Rossby Waves Research Papers - Academia.edu (original) (raw)

Samenvatting Περίληψη Acknowledgements Curriculum vitae viii Chapter 1.2.2 Rossby waves and their propagation characteristics Atmospheric Rossby waves are disturbances from the zonal symmetry in the atmospheric circulation that move westward under the inuence of the rotation of the Earth. In this thesis we study Rossby waves that are excited by tropical Rossby wave sources and travel into the extratropics, creating so-called teleconnection patterns. Rossby waves are named after Carl-Gustaf Arvid Rossby (Rossby (1939)) who identied them and explained their westward propagation in 1939. Rossby waves are dispersive, the longer the wavelength the faster the westward propagation. Their propagation is oset by the background winds such that in the region of the westerly winds, only the longest waves retrograde, move against the ow westward, while the smaller waves are carried eastward. The largest waves are called planetary waves. The planetary Rossby waves determine β * = β − ∂ 2Ū ∂y 2 (1.6) The jet stream is a fast westerly current owing around the globe in the upper troposphere with a sharp meridional prole, that is a consequence of the Earth's rotation and its dierential heating (section 1.2.1). Due to its sharp prole β * tends to have a larger relative maximum thanŪ (see Eq. 1.6). Therefore K s (Eq. 1.7) takes its maximum values along the jet core and reduces quickly toward the anks of the jet. This curvature and the attribute of the rays to bend towards larger K-values allows the changes the waveguiding properties and hence inuence the formation and propagation characteristics of Rossby waves. The pattern is most vigorous in the boreal winter when the circulation is strongest. The eect of NAO variability on the local weather conditions are distinct over Eastern North America, North Atlantic and Northern Europe and often across Southern Europe and the Middle East. Contrary to the extratropical mean ow, the NAO has a pronounced 1.4.2 The Indian Ocean Dipole Another important source of IO variability is the atmosphere-ocean coupled phenomenon widely known as the Indian Ocean Dipole (IOD) (Saji et al (1999)). On a regional scale the IOD has a signicant impact on the atmospheric circulation and precipitation anomalies and on a larger scale the phenomenon correlates with temperature and precipitation anomalies in remote regions, such as Europe, America, South Africa and Northeast Asia (Saji and Yamagata (2003a)). The IOD is characterized by an eastwest SST gradient along the central tropical IO that strongly impacts the atmospheric convection over the basin. When the Central-West IO is warmer than the Eastern IO the event is called a positive event and vice-Along the equator the strong east to southeasterly trade winds allow the cold deep currents, that ow from Antarctica to the west coast of South America, to upwell and keep the SSTs relatively cool along the equatorial cold tongue. In the Western Pacic the trade winds deepen the thermocline and the surface water is warmer. This area of warm ocean waters, the Pacic warm pool, is a major source for atmospheric convection program, which is nanced by the Netherlands Organization for Scientic Research (NWO). We also thank the two anonymous reviewers that provided valuable suggestions and remarks that improved the outcome of this work.

- by iris manola
- •
- Numerical Modeling, El Niño, North Atlantic Oscillation, Rossby Waves
- by Elise Ralph
- •
- Hydraulics, Behavior, Theory, Stability

ABSTRACT Symbiotic diatom-diazotrophic cyanobacteria association (DDA) of Rhizosolenia hebetata and Rhizosolenia formosa with endosymbiotic cyanobacteria Richelia intracellularis was noticed and documented for the first time during a bloom of the cyanobacterium Trichodesmium erythraeum in the oligotrophic shelf waters along Kochi and Mangalore transects, southeastern Arabian Sea (SEAS), during spring intermonsoon (April 2012). Although the host is frequently seen, the symbiont is rarely reported in the Indian EEZ. The presence of nitrogen-fixing symbiotic association of Rhizosolenia-Richelia along with Trichodesmium erythraeum highlights the significance of DDAs on the nutrient and energy budgets of phytoplankton in the oligotrophic environments of the Arabian Sea during spring intermonsoon.

- by Gupta GVM
- •
- Oceanography, Rossby Waves, Chlorophyll, Zooplankton

The existence of an upstream (eastward) group velocity for African easterly waves (AEWs) is shown based on single-point lag regressions using gridded reanalysis data from 1990 to 2010. The eastward energy dispersion is consistent with the direction of ageostrophic geopotential flux vectors. A local eddy kinetic energy (EKE) budget reveals that, early in the life cycle of AEWs, growth rate due to geopotential flux convergence exceeds baroclinic and barotropic growth rates. Later in the life cycle, EKE decay due to geopotential flux divergence cancels or exceeds baroclinic and barotropic growth. A potential vorticity (PV) budget is used to diagnose tendencies related to group propagation. Although both upstream and downstream group speeds are possible because of the reversal in the mean meridional PV gradient, upstream propagation associated with the positive poleward PV gradient dominates wave packet evolution. Analogous to the concept of downstream development of midlatitude barocli...

- by Michael jhon Diaz
- •
- Rossby Waves, Atmospheric sciences, Dynamics

Finite-amplitude Rossby wave activity (FAWA) proposed by Nakamura and Zhu measures the waviness of quasigeostrophic potential vorticity (PV) contours and the associated modification of the zonal-mean zonal circulation, but it does not distinguish longitudinally localized weather anomalies, such as atmospheric blocking. In this article, FAWA is generalized to local wave activity (LWA) to diagnose eddy–mean flow interaction on the regional scale. LWA quantifies longitude-by-longitude contributions to FAWA following the meridional displacement of PV from the circle of equivalent latitude. The zonal average of LWA recovers FAWA. The budget of LWA is governed by the zonal advection of LWA and the radiation stress of Rossby waves. The utility of the diagnostic is tested with a barotropic vorticity equation on a sphere and meteorological reanalysis data. Compared with the previously derived Eulerian impulse-Casimir wave activity, LWA tends to be less filamentary and emphasizes large isolated vortices involving reversals of meridional gradient of potential vorticity. A pronounced Northern Hemisphere blocking episode in late October 2012 is well captured by a high-amplitude, near-stationary LWA. These analyses reveal that the nonacceleration relation holds approximately over regional scales: the growth of phase-averaged LWA and the deceleration of local zonal wind are highly correlated. However, marked departure from the exact nonacceleration relation is also observed during the analyzed blocking event, suggesting that the contributions from nonadiabatic processes to the blocking development are significant.

- by Clare S. Y. Huang
- •
- Rossby Waves, Climate Dynamics, Conservation Laws, Blocking
- by Juliet Hermes
- •
- Indian Ocean World, Rossby Waves, Upwelling
- by Nkanyiso Mbatha
- •
- Geography, Gravitational Waves, Rossby Waves
- by Nathan Paldor
- •
- Oceanography, Differential Geometry, Geophysical Fluid Dynamics, High Frequency

We revise the notion of the cascade "arrest" in a βplane turbulence in the context of continuously forced flows using both theoretical analysis and numerical simulations. We demonstrate that the upscale energy propagation cannot be stopped by a β-effect and can only be absorbed by friction. A fundamental dimensional parameter in flows with a β-effect, the Rhines scale, L R , has traditionally been associated with the cascade "arrest" or with the scale separating turbulence and Rossby wave dominated spectral ranges. We show that rather than being a measure of the inverse cascade arrest, L R is a characteristic of different processes in different flow regimes. In unsteady flows, L R can be identified with the moving "energy front" propagating towards the decreasing wavenumbers. When large-scale energy sink is present, β-plane turbulence may attain several steady-state regimes. Two of these regimes are highlighted, friction-dominated and zonostrophic. In the former, L R does not have any particular significance, while in the latter, the Rhines scale nearly coincides with the characteristic length associated with the large-scale friction. Spectral analysis in the fre- * semion@bgu.ac.il quency domain demonstrates that Rossby waves coexist with turbulence on all scales including those much smaller than L R thus indicating that the Rhines scale cannot be viewed as a crossover between turbulence and Rossby wave ranges.

- by Boris Galperin and +1
- •
- Turbulence, Rossby Waves, Atmospheric sciences, Spectral analysis

The three-dimensional (3D) Rossby wave energy dispersion of a tropical cyclone (TC) is studied using a baroclinic primitive equation model. The model is initialized with a symmetric vortex on a beta plane in an environment at rest. The vortex intensifies while becoming asymmetric and moving northwestward because of the beta effect. A synoptic-scale wave train forms in its wake a few days later. The energy-dispersioninduced Rossby wave train has a noticeable baroclinic structure with alternating cyclonic-anticycloniccyclonic (anticyclonic-cyclonic-anticyclonic) circulations in the lower (upper) troposphere.

- by Yuqing Wang
- •
- Wave Energy, Rossby Waves, Atmospheric sciences, Energy Budget

An interpretation is made of interannual changes in acoustic travel time between Oahu and seven receivers at distances of 3000-4000 km. Measurements were made in late 1983, and over two 5-month intervals between 1987 and 1989. Previous publications demonstrated that these changes stem from variations in temperature. Two hydrodynamic ocean models are used to identify plausible oceanic features that could cause these variations. They are from the Naval Ž. Ž. Research Laboratory and the Florida State University at 1r8 8 and 1r6 8 resolution, respectively, and are forced with different interannual wind sets for more than a decade. Modelled El Nino's and La Nina's generate poleward travelling Kelvin waves on the eastern boundary of thẽP acific. These excite Rossby waves that propagate westward at mid-latitudes. Rossby waves are the dominant model features which affect the modelled acoustic travel times, and hence sectionaveraged temperatures in the eastern North Pacific. These waves yield travel times whose standard deviations and rates of changes are similar to the measurements. In the observations, some sections separated by less than 500 km exhibit trends in heat content with opposite signs. Similar variability can be explained with modelled Rossby waves. Model wavelengths less than 500 km, eddies, and seasonal cycles induced by seasonal winds yield travel times that are two orders of magnitude too small to account for the data. Published by Elsevier Science B.V.

- by Mark Johnson
- •
- Oceanography, Rossby Waves, Seasonality, Atmospheric sciences
- by Henrique Barbosa
- •
- Complex Networks, Multidisciplinary, Rossby Waves

A genesis mechanism for African easterly waves (AEWs) is proposed. In the same manner that new troughs and ridges in the midlatitudes form downstream of existing ones through a mechanism known as downstream development, it is proposed that new AEWs can be generated upstream of existing AEWs. A local eddy kinetic energy budget of the AEW that ultimately became Hurricane Alberto (2000) demonstrates that upstream development explains its genesis more convincingly than previous theories of AEW genesis. The energetics and ageostrophic secondary circulation of a composite AEW are consistent with a new AEW forming as a result of this mechanism. Some strengths and weaknesses of upstream development as a paradigm for AEW genesis are discussed with respect to other potential mechanisms.

- by Michael jhon Diaz
- •
- Rossby Waves, Atmospheric sciences

Two numerical ocean models are used to study the baroclinic response to forcing by localized wind stress curl (i.e., a wind-forced β plume, which is a circulation cell developing to the west of the source region and composed of a set of zonal jets) with implications for the Hawaiian Lee Countercurrent (HLCC): an idealized primitive equation model [Regional Ocean Modeling System (ROMS)], and a global, eddy-resolving, general circulation model [Ocean General Circulation Model for the Earth Simulator (OFES)]. In addition, theoretical ideas inferred from a linear continuously stratified model are used to interpret results. In ROMS, vertical mixing preferentially damps higher-order vertical modes. The damping thickens the plume to the west of the forcing region, weakening the near-surface zonal jets and generating deeper zonal currents. The zonal damping scale increases monotonically with the meridional forcing scale, indicating a dominant role of vertical viscosity over diffusion, a con...

- by Ali Belmadani and +1
- •
- Oceanography, Physical Oceanography, Mixing, Rossby Waves

An equatorial β-plane model of the tropical stratosphere is used to examine the effects of ozone on Kelvin, Rossby–gravity, equatorial Rossby, inertia–gravity, and smaller-scale gravity waves. The model is composed of coupled equations for wind, temperature, and ozone volume mixing ratio, which are linearized about a zonally averaged background state. Using the Wentzel–Kramers–Brillouin (WKB) formalism, equations are obtained for the vertical spatial scale, spatial damping rate, and amplitude of the waves. These equations yield an analytical expression for the ozone-modified wave driving of the zonal-mean circulation. The expression for the wave driving provides an efficient parameterization that can be implemented into models that are unable to spontaneously generate the ozone-modified, convectively coupled waves that drive the quasi-biennial and semiannual oscillations of the tropical stratosphere. The effects of ozone on the wave driving, which are strongly modulated by the Doppl...

- by Eugene Cordero
- •
- Rossby Waves, Atmospheric sciences, Gravity Waves, Ozone

A 30-yr climatology of Rossby wave breaking (RWB) on the Southern Hemisphere (SH) tropopause is formed using 30 yr of reanalyses. Composite analysis of potential vorticity and meridional fluxes of wave activity show that RWB in the SH can be divided into two broad categories: anticyclonic and cyclonic events. While there is only weak asymmetry in the meridional direction and most events cannot be classified as equatorward or poleward in terms of the potential vorticity structure, the position and structure of the fluxes associated with equatorward breaking differs from those of poleward breaking. Anticyclonic breaking is more common than cyclonic breaking, except on the lower isentrope examined (320 K). There are marked differences in the seasonal variations of RWB on the two surfaces, with a winter minimum for RWB around 350 K but a summer minimum for RWB around 330 K. These seasonal variations are due to changes in the location of the tropospheric jets and dynamical tropopause. During winter the subtropical jet and tropopause at 350 K are collocated in the Australian-South Pacific Ocean region, resulting in a seasonal minimum in the 350-K RWB. During summer the polar front jet and 330-K tropopause are collocated over the Southern Atlantic and Indian Oceans, inhibiting RWB in this region.

- by Thando Ndarana
- •
- Rossby Waves, Atmospheric sciences, Tropopause, Wave Breaking

The authors show that a simple three-dimensional ocean model linearized about a resting basic state can accurately simulate the dynamical ocean response to wind forcing by the Madden-Julian oscillation (MJO). This includes the propagation of equatorial waves in the Indian Ocean, from the generation of oceanic equatorial Kelvin waves to the arrival of downwelling oceanic equatorial Rossby waves in the western Indian Ocean, where they have been shown to trigger MJO convective activity. Simulations with idealized wind forcing suggest that the latitudinal width of this forcing plays a crucial role in determining the potential for such feedbacks. Forcing the model with composite MJO winds accurately captures the global ocean response, demonstrating that the observed ocean dynamical response to the MJO can be interpreted as a linear response to surface wind forcing. The model is then applied to study ''primary'' Madden-Julian events, which are not immediately preceded by any MJO activity or by any apparent atmospheric triggers, but have been shown to coincide with the arrival of downwelling oceanic equatorial Rossby waves. Case study simulations show how this oceanic equatorial Rossby wave activity is partly forced by reflection of an oceanic equatorial Kelvin wave triggered by a westerly wind burst 140 days previously, and partly directly forced by easterly wind stress anomalies around 40 days prior to the event. This suggests predictability for primary Madden-Julian events on times scales of up to five months, following the reemergence of oceanic anomalies forced by winds almost half a year earlier.

- by David Stevens
- •
- Oceanography, Climate, Rossby Waves, Atmospheric sciences

Global sea level has risen during the past decades as a result of thermal expansion of the warming ocean and freshwater addition from melting continental ice. However, sea-level rise is not globally uniform. Regional sea levels can be affected by changes in atmospheric or oceanic circulation. As long-term observational records are scarce, regional changes in sea level in the Indian

- by Aixue Hu
- •
- Geology, Geophysics, Climate, Risk assessment
- by A. Gelfgat
- •
- Engineering, Physics, Hydrology, Meteorology

A nonlinear, two-layer, vortex-tracking semispectral model (i.e., Fourier transformed in azimuth only) is used to study the evolution of dry, but otherwise hurricane-like, initially tilted vortices in quiescent surroundings on f and β planes. The tilt projects onto vorticity asymmetries that are dynamically vortex Rossby waves. Since the swirling wind in the principal mean vortex used here decays exponentially outside the eyewall, it has an initial potential vorticity (PV) minimum. The resulting reversal of PV gradient meets the necessary condition for inflectional (i.e., barotropic or baroclinic) instability. Thus, the vortex may be inflectionally stable or unstable. On an f plane, the tilt precesses relatively slowly because the critical radius, where the phase speeds of the waves match the mean swirling flow, is far from the center. An alternative Gaussian-like PV monopole that has a monotonic outward decrease of PV is stable to inflectional instability. It has a smaller critical...

- by Michael Montgomery
- •
- Physics, Rossby Waves, Fourier Analysis, Atmospheric sciences

The Kelvin Helmholtz ͑KH͒ problem, with zero stratification, is examined as a limiting case of the Rayleigh model of a single shear layer whose width tends to zero. The transition of the Rayleigh modal dispersion relation to the KH one, as well as the disappearance of the supermodal transient growth in the KH limit, are both rationalized from the counterpropagating Rossby wave perspective.

- by E. Kit
- •
- Hydrology, Meteorology, Stratification, Stability

Numerical models demonstrate that a broad class of geophysical vortices freely evolve toward vertically aligned, axisymmetric states. In principle, this intrinsic drive toward symmetry opposes destructive shearing by the environmental flow. This article examines the case in which a discrete vortex-Rossby-wave dominates a perturbation from symmetry, and symmetrization occurs by decay of the wave. The wave is damped by a resonance with the fluid rotation frequency at a critical radius, r *. The damping rate is proportional to the radial derivative of potential vorticity at r *. Until now, the theory of resonantly damped vortex-Rossby-waves (technically quasi-modes) was formally restricted to slowly rotating vortices, which obey quasigeostrophic (QG) dynamics. This article extends the theory to rapidly rotating vortices. The analysis makes use of the asymmetric balance (AB) approximation. Even at a modest Rossby number (unity), AB theory can predict damping rates that exceed extrapolated QG results by orders of magnitude. This finding is verified upon comparison of AB theory to numerical experiments, based on the primitive equations. The experiments focus on the decay of low azimuthal wave-number asymmetries. A discrete vortex-Rossby-wave can also resonate with an outward propagating inertia-buoyancy wave (Lighthill radiation), inducing both to grow. At large Rossby numbers, this growth mechanism can be dynamically relevant. All balance models, including AB theory, neglect inertia-buoyancy waves, and therefore ignore the possibility of a Rossby-inertia-buoyancy (RIB) instability. This article shows that a large potential vorticity gradient (of the proper sign) at the critical radius r * can suppress the RIB instability, and thereby preserve balanced flow, even at large Rossby numbers.

- by Michael Montgomery
- •
- Oceanography, Vortex dynamics, Geophysical Fluid Dynamics, Rossby Waves

The stability of the African easterly jet (AEJ) is examined using idealized numerical simulations. It is found that a zonally homogeneous representation of the AEJ can support absolute instability in the form of African easterly waves (AEWs). This finding is verified through a local energy budget, which demonstrates the presence of both upstream and downstream energy fluxes. These energy fluxes allow unstable wave packets to spread upstream and downstream relative to their initial point of excitation. This finding is further verified by showing that the ground-relative group velocity of these wave packets has both eastward and westward components. In contrast with normal-mode instability theory, which emphasizes wave growth through energy extraction from the basic state, the life cycle of the simulated AEWs is strongly governed by energy fluxes. Convergent fluxes at the beginning of the AEW storm track generate new AEWs, whereas divergent fluxes at the end of the storm track lead to...

- by Michael jhon Diaz
- •
- Rossby Waves, Atmospheric sciences, Instability

At low-resolution, idealized ocean circulation models forced by prescribed differential surface heat fluxes show spontaneous multidecadal variability depending critically on eddy diffusivity coefficients. The existence of this critical threshold in the range of observational estimates legitimates some doubt on the relevance of such intrinsic oscillations in the real ocean. Through a series of numerical simulations with increasing resolution up to eddy-resolving ones (10 km) and various diapycnal diffusivities, this multidecadal variability proves a generic ubiquitous feature, at least in model versions with a flat bottom. The mean circulation largely changes in the process of refining the horizontal grid (along with the associated implicit viscosity and diffusivity), and the spatial structure of the variability is largely modified, but there is no clear influence of the resolution on the main oscillation period. The interdecadal variability appears even more robust to low vertical d...

- by Thierry Huck
- •
- Oceanography, Turbulence, Physical Oceanography, Rossby Waves
- by Paolo Cipollini
- •
- Remote Sensing, Biogeochemistry, Sea Level, Multidisciplinary

, two very powerful and devastating extratropical cyclones hit Western Europe. These two storms were associated with an upper-level zonal jet, remarkable in its intensity and its large extension over the Atlantic Ocean. In this study, we focus on the relationship between the exceptional features of this jet stream and some heavy precipitation that occurred over Central America during mid-December. A Rossby-wave train excited by strong convective activity over this area is likely to play a key role in this teleconnection. To assess this tropical-extratropical interaction, several numerical experiments have been performed with the French global model ARPEGE. Firstly, the effects of convection have been neutralized within a domain localized over Central America. Then, to combine the model with observations, a new strategy has been devised. Within a selected window, the parametrized latent heat release has been deduced from Tropical Rainfall Measuring Mission (TRMM) precipitation data, translated in terms of a potential vorticity (hereafter PV) source and incorporated into the model using PV inversion. These simulations show, in particular, that the convective activity over Central America triggers a wave train that moves poleward and eastward with the group velocity, finally to strengthen the North Atlantic jet stream, leading to windstorms Lothar and Martin.

- by Alain Joly
- •
- Oceanography, Rossby Waves, Atmospheric sciences, Potential Vorticity

A hydrostatic global weather prediction model based on an icosahedral horizontal grid and a hybrid terrain-following/isentropic vertical coordinate is described. The model is an extension to three spatial dimensions of a previously developed, icosahedral, shallow-water model featuring user-selectable horizontal resolution and employing indirect addressing techniques. The vertical grid is adaptive to maximize the portion of the atmosphere mapped into the isentropic coordinate subdomain. The model, best described as a stacked shallow-water model, is being tested extensively on real-time medium-range forecasts to ready it for possible inclusion in operational multimodel ensembles for medium-range to seasonal prediction.

- by Rainer Bleck
- •
- Rossby Waves, Atmospheric sciences, Potential Vorticity, Wave Breaking

This paper derives a system of equations that approximately govern small-amplitude perturbations in a nonprecipitating cloudy vortex. The cloud coverage can be partial or complete. The model is used to examine moist vortex Rossby wave dynamics analytically and computationally. One example shows that clouds can slow the growth of phase-locked counter-propagating vortex Rossby waves in the eyewall of a hurricane-like vortex. Another example shows that clouds can (indirectly) damp discrete vortex Rossby waves that would otherwise grow and excite spiral inertia–gravity wave radiation from a monotonic cyclone at high Rossby number.

- by Michael Montgomery
- •
- Rossby Waves, Atmospheric sciences, Dynamics, Phase Locking

1] Prevailing hypotheses for secondary eyewall formation are examined using data sets from two high-resolution mesoscale numerical model simulations of the long-time evolution of an idealized hurricane vortex in a quiescent tropical environment with constant background rotation. The modeled hurricanes each undergo a secondary eyewall cycle, casting doubt on a number of other authors' hypotheses for secondary eyewall formation due to idealizations present in the simulation formulations. A new hypothesis for secondary eyewall formation is proposed here and is shown to be supported by these high-resolution numerical simulations. The hypothesis requires the existence of a region with moderate horizontal strain deformation and a sufficient low-level radial potential vorticity gradient associated with the primary swirling flow, moist convective potential, and a wind-moisture feedback process at the air-sea interface to form the secondary eyewall. The crux of the formation process is the generation of a finite-amplitude lower-tropospheric cyclonic jet outside the primary eyewall with a jet width on the order of a local effective beta scale determined by the mean low-level radial potential vorticity gradient and the root-mean square eddy velocity. This jet is hypothesized to be generated by the anisotropic upscale cascade and axisymmetrization of convectively generated vorticity anomalies through horizontal shear turbulence and sheared vortex Rossby waves as well as by the convergence of system-scale cyclonic vorticity by the low-level radial inflow associated with the increased convection. Possible application to the problem of forecasting secondary eyewall events is briefly considered.

- by Michael Montgomery
- •
- Convection, Turbulence, Forecasting, Multidisciplinary

1] A medium-resolution ocean general circulation model (OGCM) simulation is used to investigate the subthermocline interannual variability in the eastern South Pacific. The focus is on isotherm vertical displacement variability associated with extratropical Rossby waves (ETRW) and their connection with equatorial Kelvin waves (EKW). The WKB theory is used to interpret the modeled subsurface variability. The analysis reveals vertical propagation of energy associated with the 1997-1998 El Niño near the coast at various latitudes. Consistent with theory, WKB raypaths are steeper southward and do not extend more than 600kmfromthecoastat600 km from the coast at 600kmfromthecoastat2000 m depth. A vertical mode decomposition of model variability showed that vertical propagation mostly involves the first three baroclinic modes. The vertical isotherm displacements along the raypaths, associated with the downwelling EKW of the 1997-1998 El Niño, are negative (rising isotherms) and peak (minimum isotherm) as El Niño reverses to La Niña conditions. The relationship between vertical propagation of ETRW and equatorial variability is interpreted in the light of EKW sequence. The evolution of the vertical isotherm displacements along the raypath at extratropical latitudes is controlled by the vertical structure of the EKW. As El Niño develops, high-order baroclinic mode contributions to the EKW progressively increase, leading to the dominance of finer vertical scales of variability along the coast, triggering the vertical propagation of ETRW. This mechanism of connection between equatorial variability and subsurface variability at the extratropical latitudes illustrates how sensitive the ETRW characteristics in the eastern South Pacific are to equatorial forcing.

- by Oscar Pizarro and +1
- •
- Energy, Theory, Decomposition, Sea Level

The location of the Antarctic Polar Front (PF) was mapped over a 7-year period (1987)(1988)(1989)(1990)(1991)(1992)(1993) within images of satellite-derived sea surface temperature. The mean path of the PF is strongly steered by the topographic features of the Southern Ocean. The topography places vorticity constraints on the dynamics of the PF that strongly affect spatial and temporal variability. Over the deep ocean basins the surface expression of the PF is weakened, and the PF meanders over a wide latitudinal range. Near large topographic features, width and temperature change across the front increase, and largescale meandering is inhibited. Elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the PF encounters large topographic features. The strong correlations between topography and PF dynamics can be understood in the context of the planetary potential vorticity (PPV or f/H) field. Mean PPV at the PF varies by more than a factor of 2 along its circumpolar path. However, at the mesoscale the PF remains within a relatively narrow range of PPV values around the local mean. Away from large topographic features, the PF returns to a preferred PPV value of ϳ25 ϫ 10 Ϫ9 m Ϫ1 s Ϫ1 despite large latitudinal shifts. The mean paths of the surface and subsurface expressions of the PF are closely coupled over much of the Southern Ocean.

- by Mark Abbott
- •
- Multidisciplinary, Sea surface temperature, Vorticity, Rossby Waves

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

- by Marcel Ramos
- •
- Energy, Theory, Decomposition, Sea Level

This study examines interannual to decadal variability of the Kuroshio Extension (KE) jet using satellite altimeter observations from 1993 to 2010. The leading empirical orthogonal function (EOF) mode of sea level variability in the KE region represents the meridional shift of the KE jet, followed by its strength changes with a few month lag. This shift of the KE jet lags atmospheric fluctuations over the eastern North Pacific by about three years. Broad sea level anomalies (SLAs) emerge in the eastern North Pacific 3–4 years before the upstream KE jet shift, and propagate westward along the KE jet axis. In the course of the propagation, the meridional scale of the SLAs gradually narrows, and their amplitude increases. This westward propagation of SLAs with a speed of about 5 cm s−1 is attributed to the westward propagation of the meridional shift of the jet, consistent with the thin-jet theory, whose importance has been suggested by previous numerical studies. In addition, the west...

- by Niklas Schneider
- •
- Geology, Oceanography, Physics, Physical Oceanography

The dynamics of convectively coupled equatorial waves (CCEWs) is analyzed in an idealized model of the large-scale atmospheric circulation. The model is composed of a linear rotating shallow-water system with a variable equivalent height, or equivalent gravity wave speed, which varies in space. This model is based on the hypothesis that moist convection acts to remove convective instability, therefore modulating the equivalent height of a shallow-water system. Asymptotic solutions are derived in the case of a small perturbation around a constant coefficient, which is assumed to be a mean moist equivalent height derived from satellite observations. The first-order solutions correspond to the free normal modes of the linear shallow-water system and the second-order flow is derived solving a perturbation eigenvalue problem. The asymptotic solutions are documented in the case of a zonally varying equivalent height and for wavenumbers and frequencies that are consistent with observations of CCEWs. This analysis shows that the dynamics of the secondary divergence and its impact on the full divergence varies mode by mode. For instance, for a negative equivalent height anomaly, which is interpreted as a moister background, the secondary divergence is nearly in phase with the primary divergence in the case of Kelvin waves-in contrast to mixed Rossby-gravity waves where the secondary divergence acts to attenuate the primary divergence. While highly idealized, the modeled waves share some features with observations, providing a mechanism for the relationship between CCEWs phase speed, amplitude, and horizontal structure.

- by P. Dias
- •
- Rossby Waves, Atmospheric sciences, Kelvin waves, Shallow water equations

A common bias among global climate models (GCMs) is that they exhibit tropospheric southern annular mode (SAM) variability that is much too persistent in the Southern Hemisphere (SH) summertime. This is of concern for the ability to accurately predict future SH circulation changes, so it is important that it be understood and alleviated. In this two-part study, specifically targeted experiments with the Canadian Middle Atmosphere Model (CMAM) are used to improve understanding of the enhanced summertime SAM persistence. Given the ubiquity of this bias among comprehensive GCMs, it is likely that the results will be relevant for other climate models. Here, in Part I, the influence of climatological circulation biases on SAM variability is assessed, with a particular focus on two common biases that could enhance summertime SAM persistence: the too-late breakdown of the Antarctic stratospheric vortex and the equatorward bias in the SH tropospheric midlatitude jet. Four simulations are us...

- by Theodore Shepherd
- •
- Oceanography, Climate, Rossby Waves, Atmospheric sciences

A new statistical approach to analyzing large-scale oceanic motions characterized by strong statistical inhomogeneity in the meridional direction is developed and applied to a 9-year series of satellite altimeter observations in the Tropical Pacific between 20° S and 20°N. A special treatment of meridional variations permits quantitative characterization of latitudinally alternating, zonally elongated features of the sea surface height field.

- by Alexandra Piryatinska
- •
- Oceanography, Rossby Waves, Atmospheric sciences, Satellite Altimetry

A method for capturing the different dynamical components of the Madden–Julian oscillation (MJO) is presented. The tropical wind field is partitioned into three components using free-space Green’s functions: 1) a nondivergent component, 2) an irrotational component, and 3) a background or environmental flow that is interpreted as the influence on the tropical flow due to vorticity and divergence elements outside of the tropical region. The analyses performed in this study show that this background flow is partly determined by a train of extratropical waves. Space–time power spectra for each flow component are calculated. The strongest signal in the nondivergent wind spectrum corresponds to equatorial Rossby, mixed Rossby–gravity, and easterly waves. The strongest signal in the irrotational winds corresponds to Kelvin and inertia–gravity modes. The strongest signal in the power spectrum of the background flow corresponds to the wave band of extratropical Rossby waves. Furthermore, a ...

- by Ralph Foster
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- Vorticity, Rossby Waves, Atmospheric sciences, Madden Julian Oscillation
- by Paolo Cipollini
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- Rossby Waves, Geomatic Engineering, SST, SSH

A medium-resolution ocean general circulation model (OGCM) simulation is used to investigate the subthermocline interannual variability in the eastern South Pacific. The focus is on isotherm vertical displacement variability associated with extratropical Rossby waves (ETRW) and their connection with equatorial Kelvin waves (EKW). The WKB theory is used to interpret the modeled subsurface variability. The analysis reveals vertical propagation of energy associated with the 1997-1998 El Niño near the coast at various latitudes. Consistent with theory, WKB raypaths are steeper southward and do not extend more than 600kmfromthecoastat600 km from the coast at 600kmfromthecoastat2000 m depth. A vertical mode decomposition of model variability showed that vertical propagation mostly involves the first three baroclinic modes. The vertical isotherm displacements along the raypaths, associated with the downwelling EKW of the 1997-1998 El Niño, are negative (rising isotherms) and peak (minimum isotherm) as El Niño reverses to La Niña conditions. The relationship between vertical propagation of ETRW and equatorial variability is interpreted in the light of EKW sequence. The evolution of the vertical isotherm displacements along the raypath at extratropical latitudes is controlled by the vertical structure of the EKW. As El Niño develops, high-order baroclinic mode contributions to the EKW progressively increase, leading to the dominance of finer vertical scales of variability along the coast, triggering the vertical propagation of ETRW. This mechanism of connection between equatorial variability and subsurface variability at the extratropical latitudes illustrates how sensitive the ETRW characteristics in the eastern South Pacific are to equatorial forcing.

- by Oscar Pizarro
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- Energy, Theory, Decomposition, Sea Level

Previous studies have suggested the importance of blocking high (BH) development for the occurrence of stratospheric sudden warming (SSW), while there is a recent study that failed to identify their statistical linkage. Through composite analysis applied to high-amplitude anticyclonic anomaly events observed around every grid point over the extratropical Northern Hemisphere, the present study reveals a distinct geographical dependence of BH influence on the upward propagation of planetary waves (PWs) into the stratosphere. Tropospheric BHs that develop over the Euro-Atlantic sector tend to enhance upward PW propagation, leading to the warming in the polar stratosphere and, in some cases, to major SSW events. In contrast, the upward PW propagation tends to be suppressed by BHs developing over the western Pacific and the Far East, resulting in the polar stratospheric cooling. This dependence is found to arise mainly from the sensitivity of the interference between the climatological P...

- by Hisashi Nakamura
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- Oceanography, Climate, Rossby Waves, Atmospheric sciences

Vortex–Rossby waves (VRWs) and inertial gravity waves (IGWs) have been proposed to explain the propagation of spiral rainbands and the development of dynamical instability in tropical cyclones (TCs). In this study, a theory for mixed vortex–Rossby–inertia–gravity waves (VRIGWs), together with VRWs and IGWs, is developed by including both rotational and divergent flows in a shallow-water equations model. A cloud-resolving TC simulation is used to help simplify the radial structure equation for linearized perturbations and then transform it to a Bessel equation with constant coefficients. A cubic frequency equation describing the three groups of allowable (radially discrete) waves is eventually obtained. It is shown that low-frequency VRWs and high-frequency IGWs may coexist, but with separable dispersion characteristics, in the eye and outer regions of TCs, whereas mixed VRIGWs with inseparable dispersion and wave instability properties tend to occur in the eyewall. The mixed-wave in...

- by Da-Lin Zhang
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- Rossby Waves, Atmospheric sciences, Instability, Vortex

Idealized laboratory experiments reveal the existence of forced–dissipative hybrid Rossby-shelf modes. The laboratory ocean consists of a deeper ocean (accommodating basin-scale Rossby modes) and a coastal step shelf (accommodating trapped shelf modes). Planetary Rossby modes are mimicked in the laboratory via a uniform topographic slope in the north–south direction. Hybrid modes are found as linear modes in numerical calculations, and similar streamfunction patterns exist in streak photography of the rotating tank experiments. These numerical calculations are based on depth-averaged potential vorticity dynamics with Ekman forcing and damping. Preliminary nonlinear calculations explore the deficiencies observed between reality and the linear solutions. The aim of the work is twofold: to show that idealized hybrid Rossby-shelf modes exist in laboratory experiments and to contribute in a general sense to the discussion on the coupling and energy exchange associated with hybrid modes b...

- by Vijaya Ambati
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- Oceanography, Physical Oceanography, Rossby Waves, Damping

1] A synthesis product of the surface geostrophic circulation is used to quantify the interannual variability of the wind-driven North Equatorial Countercurrent (NECC). The first mode of a complex empirical orthogonal function (CEOF) decomposition of zonal geostrophic velocity in the NECC region reveals pronounced latitudinal displacements in addition to variations in current strength. While north-south migrations of the NECC are mainly captured by the real pattern, the imaginary pattern accounts for variations in its strength. Associated with these spatial patterns is prevailing propagation toward the northwest that is most pronounced northward of the mean NECC position ($6 N). There is evidence that the zonal propagation characteristics are consistent with long Rossby waves forced in the northeastern tropical Atlantic. The relationship between interannual NECC variations and the tropical Atlantic climate modes is investigated through regression and composite analyses. Sea surface temperature and wind stress patterns resembling the meridional and zonal modes are found for the CEOF regression. Composite analysis further shows consistent patterns for warm phases of the meridional mode and cold phases of the zonal mode; the response of the NECC to a positive meridional and negative zonal mode event may be viewed as a northward shift of its core and a current strengthening, respectively. These results support a link between the two dominant tropical Atlantic climate modes and show that the relation between interannual NECC variability and the meridional and zonal modes can primarily be regarded as a response to changes in the wind field.

- by R. Lumpkin
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- Multidisciplinary, Rossby Waves, Geophysical

Although prior studies have established that the extratropical flow pattern often amplifies downstream of recurving tropical cyclones (TCs), the extratropical flow response to recurving TCs has not to the authors' knowledge been systematically examined from a climatological perspective. In this study, a climatology of the extratropical flow response to recurving western North Pacific TCs is constructed from 292 cases of TC recurvature during 1979–2009. The extratropical flow response to TC recurvature is evaluated based on a time-lagged composite time series of an index of the North Pacific meridional flow surrounding TC recurvature. Similar time series are constructed for recurving TCs stratified by characteristics of the large-scale flow pattern, the TC, and the phasing between the TC and the extratropical flow to assess factors influencing the extratropical flow response to TC recurvature. Results reveal that following TC recurvature, significantly amplified flow develops ove...

- by Lance Bosart
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- Rossby Waves, Atmospheric sciences, Dynamics, Teleconnections
- by Nikolai Maximenko
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- Oceanography, Physical Oceanography, Mixing, Rossby Waves

Global sea level has risen during the past decades as a result of thermal expansion of the warming ocean and freshwater addition from melting continental ice 1 . However, sea-level rise is not globally uniform 1-5 . Regional sea levels can be affected by changes in atmospheric or oceanic circulation. As long-term observational records are scarce, regional changes in sea level in the Indian Ocean are poorly constrained. Yet estimates of future sea-level changes are essential for effective risk assessment 2 . Here we combine in situ and satellite observations of Indian Ocean sea level with climate-model simulations, to identify a distinct spatial pattern of sea-level rise since the 1960s. We find that sea level has decreased substantially in the south tropical Indian Ocean whereas it has increased elsewhere. This pattern is driven by changing surface winds associated with a combined invigoration of the Indian Ocean Hadley and Walker cells, patterns of atmospheric overturning circulation in the north-south and east-west direction, respectively, which is partly attributable to rising levels of atmospheric greenhouse gases. We conclude that-if ongoing anthropogenic warming dominates natural variability-the pattern we detected is likely to persist and to increase the environmental stress on some coasts and islands in the Indian Ocean.

- by Alan Wallcraft
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- Geology, Geophysics, Climate, Risk assessment
- by Ricardo Letelier and +1
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- Time Series, Satellite remote sensing, Sea Level, Multidisciplinary