Decadal variability in an OGCM Southern Ocean: Intrinsic modes,forced modes and metastable states

An ocean general circulation model (OGCM) is used to identify a Southern Ocean southeast Pacific intrinsic mode of low frequency variability. Using CORE data a comprehensive suite of experiments were carried out to elucidate excitation and amplification responses of this intrinsic mode to low frequency forcing (ENSO, SAM) and stochastic forcing due to high frequency winds. Subsurface anomalies were found to teleconnect the Pacific and Atlantic regions of the Antarctic Circumpolar Current (ACC) thermocline. The Pacific region of the ACC is characterised by intrinsic baroclinic disturbances that respond to both SAM and ENSO, while the Atlantic sector of the ACC is sensitive to higher frequency winds that act to amplify thermocline anomalies propagating downstream from the Pacific. Non-stationary cluster analysis was used to identify the system’s dynamical regimes and characterise meta-stability, persistence and transitions between the respective states. This analysis reveals significant trends, indicating fundamental changes to the meta-stability of the ocean dynamics in response to changes in atmospheric forcing. Intrinsic variability in sea-ice concentration was found to be coupled to thermocline processes. Sea-ice variability localised in the Atlantic was most closely associated with high frequency weather forcing. The SAM was associated with a circumpolar sea-ice response whereas ENSO was found to be a major driver of sea-ice variability only in the Pacific. This simulation study identifies plausible mechanisms that determine the predictability of the Southern Ocean climate on multi-decadal timescales.     

Kelvin Waves Excited by Intraseasonal Wind Forcing

The Kelvin wave excited by an intraseasonal wind forcing with a 40-day period over the western Pacific Ocean was simulated using an ocean general circulation model, and was investigated by the use of spectral analysis. The amplitude of the temperature has two peaks north and south of the equator at the depth of the thermocline, and the amplitude of zonal velocity also has two peaks on the equator above and below the thermocline. The phase shows the upward propagation of the wave. It was queried why this wave, which appears to be transient rather than modelike, is formed quickly and always propagates with a phase velocity of about 3 m/s. The vertical one-dimensional forcing problem was studied, where the external forcing of up and down motions moving eastward is imposed at the surface. The growth time is estimated from the resonant solution. The first mode can resonate quickly, but the second cannot. The response in the infinitely deep ocean was also studied to focus on the transiency, where the reflection from the bottom is inhibited. The wave response to the forcing with a speed of about 3 m/s has a large amplitude, i.e. quasi-resonance occurs. In this case, the thermocline plays the role of a reflector, and the upper ocean between the sea surface and the thermocline behaves as a duct. Here, the small resonant cavity explains why the wave is formed so quickly, and the special value of the wave velocity is interpreted as a resonance condition in the duct. The wave corresponding to the second baroclinic mode could not be excited easily by the short-lived forcing at the surface, since this mode is mainly structured under the thermocline. It was found that the wave damps in consequence of leaking energy downward, and the damping rate depends on the period of the wave. This revised version was published online in August 2006 with corrections to the Cover Date.     

Vertical structure variability in a seasonal simulation of a medium-resolution regional model of the Eastern South Pacific

A seasonal simulation from a medium-resolution ocean general circulation mode (OGCM) is used to investigate the vertical structure variability of the Southeast Pacific (SEP). The focus is on the extra-tropical Rossby wave (ETRW) variability and associated forcing mechanism. Some aspects of the model mean state are validated from available observations, which justifies a vertical mode decomposition of the model variability. The analysis of the baroclinic mode contributions to sea level indicates that the gravest mode is dominant over most of the domain at all frequencies. Annual variability is on average twice as large as the semi-annual variability which is confined near the coast for all the modes. The first baroclinic mode contribution to the annual cycle exhibits a clear westward propagation north of the critical latitude. The higher-order modes only contribute near the coast where they are associated with vertically propagating energy. The residual variability, which is the energy at all timescales other than annual and semi-annual periods peaks offshore between 20°S and 30°S for all baroclinic modes. The third baroclinic mode also exhibits a relative maximum variability off the coast of Peru south of the critical latitude of the annual cycle (13°S), where the Peru–Chile Undercurrent is the most intense. Sensitivity experiments to the atmospheric and boundary forcing suggest that the residual variability results from the non-linear interaction between annual Rossby waves and the mean flow, while the annual ETRWs in the model result from the summed-contribution from both the local wind stress and remote equatorial forcing. Overall the study extends the classical analysis of sea level variability in the SEP based on linear theory, and suggests that the peculiarities of the baroclinic modes need to be taken into account for interpreting the sea level variability and understanding its connection with the equatorial variability.     

Decadal-Scale Climate and Ecosystem Interactions in the North Pacific Ocean

Decadal-scale climate variations in the Pacific Ocean wield a strong influence on the oceanic ecosystem. Two dominant patterns of large-scale SST variability and one dominant pattern of large-scale thermocline variability can be explained as a forced oceanic response to large-scale changes in the Aleutian Low. The physical mechanisms that generate this decadal variability are still unclear, but stochastic atmospheric forcing of the ocean combined with atmospheric teleconnections from the tropics to the midlatitudes and some weak ocean-atmosphere feedbacks processes are the most plausible explanation. These observed physical variations organize the oceanic ecosystem response through large-scale basin-wide forcings that exert distinct local influences through many different processes. The regional ecosystem impacts of these local processes are discussed for the Tropical Pacific, the Central North Pacific, the Kuroshio-Oyashio Extension, the Bering Sea, the Gulf of Alaska, and the California Current System regions in the context of the observed decadal climate variability. The physical ocean-atmosphere system and the oceanic ecosystem interact through many different processes. These include physical forcing of the ecosystem by changes in solar fluxes, ocean temperature, horizontal current advection, vertical mixing and upwelling, freshwater fluxes, and sea ice. These also include oceanic ecosystem forcing of the climate by attenuation of solar energy by phytoplankton absorption and atmospheric aerosol production by phytoplankton DMS fluxes. A more complete understanding of the complicated feedback processes controlling decadal variability, ocean ecosystems, and biogeochemical cycling requires a concerted and organized long-term observational and modeling effort. This revised version was published online in July 2006 with corrections to the Cover Date.     

热带大西洋年际和年代际变率的时空结构模拟

使用美国夏威夷大学发展的中等复杂程度海洋模式(IOM)在给定表面强迫条件下模拟了热带大西洋上层海洋年际和年代际变率的时空结构.利用NCEP的41a(1958～1998年)逐月平均表面资料作为强迫场,积分海洋模式41a作为控制试验,并利用模式分别做动量(风应力)通量和热量通量无异常变化的平行试验,与控制试验作比较.对3组试验模拟上层海洋变率状况的比较,并按年际和年代际时间尺度分别分析,揭示表面风应力和热通量异常对海表面温度和温跃层深度变化的影响,并比较了其影响的相对重要性.结果表明模式成功地模拟出了热带大西洋上层海洋的变率.模式模拟的海表面温度年际变化主要表现为弱ENSO型,年代际变化表现为南、北大西洋变化相反的偶极子型.在年际时间尺度上,热力强迫和动力强迫对海表温度变化都有贡献,其中赤道外海表面温度异常(SSTA)变化主要由热通量异常引起,而近赤道SSTA的变化主要由动量异常强迫引起.在年代际时间尺度上,热通量强迫的作用远比动量强迫重要.模式不仅能够模拟SST在年际和年代际时间尺度上的变率,还能够模拟温跃层深度在年际和年代际时间尺度上的变率.年际和年代际时间尺度上,温跃层深度的变率主要由动量异常决定,热通量异常强迫的贡献很小.     

Interdecadal temperature variations in the North Pacific Central Mode Water simulated by an OGCM

The North Pacific Central Mode Water (CMW) is a water mass that forms in the Kuroshio-Oyashio Extension (KOE) region with characteristic low potential vorticity. Recent studies have suggested that the CMW, as low potential vorticity water, plays an important role in the adjustment of the subtropical gyre and subsurface variability on decadal to interdecadal timescales. We have forced a realistic ocean general circulation model (OGCM) with observed wind stress and sea surface temperature (SST) forcing to investigate the decadal variations of the CMW. Associated with the large atmospheric changes after the mid-1970s climate regime shift, the upper thermocline experiences a cooling as negative SST anomalies in the central North Pacific are subducted and advected southward. In addition to this thermodynamic response, the CMW’s path shifts anomalously eastward in response to anomalous Ekman pumping. This eastward shift of the core of the CMW produces a lowering of the isotherms, and a consequent warming, on the path of the CMW core. This warming partially counteracts the cooling associated with subducted surface anomalies, and it may be responsible for the reduced temperature variations at the climatological position of the CMW when both anomalous wind and heat fluxes are given. Lateral induction across the sloping bottom of the winter mixed layer in the KOE is critical to the formation of the low potential vorticity CMW. Coarse resolution models, which are widely used in climate modeling, underestimate the horizontal gradient of the mixed layer depth and form only a weak CMW or none at all. We have conducted a coarse resolution experiment with the same OGCM, showing that the subsurface response is much reduced. In particular, there is no dynamic warming in the CMW and the thermodynamic response to the SST cooling dominates. The resultant total response differs substantially from that in the finer resolution run where a strong CMW forms. This sensitivity to the model resolution corroborates the important dynamical role that the CMW may play with its distinctive low potential vorticity character and calls for its improved simulation.     

Observed intra-seasonal to interannual variability of the upper ocean thermal structure in the southeastern Arabian Sea during 2002–2008

The southeastern Arabian Sea (SEAS), located in the Indian Ocean warm pool, is a key-region of the regional climate system. It is suspected to play an important role in the dynamics of the Asian summer monsoon system. The present study reports the salient features derived from a newly harvested observational dataset consisting of repeated fortnightly XBT transects in the SEAS over the period 2002–2008. The fortnightly resolution of such a multi-year record duration is unprecedented in this part of the world ocean and provides a unique opportunity to examine the observed variability of the near-surface thermal structure over a wide spectrum, from intra-seasonal to interannual timescales. We find that most of the variability is trapped in the thermocline, taking the form of upwelling and downwelling motions of the thermal stratification. The seasonal variations are consistent with past studies and confirm the role of the monsoonal wind forcing through linear baroclinic waves (coastally-trapped Kelvin and planetary Rossby waves). Sub-seasonal variability takes the form of anomalous events lasting a few weeks to a few months and occurs at two preferred timescales: in the 30–110 day band, within the frequency domain of the Madden–Julian oscillation and in the 120–180 day band. While this sub-seasonal variability appears fairly barotropic in the offshore region, the sign of the anomaly in the upper thermocline is opposite to that in its lower part on many occasions along the coast. Our dataset also reveals relatively large interannual temperature variations of about 1 °C from 50 to 200 m depth that reflect a considerable year-to-year variability of the magnitude of both upwelling and downwelling events. This study clearly demonstrates the necessity for sustained long-term temperature measurements in the SEAS.     

Modeling of El Niño events using a simple model

In this paper, we present the results of modeling of El Niño events using a simple model of a classical oscillator with decay under external forcing. The sea surface temperature in the East Pacific and the mean thermocline depth in the Equatorial Pacific correspond to the roles of the momentum and position, respectively. The external forcing of the system is determined by two factors: the short-period meridional mass fluctuations in the Pacific sector of the Southern Ocean due to the joint effect of the atmospheric variability over the Antarctic Circumpolar Current, bottom topography, and coastlines; and the variability of western winds in the tropics. Under such conditions, oscillations of El Niño type arise in the model as a result of propagation of signals generated in the Southern Ocean (due to the fluctuations of meridional transport fluxes and the variability of western winds in the tropics). These signals propagate over the Equatorial Pacific as fast wave processes. It is shown that external forcing is the main factor in establishing the oscillation pattern of the model characteristics variability.     

A coupled ice–ocean model for the Greenland,Iceland and Norwegian Seas

Simulations from a coupled ice–ocean model that highlight the importance of synoptic forcing on sea-ice dynamics are described. The ocean model is a non-hydrostatic primitive equation model coupled to a dynamic thermodynamic sea ice model. The ice modelling sensitivity study presented here is part of an ongoing research programme to define the role played by sea ice in the energy balance of the Greenland Sea. The different categories of sea ice found in the subpolar regions are simulated through the use of equations for thin ice, thick ice and the Marginal Ice Zone. A basin scale numerical model of the Greenland, Iceland and Norwegian Seas has a horizontal resolution of 20 km and a vertical grid spacing of 50 m. This resolution is adequate for resolving the mesoscale topographic structures known to control the circulation in this region. The spin-up reproduces the main features of the circulation, including the cyclonic gyres in the Norwegian and Greenland Basins and Iceland Plateau. Topographic steering of the flow is evident. The baroclinic Rossby radius of deformation is between 5 and 10 km so that the model is not eddy-resolving. The coupled ice–ocean model was run for a period of two weeks. The influence of horizontal resolution of the atmospheric model was tested by comparing simulations using six hourly wind fields from the ECMWF with those generated using six hourly fields from a HIRLAM, with horizontal resolutions of 1° and 0.18° respectively. The simulations show reasonable agreement with satellite ice compactness data and data of ice transports across sections at 79°N, 75°N and Denmark Strait.     

Mesoscale, seasonal and interannual variability in the Mediterranean Sea using a numerical ocean model

In this paper, we present the results from a 1/8° horizontal resolution numerical simulation of the Mediterranean Sea using an ocean model (DieCAST) that is stable with low general dissipation and that uses accurate control volume fourth-order numerics with reduced numerical dispersion. The ocean model is forced using climatological monthly mean winds and relaxation towards monthly climatological surface temperature and salinity. The variability of the circulation obtained is assessed by computing the volume transport through certain sections and straits where comparison with observations is possible. The seasonal variability of certain currents is reproduced in the model simulations. More important, an interannual variability, manifested by changes in currents and water mass properties, is also found in the results. This may indicate that the oceanic internal variability (not depending on external atmospheric forcing), is an important component of the total variability of the Mediterranean circulation; variability that seems to be very significant and well documented by in situ and satellite data recovered in the Mediterranean Sea during the last decade.     

Reducing systematic errors by empirically correcting model errors

A methodology for the correction of systematic errors in a simplified atmospheric general‐circulation model is proposed. First, a method for estimating initial tendency model errors is developed, based on a 4‐dimensional variational assimilation of a long‐analysed dataset of observations in a simple quasi‐geostrophic baroclinic model. Then, a time variable potential vorticity source term is added as a forcing to the same model, in order to parameterize subgrid‐scale processes and unrepresented physical phenomena. This forcing term consists in a (large‐scale) flow dependent parametrization of the initial tendency model error computed by the variational assimilation. The flow dependency is given by an analogues technique which relies on the analysis dataset. Such empirical driving causes a substantial improvement of the model climatology, reducing its systematic error and improving its high frequency variability. Low‐frequency variability is also more realistic and the model shows a better reproduction of Euro‐Atlantic weather regimes. A link between the large‐scale flow and the model error is found only in the Euro‐Atlantic sector, other mechanisms being probably the origin of model error in other areas of the globe.     

Evaluation of a dynamically downscaled atmospheric reanalyse in the prospect of forcing long term simulations of the ocean circulation in the Gulf of Lions

The paper evaluates atmospheric reanalysis as possible forcing of model simulations of the ocean circulation inter-annual variability in the Gulf of Lions in the Western Mediterranean Sea between 1990 and 2000. The sensitivity of the coastal atmospheric patterns to the model resolution is investigated using the REMO regional climate model (18 km, 1 h), and the recent global atmospheric reanalysis ERA40 (125 km, 6 h). At scales from a few years to a few days, both atmospheric data sets exhibit a very similar weather, and agreement between REMO and ERA40 is especially good on the seasonal cycle and at the daily variability scale. At smaller scales, REMO reproduces more realistic spatio-temporal patterns in the ocean forcing: specific wind systems, particular atmospheric behaviour on the shelf, diurnal cycle, sea-breeze. Ocean twin experiments (1990–1993) clearly underline REMO skills to drive dominant oceanic processes in this microtidal area. Finer wind patterns induce a more realistic circulation and hydrology of the shelf water: unique shelf circulation, upwelling, temperature and salinity exchanges at the shelf break. The hourly sampling of REMO introduces a diurnal forcing which enhances the behaviour of the ocean mixed layer. In addition, the more numerous wind extremes modify the exchanges at the shelf break: favouring the export of dense shelf water, enhancing the mesoscale variability and the interactions of the along slope current with the bathymetry.     

Seasonal variability of the Antarctic Coastal Current and its driving mechanisms in the Weddell Sea

Insight into the dynamics of the Antarctic Coastal Current (ACoC) is achieved by quantifying the contributions of its driving mechanisms to the seasonal variability of its barotropic and baroclinic components. These mechanisms are sought out in the local wind, the sea-ice concentration, wind curl of the Weddell Gyre (Sverdrup transport) and the thermohaline forcing related to warming/cooling and ice melting and freezing. These driving mechanisms induce most of the seasonal variability of both the barotropic and baroclinic components of the ACoC by deepening the pycnocline towards the coast and sharpening the baroclinic profile following thermal wind balance. The resulting coastal current has mainly a barotropic transport (82%) and a major annual cycle, which explains 37% of this component's variability (tides and other high-frequency events generate 40%). The wind contributes with 58% of the seasonal variability of the barotropic component and 23% of the baroclinic; the sea-ice concentration contributes with 8% and 18%, respectively; Sverdrup transport with 4% and 30% and the thermohaline forcing with 30% and 29%. The results of this study are obtained with analysis of fifteen CTD sections (potential density and geostrophic velocities) of RV-Polarstern obtained between 1992 and 2005, as well as composite, spectral and harmonic analyses of 9 years of time series from moored instruments (current speed and temperature), wind speed, atmospheric pressure and sea-ice concentration of satellite imagery.     

First and second baroclinic mode responses of the tropical Indian Ocean to interannual equatorial wind anomalies

The combined and individual responses of the first and second baroclinic mode dynamics of the tropical Indian Ocean to the well-known Indian Ocean Dipole mode (IOD) wind anomalies are investigated. The IOD forced first baroclinic Rossby waves arrive at the western boundary in three months, while the reflected component from the eastern boundary with opposite phase arrives in five to six months, both carry input energy to the west. The inclusion of the second baroclinic mode slows down the wave propagation by mode coupling and stretches the energy spectrum to a relatively longer time scale. The total energy exists in the equatorial wave guide for at least five months from the forcing, as much as 10% of that of the atmospheric input, which mainly dissipates at the western boundary. The individual responses of the ocean to IOD interannual wind anomaly show that the significant modes of oceanic anomalies are confined to a wave guide of 10° on either side of the equator.     

Effects of horizontal mixing on the upper ocean temperature in the equatorial Pacific Ocean

The influence of horizontal mixing on the thermal structure of the equatorial Pacific Ocean is examined based on a sigma coordinate model.In general,the distributions of the temperature and currents si...     

An application of a two-layer model to wind driven sub-tidal currents in Puget Sound

A two layer model of an infinitely long channel, with one end closed, is applied to study the sub-tidal response to wind forcing of Puget Sound. The model uses a linear friction parameterization. Data show that the acceleration of current near the surface responds to the wind event almost instantaneously, however, acceleration tends to start decreasing at later times and eventually changes sign even though the wind blows in one direction throughout. Analysis of the model results show that when the forcing frequency is high, the phase lag between forcing and friction causes this phenomena, and as forcing frequency increases, phase lag between forcing and friction approaches /2. When the forcing frequency is low, phase lag between forcing and friction decreases almost linearly with forcing frequency and at extremely low frequency, they almost balance each other. Analysis of the model results show also that the amplitude of baroclinic pressure gradient increases rapidly as forcing frequency decreases and when the forcing frequency is low, the baroclinic pressure gradient becomes important. Effects of baroclinic pressure gradient propagate as a wave from the boundaries and it takes about one day to take effect at the point where the observations were made.     

Observation of ocean current response to 1998 Hurricane Georges in the Gulf of Mexico

1 Introduction Hurricane is an extremely high wind event, which injects momentum into the oceanic mixed layer along its passage for a very short duration. If our interest is not at the surface, but in a depth away from the imme-diate surface wave influenc…     

Water exchange of the Stockholm archipelago—a cascade framework modelling approach

The Stockholm archipelago spans roughly a semicircular area with a radius of approximately 60 km, traditionally partitioned into three parts: the inner, the middle and the outer archipelago. This subdivision coincides with differing water exchange regimes. The inner and middle archipelagos are characterised by comparatively larger basins which are interconnected by a limited number of straits. This configuration is well suited for a discrete basin (DB-) model approach by partitioning the area into a set of sub-basins that are only resolved vertically. The advantage of this approach over 3D-models is the possibility for enhanced vertical resolution and improved strait exchange formulation, outweighing the disadvantage of neglected horizontal gradients within the basins. In the inner archipelago the dominating exchange process is estuarine circulation, induced by the marked freshwater discharge and the vertical mixing. In the outer and middle archipelagos the density fluctuations due to Ekman pumping along the Baltic boundary interface produce another type of baroclinic process that clearly dominates. Measurements to adequately resolve these density variations do not exist. Missing forcing data are provided by linking the middle archipelago's boundary straits to a 3D-model of the Baltic with a grid resolution of 0.5 nautical miles (n.m.). This fine resolution model (FR-domain) is in turn driven by the atmospheric forcing and the density variation at the rectangular boundary of the FR-domain which acceptably resolves both the interfacial straits and the outer archipelago's complex hypsography. Massive computing resources would be demanded if the FR-domain were extended to comprise the entire Baltic. The FR-domain is thus interfaced with an existing coarse resolution model of the entire Baltic (CR-domain) with a grid size of 5 n.m., the open boundary of which is located in the Kattegat. This 3-fold model set-up has been run for one whole year (1992) with a one-year spin-up time to make up for the lack of initial data. The model concept is at this stage to be regarded as a framework for further development in anticipation of improved formulations, particularly for the strait exchange formulation. Therefore only primary validation experiments and a few sensitivity analyses have been performed.     

Variability of the North Pacific Current and its bifurcation

The North Pacific Current (NPC) bifurcates approaching the west coast of North America into a subpolar branch that forms the Alaska Current, and a subtropical branch that includes the California Current. The variability of this current system is discussed using numerical results from a wind-driven, reduced-gravity model. Indices of the strength of the subpolar and subtropical components of the NPC are examined based on output from multi-decadal simulations with the numerical model. This shows periods of both correlated and anti-correlated variability of the subpolar and subtropical gyres. A decomposition of the gyre transport time series indicates that the dominant mode of variability is a “breathing” mode in which the subpolar and subtropical gyres co-vary in response to fluctuations in the strength of the NPC. This finding is consistent with an analysis of dynamic height data of limited duration from the array of Argo drifting floats.The variability of the NPC is also examined using sea surface height (SSH) data from satellite altimetry over the period 1993-2005. The leading mode of SSH over the northeast Pacific dominates the variability of the NPC and is shown to be associated with in-phase variations in the transport of the subtropical and subpolar gyres. A strong correlation is found between time-dependent fluctuations in SSH across the NPC and variations in the strength of the transport of the NPC in the model. This agreement provides evidence for variability of the NPC occuring in direct response to large-scale atmospheric forcing.     

Numerical analysis of the mesoscale features of circulation in the Black Sea coastal zone

The results of simulating the hydrophysical fields of the Black Sea with a resolution of 1.64 × 1.64 km for January–September 2006 with the use of real atmospheric forcing are analyzed. Both vertical turbulent momentum exchange and vertical turbulent heat and salt diffusions are parameterized using the Mellor-Yamada level 2.5 scheme. The results of this numerical experiment are compared with similar data obtained with a horizontal resolution of 5 km. The features of the meso- and submesoscale dynamics of waters in individual sea regions are given. Possible physical mechanisms of forming meso- and submesoscale vortices are studied on the basis of energy analysis. It is shown that, in the absence of significant wind forcing, the main contribution to kinetic energy is made by the buoyancy force and wind-field inhomogeneities result in significant variations in both total vertical viscosity and total vertical diffusion.