Mode Rossby Waves Research Articles

Spectral characteristics of Rossby waves in the Northwestern Pacific based on satellite altimetry

Using satellite altimetry measurement data for 1993–2013, we study the spectral characteristics of Rossby waves in the Northwestern Pacific (25°–50° N, 140°–180° E). For each latitude degree, we draw integral plots of spectral power density calculated with a two-dimensional Fourier transform (2D-FFT). We compare the dispersion equations of Rossby waves calculated from the WKB-approximation and an approximation of a two-layer ocean model with the empirical velocities determined by the slope of isopleths by the Radon method; also, we compare the dispersion equations with the spectral distributions of level variations. It is shown that the main energy of Rossby waves in the Northwestern Pacific corresponds to the first baroclinic mode. At almost all latitudes, there is good agreement between the empirical phase velocities calculated by isopleths by the Radon method and the theoretical values; also, the spectral peaks correspond to graphs of the dispersion equations for the first baroclinic mode Rossby waves, except for the Kuroshio region, where some peaks correspond to the second mode.

Wave–Mean Flow Interactions and the Maintenance of Superrotation in a Terrestrial Atmosphere

Abstract The interplay between mean meridional circulation and transient eddies through wave–mean flow interaction processes defines the general behavior of any planetary atmospheric circulation. Under a higher-Rossby-number regime, equatorward momentum transports provided by large-scale disturbances generate a strong zonal flow at the equatorial region. At intermediate Rossby numbers, equatorial Kelvin waves play a leading role in maintaining a superrotating jet over the equator. However, at high Rossby numbers, the Kelvin wave only provides equatorward momentum fluxes during spinup, and the wave–mean flow process that maintains this strongly superrotating state has yet to be identified. This study presents a comprehensive analysis of the tridimensional structure and life cycle of atmospheric waves and their interaction with the mean flow, which maintains the strong, long-lived superrotating state in a higher-Rossby-number-regime atmosphere. The results show that the mean zonal superrotating circulation is maintained by the dynamical interaction between mixed baroclinic–barotropic Rossby wave modes via low-frequency variations of the zonal-mean state in short and sporadic periods of stronger instability. The modulation of amplitude of the equatorial and extratropical Rossby waves suggests a nonlinear mechanism of eddy–eddy interaction between these modes.

The Instabilities and Multiscale Energetics Underlying the Mean–Interannual–Eddy Interactions in the Kuroshio Extension Region

AbstractUsing a recently developed energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), this study investigates the intricate nonlinear mutual interactions among the decadally modulating mean flow, the interannual fluctuations, and the transient eddies in the Kuroshio Extension region. It is found that the mean kinetic energy maximizes immediately east of the Izu–Ogasawara Ridge, while the transient eddy kinetic energy does not peak until 400 km away downstream. The interannual variabilities, which are dominated by a jet-trapped Rossby wave mode, provide an energy reservoir comparable to the other counterparts. In the upstream, strong localized barotropic and baroclinic transfers from the mean flow to the eddies are observed, whereas those from the interannual variabilities are not significant. Besides fueling the eddies, the unstable mean jet also releases energy to the interannual-scale processes. Between 144° and 154°E, both transfers from the mean flow and the interannual variabilities are important for the eddy development. Farther downstream, eddies are found to drive the mean flow on both the kinetic energy (KE) and available potential energy (APE) maps. They also provide KE to the interannual variabilities but obtain APE from the latter. The gained eddy APE is then converted to eddy KE through buoyancy conversion. Upscale energy transfers are observed in the northern and southern recirculation gyre (RG) regions. In these regions, the interannual–eddy interaction exhibits different scenarios: the eddies lose KE to the interannual processes in the northern RG region, while gaining KE in the southern RG region.

Dominant role of eddies and filaments in the offshore transport of carbon and nutrients in the California Current System

AbstractThe coastal upwelling region of the California Current System (CalCS) is a well‐known site of high productivity and lateral export of nutrients and organic matter, yet neither the magnitude nor the governing processes of this offshore transport are well quantified. Here we address this gap using a high‐resolution (5 km) coupled physical‐biogeochemical numerical simulation (ROMS). The results reveal (i) that the offshore transport is a very substantial component of any material budget in this region, (ii) that it reaches more than 800 km into the offshore domain, and (iii) that this transport is largely controlled by mesoscale processes, involving filaments and westward propagating eddies. The process starts in the nearshore areas, where nutrient and organic matter‐rich upwelled waters pushed offshore by Ekman transport are subducted at the sharp lateral density gradients of upwelling fronts and filaments located at ∼25–100 km from the coast. The filaments are very effective in transporting the subducted material further offshore until they form eddies at their tips at about 100–200 km from the shore. The cyclonic eddies tend to trap the cold, nutrient, and organic matter‐rich waters of the filaments, whereas the anticyclones formed nearby encapsulate the low nutrient and low organic matter waters around the filament. After their detachment, both types of eddies propagate further in offshore direction, with a speed similar to that of the first baroclinic mode Rossby waves, providing the key mechanism for long‐range transport of nitrate and organic matter from the coast deep into the offshore environment.

Basin-Wavelength Equatorial Deep Jet Signals across Three Oceans

AbstractEquatorial deep jets (EDJs) are equatorially trapped, stacked, zonal currents that reverse direction every few hundred meters in depth throughout much of the water column. This study evaluates their structure observationally in all three oceans using new high-vertical-resolution Argo float conductivity–temperature–depth (CTD) instrument profiles from 2010 to 2014 augmented with historical shipboard CTD data from 1972 to 2014 and lower-vertical-resolution Argo float profiles from 2007 to 2014. The vertical strain of density is calculated from the profiles and analyzed in a stretched vertical coordinate system determined from the mean vertical density structure. The power spectra of vertical strain in each basin are analyzed using wavelet decomposition. In the Indian and Pacific Oceans, there are two distinct peaks in the power spectra, one Kelvin wave–like and the other entirely consistent with the dispersion relation of a linear, first meridional mode, equatorial Rossby wave. In the Atlantic Ocean, the first meridional mode Rossby wave signature is very strong and dominates. In all three ocean basins, Rossby wave–like signatures are coherent across the basin width and appear to have wavelengths the scale of the basin width, with periods of about 5 yr in the Indian and Atlantic Oceans and about 12 yr in the Pacific Ocean. Their observed meridional scales are about 1.5 times the linear theoretical values. Their phase propagation is downward with time, implying upward energy propagation if linear wave dynamics hold.

Decadal change in factors affecting winter precipitation over eastern China

The temporal and spatial distributions of winter precipitation variability over eastern China were analyzed on the basis of the empirical orthogonal function method. The results showed that the primary mode of winter precipitation variability over this area presented a homogeneous change during the study period, with a significant decadal change around the late 1980s. The factors that influenced winter precipitation variability over eastern China changed over different interdecadal periods. Before the late 1980s, the Eurasian (EU) mode and North Pacific Oscillation (NPO) mode were the two major atmospheric factors. After the late 1980s, the influence of the EU mode remained. However, the impact of the NPO weakened significantly, and a new Rossby Wave (RW) pattern became a key factor. Further analyses of both observations and numerical simulations indicated that the convective activity over the western tropical Pacific strengthened significantly around the late 1980s; the convection encouraged the RW mode and ultimately contributed to the anomalous winter precipitation over eastern China after the late 1980s. The results imply that the prediction of winter precipitation should consider different interdecadal backgrounds; otherwise, the changing factors could result in failure of the prediction over some decadal periods.

The Vorticity Balance of the Ocean Surface in Hawaii from a Regional Reanalysis

AbstractThe ocean surface vorticity budget around the Hawaiian Islands is examined using an 18-month model reanalysis generated using four-dimensional variational state estimation with all available observations (satellite, in situ, and high-frequency radio). To better resolve the ocean surface currents and reduce the representation error of the radio-measured surface currents, this study developed a new vertical scheme for the Regional Ocean Modeling System. A new, detailed description of the ocean surface vorticity is created, revealing a region dominated by cyclonic (anticyclonic) vorticity to the north (south) of the mean position of the Hawaii Lee Countercurrent. Advection of vorticity is the primary process that transports the vorticity generated in the lee of the islands by the wind wake. In this island lee, the zonal wavenumber spectra show a cascade of vorticity/energy from the submesoscale toward the larger scales. Latitudinal differences in the advection of vorticity spectra indicate the propagation of a first baroclinic mode Rossby wave in the region dominated by anticyclonic vorticity to the west of the island of Hawaii, while the region dominated by cyclonic vorticity is disrupted by the wake of the smaller islands.

Thermocline spiciness variations in the tropical Indian Ocean observed during 2003–2014

Spiciness variability in the thermocline is believed to be an important subsurface ocean process affecting sea surface temperature (SST) and climate variability over the tropical oceans. Analysis of in-situ observations during 2003–2014 reveals pronounced spiciness variations at interannual-to-decadal timescale within the thermocline of the tropical Indian Ocean (TIO). Isopycnal potential temperature and salinity anomalies between σθ=24.0–25.0kgm−3 have typical magnitudes of 0.2°C and 0.08psu in the southeastern Arabian Sea and the southern TIO, comparable with those observed in the Pacific basin. In the southeastern Arabian Sea, spiciness variations are dominated by a decadal trend, showing positive (warm, salty) anomalies in 2003–2006 and negative (cold, fresh) anomalies in 2009–2013. The major cause is the mixed-layer property change in the northern Arabian Sea, which induces variation in both spiciness and amount of water detrained down to the thermocline. In the southern TIO, largest variations occur at two zonal spiciness fronts where different thermocline water masses converge. These signals are primarily produced by wind-driven geostrophic advection. Anomalies at the northern front (11–6°S) exhibit westward spreading tendency and quick diffusion, which reflect mainly the signatures of the 1st baroclinic mode Rossby waves. Anomalies at the southern front (18–13°S) move westward to the western TIO via the advection of the South Equatorial Current (SEC). These low-frequency subsurface spiciness variations can alter the background vertical thermal gradient in the thermocline ridge region (55–85°E, 12–4°S), although such impact on the SST variability is generally small.

Origin of Decadal-Scale, Eastward-Propagating Heat Content Anomalies in the North Pacific*

Abstract Upper ocean heat content (OHC) is at the heart of natural climate variability on interannual-to-decadal time scales, providing climate memory and the source of decadal prediction skill. In the midlatitude North Pacific Ocean, OHC signals are often found to propagate eastward as opposed to the frequently observed westward propagation of sea surface height, another variable that represents the ocean subsurface state. This dichotomy is investigated using a 150-yr coupled GCM integration. Simulated OHC signals are distinguished in terms of two processes that can support eastward propagation: higher baroclinic Rossby wave (RW) modes that are associated with density perturbation, and spiciness anomalies due to density-compensated temperature and salinity anomalies. The analysis herein suggests a unique role of the Kuroshio–Oyashio Extension (KOE) region as an origin of the spiciness and higher mode RW signals. Wind-forced, westward-propagating equivalent barotropic RWs cause meridional shifts of the subarctic front in the KOE region. The associated anomalous circulation crosses mean temperature and salinity gradients and thereby generates spiciness anomalies. These anomalies are advected eastward by the mean currents, while the associated surface temperature anomalies are damped by air–sea heat exchange. The accompanying surface buoyancy flux generates higher baroclinic, eastward-propagating RWs. The results suggest that the large OHC variability in the western boundary currents and their extensions is associated with the spiciness gradients and axial variability of oceanic fronts.

The role of tilted heating in the evolution of the MJO

There has been much discussion about the role of tilted heating in the evolution of the MJO. It is believed that the inability of many general circulation models to produce a robust MJO is directly related to deficiencies in the model-produced tropical heating distributions. Given the MJO's importance in tropical climate, we need to better understand what heating distributions are required in climate models to produce strong MJOs. This study addresses the role of tilted heating in the Community Atmospheric Model version 4 (CAM4). Idealized and observed heating distributions are added to the tropical atmosphere in CAM4. These include heating that tilts westward with height in the lower and upper atmosphere, as well as various tilted and untilted heating in the form of either idealized blobs or observations derived from the Tropical Rainfall Measuring Mission Precipitation Radar and the International Satellite Cloud Climatology Project. We find that low-level heating ahead of the MJO convective center is critical for the initial strengthening and later maintenance of the MJO. However, tilted heating is not necessary to simulate a realistic MJO. In addition, excess upper level heating, whether tilted or not, appears to degrade the MJO signal, although Rossby wave modes are evident only in runs with additional upper level heating. Kelvin waves are evident during strong MJOs for all simulations but are only visible over the central Pacific. Results suggest that accurate shallow convective parameterizations may be more important than deep convective ones in the evolution of the MJO.

A mechanism for the latitudinal dependence of peak‐spectrum sea surface height variability

AbstractPrevious studies have shown that the power spectrum of satellite‐observed sea surface height (SSH) variability peaks at a certain frequency (or a wave number) band at a given latitude. Lin et al. (2008) attributed this latitudinal dependence to the critical frequency of the first baroclinic mode Rossby waves in the tropical and subtropical oceans. Their study was based on the linear Rossby wave theory and focused on SSH variability in the tropical and subtropical oceans since the altimetry data do not adequately resolve lengths of baroclinic Rossby waves at and near the critical frequency in high latitudes. In this study, we expand their analysis to high‐latitude oceanic basins and to include nonlinear eddy effects, by using a linear wave model and a high‐resolution model output from the OGCM for the Earth Simulator (OFES). It is found that the linear wave mechanism by and large remains valid in the tropical and subtropical oceans. In higher latitudes as well as in some regions in the western tropical and subtropical oceans, other mechanisms, like nonlinear eddy, play more important role in determining the SSH variability.

Dynamics of satellite-derived interannual ocean bottom pressure variability in the western tropical North Pacific

[1] Ocean bottom pressure variability, derived from Release-05 Gravity Recovery and Climate Experiment time-variable gravity coefficients over the ocean, is investigated along the tropical North Pacific for the case of long time scales (>1 yr) and large space scales (>750 km). To interpret the observations, a linear model of the bottom pressure response to interior wind stress curl is derived on the basis of normal vertical modes; the adjustment comprises contributions from barotropic Sverdrup dynamics as well as first baroclinic mode Rossby waves. Model solutions are evaluated numerically using time-mean ocean stratification from the Ocean Comprehensible Atlas and time-varying surface wind stress from the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis. In the western tropical North Pacific, model and data compare favorably; simulated and observed time series are significantly correlated, and the model generally explains more than half of the data variance; the good correspondence between model and data speaks to the good quality of the satellite-derived fields. In the central and eastern tropical North Pacific, findings are more ambiguous; model and data time series are mostly not significantly correlated, and simulations generally explain less than half of the data variance; discrepancies between model and data could point to physics absent from the model, for example, signals generated at the eastern boundary. Results provide observational demonstration that baroclinic contributions to bottom pressure changes can be important at low latitudes and low frequencies; findings hint at a basin-scale influence of tropical climate modes on the ocean bottom pressure field.

Interannual sea level variability in the tropical Pacific Ocean from 1993 to 2006

Three net surface heat flux products, namely from 1) version 2 of Common Ocean Reference Experiment (CORE.2), 2) Objectively Analyzed Air–Sea Fluxes (OAFlux), and 3) the European Centre for Medium-Range Weather Forecasts operational ocean analysis/reanalysis system (ECMWF ORA-S3), and three wind stress products, namely from 1) CORE.2, 2) Simple Ocean Data Assimilation Reanalysis, version 2.1.6 (SODA 2.1.6), and 3) ECMWF ORA-S3 are used to investigate the abilities of four simple oceanic mechanisms in explaining the interannual variance of altimetry-derived sea surface height (SSH) anomalies in the tropical Pacific Ocean over the period 1993–2006. It is found that local response to surface heating plays an important role in sea level rise along the western equatorial Pacific (150°–180°E). The dominant processes affecting interannual variability of observed SSH anomalies vary regionally in the tropical Pacific; local response to surface heating, local Ekman pumping, wind-induced first baroclinic mode Rossby waves and the eastern boundary forcing are all important. Both the local response to surface heating and the eastern boundary forcing are important in explaining the interannual variance of observed SSH anomalies in the northeastern tropical Pacific; while the dominant contribution to interannual sea level variability in the southeastern tropical Pacific is from the eastern boundary forcing, the local Ekman pumping plays a relatively minor role in the interannual SSH change there. The wind-induced first baroclinic mode Rossby waves dominate interannual SSH variability in the western tropical Pacific, excluding the area of 2°–10°N, west of 170°E. Although a large part of the interannual sea level variability in the western tropical Pacific is related to the oceanic remote adjustment to wind stress forcing, the contributions of local responses to surface heating and wind forcing cannot be overlooked.

Buoyancy-Driven Interannual Sea Level Changes in the Tropical South Atlantic

Abstract Linear models of dynamical ocean adjustment to wind field changes, local atmospheric driving, and eastern boundary forcing are often invoked to explain observed patterns of interannual regional sea level variability. While skillful in some regions, these processes alone cannot explain low levels of interannual sea level variability observed in the tropical Atlantic. In this study, through a set of modeling approaches, interannual sea level changes in the tropical South Atlantic are attributed and the dynamical influence of buoyancy forcing is elucidated. Similar to recent findings in the southeast tropical Pacific, sea level patterns in the tropical South Atlantic (as estimated from a data-constrained ocean general circulation model) are found to result from the action of both surface wind and buoyancy forcing; in addition to static local effects, the buoyancy-driven changes comprise important nonlocal ocean dynamical processes. It is shown that the buoyancy-driven sea level changes can be understood within the framework of a linear first baroclinic mode Rossby wave model forced by atmospheric fields and variability along the eastern boundary. To lowest order, the linear model framework also reproduces qualitative patterns of basinwide compensation between wind- and buoyancy-driven sea level changes, which are mostly tied to the anticorrelation of both surface and boundary forcing. Results suggest that the ocean’s dynamical adjustment to buoyancy forcing exerts an important influence on interannual sea level changes across all tropical oceans.

The dynamics of wind‐driven intraseasonal variability in the equatorial Indian Ocean

Variability in the equatorial Indian Ocean on intraseasonal time scales (defined as periods of 30–110 days) is investigated using satellite and in situ observations and a simple analytical linear long‐wave equatorial β‐plane model. Despite the extreme simplicity of the model, which includes just the two gravest baroclinic mode Kelvin waves and first meridional mode Rossby waves, simulated surface zonal velocity and sea surface height compare very well with observations. Both observations and model are characterized by a red shift in the velocity spectrum relative to the wind forcing spectrum, which is attributable to a combination of factors, including (1) the near resonant excitation of Kelvin waves by eastward propagating winds, (2) constructive interference between wind‐forced waves and Rossby waves reflected from the eastern boundary, and (3) the favored excitation of low‐frequency waves whose zonal wavelengths are long compared to the zonal fetch of the wind. We decomposed variability in two broad period bands, namely, 30–70 days and 70–110 days, for detailed analysis. At periods of 30–70 days, zonal velocity tends to be stationary in the directly forced region along the equator owing to the competing contributions of Kelvin and Rossby waves. In contrast, at 70–110 day periods, zonal velocity propagates westward despite eastward propagation of zonal wind stress because of the combined influence of eastern boundary generated and wind‐forced Rossby waves. Kelvin waves reflected from the western boundary are negligibly small, indicating that basin mode resonances are not prominent as has been previously suggested.

Abrupt Transition to Strong Superrotation Driven by Equatorial Wave Resonance in an Idealized GCM

Abstract Persistent superrotation is seen in the atmospheres of other terrestrial bodies (Venus, Titan) but not in that of present Earth, which is distinguished by equatorial easterlies. Nevertheless, superrotation has appeared in numerical simulations of Earth’s atmosphere, from two-layer models to multilevel comprehensive GCMs. Simulations of warm climates that generate enhanced tropical convective variability seem particularly prone to superrotation, which has led to hypotheses that the warmer atmospheres of the early Pliocene and Eocene may have been superrotating, and that the phenomenon may be relevant to future climate projections. This paper considers a positive feedback leading to superrotation based on an equatorial wave resonance that occurs in a westerly background flow. The authors present simulations with an idealized multilevel GCM forced with a zonally varying equatorial heating, which show abrupt transitions to strongly superrotating states. Linear shallow water theory is used to show that these transitions occur as the superrotating jet velocity approaches the phase speed of free equatorial Rossby wave modes, leading to a resonant amplification of the response to eddy heating and its associated equatorward momentum flux. The resonance and transition are most prominent in simulations where the meridional temperature gradient has been reduced, and hysteresis behavior is seen when the gradient is eliminated completely. No evidence is found in these simulations for the midlatitude wave feedback believed to drive abrupt transitions in two-layer models, and there is only a minor role for the axisymmetric feedback based on vertical advection by the Hadley circulation.

Stratospheric Sudden Warmings as Self-Tuning Resonances. Part II: Vortex Displacement Events

Abstract Vortex displacement stratospheric sudden warmings (SSWs) are studied in an idealized model of a quasigeostrophic columnar vortex in an anelastic atmosphere. Motivated by the fact that observed events occur at a fixed orientation to the earth’s surface and have a strongly baroclinic vertical structure, vortex Rossby waves are forced by a stationary topographic forcing designed to minimize excursions of the vortex from its initial position. Variations in the background stratospheric “climate” are represented by means of an additional flow in solid body rotation. The vortex response is determined numerically as a function of the forcing strength M and the background flow strength Ω. At moderate M it is found that a large response, with many features resembling observed displacement SSWs, occurs only for a narrow range of Ω. Linear analysis reveals that for this range of Ω the first baroclinic azimuthal wave-1 Rossby wave mode is close to being resonantly excited. A forced nonlinear oscillator equation is proposed to describe the nonlinear behavior, and a method for determining the relevant coefficients numerically, using unforced calculations of steadily propagating vortex “V states,” is adopted. The nonlinear equation predicts some qualitative details of the variation in the response at finite M. However, it is concluded that strongly nonlinear processes, such as wave breaking and filament formation, are necessarily quantitatively important in determining the amplitude of the near-resonant response at finite M.

The Relative Roles of Different Physical Processes in Unstable Midlatitude Ocean–Atmosphere Interactions

Abstract Following Goodman and Marshall (hereinafter GM), an improved intermediate midlatitude coupled ocean–atmosphere model linearized around a basic state is developed. The model assumes a two-layer quasigeostrophic atmosphere overlying a quasigeostrophic upper ocean that consists of a constant-depth mixed layer, a thin entrainment layer, and a thermocline layer. The SST evolution is determined within the mixed layer by advection, entrainment, and air–sea flux. The atmospheric heating is specified at midlevel, which is parameterized in terms of both the SST and atmospheric temperature anomalies. With this coupled model, the dynamical features of unstable ocean–atmosphere interactions in the midlatitudes are investigated. The coupled model exhibits two types of coupled modes: the coupled oceanic Rossby wave mode and the SST-only mode. The SST-only mode decays over the entire range of wavelengths, whereas the coupled oceanic Rossby wave mode destabilizes over a certain range of wavelengths (∼10 500 km) when the atmospheric response to the heating is equivalent barotropic. The relative roles of different physical processes in destabilizing the coupled oceanic Rossby wave are examined. The main processes emphasized are the influence of entrainment and advection for determining SST evolution, and the atmospheric heating profile. Although either entrainment or advection can lead to unstable growth of the coupled oceanic Rossby wave with similar wavelength dependence for each case, the advection process is found to play the more important role, which differs from GM’s results in which the entrainment process is dominant. The structure of the unstable coupled mode is sensitive to the atmospheric heating profile. The inclusion of surface heating largely reduces the growth rate and stabilizes the coupled oceanic Rossby wave. In comparison with observations, it is demonstrated that the structure of the growing coupled mode derived from the authors’ model is closer to reality than that from the previous model.

On the Dynamics of Two-Dimensional Hurricane-Like Vortex Symmetrization

Abstract Despite the fact that asymmetries in hurricanes, such as spiral rainbands, polygonal eyewalls, and mesovortices, have long been observed in radar and satellite imagery, many aspects of their origin, space–time structure, and dynamics still remain unsolved, particularly their role on the vortex intensification. The underlying inner-core dynamics need to be better understood to improve the science of hurricane intensity forecasting. To fill this gap, a simple 2D barotropic “dry” model is used to perform two experiments starting respectively with a monopole and a ring of enhanced vorticity to mimic hurricane-like vortices during incipient and mature stages of development. The empirical normal mode (ENM) technique, together with the Eliassen–Palm (EP) flux calculations, are used to isolate wave modes from the model datasets to investigate their space–time structure, kinematics, and the impact on the changes in the structure and intensity of the simulated hurricane-like vortices. From the ENM diagnostics, it is shown in the first experiment how an incipient storm described by a vortex monopole intensifies by “inviscid damping” of a “discrete-like” vortex Rossby wave (VRW) or quasi mode. The critical radius, the structure, and the propagating properties of the quasi mode are found to be consistent with predictions of the linear eigenmode analysis of small perturbations. In the second experiment, the fastest growing wavenumber-4 unstable VRW modes of a vortex ring reminiscent of a mature hurricane are extracted, and their relation with the polygonal eyewalls, mesovortices, and the asymmetric eyewall contraction are established in consistency with results previously obtained from other authors.

The Dynamics of the West African Monsoon. Part V: The Detection and Role of the Dominant Modes of Convectively Coupled Equatorial Rossby Waves

Abstract This study is the last in a series of papers addressing the dynamics of the West African summer monsoon at intraseasonal time scales between 10 and 90 days. The signals of convectively coupled equatorial Rossby (ER) waves within the summer African monsoon have been investigated after filtering NOAA outgoing longwave radiation (OLR) data within a box delineated by the dispersion curves of the theoretical ER waves. Two families of waves have been detected in the 10–100-day periodicity band by performing a singular spectrum analysis on a regional index of ER-filtered OLR. For each family the first EOF mode has been retained to focus on the main convective variability signal. Within the periodicity band of 30–100 days, an ER wave pattern with an approximate wavelength of 13 500 km has been depicted. This ER wave links the MJO mode in the Indian monsoon sector with the main mode of convective variability over West and central Africa. This confirms the investigations carried out in previous studies. Within the 10–30-day periodicity band, a separate ER wave pattern has been highlighted in the African monsoon system with an approximate wavelength of 7500 km, a phase speed of 6 m s−1, and a period of 15 days. The combined OLR and atmospheric circulation pattern looks like a combination of ER wave solutions with meridional wavenumbers of 1 and 2. Its vertical baroclinic profile suggests that this wave is forced by the deep convective heating. Its initiation in terms of OLR modulation is detected north of Lake Victoria, extending northward and then propagating westward along the Sahel latitudes. The Sahel mode identified in previous studies corresponds to the second main mode of convective variability within the 10–30-day periodicity band, and this has also been examined. Its pattern and evolution look like the first-mode ER wave pattern and they are temporally correlated with a coefficient of +0.6. About one-third of the Sahel mode events are concomitant with an ER wave occurrence. The main difference between these two signals consists of a stronger OLR and circulation modulation of the Sahel mode over East and central Africa. Thus, the Sahel mode occurrence and its westward propagation could be explained in part by atmospheric dynamics associated with the ER waves and in part by land surface interactions, as shown in other studies.