Synoptic-scale Eddies Research Articles

A Barotropic Envelope Rossby Soliton Model for Block–Eddy Interaction. Part I: Effect of Topography

AbstractA new forced envelope Rossby soliton model in an equivalent barotropic beta-plane channel is proposed to describe the interaction between an incipient block (planetary scale) and short synoptic-scale eddies. This model is based on two assumptions, motivated by observations that (i) there exists a zonal scale separation between the planetary-scale and synoptic-scale waves and (ii) that the range of synoptic-scale zonal wavenumber is comparable to the planetary-scale zonal wavenumber. These assumptions allow an analytical treatment. The evolution of the planetary-scale block under the influence of synoptic-scale eddies is described by a forced nonlinear Schrödinger equation that is solved numerically, while the feedback of block development on the preexisting synoptic-scale eddies is derived analytically. It is shown that the planetary-scale projection of the nonlinear interaction between synoptic-scale eddies is the most important contributor to the amplification and decay of the planetary-scale blocking dipole or anticyclone, while the synoptic–planetary-scale interaction contributes significantly to the downstream development of preexisting synoptic-scale eddies. Large-scale topography plays a secondary role compared to the synoptic-scale eddies in exciting the block. However, it plays a role in inducing a standing planetary-scale ridge prior to block onset, which fixes the geographical location of the block and induces meridional asymmetry in the flow. In particular, the topographically induced planetary-scale ridge that is almost in phase with a dipole component of blocking flow is found to be a controlling factor for the northward deflection of storm tracks associated with blocking anticyclones.

Interaction between envelope soliton vortex pair blocks and synoptic-scale eddies in an inhomogeneous baroclinicity environment

In this paper, the importance of an inhomogeneous baroclinicity in the onset of an isolated vortex pair block associated with synoptic-scale eddies is examined theoretically in a two-layer model. The vertical shear of basic zonal wind is assumed to vary slowly in the zonal direction, to permit the representation of zonally localized regions of high baroclinicity. It is found that the nonlinear resonance between an incipient dipole block and the eddy forcing arising from pre-existing synoptic-scale eddies is more easily met in an inhomogeneous baroclinicity environment than in a barotropic environment. An isolated barotropic vortex pair block excited by the synoptic eddies can be remarkably enhanced when the position of the maximum baroclinicity is within a moderate range upstream of a weak diffluent dipole flow prior to block onset. The high baroclinicity upstream tends to make the intensified barotropic vortex pair structure more persistent and shift more westward than in a barotropic system; but during the decay phase of a vortex pair block the high baroclinicity will promote the development of synoptic weather systems downstream of the vortex pair block, which becomes particularly noticeable under unstable eddy forcing. In contrast, high baroclinicity will prevent the onset and persistence of a vortex pair block if the high baroclinicity is very close to an initial diffluent dipole flow. On the other hand, strong meridional eddy heat fluxes are found to precede the full establishment of a vortex pair block under certain conditions. Thus, it is suggested that a moderately high baroclinicity is a favourable environment for the onset and maintenance of a vortex pair block. Copyright © 2005 Royal Meteorological Society

A Barotropic Envelope Rossby Soliton Model for Block–Eddy Interaction. Part II: Role of Westward-Traveling Planetary Waves

Abstract The role of westward-traveling planetary waves in the block onset and the deformation of eddies during the interaction between synoptic-scale eddies and an incipient block is first examined by constructing an incipient block that consists of a stationary dipole wave for zonal wavenumber 2 and a westward-traveling monopole wave with constant amplitude (C wave) for zonal wavenumber 1 or 2. It is shown that the C-wave can affect the onset and strength of blocking through influencing the preblock (diffluent) flow even though it does not affect the amplification of the dipole wave associated with the synoptic-scale eddies. Whether the storm tracks organized by the deformed eddies deflect northward depends upon the zonal wavenumber, amplitude, and phase of the C wave relative to the stationary dipole wave. A typical retrograde blocking anticyclone can arise through the interaction of an incipient block with synoptic-scale perturbations when the C-wave ridge with zonal wavenumber 1 shifts westward from the east of the dipole wave in an incipient block. In this process, a slight northward deflection of organized storm tracks is also observed, particularly under the condition of a large-amplitude C wave. In addition, the interaction between a diffluent flow, consisting of a coupled dipole and monopole waves, and upstream synoptic-scale eddies is investigated. It is found that the eddy forcing tends to not only periodically amplify the dipole soliton and to retard its eastward movement, but to make the monopole wave break up. The breaking of the traveling monopole wave will suppress the eddy-induced blocking ridge that exhibits a surf zone structure where the negative meridional gradient of planetary-scale potential vorticity exists and cause the planetary-scale blocking field to lose its closed circulation compared to that without coupling.

Validation of a global reanalysis model in representing synoptic–scale eddies using scatterometer data: A case study

[1] A synoptic-scale cyclonic eddy was captured in the National Centers for Environmental Prediction (NCEP) global reanalysis surface winds in the Bay of Bengal and the Arabian Sea in October 1996. The eddy was formed in the Bay of Bengal in the middle of October 1996. It migrated across southern India, entered the Arabian Sea, and moved along the west coast of India before it turned and propagated across the Arabian Sea toward the Somali coast. The eddy eventually dissipated near Socotra in early November. The performance of the model winds is validated using the National Aeronautics and Space Administration Scatterometer (NSCAT) data along swath measurements. The model is able to resolve the eddy. The results compare well with observations in circulation intensity but differ in eddy center locations and, therefore, in westward propagation speeds in the late stage of the eddy life cycle.

FICKIAN DIFFUSION AND NEWTONIAN COOLING: A CONCEPT FOR NOISE INDUCED CLIMATE VARIABILITY WITH LONG-TERM MEMORY?

Observed near surface air and soil temperature time series reveal a long-term memory, which is associated with a power-law scaling of the frequency spectra, S(ω) ~ ω- β with β ~ 0.6, lying between white and flicker noise, 0 < β < 1. As this power law scaling is not consistent with the Brownian motion concept of climate variability, Fickian diffusion is added to a Newtonian cooling relaxation to provide a more suitable analog of climatic fluctuations: (i) Diffusive plus random heat fluxes parametrise the turbulent mixing by synoptic scale eddy life cycles, affect tropospheric and near surface temperatures and excite a long-term memory regime with a β ~ 0.5 scaling. (ii) Newtonian cooling describes the near surface temperatures relaxing towards a global mean deep soil temperature and stabilises the system to a white noise response at very low frequencies. The long-term memory regime emerges from the high frequency scaling (β ~ 1.5), once temperatures become correlated in space due to diffusion, so that spatially averaged fluctuations correlate for times beyond the diffusion time scale. The long-term memory regime disappears into a white noise plateau (β ~ 0), when low frequencies exceed the damping time scale of Newtonian cooling. This system may be interpreted as a diffusive system relaxing towards the deep soil restoration temperature with an almost infinitely large time scale.

Life cycles of blocking flows associated with synoptic—scale eddies: Observed results and numerical experiments

An observational study is presented to confirm that the life cycle of blocking associated with synoptic—scale eddies is a transfer process between dispersion and nondispersion. In addition, numerical experiments are conducted. It is found that the synoptic—scale eddies seem to play a dominant role in the amplification of blocking, while the topography effect appears to play a phase-locking role. At the same time, the synoptic eddies tend to split into two branches during the onset of blocking. This supports the observational results obtained.

ANTICYCLONIC EDDIES OVER THE NORTHWESTERN CONTINENTAL SLOPE IN THE BLACK SEA AND TRANSPORT OF CHLOROPHYLL-RICH WATERS INTO ITS ABYSSAL BASIN

A series of infrared NOAA satellite images of the northwestern Black Sea (resolution 1 km) and corresponding maps of chlorophyll “a” (SeaWiFS data, resolution 9 km) are analyzed for the period 1 June-28 August 1998. A major objective was detection of the movement of three synoptic-scale anticyclonic eddies over the continental slope and the entrainment of photopigment-rich coastal waters by them. The study examines the effects of wind direction on the visibility of eddies in the chlorophyll field, and attempts to identify conditions favoring the transport of chlorophyll-rich shelf waters into the sea's abyssal basin.

Barotropic interaction between planetary- and synoptic-scale waves during the life cycles of blockings

In this paper, in an equivalent barotropic framework a new forced nonlinear Schroedinger equation is proposed to examine the interaction between the planetary-scale waves and the localized synoptic-scale eddies upstream. With the help of the perturbed inverse scattering transform method, nonlinear parameter equations can be derived to describe the evolution of the dipole soliton amplitude, frequency, group velocity and phase under the forcing of localized synoptic-scale eddies. The numerical solutions of these equations predict that in the interaction between the weak dipole soliton (weak incipient dipole anomaly) and the synoptic-scale eddies, only when the high-frequency eddies themselves have a moderate parameter match they can near resonantly enhance a quasi-stationary large-amplitude split flow. The instantaneous total streamfunction field (the sum of background westerly wind, envelope Rossby soliton and synoptic-scale waves) is found to be very similar to the observed Berggren-type blocking on the weather map(Berggren et al. 1949). The role of synoptic-scale eddies is to increase the amplitude of large-scale dipole anomaly flow, and to decrease its group velocity, phase velocity and zonal wavenumber so that the dipole anomaly system can be amplified and transferred from dispersive system to very weak dispersive one. This may explain why and how the synoptic-scale eddies can reinforce and maintain vortex pair block. Furthermore, it is clearly found that during the prevalence of the vortex pair block the synoptic-scale eddies are split into two branches around the vortex pair block due to the feedback of amplified dipole block.

Planetary-scale baroclinic envelope Rossby solitons in a two-layer model and their interaction with synoptic-scale eddies

In this paper, planetary-scale baroclinic envelope Rossby solitons for zonal wavenumber 2 in a two-layer model are first investigated. It is found that when the shear of basic state westerly winds between the upper and lower layers is weak, both the upper- and lower-layer envelope Rossby solitons are almost in phase and exhibit vortex pair block structures which have a weak baroclinicity. The possibility of the application of planetary-scale envelope Rossby soliton to observed dipole blocks is discussed. Second, a highly idealized model having weak vertical shear is proposed to investigate the interaction between baroclinic planetary-scale dipole soliton (weak incipient dipole block) and a train of synoptic-scale waves (eddies) upstream. It is shown that in the interaction process, both the planetary-scale dipole soliton and the synoptic-scale eddies are deformed simultaneously. The amplitude of the dipole soliton block can be amplified through the near-resonant forcing of synoptic-scale eddies. In the intensification process of the dipole soliton block, its zonal scale is prolonged, and its phase velocity gradually tends to be equal to its group velocity so that the block envelope and carrier wave can be phase-locked at a certain time. This process reflects a transfer of dipole blocking system from dispersion to nondispersion. This may explain why the synoptic-scale eddies can reinforce and maintain blocking dipole. Conversely, in the decay process, the zonal scale of the dipole soliton block is shortened, and the discrepancy between its phase velocity and group velocity gradually increases. This process may describe a transfer of dipole blocking system from nondispersion to dispersion, which leads to the break-down of the dipole block. Due to the feedback of the intensified dipole block on the synoptic-scale eddies, the upper-layer eddies are split into two branches, and the lower-layer eddies are split into three branches. Moreover, the instantaneous total streamfunction fields (the streamfunction field of planetary-scale envelope Rossby soliton plus the streamfunction field of the deformed synoptic-scale eddies) that describe the coupling between baroclinic dipole envelope soliton and synoptic-scale eddies are found to bear a remarkable resemblance to the synoptic maps observed during a blocking episode. It appears that the eddy-forced envelope soliton model established here could describe the initiation, maintenance and decay of dipole blocking by the baroclinic synoptic-scale eddies. Therefore, this theoretical model seems to provide a theoretical framework for further studying the interaction between synoptic-scale eddies and dipole block.

Study of Block Onset Using Sensitivity Perturbations in Climatological Flows

This work describes the dynamics of adjoint sensitivity perturbations that excite block onsets over the Pacific and Atlantic Oceans. Appropriate functions are derived for the blocking indices for these two regions and the model basic flow is constructed from Northern Hemisphere climatological data. The concepts of sensitivity analysis are extended to forced problems. This tool is used to investigate block onset due to atmospheric forcing, such as that resulting from tropical sea surface temperature anomalies. These linear studies are carried out in a hemispherical, primitive equations, θ-coordinate, two-layer model. Results show that wind sensitivity perturbations less than 10 m s−1 and sensitivity forcing of vorticity sources of the order of 1.5 × 10−10 s−2 are sufficient to excite block onset. Both for the Pacific and Atlantic blocking, sensitivity perturbations and forcing perturbations, when expressed in terms of vertical vorticity, display a Rossby wave train structure mainly found on the southward flanks of the Pacific and Atlantic jets, that is, near the Philippines and the Caribbean regions. From inferences based on the flow evolution of these sensitivity perturbations and with the help of potential vorticity analyses on the two constant potential temperature surfaces in this model, a dynamical framework that may explain Pacific and Atlantic block onsets is proposed. The nonuniform potential vorticity distribution in the jets, in particular the concentration of these gradients on potential vorticity waveguides, and the Lagrangian advection of potential vorticity by the eddies making up the stationary Rossby wave train and their energy propagation and convergence all conspire to play a key role in the growth of the synoptic-scale eddies supported by baroclinic as well as barotropic processes. It is proposed that the structural modification of the eddies in the wave train leads to the planetary structures that become associated with block onset. More specifically, the wave train in the Pacific evolves into a blocking dipole while the Atlantic block is found at the leading edge of the Rossby wave train across the Atlantic. Furthermore, this study shows that at the initial time the Pacific block displays a clear baroclinic structure while the wave train associated with the Atlantic block has a much more barotropic structure. The significance of these results and their potential applications to predictions of blocking are discussed.

North Atlantic wintertime intraseasonal variability and its sensitivity to GCM horizontal resolution

The wintertime extratropical intraseasonal variability in the North Atlantic-European regionhas been analysed, and the ability of an atmospheric general circulation model to simulate itat different horizontal resolutions is discussed. Three runs of the French global spectral model(ARPEGE) are investigated: both medium (T42) and high (T106) resolution, and a conformallystretched T63 version with maximum resolution over the Mediterranean. Circulation features arevalidated against ECMWF analyses. The systematic errors in the simulated variability have beenisolated using space-time spectral and complex principal component analyses. The time-meanstructure of the 500 hPa geopotential height indicates that the model is too zonal, especially athigh resolution, with a stronger than normal synoptic-scale activity. The Atlantic storm trackpenetrates too far eastward over Europe, is too far south and does not veer to the northeast at theexit of the jet. These features may be in part associated with too strong barotropic forcing of themean flow by the synoptic-scale eddies on the equatorward side of the jet. Despite the overly strongzonal mean flow, the high resolution simulation has more realistic storm tracks than the other twoversions. Low-frequency intraseasonal variance is underestimated at each resolution, mainly northof 50° N. Furthermore, the model lacks planetary-scale slow westward travelling waves and doesnot show realistic spatial modes of low-frequency variability at any resolution. Blocking frequencyis underestimated and the blocking patterns are shifted eastward, following the excess of stormtrack penetration over Europe. However, these drawbacks are slightly improved at higher (T106)resolution, especially for blocking.

A Linear Theory of Extratropical Synoptic Eddy Statistics

Abstract This paper investigates the extent to which the statistics of extratropical synoptic eddies may be deduced from the assumption that the eddies are stochastically forced disturbances evolving on a baroclinically stable background flow. To this end, a two-level hemispheric quasigeostrophic model is linearized about the observed long-term mean flow and forced with Gaussian white noise. The mean flow is baroclinically stable for a reasonable choice of dissipation parameters. Synoptic-scale eddy disturbances can still grow on such a flow, albeit for a finite time, either in response to the stochastic forcing or through baroclinic and barotropic energy interactions with the background flow. In a statistically steady state, a fluctuation–dissipation relation (FDR) links the covariance of the eddies to the spatial structure of the background flow and the covariance of the forcing. Although not necessary, in this study the forcing is assumed to have always the same trivial statistics (white in both space ...

Clarifying the Dynamics of the General Circulation: Phillips's 1956 Experiment

In the mid-1950s, amid heated debate over the physical mechanisms that controlled the known features of the atmosphere's general circulation, Norman Phillips simulated hemispheric motion on the high-speed computer at the Institute for Advanced Study. A simple energetically consistent model was integrated for a simulated time of approximately 1 month. Analysis of the model results clarified the respective roles of the synoptic-scale eddies (cyclones-anticyclones) and mean meridional circulation in the maintenance of the upper-level westerlies and the surface wind regimes. Furthermore, the modeled cyclones clearly linked surface frontogenesis with the upper-level Charney–Eady wave. In addition to discussing the model results in light of the controversy and ferment that surrounded general circulation theory in the 1940s–1950s, an effort is made to follow Phillips's scientific path to the experiment.

The Role of High- and Low-Frequency Dynamics in Blocking Formation

Time evolutions of prominent blocking flow configurations over the North Pacific and Europe are compared based upon composites for the 30 strongest events observed during 27 recent winter seasons. Fluctuations associated with synoptic-scale migratory eddies have been filtered out before the compositing. A quasi-stationary wave train across the Atlantic is evident during the blocking amplification over Europe, while no counterpart is found to the west of the amplifying blocking over the North Pacific. Correlation between the tropopause-level potential vorticity (PV) and meridional wind velocity associated with the amplifying blocking is found to be negative over Europe in association with the anticyclonic evolution of the low-PV center, but it is almost zero over the North Pacific. Feedback from the synoptic-scale eddies, as evaluated in the form of 250-mb geopotential height tendency due to the eddy vorticity flux convergence, accounts for more than 75% of the observed amplification for the Pacific blocking and less than 45% for the European blocking. This difference is highlighted in two types of ‘‘contour advection with surgery’’ experiments. In one of them PV contours observed four days before the peak blocking time were advected by composite time series of the low-pass-filtered observational wind, and in the other experiment they were advected by the low-pass-filtered wind from which the transient eddy feedback evaluated as above had been removed at every time step. Hence, the latter data should be dominated by low-frequency dynamics. For the European blocking both experiments can reproduce the anticyclonic evolution of low-PV air within a blocking ridge as observed. For the Pacific blocking, in contrast, the observed intrusion of low-PV air into the higher latitudes cannot be reproduced without the transient feedback. Furthermore, in a barotropic model initialized with the composite 250-mb flow observed three days before the peak time, a simulated blocking development over the North Pacific is more sensitive to the insertion of the observed transient feedback than that over Europe. These results suggest that the incoming wave activity flux associated with a quasi-stationary Rossby wave train is of primary importance in the blocking formation over Europe, whereas the forcing by the synoptic-scale transients is indispensable to that over the North Pacific.

On the modification of the high- and low-frequency eddies associated with the PNA anomaly: an observational study

A 24-year NMC data set was used to study the interannual fluctuations in the extratropical atmospheric flow and the interactions between transients of different frequencies. Significant differences were found in the transient activities between winters with positive and negative Pacific/North American (PNA) seasonal anomalies. During the winters with an enhanced positive PNA pattern, the eddy activity is reduced over the North Pacific. The reduction of eddy activity is dominated by the low-frequency transients (periods from 10 days to a season). The occurrence of persistent anomalies in this region is also less frequent. The high-frequency baroclinic waves are shifted south-eastward of their normal position in the Pacific. The weak low-frequency activity over the North Pacific during positive PNA winters is linked to two processes. Firstly, the large-scale seasonal mean flow makes a smaller contribution to the low-frequency height variance. Secondly, the strong seasonal-mean Aleutian low tends to keep the baroclinic synoptic-scale eddies moving along its southern side, causing only a weak interaction with the low-frequency eddies over the North Pacific, and thus a smaller synoptic-scale eddy forcing of the low-frequency flow. The low-frequency activity is closely associated with the atmospheric natural variability which causes error growth in medium- and extended-range numerical predictions. Our result suggests that during positive PNA winters a better predictive skill for medium- and extended-range numerical predictions may be possible.

Simulated Life Cycles of Persistent Anticyclonic Anomalies over the North Pacific: Role of Synoptic-Scale Eddies

This study examines the role of synoptic-scale eddies during the development of persistent anticyclonic height anomalies over the central North Pacific in a general circulation model under perpetual January conditions. The GCM replicates the basic characteristics of the evolution of the anomaly patterns found in observations. The life cycle is characterized by the rapid establishment of the major anomaly center and considerably longer maintenance and decay phases, which include the development of downstream anomaly centers. The simulation also shows a realistic evolution of synoptic-scale activity beginning with enhanced activity off the east coast of Asia prior to onset, followed by a northward shift of the Pacific storm track, which lasts throughout the maintenance phase. The initial enhancement of synoptic-scale eddy activity is associated with a large-scale cyclonic anomaly that develops over Siberia several days prior to the onset of the main anticyclonic anomaly over the central North Pacific. The observations, however, show considerable interdecadel variability in the details of the composite onset behavior; it is unclear whether this variability is real or whether it reflects differences in the data assimilation systems. The role of the time mean flow and synoptic-scale eddies in the development of the persistent Pacific anomalies is studied within the context of a kinetic energy budget in which the flow is decomposed into the time-mean, low-frequency (timescales longer than 10 days), and synoptic (timescales less than 6 days) components. The budget, which is carried out for the simulation at 500 mb, shows that the initial growth of the persistent anticyclonic anomalies is associated with barotropic conversions of energy, with approximately equal contributions coming from the mean flow and the synoptic-scale eddies. After onset the barotropic conversion from the mean flow dominates, whereas the decay phase is associated with baroclinic processes within the low-frequency flow.

Rotational evolution of potential vorticity associated with a strong blocking flow configuration over Europe

Time evolution of a prominent tropospheric blocking flow configuration over Europe is examined based upon composites for the 15 strongest events observed during the last 27 winter seasons, from which the fluctuations associated with synoptic‐scale eddies have been filtered out. The blocking, initiated in the exit region of the Atlantic jet stream, forms into a domain of the very weak westerlies. The tropopause‐level potential vorticity (PV) exhibits a distinct, clockwise evolution within this domain, so that the negative correlation between PV and the meridional wind velocity associated with the amplifying blocking becomes positive as it decays, which signifies the convergence of the wave activity density flux and its subsequent divergence. Indeed, a quasi‐stationary wavetrain across the Atlantic is evident during the blocking amplification, and then another wavetrain becomes apparent to the east of the decaying ridge. It appears that the wave activity propagating across the Atlantic is obstructed and accumulated in the weak‐westerly domain associated with the developing blocking ridge until it is saturated, followed by the release of the activity to the east. The result suggests that the low‐frequency dynamics of the atmospheric flow plays an important role in the formation of a certain type of blocking.

A Diagnostic and Modeling Study of the Monthly Mean Wintertime Anomalies Appearing in a 100-Year GCM Experiment

Abstract The nature of simulated atmospheric variability on monthly time scales has been investigated by analyzing the output from a 100-year integration of a spectral GCM with rhomboidal wavenumber 15 truncation. In this experiment, the seasonally varying, climatological sea surface temperature was prescribed throughout the world oceans. The principal modes of variability in the model experiment were identified by applying a rotated empirical orthogonal function (EOF) analysis to the Northern Hemisphere monthly averaged 515-mb geopotential height for the winter season (November through March). The individual leading spatial modes are similar to the observed north-south dipoles over the North Atlantic and North Pacific, as well as wavelike patterns in the Pacific/North American and Northern Asian sectors. Quasigeostrophic geopotential tendencies forced by synoptic-scale (2.5–6 day) eddy vorticity and heat fluxes were computed for those months when the individual EOF modes are particularly active. The comp...

Global Chaotic Mixing on Isentropic Surfaces

Abstract The general nature of two-dimensional mixing on isentropic surfaces in the troposphere has been investigated. The daily time series of isentropic winds is obtained from a global general circulation model and is used to drive a high-resolution fully Lagrangian passive tracer model. Results are compared with the extremes of chaotic mixing by organized waves on the one hand and classical diffusion on the other and are found to lie in the middle ground. Advection by planetary-and synoptic-scale eddies generates small scales from an initially smooth tracer field exponentially fast, but given a modest degree of smoothing the tracer field evolving from a localized release rapidly attains the form of an algebraically spreading cloud. The zonal size of the cloud increases linearly with time (superdiffusively), owing to the systematic shear in the extratropical zonal jets, while the meridional spread has the square-root-of-time increase characteristic of classical diffusion. It is argued, however, that the...

Dynamic Forcing of the Slow Transients by Synoptic-Scale Eddies: An Observational Study

The interactions between low-frequency transients and synoptic-scale eddies are examined. The 300-mb data from 1981 to 1986 analyzed by the ECMWF are used to calculate the height tendency of the slow transients due to vorticity forcing by synoptic-scale eddies. It is found that the low-frequency transients are correlated with the forcing by the synoptic-scale eddies, most notably in the eastern part of the Pacific and Atlantic basins, where a significant portion of the time variance of the slow transient height field can be explained by the forcing. The lag correlation coefficients between the forcing and the slow transients indicate that the former leads the latter by a small phase difference (about one day). The structure of the forcing is studied by an EOF analysis. The leading EOF mode in both oceanic sectors has a dipole structure. The pattern of the geopotential height field associated with each EOF mode of the forcing is also identified. The height pattern associated with the first Pacific forcing mode has a wave train structure, and the one associated with the first Atlantic forcing mode is a localized dipole. The correlation between the height pattern and the corresponding EOF mode in each sector is then examined, and no significant time phase shift can be found between the two. Finally, a simple theoretical explanation is proposed to account for the phase relationship between the slow transients and their forcing by the synoptic-scale eddies.