Baroclinic Waves Research Articles

Frontogenesis and Turbulence: A Numerical Simulation

Abstract A numerical simulation is analyzed that resolves the full range of motions from rotationally dominated, growing baroclinic waves to quasi-isotropic, three-dimensional shear instabilities. The results confirm a 40-yr-old prediction, made by B. Hoskins and F. Bretherton, that frontogenetic collapse of cross-frontal spatial scales, driven by baroclinic-wave deformation fields, will continue to the Kelvin–Helmholtz (K–H) turbulent transition. This process of frontal collapse followed by K–H transition provides a mechanism for spontaneous loss of balance in an initially geostrophic flow, and a direct, spectrally nonlocal pathway for downscale energy transfer that is phenomenologically distinct from traditional concepts of turbulent cascades and can contribute substantially to total kinetic energy dissipation. These results, which neglect surface drag and several other potentially relevant atmospheric processes, would suggest that the turbulence associated with collapsing fronts in the atmosphere can extend upward from the surface through roughly one-third of the troposphere.

Oceanic Forcings of the Interdecadal Variability in East Asian Summer Rainfall

Abstract In this study, it was found that the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) are shown to be the two major drivers of the interdecadal variability of summer rainfall over East Asia. The first leading mode (PC1) of this interdecadal variability—associated with an in-phase variation of rainfall anomalies along the Yangtze River valley and Huanghe–Huaihe River valley in China—is attributed to the PDO, while the second leading mode (PC2)—associated with seesawlike rainfall anomalies between the Yangtze River valley and Huanghe–Huaihe River valley—is attributed to the AMO. The AMO teleconnects its influence to the East Asian region, and beyond, through a circumglobal stationary baroclinic wave train extending from the Atlantic Ocean, through the Eurasian continent, and extending to North America. The AMO also altered the nature of the PDO through this atmospheric teleconnection, resulting in the occurrence of a different PDO pattern (“pseudo-PDO”) between the 1960s and 2010s. The pseudo-PDO has a different anomalous SST pattern in both the tropical and midlatitude Pacific compared to the conventional PDO. The pseudo-PDO causes a distinct atmospheric response in East Asia leading to an opposite relationship with the PC1 compared to the conventional PDO, thus leading to a change in the direction of the influence of the PDO on PC1 between the 1880s–1950s and the 1960s–2010s.

Remote sources for year‐to‐year changes in the seasonality of the Florida Current transport

AbstractThe seasonal variability of the Florida Current (FC) transport is often characterized by the presence of an average annual cycle (8% of the variance) of ∼3 Sv range peaking in boreal summer. However, the seasonality displayed by the FC transport in any individual year may have very distinct characteristics. In this study, the analysis focuses on seasonal changes (73–525 day frequency band) in the FC transport that are associated with a variable annual phase, which is defined as the transient seasonal component (FCt, 27% of the variance). It is shown that the FCt is largely modulated by westward propagating sea height anomaly (SHA) signals that are formed in the eastern North Atlantic 4–7 years earlier than observed at 27°N in the Florida Straits. These westward propagating SHA signals behave approximately like first baroclinic Rossby waves that can modulate changes in the FC seasonal variability upon arrival at the western boundary. The main finding from this study is that changes in coastal sea‐level between 25°N and 42°N linked with westward propagating signals account for at least 50% of the FCt. The integrated changes in the coastal sea‐level between 25°N and 42°N, in turn, drive adjustments in the geostrophic transport of the FC at 27°N. Results reported here provide an explanation for previously reported year‐to‐year changes in the FC seasonality, and suggest that large sea‐level variations along the coast of Florida may be partially predictable, given that these Rossby‐wave‐like signals propagate approximately at fixed rates in the open ocean along 27°N.

Anticipation of ENSO: what teach us the resonantly forced baroclinic waves

The El Niño-Southern Oscillation (ENSO) is characterized based on the date the events are mature. Their time lag defined relative to the central value of successive intervals of 4 years length, e.g. 01/1868–01/1872, 01/1872–01/1876 …, 01/1996–01/2000 … affects their evolution and, for a given amplitude, their variability. It specifies the dynamics of the quadrennial Quasi-Stationary Wave (QSW) in the tropical Pacific since ENSO always occurs at the end of the eastward phase propagation of that QSW. A third of events are unlagged with very low variability, SST anomalies being nearly concomitant between the extreme eastern and the central-eastern Pacific. A third of events are weakly lagged, in phase with the annual QSW, whose variability is much greater. Several months may elapse between the maximum SST anomalies east of the basin and along its equatorial central part. The last third of the events exhibits considerable variability, whether they are out of phase with the annual QSW or strongly lagged. The SST anomaly between 5°N and 20°N plays a key role in the maturation of the events out of phase. The events in phase (10% of the total population) are characterized by a negative SST anomaly in the central-eastern Pacific that reverses just before the maturation stage of ENSO. Sea water temperature 125 m deep in the central-eastern Pacific carries the earliest relevant information with a lead time of one year for forecasting the amplitude of unlagged ENSO while reporting how SST anomalies will develop until ENSO is fully developed. Besides, long-term forecast of the resumption of La Niña can be achieved accurately when weakly lagged events in phase with the annual QSW occur. The well differentiated typology of events vs. their time lag is the best clue to prove the leading role of the quadrennial QSW in the genesis of ENSO, while the related dynamic of the atmosphere ensues.

The statistical structure of synoptic variability ocean currents at the continental slope of the Laptev Sea and features of their generation by anemobaric forces

Based on vector-algebraic analysis of random processes, we study the statistical structure of the synoptic variability of currents measured by an ADCP in the upper mixed layer in the central part of the continental slope of the Laptev Sea in 2006–2007. The results of statistical analysis show that in some cases the synoptic currents in the surface layer of the sea are signs of wind drift currents. This is indicated by the high correlation between the tangential friction of wind and currents, as well as the reversal of the depth of current vectors and the major axes of the ellipses of the mean-square deviation of the Ekman spiral. Due to the large variability of wind flows and stratification of water masses, the penetration depth of these currents is small and varies from 6 to 30 m, with pronounced seasonal variation. In deeper layers, no relationship between the currents and anemobaric forces is traced. It is concluded that the fluctuations of synoptic scale currents in the area of the continental slope of the Laptev Sea represent a superposition of Ekman drift currents and movements associated with free baroclinic Kelvin waves. These currents are the dominant contributor in the upper 30-m layer of the ocean, while waves play a key role in deeper waters.

Cloud Radiative Effects and Precipitation in Extratropical Cyclones

AbstractClouds associated with extratropical cyclones complicate the well-developed theory of dry baroclinic waves through feedback on their dynamics by precipitation and cloud-altered radiative heating. The relationships between cyclone characteristics and the diabatic heating associated with cloud radiative effects (CREs) and latent heat release remain unclear. A cyclone tracking algorithm [NASA’s Modeling, Analysis, and Prediction (MAP) Climatology of Midlatitude Storminess (MCMS)] is used to identify over 106 cyclones in 33 years of the ERA-Interim and collect the properties of each disturbance. Considering storm intensity as related to wind speeds, which depend on the pressure gradient, the distribution of cyclone properties is investigated using groups defined by their depth (local pressure anomaly) and the radius of the region within closed pressure contours to investigate variations with longitude (especially ocean and land), hemisphere, and season. Using global data products of cloud radiative effects on in-atmosphere net radiation [the ISCCP radiative flux profile dataset (ISCCP-FD)] and precipitation (GPCP), composites are assembled for each cyclone group and for “nonstormy” locations. On average, the precipitation rate and the CRE are approximately the same among all cyclone groups and do not strongly differ from nonstormy conditions. The variance of both precipitation and CRE increases with cyclone size and depth. In larger, deeper storms, maximum precipitation and CRE increase, but so do the amounts of nonprecipitating and clear-sky conditions.

Effects of surface drag on upper-level frontogenesis within a developing baroclinic wave

This paper investigates the effects of surface drag on upper-level front with a three-dimensional nonhydrostatic mesoscale numerical model (MM5). To this end, a new and simple potential vorticity intrusion (PVI) index is proposed to quantitatively describe the extent and path that surface drag affects upper-level front. From a PV perspective, the formation of the upper-level front is illustrated as the tropopause folding happens from the stratosphere. The PVI index shows a good correlation with the minimum surface pressure, and tends to increase with the deepening of the surface cyclone and upper-level front. The surface drag acts to damp and delay the development of upper-level front, which could reduce the growth rate of the PVI index. However, the damping presents different effects in different development stages. It is the most significant during the rapid development stage of the surface cyclone. Compared with no surface drag cases, the tropopause is less inclined to intrude into the troposphere due to the surface drag. Positive feedback between the surface cyclone and upper-level front could accelerate the development of the frontal system.

An Analysis of Gravity Wave Spectral Characteristics in Moist Baroclinic Jet–Front Systems

Abstract This study investigates gravity wave spectral characteristics based on high-resolution mesoscale simulations of idealized moist baroclinic jet–front systems with varying degrees of convective instability, with the intent of improving nonorographic gravity wave parameterizations. In all experiments, there is a clear dominance of negative vertical flux of zonal momentum. The westward momentum flux is distributed around the estimated ground-based baroclinic wave phase velocity along the zonal direction, while strong moist runs indicate a dipole structure pattern with stronger westward momentum flux centers at slower phase velocity and weaker eastward momentum flux centers at faster phase velocity. The spectral properties of short-scale wave components (50–200 km) generally differ from those of medium-scale ones (200–600 km). Compared to the medium-scale wave components, the momentum flux phase speed spectra for the short-scale ones appear to be more sensitive to the increasing initial moisture content. The spectral behavior in horizontal wavenumber space or phase velocity space is highly anisotropic, with a noticeable preference along the jet direction, except for the short-scale components in strong moist runs. It is confirmed that the dry gravity wave source (i.e., upper jet and/or surface front) generates a relatively narrow and less symmetrical power spectrum (dominated by negative momentum flux) centered around lower phase velocity and horizontal wavenumber, whereas the moist gravity wave source (i.e., moist convection) generates a broader and more symmetrical power spectrum, with a broader range of phase speeds and horizontal wavenumbers. This study also shows that the properties of gravity wave momentum flux depend on the location relative to the baroclinic jet.

Validating weather and climate models at small Rossby numbers: including a boundary layer

Ideally, the validation of weather and climate models requires that the predictions remain close to an exact solution of the governing equations. The complexity of weather and climate models means that it is not possible to compute exact solutions except in trivial cases. However, in the limit of small Rossby number, the exact solution of the Euler equations can be shown to be close to that of a semi‐geostrophic model, which can be computed. Previous studies have used the small‐Rossby‐number limit to validate numerical methods for a baroclinic wave without sub‐grid physics. However, the method of coupling to the sub‐grid physics plays an important role in the performance of weather and climate models. The aim of this article is thus to extend the previous studies to include a boundary‐layer parametrization. We use a balanced model that includes a known boundary‐layer parametrization, the semi‐geotriptic model. We then demonstrate that the semi‐geotriptic model is the appropriate small‐Rossby‐number limit of the solution of the Euler equations with the same boundary‐layer representation. The semi‐geotriptic model is then used to expose weaknesses in the numerical methods for coupling the boundary layer to the rest of the model.

A Rossby whistle: A resonant basin mode observed in the Caribbean Sea

AbstractWe show that an important source of coastal sea level variability around the Caribbean Sea is a resonant basin mode. The mode consists of a baroclinic Rossby wave which propagates westward across the basin and is rapidly returned to the east along the southern boundary as coastal shelf waves. Almost two wavelengths of the Rossby wave fit across the basin, and it has a period of 120 days. The porous boundary of the Caribbean Sea results in this mode exciting a mass exchange with the wider ocean, leading to a dominant mode of bottom pressure variability which is almost uniform over the Grenada, Venezuela, and Colombia basins and has a sharp spectral peak at 120 day period. As the Rossby waves have been shown to be excited by instability of the Caribbean Current, this resonant mode is dynamically equivalent to the operation of a whistle.

Dynamics of Upper-Level Frontogenesis in Baroclinic Waves

Abstract This paper reports a diagnosis of the structure and dynamics of upper-level fronts (ULFs) simulated with a high-resolution Weather Research and Forecasting Model with diabatic heating versus one without diabatic heating. The ULFs of both simulations develop in about 6 days as integral parts of intensifying baroclinic waves. Each has a curvilinear structure along the southern edge of a relatively narrow long tongue of high potential vorticity in which stratospheric air is subducted to different tropospheric levels by synoptic-scale subsidence. It resembles a veil in the sky of varying thickness across the midsection upstream of the trough of the baroclinic wave. The 3D frontogenetical function is shown to be a necessary and sufficient metric for quantifying the rate of development of ULFs. Its value is mostly associated with the contribution of the 3D ageostrophic velocity component. Upper-level frontogenesis is attributable to the joint direct influence of the vortex-stretching process and the deformation property of the 3D ageostrophic flow component. The model also generates a spectrum of vertically propagating mesoscale gravity waves. The ULFs simulated with and without diabatic heating processes are qualitatively similar. The ULF is considerably more intense when there is heating. The heating, however, does not make a significant direct contribution to but indirectly does so through its impacts on the subsidence field of the baroclinic wave.

A wavelet-based approach to detect climate change on the coherent and turbulent component of the atmospheric circulation

Abstract. The modifications of atmospheric circulation induced by anthropogenic effects are difficult to capture because wind fields feature a complex spectrum where the signal of large-scale coherent structures (planetary, baroclinic waves and other long-term oscillations) is mixed up with turbulence. Our purpose is to study the effects of climate changes on these two components separately by applying a wavelet analysis to the 700 hPa wind fields obtained in climate simulations for different forcing scenarios. We study the coherent component of the signal via a correlation analysis to detect the persistence of large-scale or long-lasting structures, whereas we use the theory of autoregressive moving-average stochastic processes to measure the spectral complexity of the turbulent component. Under strong anthropogenic forcing, we detect a significant climate change signal. The analysis suggests that coherent structures will play a dominant role in future climate, whereas turbulent spectra will approach a classical Kolmogorov behaviour.

Wave analysis in the atmosphere of Venus below 100-km altitude, simulated by the LMD Venus GCM

A new simulation of Venus atmospheric circulation obtained with the LMD Venus GCM is described and the simulated wave activity is analyzed. Agreement with observed features of the temperature structure, static stability and zonal wind field is good, such as the presence of a cold polar collar, diurnal and semi-diurnal tides. At the resolution used (96 longitudes × 96 latitudes), a fully developed superrotation is obtained both when the simulation is initialized from rest and from an atmosphere already in superrotation, though winds are still weak below the clouds (roughly half the observed values). The atmospheric waves play a crucial role in the angular momentum budget of the Venus’s atmospheric circulation. In the upper cloud, the vertical angular momentum is transported by the diurnal and semi-diurnal tides. Above the cloud base (approximately 1 bar), equatorward transport of angular momentum is done by polar barotropic and mid- to high-latitude baroclinic waves present in the cloud region, with frequencies between 5 and 20 cycles per Venus day (periods between 6 and 23 Earth days). In the middle cloud, just above the convective layer, a Kelvin type wave (period around 7.3 Ed) is present at the equator, as well as a low-latitude Rossby-gravity type wave (period around 16 Ed). Below the clouds, large-scale mid- to high-latitude gravity waves develop and play a significant role in the angular momentum balance.

Metrics for the Diurnal Cycle of Precipitation: Toward Routine Benchmarks for Climate Models

Abstract Metrics are proposed—that is, a few summary statistics that condense large amounts of data from observations or model simulations—encapsulating the diurnal cycle of precipitation. Vector area averaging of Fourier amplitude and phase produces useful information in a reasonably small number of harmonic dial plots, a procedure familiar from atmospheric tide research. The metrics cover most of the globe but down-weight high-latitude wintertime ocean areas where baroclinic waves are most prominent. This enables intercomparison of a large number of climate models with observations and with each other. The diurnal cycle of precipitation has features not encountered in typical climate model intercomparisons, notably the absence of meaningful “average model” results that can be displayed in a single two-dimensional map. Displaying one map per model guides development of the metrics proposed here by making it clear that land and ocean areas must be averaged separately, but interpreting maps from all models becomes problematic as the size of a multimodel ensemble increases. Global diurnal metrics provide quick comparisons with observations and among models, using the most recent version of the Coupled Model Intercomparison Project (CMIP). This includes, for the first time in CMIP, spatial resolutions comparable to global satellite observations. Consistent with earlier studies of resolution versus parameterization of the diurnal cycle, the longstanding tendency of models to produce rainfall too early in the day persists in the high-resolution simulations, as expected if the error is due to subgrid-scale physics.

Stationary nonlinear waves, superposition modes and modulational instability characteristics in the AB system

We study the AB system describing marginally unstable baroclinic wave packets in geophysical fluids and also ultra-short pulses in nonlinear optics. We show that the breather can be converted into different types of stationary nonlinear waves on constant backgrounds, including the multi-peak soliton, M-shaped soliton, W-shaped soliton and periodic wave. We also investigate the nonlinear interactions between these waves, which display some novel patterns due to the non-propagating characteristics of the solitons: (1) Two antidark solitons can produce a W-shaped soliton instead of a higher-order antidark one, (2) The interaction between an antidark soliton and a W-shaped soliton can not only generate a higher-order antidark soliton, but also form a W-shaped solion pair, (3) The interactions between an oscillation W-shaped soliton and an oscillation M-shaped soliton show the multipeak structures. We find that the transition occurs at a modulational stability region in a low perturbation frequency region.

Adjoint sensitivity study on idealized explosive cyclogenesis

The adjoint sensitivity related to explosive cyclogenesis in a conditionally unstable atmosphere is investigated in this study. The PSU/NCAR limited-area, nonhydrostatic primitive equation numerical model MM5 and its adjoint system are employed for numerical simulation and adjoint computation, respectively. To ensure the explosive development of a baroclinic wave, the forecast model is initialized with an idealized condition including an idealized two-dimensional baroclinic jet with a balanced three-dimensional moderate-amplitude disturbance, derived from a potential vorticity inversion technique. Firstly, the validity period of the tangent linear model for this idealized baroclinic wave case is discussed, considering different initial moisture distributions and a dry condition. Secondly, the 48-h forecast surface pressure center and the vertical component of the relative vorticity of the cyclone are selected as the response functions for adjoint computation in a dry and moist environment, respectively. The preliminary results show that the validity of the tangent linear assumption for this idealized baroclinic wave case can extend to 48 h with intense moist convection, and the validity period can last even longer in the dry adjoint integration. Adjoint sensitivity analysis indicates that the rapid development of the idealized baroclinic wave is sensitive to the initial wind and temperature perturbations around the steering level in the upstream. Moreover, the moist adjoint sensitivity can capture a secondary high sensitivity center in the upper troposphere, which cannot be depicted in the dry adjoint run.

A Semi-Implicit Modification to the Lorenz N-Cycle Scheme and Its Application for Integration of Meteorological Equations

Abstract The Lorenz N-cycle is an economical time integration scheme that requires only one function evaluation per time step and a minimal memory footprint, but yet possesses a high order of accuracy. Despite these advantages, it has remained less commonly used in meteorological applications, partly because of its lack of semi-implicit formulation. In this paper, a novel semi-implicit modification to the Lorenz N-cycle is proposed. The advantage of the proposed new scheme is that it preserves the economical memory use of the original explicit scheme. Unlike the traditional Robert–Asselin (RA) filtered semi-implicit leapfrog scheme whose formal accuracy is only of first order, the new scheme has second-order accuracy if it adopts the Crank–Nicolson scheme for the implicit part. A linear stability analysis based on a univariate split-frequency oscillation equation suggests that the 4-cycle is more stable than other choices of N. Numerical experiments performed using the dynamical core of the Simplified Parameterizations Primitive Equation Dynamics (SPEEDY) atmospheric general circulation model under the framework of the Jablonowski–Williamson baroclinic wave test case confirms that the new scheme in fact has second-order accuracy and is more accurate than the traditional RA-filtered leapfrog scheme. The experiments also give evidence for Lorenz’s claim that the explicit 4-cycle scheme can be improved by running its two “isomeric” versions in alternating sequences. Unlike the explicit scheme, however, the proposed semi-implicit scheme is not improved by alternation of the two versions.

Internal tides recorded at ocean bottom off the coast of Southeast Taiwan

AbstractAn ocean‐bottom experiment consisting of an array of four ocean‐bottom seismometers (OBS) was conducted off the coast of southeast Taiwan during May–July 2011. We develop comprehensive analyses of the space‐time kinematics of the tidal signals recorded in the compact high‐sensitivity temperature loggers (CHTL) and the OBS geophones at the ocean bottom with depths ranging from 1254 to 1610 m. The evidence suggests that internal tides are responsible for the recorded signals: baroclinic internal waves (mainly the M2 tide) are generated by barotropic tidal currents in the Luzon Strait. The internal tides exhibit gradual phase changing and irregularly fluctuating strength, leaving signatures in the CHTL as ambient temperature variations, signifying low‐mode wave motions within the stratified water layers; and in OBS geophones as intermittent “tremor” agitations, signifying high‐mode turbulent flows on the seafloor. The M2 internal tides across our array are found to propagate in the northeast direction at speeds ranging from 1 to 2+ m s−1. Furthermore, the internal tides are identified at the ocean‐bottom based on an operational hydrodynamic hindcast/forecast model. The simulations show good agreement with the observed temperature variation on the seafloor and substantiate the vertical velocity and displacement of the water parcel driven by the internal tides. The joint detection of the temperature and tremor signals provides further information about the interactions of internal tides with the seafloor topography and the associated energy dissipation. Our results elucidate the space‐time ubiquity of the internal tides at the ocean bottom, which is an important interface of dynamic oceanography.

The role of heating, winds, and topography on sea level changes in the North Atlantic

AbstractSeasonal and interannual‐to‐decadal variations of large‐scale altimetric sea surface height (SSH) owing to surface heating and wind forcing in the presence of topography are investigated using simplified models. The dominant forcing mechanisms are time scale dependent. On the seasonal time scale, locally forced thermosteric height explains most of the SSH variance north of 18°N. First‐mode linear long baroclinic Rossby waves forced by changes in the winds and eastern boundary conditions explain most of the variance between 10°N and 15°N and are also important east of Greenland. On interannual‐to‐decadal time scales, local thermosteric height remains important at several locations in the middle and high latitudes. A topographic Sverdrup response explains interannual‐to‐decadal SSH between 53°N and 63°N east of Greenland. Farther south, the linear Rossby wave model explains SSH variations on interannual‐to‐decadal time scales between 30°N and 50°N from mid‐basin to the eastern boundary. Propagation of the eastern boundary condition into the interior dominates the interannual‐to‐decadal SSH signals south of 30°N. The effect from NAO‐related heat flux on SSH is small, but forcing the topographic Sverdrup models with NAO‐regressed winds gives slightly better agreement with the observed SSH in the subpolar gyre on interannual‐to‐decadal time scales than using the full winds.

Characteristics of gravity waves generated in the jet-front system in a baroclinic instability simulation

Abstract. An idealized baroclinic instability case is simulated using a ∼ 10 km resolution global model to investigate the characteristics of gravity waves generated in the baroclinic life cycle. Three groups of gravity waves appear around the high-latitude surface trough at the mature stage of the baroclinic wave. They have horizontal and vertical wavelengths of 40–400 and 2.9–9.8 km, respectively, in the upper troposphere. The two-dimensional phase-velocity spectrum of the waves is arc shaped with a peak at 17 m s−1 eastward. These waves have difficulty in propagating upward through the tropospheric westerly jet. At the breaking stage of the baroclinic wave, a midlatitude surface low is isolated from the higher-latitude trough, and two groups of quasi-stationary gravity waves appear near the surface low. These waves have horizontal and vertical wavelengths of 60–400 and 4.9–14 km, respectively, and are able to propagate vertically for long distances. The simulated gravity waves seem to be generated by surface fronts, given that the structures and speeds of wave phases are coherent with those of the fronts.