#### Abstract

Using the method of multiple scales and perturbation method, a set of coupled models describing the envelope Rossby solitary waves in ()-dimensional condition are obtained, also can be called coupled NLS (CNLS) equations. Following this, based on trial function method, the solutions of the NLS equation are deduced. Moreover, the modulation instability of coupled envelope Rossby waves is studied. We can find that the stable feature of coupled envelope Rossby waves is decided by the value of . Finally, learning from the concept of chirp in the optical soliton communication field, we study the chirp effect caused by nonlinearity and dispersion in the propagation of Rossby waves.

#### 1. Introduction

Wave phenomenon exists widely in nature. As a special and important branch of waves, Rossby solitary waves have important theoretical significance and research value. Meanwhile, with the intercross and penetration of different knowledge, the Rossby solitary waves theory has applied to many other fields successfully, such as physical oceanography, atmospheric physics, hydraulic engineering, communication engineering, and thermal power engineering. In the case of application of solitary waves in engineering, the application of optical soliton in communication engineering is the most representative. Meanwhile, the Rossby waves theory was widely used in the study of mesoscale eddies and the interaction of large and medium scale motions. Rossby solitary waves in the westerly shear flow were first found by Long [1]. Afterward, Benny [2] amplified this research and found that velocity and amplitude of Rossby waves were proportional and depicted the importance of nonlinearity. In view of the barotropic fluid and stratified fluid model, the KdV and mKdV equation are also generated to describe the generation and evolution of Rossby solitary waves by Redekopp [3]. Compared to KdV equation, the mKdV equation is more suitable to express the condition with stronger perturbation. With the development of solitary waves, a variety of equation models for describing the Rossby solitary waves such as ILW-Burgers equation and ZK-Burgers equation were discussed by Yang et al. [4, 5]. Moreover, the generation and evolution of solitary waves in different topography condition and different fluid depths were discussed. Recently, the Rossby parameters along with the changes of latitude were discussed by Luo [6] and plane approximation was obtained. These Rossby solitary waves are called classical solitary waves which related to the KdV-type equations. Later, many researchers have studied the Rossby wave equation in many aspects [7, 8], such as integrable system [9, 10], the integrable coupling of equations [11], and Hamiltonian structures [12]. Unlike the KdV-type equation, the nonlinear Schrödinger equations were used to study the evolution of envelope classical Rossby solitary waves. From the end of the 70’s to the 80’s, driven by the study of atmospheric blocking dynamics [13], nonlinear Rossby wave theory had been developing rapidly and gradually formed Rossby solitary waves theory and dipole waves theory. In addition, beside the above two theories, envelope Rossby solitary waves also dropped in the research scope of the theme. The envelope Rossby solitons in the barotropic shear and uniform flows were first investigated by Benney [14] and Yamagata [15]. Afterward, Luo [16] tried to use this envelope Rossby solitons to explain atmospheric blocking phenomenon. Later, dissipative NLS equation in rotational stratified fluids and its solution were obtained by Shi et al. [17]. As we all know, using the nonlinear Schrödinger equation describing the Rossby solitary waves, we can introduce the concept of chirp in optical soliton communication [18], to study the influence of dispersion and nonlinearity on solitary waves propagation process. In the optical soliton communication field, the concept of chirp [19, 20] is the phenomenon that the central wavelength shifts when the pulse is transmitted. It is helpful to analyze the propagation characteristics and the formation mechanism of solitary waves.

In the domain of solitary wave models, it is necessary to obtain the exact solutions of solitary wave models by all kinds of solving methods and analyze the feature of solitary waves in propagation process based on the exact solutions. Many methods to solve the equations are proposed, such as traveling wave method [21], Darboux transformation method [22–24], Hirota method [25, 26], homogeneous balance method [27], Jacobi elliptic function method [28], Symmetry method [29, 30], Rational solutions [31–33]; meanwhile the features of equations are also discussed [34–36]. In this paper, we plan to adopt the trial function method and derive the exact solution of model. The difference between the two-dimensional and three-dimensional model will be given and some features of three-dimensional NLS equation will be discussed.

We note that the above researches commonly considered two-dimensional model or single ()-dimensional model to reflect the evolution of envelope Rossby solitary waves. There are two disadvantages:(1)The two-dimensional model can only be applied to describe the evolution of envelope Rossby solitary waves in a line.(2)The velocity of the KdV-type soliton is larger than the real observation.

While, as we know, the ()-dimensional model can be applied to reflect the evolution of envelope Rossby solitary waves in a plane, which is more suitable for the real ocean and atmosphere, in this paper, by using the method of multiple scales and perturbation method, starting from the barotropic atmospheric vorticity equation, we will derive the coupled ()-dimensional nonlinear Schrödinger equations for envelope Rossby solitary waves in Section 2. Not only is the model ()-dimensional and more suitable to describe the feature of two envelope Rossby solitary waves in a plane, particularly, but also it is a coupled model and can show the interaction process between two waves. Then, based on trial function method, we will deduce the solution of the CNLS equations group and the envelope solitary waves characteristics in Section 3. Thirdly, we study the modulation instability of Rossby waves trains in ()-dimensional condition in Section 4. Finally, the concept of chirp in optical soliton communication is introduced, and the chirp effect caused by dispersion and nonlinearity is also discussed in Section 5.

#### 2. Derivation of the (2 + 1)-Dimensional CNLS Equations

The tropical atmospheric motion is quasihorizontal and quasiconvergent. The governing equation is the quasigeostrophic barotropic vorticity equation [37].and the boundary condition iswhere and are the local Cartesian coordinates pointing east and north. In this , is Rossby parameter. is characteristic velocity, is characteristic length, and is the width of the beta-channel. is small parameter, on behalf of nonlinear strength. is the Jacobian of .and is Laplace operatorIn general, it is difficult to get analytic solution of (1). But, because of the nonlinear term with a small parameter , we use the multiple scales method and perturbation method to obtain the nonlinear solution of it. As a preliminary study, we will consider two waves.

Let us introduce the slow time and space variablesso long time and space scales are defined asSubstituting (6) into (1), we obtainFurtherand the boundary condition isThe stream function is expanded according to the small parameter and, substituting (9) into (8), we get the multiple order questions of the stream function .

First, one introduces an operatorso thatAssumeand, substituting (13) into , we haveand furtherand the boundary condition iswhereFrom (15), we can get the solution of :Formula (15) under the boundary condition poses a standard Sturm-Liouville problem. The effects of zonal flows on linear equation trapped waves were treated in detail by many researchers. The analytic solution of (15) can be obtained when takes some specific functions. Under normal circumstance one can only seek numerical solution, so we consider the higher order questionand, further, we getwhereand the special solution corresponding to the second, third, and forth item of (21) right side iswhere , , and satisfied and boundary conditionWe assume the special solution corresponding to the first item of the right side of (20) iswhere satisfied The following operators are introduced:Substituting (28) into (27) and because is infinitesimal, we can ignore the first item Obviously the solution for may be expressed in the following form:So we obtained the solution of (20):So as to obtain the solution of , we continue to consider the question of . Substituting (19) and (30) into and collecting the secular-producing items proportional to , we haveFor the sake of obtaining the evolution equation of , we consider another class of nonhomogeneous solutions, assumingintroducing (33) into (32), whereWe assume two special nonhomogeneous solutions of (33) areand, substituting (35) into (34), we getmultiplying (35) by , and integrating on from to . The two sides of (35) are equal to zero, so we get the solution conditions:We get the evolution equations group of wave amplitude, that is, the coupled equations group. To simplify, we introduce the following transformation:and then (37) can be written aswhere In (38), coefficients , , , and are dispersion coefficients, , are Landau coefficients, , are interaction coefficients, and from their expressions it can be seen that their values are related to the base flow function . The (38) is called CNLS equations group.

#### 3. The Solutions of the (2 + 1)-Dimensional CNLS Equations

In this chapter, we will discuss the solutions of the ()-dimensional CNLS equation. Based on our experience, we should transform the coupled equations into two independent equations. Inspired by [38, 39], (39) can be written with the following form:where , represent the coefficients of dispersion term. is the nonlinear coupling term coefficient, which can be positive or negative. In order to obtain the traveling wave solutions of the CNLS equations, we define the transformation which is the complex number envelope solution:where is the amplitude portion of the soliton solutions and is the phase portion of the soliton solution, which is given asSubstituting (42) into (41), let real part and imaginary part be zero, and we can get the following coupled equations:Further, replacing with (43)Using the traveling wave transformation, this pair of equations will be analyzed further, letand we haveFrom the first term of (47), we getFurther, according to the balance principle in trial function method, we will balance with . Using the solution procedure of the trial function method, we will obtain the system of algebraic equations as follows:From the equations above, we have the results of the systemTherefore, we know that satisfiedIf we set in (51) and integrating with respect to , we will obtain the following soliton solutions of (41) as follows:and these solutions are the soliton solutions for CNLS equations, when .

#### 4. Modulation Instabilities of Coupled Envelope Rossby Waves

For coupled envelope nonlinear Rossby waves, they meet the CNLS equations (39). We set the NLS equation of the ()-dimension with constant coefficients asFurther, introducing(53) reduces towhere .

From Section 3, we know the exact periodic wave solutions, taking the simple form as follows:where , , , and satisfyWe assume that , are real numbers and , represent wave number. Equation (54) shows that each nonlinear Rossby wave dispersion not only contains itself wave number and amplitude, but also contains another wave amplitude, which is characteristic of the interaction between wave and wave.

Next, we will analyze the stability of waves solution below. Assume the solutions for the disturbance as follows:Substituting (58) into (53), we can get the linear equations as follows:Further, assumeand, substituting (60) into (59), we can getThe above equations are linear homogeneous equations for , , , and . If there are nonzero solutions, the coefficients determinant must be zero. So we can get the next type:and these are the four algebraic equations of . When the parameters are certain, the value of makes . But, for the influence of the interaction between wave and wave on the stability of wave, we will give a special case for discussion. Assume the number of waves satisfiesand, moreover, setClearly, when the first wave is stable and there is no interaction and the contrary occurs when is instable. Similarly, when the second wave is stable and there is no interaction, and the contrary occurs when is instable. From (62), we getwhere represents the gain for the frequency shift, which has been described in Figure 1. Equation (65) shows that when and , that is, , no matter what value takes, at least there is one which satisfies ; therefore, the waves have instability. This conclusion shows that when , two modulated unstable waves, the interaction of the two waves is still unstable.

When , , that is, , corresponding to no interaction, the two waves are stable. If , when , two waves are stable after interaction. When , two waves are unstable after interaction. If , when , the two waves are stable. When or two waves are unstable after interaction. From the above analysis, we can find that when and , even with two stable nonlinear waves through interaction, the stable feature is decided by the value of .

When and , corresponding to no interaction, the first wave is stable, but the second wave is unstable, while when , , the condition is opposite.

#### 5. Chirp Effect

With summary of previous studies on Rossby waves, it is not hard to find that nonlinearity and dispersion are important factors affecting the propagation of Rossby waves. In this section, we use the concept of chirp in the field of optical soliton communication to study the chirp effect caused by nonlinearity and dispersion in the propagation of Rossby waves.

when , the NLS equation (41) for describing the characteristics of Rossby wave propagation transforms towhere is the coefficient of dispersion and is the nonlinear coefficient. Here, based on the soliton solution of the NLS equation, we take the initial wave form of ()-dimensional Rossby solitary waves as follows, setting :

##### 5.1. Chirp Effect Caused by Dispersion

Let us consider the dispersion effect of chirp, and (66) becomesReviewing the time from , where is an infinitesimal variable, and introducing (67) into (68), we can get the approximate solution of (68) as follows:so that the phase of the wave meetsand, based on (70), we can obtain the chirp effect caused by the dispersion

##### 5.2. Chirp Effect Caused by Nonlinearity

Separately considering the nonlinear effect of chirp, the NLS equation (66) becomesand, investigating the condition of time from , where is an infinitesimal variable, and introducing (67) into (72), we can get the approximate solution of (72) as follows:and the phase of the wave meetsand, based on (74), we can get the chirp effect caused by the nonlinear

##### 5.3. Discussion of Total Chirp

According to (71) and (75), the total chirp is

(1) When the dispersion and nonlinear effects cancel each other, as follows:we can get .

(2) When the dispersion effect is greater than the nonlinear effect, as follows:we get .

(3) When the dispersion is less than the nonlinear effect, as follows:we get . As we know, the total chirp effect is related to the sea conditions in the propagation region and the amplitude of the initial wave. If the propagation area and the sea condition parameter is determined, the total chirp effect is only related to the initial amplitude. The chirp effect caused by dispersion and nonlinearity is described in Figures 2 and 3. The above calculation shows that the magnitude of the initial amplitude is determined by the parameter . Combining Figure 2, Figure 3, and calculation, we can get the following conclusions.

(1) When , the dispersion and nonlinear effects cancel each other, and solitary waves propagate over long distances and keep waveforms constant.

(2) When , the nonlinear effect is greater than the dispersion effect, so that the total chirp is not zero. In this case, the isolated waves present nonlinear characteristics, and the amplitude of isolated waves is changed periodically.

(3) When , the dispersion effect is greater than the nonlinear effect, so that the total chirp is not zero. In this case, the isolated waves present dispersion property, and the amplitude of isolated waves is changed periodically.

#### 6. Conclusions

In this paper, we obtained the ()-dimensional coupled NLS (CNLS) equations and discussed the solutions of the single nonlinear Schrödinger equation. In addition, we know that, for two nonlinear Rossby waves, they can be described by CNLS equations. The equations show that no matter the two waves’ interaction process, their respective energy and momentum are conserved. Furthermore, modulational instability of coupled envelope Rossby waves in ()-dimensional condition is also discussed. we can find that the stable feature of coupled envelope Rossby waves is decided by the value of , which implies that the instability condition depends not only on the prescribed perturbation wave numbers , , but also on the amplitude of the Rossby waves. Finally, introducing the concept of chirp in the optical soliton communication field we study the chirp effect caused by nonlinearity and dispersion in the propagation of Rossby waves.

#### Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

#### Acknowledgments

The project is supported by the Scientific and Technological Innovation Project Financially Supported by Qingdao National Laboratory for Marine Science and Technology (no. 2016ASKJ12), China Postdoctoral Science Foundation Funded Project (2017M610436), Open Fund of the Key Laboratory of Meteorological Disaster of Ministry of Education (Nanjing University of Information Science and Technology) (no. KLME1507), and Open Fund of the Key Laboratory of Ocean Circulation and Waves, Chinese Academy of Sciences (no. KLOCAW1401).