The Energy Cycle Associated to the Pacific Walker Circulation and Its Relationship to ENSO

José Augusto P. Veiga, Alexandre B. Pezza, Tercio Ambrizzi, V. Brahmananda Rao, Sergio H. Franchito, Marcos C. Yoshida

Institute of Astronomy, Geophysics and Atmospheric Sciences, University of S?o Paulo, S?o Paulo, Brazil.
Institute of Technology, Amazon State University, Manaus, Brazil.
National Institute for Space Reseach, S?o José dos Campos, Brazil.
School of Earth Sciences, The University of Melbourne, Victoria, Australia.

DOI: 10.4236/acs.2013.34065                

Abstract

In this paper we study the Lorenz energy cycle of the Walker circulation associated with ENSO. The robust formulation of the energetics allows drawing a clear picture of the global energy and conversion terms associated with the three dimensional domains appropriate to qualify the large scale transfers that influence, and are influenced by, the anomalies during ENSO. A clear picture has emerged in that El Nino and La Nina years have approximately opposite anomalous energy fluxes, regardless of a non-linear response identified in the potential energy fields (zonal and eddy). During El Ninos the tropical atmosphere is characterized by an increase of zonal available potential energy, decrease of eddy available potential energy and decrease of kinetic energy fields. This results in weaker upper level jets and a slowingdown of the overall Walker cell. During La Ninas reversed conditions are triggered, with an acceleration of the Walker cell as observed from the positive anomalous kinetic energy. The potential energy in the Walker circulation domain during the cold phase is also reduced. An equally opposite behavior is also experienced by the energy conversion terms according to the ENSO phase. The energetics-anomalous behavior seem to be triggered at about the same time when ENSO starts to manifest for both the positive and negative phases, suggesting a coupled mechanism in which atmospheric and oceanic anomalies interact and feed back onto each other.

Keywords

Walker Circulation; Energetics; El Nino; La Nina; ENSO

1. Introduction

The Walker circulation (WC) can be defined as a longitudinal overturning circulation that is closely tied to the east-west SST gradient along the equatorial Pacific Ocean [1]. It consists of rising motion and easterly flow at low levels in the western Pacific and westerly flow at upper levels and sinking motions in the eastern Pacific. The intensity of the WC substantially decreases when the winds in the eastern Pacific weaken, a pattern that, according to Bjerknes, is related to the weakening of the sea surface temperature (SST) gradient. Bjerknes concluded that the WC would not exist in absence of a zonal SST gradient. However, [2] carried out a scale-analysis of the thermodynamic energy equation and verified that both radiative heating and the evaporation rate could be neglected when compared with latent heat release in driving the ascent motions in the WC. Consequently, the authors carried out a radiation budget analysis of the equatorial region and noted that there is a near balance between local evaporation and the downward solar energy flux. According to this result, the evaporation rate which Bjerknes considered as one of the main driving forces for the WC could not produce ascending motions over the western Pacific due to attenuation resulting from cloud cover.

[3] employed an Atmospheric General Circulation Model (AGCM) to evaluate changes in the spatial behavior of the WC under different zonal SST gradients and showed that when the zonal SST gradient is completely absent the spatial pattern of the WC is notably disturbed. Their results suggest that the zonal SST gradient is important to modulate spatially the WC, but not to drive it. In order to study the energy and moisture budgets of the WC, [4] designed a set of numerical experiments with an AGCM and showed that the correlation between precipitation and moisture convergence is stronger than the relationship between precipitation and local evaporation. These results support the idea that rising motions in ascending branches of the WC are related to moisture convergence instead of evaporation as proposed by Bjerknes.

Using a two-box model, representative of a cold and warm pool atmosphere denoted, respectively, by CPA and WPA, [5] suggested that the air cooling over the CPA box representative of a region free of convection would be produced by the horizontal moisture transport from this region toward higher temperature areas; i.e., the WPA which is representative of net upward motions and a region of deep convection. [6] showed that the intensity of the WC, quantified in terms of the magnitude of vertical motions, would be to a first approximation determined by subsidence-induced warming being balanced by radiative cooling over the eastern Pacific. Moreover, according to [6] the descending motions are the main driving of the WC and the upward motions that occur in western Pacific would be a consequence of the former. [7] based on numerical simulation showed that the WC has slowed down due to a decrease in the zonal SST and mean sea level pressure gradient in the last decades. According to the authors, the decrease in the zonal atmospheric overturning circulation above the tropical Pacific Ocean is presumably driven by oceanic rather than atmospheric processes.

From an annual mean analysis of the heat balance of the WC [8] showed that ascending motion in the WC’s upward branch is determined by the joint effect of latent heat and radiative cooling processes while infrared radiation loss is associated with sink motions on the WC’s descending branch. Once warm air rises and relatively cold air sinks, respectively, over the western and eastern equatorial Pacific basin there is a continuous conversion between potential and kinetic energy; however, what maintains the apparently continuous generation of the kinetic energy reservoir at the expense of available potential energy had not been quantified yet. The importance of measuring the energy reservoirs and the way they are transformed into another kind of energy (potential to kinetic or vice-versa) has been emphasized in many studies of open ([9-16]) and closed ([17-23]) domains. Thus, in order to quantify the generation of potential energy, the conversion between potential and kinetic energy, and the sources and sinks of available potential and kinetic energy, the present study focuses on the atmospheric energetics involved in the WC and its behavior for strong ENSO phases.

The paper is structured as follows: Section 2 describes the data and methodology used in this work, with an application of the Lorenz energetics technique. In Section 3.1, we present a volume integrated energy cycle, with an analysis of the energy cycle for normal and ENSO conditions. In Section 3.2, we present the climatological vertical-time distribution of the main energy components relative to Walker circulation domain, stressing the main energy mechanism responsible by the maintenance and strength of the Walker circulation. This is followed by an analysis of the energetics for the Walker circulation domain for a composite of El Niños and La Niñas focusing on the energy patterns which drove the differences in each energetic behaviors (Section 3.3). In Section 4, we present a discussion about the main findings. We conclude in Section 5 with a description of the energy cycle evolved in the maintenance of the Walker circulation and their patterns during ENSO events. Furthermore, we suggest future applications of the energetics to quantify the Walker circulation’s intensity changes from a climate change perspective.

2. Data and Methodology

To compute the energetics proposed in this study daily mean data of geopotential (Φ), air temperature (T), zonal (u), meridional (v) and vertical (w) components of the wind velocity in a regular horizontal space of 2.5˚ × 2.5˚ grid resolution from NCEP/NCAR Reanalysis 2 for 12 standard isobaric levels (1000, 925, 850, 700, 600, 500, 400, 300, 250, 200, 150 and 100 hPa) were used. The data cover a period of 33 years (1979 to 2011) and was extracted from the NOAA-CIRES Climate Diagnostic Center available on the website http://www.cdc.noaa.gov. The computation of the energetics for the Pacific WC is applied to an area limited by the latitudes of 10˚S and 5˚N, and longitudes 120˚E and 80˚W. Although the WC is observed globally with branches of varying intensity over all basins, in the present work we concentrate on the largest and strongest cell observed over the Pacific Ocean. A slight asymmetry in relation to the equator is also observed on an annual basis. Figure 1 shows the climatological values of omega and zonal wind over three different latitudinal bands within the 10˚S - 10˚N region around the Equator. As shown by this figure, our chosen range of 10˚S - 5˚N has a slightly enhanced distinction between the ascending and descending branches between the western and eastern boards of the Pacific sector of the cell. Hence, this area optimizes a balance between ascending and descending motion when calculating the energetics. The slight asymmetry occurs because most of the subsidence on the eastern branch occurs to the south of the Equator, where the waters are significantly colder. Additional tests were performed for several slight variations of the original domain, and the core of our results is unchanged.

Mass integrals required for the computation of the energetics are numerically evaluated for the whole troposphere from 1000 to 100 hPa. To compute generation of available potential energy, diabatic heating is required.

Figure 1. Annual climatology of omega and zonal wind associated with the Walker Circulation for (a) 10˚S - 10˚N, (b) 10˚S-Equator and (c) 10˚S - 5˚N relative to the period of 1979 to 2011. The W and E mark the boundaries of our environmental box that defines the energetics of the Pacific sector of the Walker Circulation.

As this variable is directly produced by the radiation and convection parameterization in numerical models and is classed as type-C variables, we instead compute the diabatic heating as a residual from the balance equations [24]. In this case the residual express the sum of all kinds of diabatic heating involved into the atmosphere, ex: diabatic heating due to the condensation of moisture, sensible heating, diabatic cooling due to thermal radiative processes and short wave radiation. The Lorenz energetics analysis, including generation, conversion and dissipations of kinetic and potential energy are shown in terms of monthly and annual means.

Lorenz Energetics

A traditional and compact form of presenting the spatial domain of atmospheric energetic was firstly suggested by [25]. In this energetic frame of reference the kinetic as well as the available potential energy are resolved into the amounts associated with the zonally averaged fields of motion and mass and the amounts associated with eddies ([25,26]). Lorenz defined zonal kinetic energy (KZ) as the amount of kinetic energy which would exist if motion where purely zonal. Naturally, both u and v are included in this definition, as a spatial average taken over a given latitude (see appendix A). On the other hand, eddy kinetic energy (KE) would be the excess of kinetic energy over KZ or additionally the kinetic energy related to the eddies in absence of zonal motion. Each energy type is connected by conversion terms, which are produced or destroyed by sources or sink terms. Furthermore, as discussed earlier, we here use the concept of limited area for the energetics where the “zonal” and “eddy” components are expressed as averages within a horizontal domain incorporating the ascending and descending branches of the Pacific WC. Within this framework the calculation of the transport across the boundaries is also relevant. As we will discuss in the next section, those term are in general at least one order of magnitude less than the energy conversion terms, adding to the robustness of our findings.

Figure 2 shows that the Lorenz energy cycle consists of four boxes denoting primary energy exchanges, including the zonal and eddy parts of the potential and kinetic energies within each box, with their connections given by energy conversion terms representing different dynamical process in the atmosphere (for instance, baroclinic and barotropic growth processes). The conversion terms are labeled as CZ, CA, CE and CK, respectively, denoting the conversion from AZ (zonal available potential energy) into KZ (zonal kinetic energy), AZ into AE (eddy available potential energy), AE into KE (eddy kinetic energy), and KE into KZ respectively. The complete set of boundary transport terms resulting from the limited area calculation are also indicated (terms starting with “B”). The magnitude of those terms will be dis-

Conflicts of Interest

The authors declare no conflicts of interest.

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