Pacific ENSO events, which appear to initiate the second leading mode of atmospheric variability in the Southern Hemisphere (PSA1). It is for this reason that previous studies have found weak correlations between ENSO and SASD. These findings can improve the climate prediction of southeastern South America and southern Africa since these regions are affected by sea surface temperature anomalies of both the Pacific and Atlantic Oceans.

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Reindert J. Haarsma, Edmo J. D. Campos, Wilco Hazeleger, Camiel Severijns, Alberto R. Piola, and Franco Molteni

Abstract

Using an atmosphere model of intermediate complexity and a hierarchy of ocean models, the dominant modes of interannual and decadal variability in the South Atlantic Ocean are studied. The atmosphere Simplified Parameterizations Primitive Equation Dynamics (SPEEDY) model has T30L7 resolution. The physical package consists of a set of simplified physical parameterization schemes, based on the same principles adopted in the schemes of state-of-the-art AGCMs. It is at least an order of magnitude faster, whereas the quality of the simulated climate compares well with those models. The hierarchy of ocean models consists of simple mixed layer models with an increasing number of physical processes involved such as Ekman transport, wind-induced mixing, and wind-driven barotropic transport. Finally, the atmosphere model is coupled to a regional version of the Miami Isopycnal Coordinate Ocean Model (MICOM) covering the South Atlantic with a horizontal resolution of 1° and 16 vertical layers.

The coupled modes of mean sea level pressure and sea surface temperature simulated by SPEEDY–MICOM strongly resemble the modes as analyzed from the NCEP–NCAR reanalysis, indicating that this model configuration possesses the required physical mechanisms for generating these modes of variability. Using the ocean model hierarchy the authors were able to show that turbulent heat fluxes, Ekman transport, and wind-induced mixing contribute to the generation of the dominant modes of coupled SST variability. The different roles of these terms in generating these modes are analyzed. Variations in the wind-driven barotropic transport mainly seem to affect the SST variability in the Brazil–Malvinas confluence zone.

The spectra of the mixed layer models appeared to be too red in comparison with the fully coupled SPEEDY–MICOM model due to the too strong coupling between SST and surface air temperatures (SATs), resulting from the inability to advect and subduct SST anomalies by the mixed layer models. In SPEEDY–MICOM anomalies in the southeastern corner of the South Atlantic are subducted and advected toward the north Brazilian coast on a time scale of about 6 yr.

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Regina R. Rodrigues, Reindert J. Haarsma, Edmo J. D. Campos, and Tércio Ambrizzi

Abstract

In this study, observations and numerical simulations are used to investigate how different El Niño events affect the development of SST anomalies in the Atlantic and how this relates to the Brazilian northeast (NE) precipitation. The results show that different types of El Niño have different impacts on the SST anomalies of the equatorial and tropical South Atlantic but a similar SST response in the tropical North Atlantic. Strong and long (weak and short) El Niños with the main heating source located in the eastern (central) Pacific generate cold (warm) anomalies in the cold tongue and Benguela upwelling regions during boreal winter and spring. When the SST anomalies in the eastern equatorial and tropical South Atlantic are cold (warm), the meridional SST gradient across the equator is positive (negative) and the ITCZ is not allowed (allowed) to move southward during the boreal spring; as a consequence, the precipitation is below (above) the average over the NE. Thus, strong and long (weak and short) El Niños are followed by dry (wet) conditions in the NE. During strong and long El Niños, changes in the Walker circulation over the Atlantic and in the Pacific–South Atlantic (PSA) wave train cause easterly wind anomalies in the western equatorial Atlantic, which in turn activate the Bjerknes mechanism, establishing the cold tongue in boreal spring and summer. These easterly anomalies are also responsible for the Benguela upwelling. During short and weak El Niños, westerly wind anomalies are present in the western equatorial Atlantic accompanied by warm anomalies in the eastern equatorial and tropical South Atlantic; a positive phase of the South Atlantic dipole develops during boreal winter. The simulations highlight the importance of ocean dynamics in establishing the correct slope of the equatorial thermocline and SST anomalies, which in turn determine the correct rainfall response over the NE.

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S. S. Drijfhout, A. Kattenberg, R. J. Haarsma, and F. M. Selten

Abstract

Three 1000-yr climate simulations with an atmospheric general circulation model (AGCM) coupled to, respectively, a slab mixed layer model, an ocean GCM, and responding to yearly repeating daily sea surface temperature (SST) and sea-ice coverage climatology derived from the fully coupled run were analyzed and compared. When coupled to a slab mixed layer, surface air temperature (SAT) and SST are strongly coupled and the reddening is significantly larger than in the case of coupling to a dynamically active ocean. A simple one-dimensional stochastic model is developed to explain the different spectra of SAT above land and ocean. It is argued that ocean advection generating SST variability that does not match the principal modes of SAT above the ocean is the main factor in damping SAT variability. The variability of SAT and 800-hPa geopotential height (GEO) and covariability of SST–SAT and SST–GEO have been analyzed, and it is found that coupling does not change the dominant patterns of atmospheric variability, but it affects the spectra. The relative importance of the dominant patterns of variability is not affected by coupling, nor do significant peaks arise in the spectra. Coupling does give rise to preferred modes of covariability between SST and SAT or GEO. A dynamically active ocean affects the spectra of these modes and occasionally gives rise to a significant spectral peak on decadal to interdecadal timescales. Also, a dynamical ocean affects SAT spectra above sea by a systematic deviation from the fitted AR(1) process.

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