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Bin Wang and Albert Barcilon

Abstract

The moist stability of a midlatitude zonal flow with a conditionally unstable layer in the presence of an Ekman layer is investigated. The vertical velocity employed in a simplified Kuo's parameterization is sustained by baroclinic wave forcing, diabatic heating and Ekman pumping. A general dispersion relation and eigenfunction are derived analytically for a class of flows with various vertical heating profiles.

The moist unstable mode may be regarded as a baroclinic wave modified by the bulk effect of the convective heating, for which the fundamental dependences of the baroclinic growth rate on the Burger number and vertical shear remain qualitatively valid. Waves longer than the Rossby radius of deformation are not appreciably affected, while the shorter waves are significantly destablized by the convective heating. The growth rates and wavelengths of the most unstable modes are nonlinear functions of the averaged specific humidity of the moist layer, and there is an optimum specific humidity that minimizes the preferred wavelength, this value being proportional to the static stability for a representative heating profile. The quasi-geostrophic constraints and baroclinity appear to be decisive factors that suppress short waves and lead to a finite preferred wavelength.

The destabilizing effect of the convective heating is considerably enhanced by the reduction of the static stability. Among the other influential parameters that affect the growth rate, relatively lower cloud top and a deep moist layer have a profound effect an the stability. Because of the cooperative interactions between favorable factors, the simultaneous occurrence of several of the mechanisms listed above may produce explosive-like growth. The relatively shallow convention and the Ekman layer will slow down the wave propagation speed.

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Bin Wang and Albert Barcilon

Abstract

The weakly nonlinear evolutions of the Green and Charney waves are compared for two regimes: (1) when internal dissipation is the dominant dissipation; (2) when Ekman friction is the dominant dissipation.

When the Ekman dissipation is dominant, we obtain a large amplitude equilibrated wave state which depends upon the initial conditions but not upon the magnitudes of the dissipation; the steady wave features a barotropic structure, and does not transport heat in the meridional direction. In sharp contrast, when internal dissipation is dominant, a small amplitude, equilibrated wave state is found, which is independent of the initial conditions but depends on the magnitude of the internal dissipation. The steady wave exhibits a westward phase tilt and transports heat poleward by an amount proportional to the internal dissipation.

The presence of a large planetary vorticity gradient stabilizes the finite amplitude evolution of the planetary waves and leads to a stable equilibrium planetary wave state.

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Xiouhua Fu and Bin Wang

Abstract

A coupled tropical ocean–atmosphere model that fills the gap between anomalous coupled models and fully coupled general circulation models is described. Both the atmosphere and ocean are represented by two and one-half layer primitive equation models, which accentuate the physical processes in the oceanic mixed layer and atmospheric boundary layer. The two media are coupled through both momentum and heat flux exchanges without explicit flux correction. The coupled model, driven by solar radiation, reproduces realistic seasonal cycles of the mixed layer temperature, currents, and depth, and the surface winds and rainfall in the tropical Pacific.

The model results indicate that the equatorial westward phase propagation of the annual warming is primarily caused by zonal temperature advection and downward solar radiation modified by clouds, whereas the wind-evaporation-SST feedback plays a minor role. The meridional wind component appears to have a stronger impact than the zonal wind component on the seasonal cycle of the eastern Pacific cold tongue, because the cross equatorial winds have stronger annual variation, which is more effective in regulation of SST through changing surface evaporation and mixed layer entrainment. The annual variation of the solar forcing is shown to have a significant impact on the long-term mean state. Without the seasonal cycle forcing, the western Pacific warm pool would shift eastward and the latitudinal climate asymmetry in the eastern Pacific would be stronger.

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Bin Wang and Xiaosu Xie

Abstract

Over the warm pool of the equatorial Indian and western Pacific Oceans, both the climatological mean state and the processes of atmosphere–ocean interaction differ fundamentally from their counterparts over the cold tongue of the equatorial eastern Pacific. A model suitable for studying the coupled instability in both the warm pool and cold tongue regimes is advanced. The model emphasizes ocean mixed layer physics and thermodynamical coupling that are essential for the warm pool regime. Different coupled unstable modes are found under each regime.

In contrast to the cold tongue basic state, which favors coupled unstable low-frequency SST mode, the warm pool regime (moderate mean surface westerlies and deep thermocline) is conducive for high-frequency (intraseasonal timescale) coupled unstable modes. The wind–mixed layer interaction through entrainment/evaporation plays a central role in the warm pool instability. The cloud-radiation feedback enhances the instability, whereas the ocean wave dynamics have little impact. The thermodynamic coupling between the atmosphere and ocean mixed layer results in a positive SST anomaly leading convection, which provides eddy available potential energy for growing coupled mode. The relatively slow mixed layer response to atmospheric forcing favors the growth of planetary-scale coupled modes. The presence of mean westerlies suppresses the low-frequency SST mode.

The characteristics of the eastward-propagating coupled mode of the warm pool system compares favorably with the large-scale features of the observed Madden–Julian Oscillation (MJO). This suggests that, in addition to atmospheric internal dynamic instability, the ocean mixed layer thermodynamic processes interacting with the atmosphere may play an active part in sustaining the MJO by (a) destabilizing atmospheric moist Kelvin waves, (b) providing a longwave selection mechanism, and (c) slowing down phase propagation and setting up the 40–50-day timescale.

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Xiaofan Li and Bin Wang

Abstract

An energetics analysis is presented to reveal the mechanisms by which the environmental flows affect hurricane beta-gyre intensity and beta-drift speed. The two-dimensional environmental flow examined in this study varies in both zonal and meridional directions with a constant shear.

It is found that a positive (negative) shear strain rate of the environmental flow accelerates (decelerates) beta drift. The horizontal shear of the environmental flow contains an axially symmetric component that is associated with vertical vorticity and an azimuthal wavenumber two component that is associated with shear strain rate. It is the latter that interacts with the beta gyres, determining the energy conversion between the environmental flow and beta gyres. A positive shear strain rate is required for transfering kinetic energy from the environmental flow to the beta gyres. As a result, the positive shear strain rate enhances the beta gyres and associated steering flow that, in turn, accelerates the beta drift.

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Bin Wang and Xiouhua Fu

Abstract

The annual reestablishment of the equatorial cold tongue (ECT) in the Pacific is signified by a remarkably rapid reversal of the warming trend from March to May. The processes responsible for this dramatic turnabout are investigated using the outputs generated by a coupled ocean–atmosphere model, which simulates realistic tropical Pacific climate. A new diagnostic equation is put forward for a budget study of the temperature tendency in a mixed layer (ML) with a variable depth. The budget study reveals that the rapid boreal spring cooling in the ML of the ECT (4°S–2°N, 120°–90°W) is primarily attributed to turbulent entrainment (54%), surface evaporation (21%), and meridional advection (14%). The spring shallowness of the ML is also a significant“implicit” contributor. Annually, the ML depth in the ECT varies nearly 180° out of phase with the SST while in phase with the ML heat content. The annual variation of the ML depth is determined by competing effects of the Ekman transport and turbulent entrainment. From March to July, the increase of the meridional wind component dominates that of the zonal component; thereby, the effect of entrainment surpasses that of upwelling, leading to mixed layer deepening. The mechanism governing the annual variation of the ML heat content is essentially the same as those governing the ML depth variation. The results suggest that accurate modeling of the ML turbulent mixing holds the key to realistic simulation of the annual cycle of the ECT.

In contrast, beneath the ITCZ (8°–12°N, 100°–120°W), the rapid spring warming is attributable to increased surface heat flux, while entrainment and thermal advection play minor roles. From February to May, the downward shortwave radiation and the surface latent heat fluxes, along with concurrent equatorial cooling, result in a northward progression of the annual warming and promote an active ITCZ–ECT interaction (including evaporation–wind feedback and cloud–radiation–SST interaction).

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Bin Wang and Xiaowei Tan

Abstract

An ensemble-based approach is proposed to obtain conditional nonlinear optimal perturbation (CNOP), which is a natural extension of linear singular vector to a nonlinear regime. The new approach avoids the use of adjoint technique during maximization and is thus more attractive. Comparisons among CNOPs of a simple theoretical model generated by the ensemble-based, adjoint-based, and simplex-search methods, respectively, not only show potential equivalence of the first two approaches in application according to their very similar spatial structures and time evolutions of the CNOPs, but also reveal the limited performance of the third measure, an existing adjoint-free algorithm, due to its inconsistent spatial distribution and weak net growth ratio of norm square of CNOP comparing with the results of the first two methods. Because of its attractive features, the new approach is likely to make it easier to apply CNOP in predictability or sensitivity studies using operational prediction models.

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Bin Wang and Zheng Fang

Abstract

Based on first principles, a theoretical model for El Niño-Southern Oscillation (ENSO) is derived that consists of prognostic equations for sea surface temperature (SST) and for thermocline variation. Considering only the large-scale, equatorially symmetric, standing basin mode yields a minimum dynamic system that highlights the cyclic, chaotic, and season-dependent evolution of ENSO.

For a steady annual mean basic state, the dynamic system exhibits a unique limit cycle solution for a fairly restricted range of air-sea coupling. The limit cycle is a stable attractor and represents an intrinsic interannual oscillation of the coupled system. The deepening (rising) of the thermocline in the eastern (western) Pacific leads eastern Pacific warming by a small fraction of the cycle, which agrees well with observation and plays a critical role in sustaining the oscillation. When the nonlinear growth of SST anomalies reaches a critical amplitude, the delayed response of thermocline adjustment provides a negative feedback, turning over warming to cooling or vice versa.

When the basic state varies annually, the limit cycle develops a strange attractor and the interannual oscillation displays inherent deterministic chaos. On the other hand, the transition phase of the oscillation tends to frequently occur in boreal spring when the basic state is most unstable. The strongest boreal spring instability is due to the weakest mean upwelling and largest vertical temperature difference across the mixed layer base. The former minimizes the negative feedback of mean upwelling, whereas the latter maximizes the positive feedback of anomalous upwelling effects on SST; both favor spring instability. It is argued that the season-dependent coupled instability may be responsible for the tendencies of ENSO phase locking with season and period-locking to integer multiples of the annual period, which, in turn, create irregularities in oscillation period and amplitude.

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Xiaosu Xie and Bin Wang

Abstract

The stability of equatorial Rossby waves in the presence of mean flow vertical shear and moisture convergence-induced heating is investigated with a primitive equation model on an equatorial β plane.

A vertical shear alone can destabilize equatorial Rossby waves by feeding mean flow available potential energy to the waves. This energy transfer necessitates unstable waves’ constant phase lines tilt both horizontally (eastward with latitude) and vertically (against the shear). The preferred most unstable wavelength increases with increasing vertical shear and with decreasing heating intensity, ranging typically from 3000 to 5000 km. The instability strongly depends on meridional variation of the vertical shear. A broader meridional extent of the shear allows a faster growth and a less-trapped meridional structure. When the shear is asymmetric relative to the equator, the unstable Rossby wave is constrained to the hemisphere where the shear is prominent. Without boundary layer friction the Rossby wave instability does not depend on the sign of the vertical shear, whereas in the presence of the boundary layer, the moist Rossby wave instability is remarkably enhanced (suppressed) by easterly (westerly) vertical shears. This results from the fact that an easterly shear confines the wave to the lower level, generating a stronger Ekman-pumping-induced heating and an enhanced meridional heat flux, both of which reinforce the instability.

The moist baroclinic instability is a mechanism by which westward propagating rotational waves (Rossby and Yanai waves) can be destabilized, whereas Kelvin waves cannot. This is because the transfer of mean potential energy to eddy requires significant magnitude of barotropic motion. The latter is a modified Rossby wave and can be resonantly excited only by the westward propagating rotational waves. The common features and differences of the equatorial Rossby wave instability and midlatitude baroclinic instability, as well as the implications of the results are discussed.

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Xiouhua Fu and Bin Wang

Abstract

An air–sea–land coupled model of intermediate complexity was used to reveal the important roles of air–sea coupling and adjacent continental monsoons (i.e., American monsoons and Asian–Australian monsoons) on the annual cycle and mean state of the equatorial Pacific.

Excluding the effects of adjacent continental monsoons, the simulated mean SST in the western Pacific displays a warm bias; the SST seasonal cycle exhibits an erroneous, dominant annual component in the western Pacific, and insufficient strength and a 2-month phase delay in the equatorial eastern Pacific. The air–sea coupling alone cannot sustain the full strength of the annual marches of the ITCZ/cold tongue complex. This is because the diabatic heating associated with the ITCZ rainfall generates both a southerly and a westerly component to its equatorward side; while the southerly cools the cold tongue establishing a positive feedback to enhance the ITCZ, and the equatorial westerly favors cold tongue warming inducing a negative feedback that offsets the effect of the southerly component.

Including the influences from the adjacent continental monsoons significantly improves the simulations of the mean state and annual cycle of the equatorial Pacific. The Asian–Australian monsoons are found to improve the mean SST through enhancing the strength of the trades and to yield a correct semiannual cycle of surface wind speed and SST in the equatorial western Pacific. However, they have little influence on the annual cycle in the eastern Pacific SST. In contrast, the South American monsoon exerts profound impacts on the annual variations of the southeast trades and SST in the eastern Pacific, but not the mean SST. The Colombian and North American continental monsoons have little impact on the annual cycle of SST in the cold tongue.

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