Baroclinic Component Research Articles

Analysis of internal tidal characteristics in the layer above 450 m from acoustic Doppler current profiler observations in the Luzon Strait

The characteristics of internal tides in the upper layer of the Luzon Strait are investigated on the basis of direct-observation current data recorded on April 25 and September 26, 2008 by an acoustic Doppler current profiler. Spectral analysis and energy estimation show that the diurnals and semidiurnals carry most of the energy of internal tides. Values of the depth-integrated total energy E for the three frequency bands of diurnal, semidiurnal, and high frequencies are 31, 6.9, and 3.4 kJ·m, respectively. Near-inertial peaks are only present in the baroclinic component. The behavior of typical tidal frequencies (i.e., O1, K1, M2, MK3, and M4) and the near-inertial frequency is basically consistent with linear internal wave theory, which predicts E+(ω)/E−(ω)=(ω−f)2/(ω+f)2 at depths above 66 m, while not all prominent tidal components coincide well with the relation of the linear internal wave field at other depths. Examinations of depth structures of the baroclinic tides and temporal variations show that the surface tides and internal tides are both of mixed type, having diurnal inequality and spring-neap fortnight periods. The K1 and O1 tides have comparable cross- and along-shelf components, while the M2 and S2 tides propagate toward the shelf in the northern South China Sea as wave beams. The amplitude and phase of internal tides vary with time, but M2 and S2 tides appear to have structures dominated by the first mode, while the K1 and O1 tides resemble second-mode structures. The minor to major axis ratios are close to expected values of f/ω in the thermocline.

Assessing decadal changes in the Deep Western Boundary Current absolute transport southeast of Cape Farewell, Greenland, from hydrography and altimetry

In earlier studies, the decadal variability of the Deep Western Boundary Current (DWBC) transport in the vicinity of Cape Farewell, Greenland, has been assessed from changes in the baroclinic velocities computed from hydrographic data and referenced to 1000 m depth. The main limitation of using such an estimate as an index for the DWBC absolute transport variability comes from the unaccounted for decadal velocity changes at the reference level (1000 m). These changes may substantially contribute to the DWBC absolute transport variability by compensating for or adding to the baroclinic transport changes. To assess this contribution to variability, we quantify the decadal velocity changes which occurred at 1000 m depth southeast of Cape Farewell since the mid‐1990s. The analysis combines estimates of the baroclinic velocity changes in the water column derived from repeat hydrography at ∼59.5°N and the velocity changes at the sea surface derived from altimetry. An increase in the southward velocity at 1000 m above the DWBC between the periods of 1994–1997 and 2000–2007 is inferred. It indicates that the increase in the DWBC absolute transport was larger than the 2 Sv (1 Sv = 106 m3 s−1) increase in its baroclinic component referenced to 1000 m. This result and the observed coherence of the DWBC absolute and baroclinic transport changes between individual observations imply that the DWBC absolute transport variability in the region is underestimated but qualitatively well represented by its baroclinic component on decadal and shorter time scales.

Two-Layer Tidal Circulation in a Frictional, Rotating Basin

Abstract The three-dimensional tidal circulation in an elongated basin of arbitrary depth is described with a coupled barotropic and baroclinic two-layer model on the f plane. As long as friction is not dominant, near-standing waves are present on the interface as well as on the surface. The surface pattern is principally determined by the product of the tidal barotropic wavenumber by the basin length. The interface deformation is determined by a baroclinic equivalent, usually a much larger number. As a result, the shape of the interface is characterized by horizontally smaller features than the surface. If the product of the tidal baroclinic wavenumber by the basin width is greater than one, both lateral and axial modes can be excited at the interface. If these modes are near resonant, large internal tides can be forced directly by the co-oscillating surface tide at the basin entrance. The amplitude and phase of the baroclinic component are sensitive functions of the density anomaly and the interface depth. As a result, the phase and amplitude of the interface vary by large amounts with comparatively small changes in those parameters. The model behavior is qualitatively consistent with observations in fjords and straits.

Estimation of heat flux through the eastern Bering Strait

We estimated the northward heat flux through the eastern channel of the Bering Strait during the ice-free seasons between 1999 and 2008. This is likely about half of the total heat flux through the strait. The net volume transport and heat flux through the eastern channel of the strait were estimated from multiple linear regression models with in-situ/satellite remotely sensed datasets and NCEP reanalysis 10 m wind. The net volume transport was well explained by the west-east slope of sea level anomaly and NNW wind component at the strait. On the heat flux, the contributions of both barotropic and baroclinic components were taken into account. Estimated volume transport and vertical profile of temperature were used to calculate northward heat flux through the eastern channel of the strait. The magnitude of the estimated heat flux is comparable to estimates from in-situ measurements. Averaged heat flux in the eastern Bering Strait between 2004 and 2007 was about 1.9 times larger than that between 2000 and 2003. Maximum heat flux occurred in 2004, and same magnitude of heat flux was estimated from 2005 to 2007. This resulted not only from the increase in northward volume transport but also anomalous warm water intrusion from the Bering Sea. Our results suggest a candidate among the important parameters controlling heat budget, which contributes to the Arctic sea ice reduction, whereas more studies are required to confirm that this mechanism is actually responsible for the interannual and longer timescale variability.

Bathymetric influences of the Emperor Seamounts upon the subarctic gyre of the North Pacific: Examining boundary current dynamics along the eastern side of the mountain ridge with an idealized numerical model

The possible influences of the Emperor Seamounts (ESs) upon the subarctic gyre of the North Pacific (NPSAG) were investigated by a series of numerical experiments. In the experiments, a two-layer ocean with a meridional mountain ridge was forced by seasonally varying wind stress. We focused on how the return ratio, the ratio of the boundary transport along the eastern side of the ridge to the interior Sverdrup transport, changes with ridge height, width and density stratification. It was found that the return ratio can be large if the ridge width is greater than the width of the viscous boundary layer. In this case, the bottom pressure torque determines the return ratio; the return ratio is almost proportional to the ridge height when the ridge height is small and some contours of planetary potential vorticity pass over the ridge. However, the return ratio is independent of the ridge height and decreases with the stratification when the ridge height is large and all the contours of planetary potential vorticity are closed. These dependences of the return ratio were understood in terms of barotropic and baroclinic components of the bottom pressure torque. Implications for the bathymetric influences of ESs on the actual NPSAG are also discussed.

Dynamics of the semi-diurnal and quarter-diurnal internal tides in the Bay of Biscay. Part 2: Baroclinic tides

A regional baroclinic model forced with several tidal constituents at different frequencies is used to investigate the internal tide of the Bay of Biscay. The regional ocean model is free surface, sigma-coordinate and it is implemented in order to accurately take into account the barotropic forcing, the strong bathymetry gradients and the temperature and salinity stratifications. In a previous paper, the barotropic component of the tides was studied in details and the boundary conditions of the three-dimensional model were extracted from the atlases. In the present paper, we focus on the baroclinic component of the tides and the simulations are validated against observations from the MINT94 experiment. The observed currents and stratification are accurately reproduced by the model. The internal tide pattern is consistent with the descriptions found in the literature. Combining wavelet and principal component analysis we extract the patterns of generation and propagation of the internal tide at the semi-diurnal and quarter-diurnal frequencies. Secondary internal wave generation areas are identified over the plain. The vertical displacements of isopycnal surfaces for the M4 internal tide are found to be locally half those induced by the semi-diurnal internal tide. A sensitivity study shows the impact of using a three-dimensional initial stratification over the direction of propagation and wavelengths of the internal tides.

Mathematical modelling of spatial spectra of atmospheric turbulence

Rayleigh–Bénard thermal convection in a doubly periodic channel with rigid walls is studied using the LES-model as an analogue of multiscale atmospheric turbulence from the viewpoint of reproduction of spectral properties. A large ratio of the horizontal size to the vertical one ensures the existence of quasi-two-dimensional large-scale flow components, and a computational grid of several tens of millions nodes allows us to reproduce explicitly the dynamics of the small-scale three-dimensional turbulent component. The decomposition of the turbulent flow into the barotropic and baroclinic components allows us to propose the transformation scheme for kinetic energy in the studied system, which explains some spectral properties of the observed atmospheric turbulence.

Increase of upper troposphere/lower stratosphere wave baroclinicity during the second half of the 20th century

Abstract. A strengthening of the equatorward temperature gradient in the upper troposphere/lower stratosphere (UTLS), at subtropics and midlatitudes, is consistently reproduced in several modelling studies of the atmospheric response to the increase of greenhouse gas radiative forcing. Some of those studies suggest an increase of the baroclinicity in the UTLS region because of the enhanced meridional temperature gradient. This study presents observational evidence of an increase of the baroclinic wave components of UTLS circulation (UTLS wave baroclinicity), during the second half of the 20th century. The evidence is given by significant positive trends in the energy of baroclinic normal modes of the NCEP/NCAR reanalysis, and significant positive trends in the UTLS eddy available potential energy of the NCEP/NCAR, ERA-40, NCEP-2 and JRA-25 reanalyses. Significant positive trends in the frequency of double tropopause events in radiosonde data are also interpreted as a manifestation of an increase of the UTLS wave baroclinicity.

Measurements of Sea Surface Height Variability in the Eastern South Atlantic from Pressure Sensor–Equipped Inverted Echo Sounders: Baroclinic and Barotropic Components

Abstract Variability in sea surface height (SSH) can be decomposed into two contributions: one from changes in mass in the water column (barotropic) and the other from purely steric changes (baroclinic). Both contributions can be determined from data recorded by a pressure sensor–equipped inverted echo sounder (PIES). PIES data from the Agulhas South Atlantic Thermohaline Experiment (ASTTEX) were used, collected in the Cape Basin off South Africa, along 1000 km of an eddy corridor where Agulhas eddies carry cores of warm, salty Indian Ocean waters into the South Atlantic. The paper presents in detail the method used to convert PIES measurements into barotropic, baroclinic, and total SSH, and discusses the error budget. The baroclinic contribution is geopotential height (reference 4500 dbar), which can be determined from the measured vertical acoustic travel time together with a lookup curve based on the regional hydrography. The main error source is scatter about this curve that depends on the extent to which water masses advecting along each geopotential streamline may derive from different ocean regions. The barotropic contribution can be determined from the bottom pressure measurements of the mass variation in the water column and has an uncertainty due to sensor calibration drift in two years corresponding to 1-cm water column height. The barotropic component accounts for 20% of the overall SSH variance and 47% during large signal intervals exceeding 15 cm. PIES data demonstrate via the two measurements that barotropic and baroclinic contributions may work independently or in concert in different mesoscale eddies. The combined structure need not be equivalent barotropic. In particular, deep barotropic eddies exhibit mesoscale spatiotemporal scales and may or may not be systematically tilted or aligned in space or time relative to baroclinic eddies.

The circulation and water masses of the Antarctic shelf and continental slope between 30 and

Abstract The circulation and water masses from the Antarctic continental shelf to 62 ∘ S between 30 and 80 ∘ E are described using hydrographic data collected on seven hydrographic sections during the Baseline Research on Oceanography, Krill and the Environment-West (BROKE-West) experiment. The eastern limb of the Weddell Gyre dominates circulation between 30 and 40 ∘ E , and is significantly cooler and fresher than the region to the east. The Antarctic Circumpolar Current (ACC) extends from the north into the survey region east of 40 ∘ E , reaching as far south as 65 . 5 ∘ S at 60 ∘ E . This results in increasing observed maximum temperature and salinities progressively towards the east, peaking at 80 ∘ E due to the intrusion of the southern ACC Front (sACCF) to 63 ∘ S . This southward extension is steered by the southern end of Kerguelen Plateau, causing a horizontal shear of over 0.15 m s - 1 between the eastward ACC and westward-flowing Antarctic Slope Current (ASC). The ASC is observed at all six meridional sections immediately north of the shelf break. It is strongly barotropic and transports a total of 15.8 ± 7.4 Sv westwards, while the bottom referenced baroclinic component only contributes 1.3 ± 0.3 Sv . At each section this current intensifies to a narrow westward ‘jet’ with absolute velocities up to 0.3 m s - 1 over the steepest shelf slope gradients. At 70 ∘ E a ‘V’ shape is observed in the ASF. This, and the nearby presence of denser shelf water and ice-shelf water, is characteristic of Antarctic Bottom Water (AABW) formation, but no new AABW is found on this section. Instead, significantly warmer, saltier and less oxygenated AABW to the east and newly formed AABW high on the continental slope immediately to the west suggest a formation region just west of 70 ∘ E . This newly formed AABW progressively becomes warmer and saltier west of 60 ∘ E and is observed extending offshore and moving westward below eastward flowing water masses. ACC frontal positions are found to be 1–2° farther north in the survey region than suggested by historical climatology.

A Model of Multiple Zonal Jets in the Oceans: Dynamical and Kinematical Analysis

Abstract Multiple alternating zonal jets observed in the ocean are studied with an idealized quasigeostrophic zonal-channel model, with the supercritical, zonal background flow imposed. Both eastward and westward background flows with vertical shear are considered. The underlying nonlinear dynamics is illuminated with analysis of the vertical-mode interactions and time-mean eddy fluxes. Interactions between the vertical modes are systematically studied. The barotropic component of the jets is maintained by both barotropic–barotropic and baroclinic–baroclinic time-mean interactions; thus, the barotropic component of the jets cannot be accurately simulated with a randomly forced barotropic model. The roles of the vertical-mode interactions in driving the baroclinic component of the jets are also characterized. Not only the first but also the second baroclinic mode is found to be important for maintaining the baroclinic component of the jets, whereas the barotropic component of the jets is maintained mostly by the barotropic and first baroclinic modes. The properties of the eddy forcing were systematically studied. It is shown that the baroclinic component of the jets is maintained by Reynolds stress forcing and resisted by form stress forcing only in the eastward background flow. In the westward background flow, the jets are maintained by form stress forcing and resisted by Reynolds stress forcing. The meridional scaling and kinematical properties of the jets are studied as well as the roles of meridional boundaries. The Rhines scaling for meridional spacing of the jets is not generally confirmed, and it is also shown that there are multiple stable equilibria with different numbers of the time-mean jets. It is also found that the jets are associated with alternating weak barriers to the meridional material transport, but the locations of these barriers are not unique and depend on the direction of the background flow and depth. Finally, if the channel is closed with meridional walls, then the jets become more latent but the eddy forcing properties do not change qualitatively.

Adjustment of the basin-scale circulation at 26° N to variations in Gulf Stream, deep western boundary current and Ekman transports as observed by the Rapid array

Abstract. The Rapid instrument array across the Atlantic Ocean along 26° N provides unprecedented monitoring of the basin-scale circulation. A unique feature of the Rapid array is the combination of full-depth moorings with instruments measuring temperature, salinity, pressure time series at many depths with co-located bottom pressure measurements so that dynamic pressure can be measured from surface to bottom. Bottom pressure measurements show a zonally uniform rise (and fall) of bottom pressure of 0.015 dbar on a 5 to 10 day time scale, suggesting that the Atlantic basin is filling and draining on a short time scale. After removing the zonally uniform bottom pressure fluctuations, bottom pressure variations at 4000 m depth against the western boundary compensate instantaneously for baroclinic fluctuations in the strength and structure of the deep western boundary current so there is no basin-scale mass imbalance resulting from variations in the deep western boundary current. After removing the mass compensating bottom pressure, residual bottom pressure fluctuations at the western boundary just east of the Bahamas balance variations in Gulf Stream transport. Again the compensation appears to be especially confined close to the western boundary. Thus, fluctuations in either Gulf Stream or deep western boundary current transports are compensated in a depth independent (barotropic) manner very close to the continental slope off the Bahamas. In contrast, compensation for variations in wind-driven surface Ekman transport appears to involve fluctuations in both western basin and eastern basin bottom pressures, though the bottom pressure difference fluctuations appear to be a factor of 3 too large, perhaps due to an inability to resolve small bottom pressure fluctuations after removal of larger zonal average, baroclinic, and Gulf Stream pressure components. For 4 tall moorings where time series dynamic height (geostrophic pressure) profiles can be estimated from sea surface to ocean bottom and bottom pressure can be added, there is no general correlation between surface dynamic height and bottom pressure. Dynamic height on each mooring is strongly correlated with sea surface height from satellite observations and the variability in both dynamic height and satellite sea surface height decrease sharply as the western boundary is approached.

Nonlinear development of inertial instability in a barotropic shear

Inertial instability is investigated numerically in a two-dimensional setting in order to understand its nonlinear stage and saturation. To focus on fundamental mechanisms, a simple barotropic shear U(y)=tanh y on the f-plane is considered. The linear stability problem is first solved analytically, and the analytical solutions are used to benchmark numerical simulations. A simple scenario of the nonlinear development of the most unstable mode was recurrently observed in the case of substantial diffusivity: while reaching finite amplitude the unstable mode spreads laterally, distorting the initially vertical instability zone. This process produces strong vertical gradients which are subsequently annihilated by diffusion, making the flow barotropic again but with the shear spread over a wider region. In the course of such evolution, unexpectedly, strong negative absolute vorticity anomalies are produced. In weakly diffusive simulations, the horizontal spreading of the unstable motions and the enhancement of the anticyclonic vorticity extremum persist, but small-scale motions/instabilities render the flow considerably more complex. It is known that the barotropic component of the final state can be predicted from the conservation of momentum. Our simulations confirm the relevance of this simple prediction in the cases investigated regardless of resolution and diffusion. The baroclinic component of the final state is also analyzed and three types of structures are identified: persistent stationary stratification layers, subinertial waves trapped in the anticyclonic shear, and freely propagating inertia-gravity waves. The subinertial waves and the stratification staircase have clear signatures and can therefore help to identify the regions that have undergone inertial instability.

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.

Intense diurnal surface currents in the Bay of La Paz, Mexico

Currents in the northern Bay of La Paz were examined using an 8-month Acoustic Doppler Current Profiler (ADCP) record collected in the upper 185 m of the water column during 2007. Flow variability was dominated by tidal motions, which accounted for 43% (33% diurnal, 10% semidiurnal) of the total kinetic energy. The tidal motions had a pronounced vertical structure dominated within a shallow (∼30 m thick) surface layer by intense counterclockwise (CCW) rotary S 1 diurnal radiational currents that were highly coherent with the counterclockwise seabreeze. Motions within the semidiurnal frequency band were primarily associated with significant counterclockwise S 2 radiational tidal currents, which were also coherent with the seabreeze. Both S 1 and S 2 tidal ellipses in the upper layer were aligned perpendicular to the bay entrance with mean semi-major axes of 55 and 20 cm/s, respectively. Below the surface layer, tidal currents decayed rapidly to relatively weak, clockwise rotary barotropic motions. In contrast to those for radiational harmonics, tidal ellipses of the gravitational constituents (M 2, K 1 and O 1) were oriented cross-bay. Energy within the diurnal frequency band in the surface layer was dominated by a coherent component (barotropic, phase-locked baroclinic and radiational), which accounted for roughly 65% (59% from S 1 alone) of the total diurnal kinetic energy. Of the remaining diurnal band energy, 18% was associated with an incoherent baroclinic component and 17% with a background noise component. Below 30 m depth, the corresponding estimates are 40%, 32% and 28%, respectively. The persistent, surface-intensified CCW rotary currents observed at the mooring site are assumed to be forced by strong CCW seabreeze winds in the presence of a “slippery” low-density surface layer. This response may be further augmented by topographic narrowing at the bay entrance and by the close proximity of the diurnal and inertial frequency bands in the region.

A Simple Atmospheric Model of the Local and Teleconnection Responses to Tropical Heating Anomalies

Abstract A minimal complexity model of both the local and remote stationary responses of the atmosphere to tropical heating anomalies is described and demonstrated. Two levels are recast as baroclinic and barotropic components with thermal advection in the tropics neglected. The model is linearized about some idealized and realistic background wind fields and forced with a localized heating for illustration. In the tropics, the baroclinic responses are familiar from the Matsuno–Gill model; these excite barotropic responses by advective interactions with vertical background wind shear. The barotropic signals are in turn transmitted to high latitudes only in the presence of barotropic background westerly winds. For an El Niño–like equatorial heating, the barotropic response has anticyclones to the north and south of the heating reinforcing (opposing) the anticyclonic (cyclonic) baroclinic gyres in the upper (lower) troposphere. With realistic background flows, the model reproduces the hemispheric asymmetry of ENSO teleconnections. Further experiments show that the winter hemisphere is favored mainly because the summer hemispheric subtropical jet is farther from the heating latitude, suggesting that the summer hemisphere can still host robust stationary Rossby waves if the heating occurs in the vicinity of the jet. As an example, it is shown that summer heating over the Atlantic warm pool (AWP) can have a remote influence on the summer climate of North America and Europe.

Flow variability in the Strait of Gibraltar: The Mediterranean adjustment to tidal forcing

The vertical structure of the flow variability through the Strait of Gibraltar is studied based on the Gibraltar Experiment and the Word Ocean Circulation Experiment data sets. An analysis of the leading modes of velocity and density variability at the Strait of Gibraltar showed an adjustment of the water masses exchanged through the Strait. Mediterranean mass variations resulting from the water exchanged by barotropic tidal oscillations generate changes of the baroclinic component of the flow that damp these mass variations. This adjustment explains the previously observed fortnightly variation of the shear. Moreover, the adjustment is found to operate for the subinertial time scale flow variability forced by the atmospheric pressure. An analytical model aimed at reproducing variations of the velocity with time and in the vertical is derived. The model includes a depth-varying parameterisation of friction and takes into account density gradient fluctuations across the Strait. The model reproduces the main features of the flow, in particular the shear and the interface depth variations with the tide phase.

Vertical structure of anthropogenic zonal‐mean atmospheric circulation change

The atmospheric circulation changes predicted by climate models are often described using sea level pressure, which generally shows a strengthening of the mid‐latitude westerlies. Recent observed variability is dominated by the Northern Annular Mode (NAM) which is equivalent barotropic, so that wind variations of the same sign are seen at all levels. However, in model predictions of the response to anthropogenic forcing, there is a well‐known enhanced warming at low levels over the northern polar cap in winter. This means that there is a strong baroclinic component to the response. The projection of the response onto a NAM‐like zonal index varies with height. While at the surface most models project positively onto the zonal index, throughout most of the depth of the troposphere many of the models give negative projections. The response to anthropogenic forcing therefore has a distinctive baroclinic signature which is very different to the NAM.

Structure, transport and potential vorticity of the Gulf Stream at 68°W: Revisiting older data sets with new techniques

The stream-coordinates mean structure of the Gulf Stream at 68°W is derived using new methods for both defining stream coordinates and interpreting bottom pressure and inverted echo sounder travel times collected during the extensive Synoptic Ocean Prediction experiment. These new analyses provide pictures of the vertical structure of Gulf Stream flows that are demonstrably dynamically consistent with the density field at all depths, in contrast to previous work that relies on simple vertical interpolations to fill gaps between sparse current meter measurements. This new view of the Gulf Stream suggests a slightly higher total mean transport, with the increases coming from both baroclinic and barotropic components, and slightly stronger recirculation cells, particularly on the southern side. The recirculation of the Gulf Stream appears to have a weak baroclinic component, perhaps 10% of the total. A significant advantage of the methodology is the ability to obtain sensible vertical and horizontal gradients of currents and density so that the vertical and cross-stream structures of the components of the mean potential vorticity can be clearly imaged. One new feature from this calculation is that the along-stream gradient of the cross-stream velocity, a term that is often ignored in potential vorticity analyses, is non-negligible (though small) and is asymmetric about the current axis. Both the derived structure and implied dynamics of the circulation can be significantly altered by small changes to the method of calculating daily stream coordinates, e.g., by carefully filtering out observations in rings or not. Arrays of pressure-equipped inverted echo sounders provide the opportunity (at reasonable cost) for properly defining the stream coordinates of energetic jets such as the Gulf Stream.

‘Explosive’ meridional migration of cyclones and anticyclones

We examine the very rapid meridional migration of barotropic eddies and show that the main balance of forces is between the meridional β -induced force and the opposing form-drag generated by the flow surrounding the rapidly moving eddy. The solution is constructed by assuming that, as the flow around a solid cylinder, the flow surrounding the eddy detaches from the eddy downstream. We find the migration speed to be (3π βV m /5) 1/2 r m , where V m is the maximum orbital eddy speed and r m is the radius where the maximum speed occurs. It is typically an order of magnitude faster than the classical westward speed induced by the baroclinic component ( βr m 2 ). The above formula gives values that are within 30–50% of the speeds measured in both our own numerical experiments, numerical experiments conducted by others, as well laboratory experiments (conducted by others). We suggest that it is the barotropic component that very rapidly (10 km d −1 ) pulls young Agulhas rings towards the equator. Poleward propagating cyclones are not very common but, during POLYMODE, an extremely rapidly propagating cyclone was observed in the North Atlantic. It is speculated here that its rapid propagation was also due to a barotropic component.