On the Effect of Surface Friction and Upward Radiation of Energy on Equatorial Waves

In theoretical models of tropical dynamics, the effects of both surface friction and upward wave radiation through interaction with the stratosphere are oft-ignored, as they greatly complicate mathematical analysis. In this study, we relax the rigid-lid assumption and impose surface drag, which allows the barotropic mode to be excited in equatorial waves. In particular, a previously developed set of linear, strict quasi-equilibrium tropospheric equations is coupled with a dry, passive stratosphere, and surface drag is added to the troposphere momentum equations. Theoretical and numerical model analysis is performed on the model in the limits of an inviscid surface coupled to a stratosphere, as well as a frictional surface under a rigid-lid. This study confirms and extends previous research that shows the presence of a stratosphere strongly shifts the growth rates of fast propagating equatorial waves to larger scales, reddening the equatorial power spectrum. The growth rates of modes that are slowly propagating and highly interactive with cloud-radiation are shown to be negligibly affected by the presence of a stratosphere. Surface friction in this model framework acts as purely a damping mechanism and couples the baroclinic mode to the barotropic mode, increasing the poleward extent of the equatorial waves. Numerical solutions of the coupled troposphere-stratosphere model with surface friction show that the stratosphere stratification controls the extent of tropospheric trapping of the barotropic mode, and thus the poleward extent of the wave. The superposition of phase-shifted barotropic and first baroclinic modes is also shown to lead to an eastward vertical tilt in the dynamical fields of Kelvin-wave like modes.

A regularity criterion for a 2D tropical climate model with fractional dissipation

Tropical climate model derived by Frierson et al. (Commun Math Sci 2:591–626, 2004) and its modified versions have been investigated in a number of papers [see, e.g., Li and Titi (Discrete Contin Dyn Syst Series A 36(8):4495–4516, 2016), Wan (J Math Phys 57(2):021507, 2016), Ye (J Math Anal Appl 446:307–321, 2017) and more recently Dong et al. (Discrete Contin Dyn Syst Ser B 24(1):211–229, 2019)]. Here, we deal with the 2D tropical climate model with fractional dissipative terms in the equation of the barotropic mode u and in the equation of the first baroclinic mode v of the velocity, but without diffusion in the temperature equation, and we establish a regularity criterion for this system.

The intra-annual variability as a potential driver for the mean deep circulation in the tropical oceans

<p>The deep tropical ocean circulation is dominated by systems of vertically and meridionally alternating zonal jets, known as the Equatorial Deep Jets (EDJs) and Extra-Equatorial Jets (EEJs) respectively. The energy sources and physical mechanisms responsible for this circulation are still poorly understood. Recent studies have suggested the importance of intra-annual equatorial waves to transfer their energy to the EDJs.</p><p>In this study, we use idealized numerical simulations forced with a wave-like surface momentum flux to investigate how intra-annual variability can be relevant to the formation of the EEJs. It is shown that the amplitude of the jets, their meridional scales and their vertical and latitudinal extensions are sensitive to the period and wavelength of the forced wave. Short intra-annual waves with periods around ~70 days and wavelength ~300 km are found to reproduce the observed circulation most realistically. Focusing on the dominant barotropic mode, the underlying physical processes are detailed. A spectral analysis reveals that the energy transfer between the forced waves and the jet-structured circulation is compatible with a decay instability occurring in waves triadic interactions.</p><p>In parallel, a statistical analysis is performed on observations of the 1000m-velocities inferred from Lagrangian Argo floats drifts to document the amplitude and scales of the deep intra-annual variability in the tropical Pacific and Atlantic oceans. It gives evidence for the presence of short intra-annual waves that share common properties with the most unstable waves found for the EEJ generation.</p>

Variation in Dipole Blocking Associated with Arctic Warming in Winter: Potential Contributions to Cold and Extremely Cold Events over Eurasia

In this study, the barotropic mode of thermal forcing responsible for the difference in temperature between the Arctic and midlatitude regions was simplified by the nonlinear Schrӧdinger equation with disturbance terms using multiscale perturbation methods. The impact of Arctic warming on dipole blocking, which results in temperature anomalies over the midlatitudes of Eurasia, was studied using the direct perturbation theory for solitons. The results showed: (1) if only nonlinear effects exist between waves and zonal flows, a dipole blocking structure can present in the westerly air flows; (2) the temperature gradient between midlatitude warming and Arctic cooling inhibits the development of dipole blocking structures; and (3) Arctic warming is theoretically more conducive to intensifying the strength of dipole blocking and meridional activities over Eurasia and is more likely to cause the southward invasion of cold air from the Arctic, thereby inducing regionally cold and even extremely cold events in the mid- and low latitudes of Eurasia, including eastern China.

Structural Changes in the Pacific–Japan Pattern in the Late 1990s

The Pacific–Japan (PJ) pattern, also known as the East Asia–Pacific pattern, is a teleconnection that significantly influences the East Asian summer climate on various time scales. Based on several reanalysis and observational datasets, this study suggests that the PJ pattern has experienced a distinct three-dimensional structural change in the late 1990s. Compared with those during 1979–98, the PJ pattern shifts eastward by approximately 20° during 1999–2015, and the intensity of its barotropic structure in the extratropics weakens significantly. As a result, its influences on the summer rainfall along the mei-yu band are weakened after the late 1990s. These observed changes can be attributed to three reasons. First, the location where the PJ pattern is excited shifts eastward. Second, the easterly shear of the background wind is very weak around the source region of the PJ pattern after the late 1990s, which prevents the convection-induced baroclinic mode from converting into barotropic mode and thereby from propagating into the extratropics. Third, the PJ pattern–induced rainfall anomalies are weak along the mei-yu band after the late 1990s. As a result, their feedbacks to the PJ pattern become weak and play a considerably reduced role in maintaining the structure of the PJ pattern in the midlatitudes. In contrast, the eddy energy conversion from the basic flow efficiently maintains the PJ pattern before and after the late 1990s and thereby contributes little to the observed change.

Investigations of non-hydrostatic, stably stratified and rapidly rotating flows

We present an investigation of rapidly rotating (small Rossby number $Ro\ll 1$) stratified turbulence where the stratification strength is varied from weak (large Froude number $Fr\gg 1$) to strong ($Fr\ll 1$). The investigation is set in the context of a reduced model derived from the Boussinesq equations that retains anisotropic inertia-gravity waves with order-one frequencies and highlights a regime of wave–eddy interactions. Numerical simulations of the reduced model are performed where energy is injected by a stochastic forcing of vertical velocity, which forces wave modes only. The simulations reveal two regimes: characterized by the presence of well-formed, persistent and thin turbulent layers of locally weakened stratification at small Froude numbers, and by the absence of layers at large Froude numbers. Both regimes are characterized by a large-scale barotropic dipole enclosed by small-scale turbulence. When the Reynolds number is not too large, a direct cascade of barotropic kinetic energy is observed, leading to total energy equilibration. We examine net energy exchanges that occur through vortex stretching and vertical buoyancy flux and diagnose the horizontal scales active in these exchanges. We find that the baroclinic motions inject energy directly to the largest scales of the barotropic mode, implying that the large-scale barotropic dipole is not the end result of an inverse cascade within the barotropic mode.