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The Longitudinally-Dependent Hadley Circulation: Seasonality and Interannual Variability

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The Longitudinally-Dependent Hadley Circulation: Seasonality and Interannual Variability The longitudinally-dependent Hadley circulation: seasonality and interannual variability Eli Galanti1, Dana Raiter1, Yohai Kaspi1, and Eli Tziperman2 submitted to Journal of Climate January 1, 2019 1Department of Earth and Planetary Sciences, moisture transport from the equatorial region to the sub- Weizmann Institute of Science, Rehovot, Israel. tropics [e.g., Trenberth and Stepaniak, 2003]. The cir- 2Department of Earth and Planetary Sciences, Harvard culation is commonly defined as the zonally-averaged University, Cambridge, MA, USA meridional circulation in the tropical region [Hartmann, 2016], and is usually calculated as an annual mean or as an average over specific months or seasons. The large longitudinal variations in the different el- Abstract ements involved in the Hadley circulation, such as the strength of the Inter Tropical Convergence Zone (ITCZ) The Hadley circulation is commonly defined as the and the location of the subtropical jets that mark the meridional circulation of the zonally averaged flow in edge of the Hadley circulation, led to the need to calcu- the tropics. In recent years, several studies looked at late the contributions to the Hadley circulation at differ- the longitudinal decomposition of the three-dimensional ent longitudes. A method for calculating localized 2D cir- atmospheric flow into local meridional and zonal flows. culations from the 3D wind field was first introduced by These studies gave useful analysis on the regionality and Keyser et al. [1989]. Decomposing the wind field into a variability of the meridional circulation, yet their spatial rotational and divergent components (Helmholtz decom- analysis remained essentially three-dimensional, and the position), the longitudinally-dependent circulation can temporal variability was strongly dependent on assump- be derived from the divergent part of the flow. This tions such as the connection to ENSO. method was implemented in several studies for the anal- Here we present an efficient representation of the ysis of the the Hadley and Walker circulations in specific longitudinally-dependent meridional circulation (LMC) longitudinal sectors. It was first used to define the hori- that is a function of longitude and latitude only. Us- zontal velocity potential and divergent wind in the upper ing the new definition we find that the effective merid- troposphere, in which both the meridional circulation ional circulation is even more longitudinally restricted (Hadley) and the zonal circulation (Walker) are man- than was found in earlier studies, stressing the impor- ifested. This definition enabled the investigation of the tance of treating the Hadley circulation as a regional global monsoon system and its relation to the Hadley and phenomenon. We then use hierarchical clustering to ob- Walker circulations on seasonal to decadal time scales jectively study both the seasonality and the interannual [Trenberth et al., 2000; Tanaka et al., 2004]. variability of the LMC. We find that the seasonal cycle Decomposing the flow field into a local meridional is defined with 3 clusters and the interannual variability and zonal circulations was used to calculate the rel- with additional 5 clusters. The most prominent interan- ative contributions of the vertical mass fluxes in the nual variability of the LMC is the shifting in the east-west middle-troposphere to the Hadley and Walker circula- direction of the circulation, which is strongly related to tions [Schwendike et al., 2014]. Examining the interan- other atmospheric variables such as the sea surface tem- nual variability of the different circulations, Schwendike perature, precipitation and air temperature. Using mul- et al. [2014] found that ENSO has a much larger effect tiple linear regression we analyze these dependencies and on the local Hadley circulation than on the local Walker discuss their implications for the climate system. circulation. This study was performed over specific lon- gitudinal and latitudinal sectors, focusing on the Mar- 1 Introduction itime continent. The same methodology was later used to study the inter-decadal trend of both the Hadley and The Hadley circulation is a key element of the climate Walker circulations [Schwendike et al., 2015], where it system [Hartmann, 1994], responsible for the energy and was shown that in order to understand the effect of cli- 1 mate variability on the tropical circulation patterns the 2 Data and methods analysis should be regional. 2.1 The longitudinally dependent The same method was also used to study specific re- gions of interest, such as the Atlantic sector of the Hadley Hadley circulation circulation during the Boreal summer and its connec- We use the European Center for Medium range Weather tion to the Atlantic tropical cyclone activity [Zhang and Forecasts ERA-interim reanalysis covering the years Wang, 2013], and the variability of the Hadley circula- 1979-2017 [Dee et al., 2013]. The 6-hourly data is av- tion in the Southern Hemisphere in different sectors of eraged over each month, producing for each variable and the world [Nguyen et al., 2018]. The method was also location a time series of 468 data points. compared to a more general 3D decomposition of global The mass-weighted global meridional circulation atmospheric circulation [Hu et al., 2017]. [Hartmann, 2016] is represented by a stream function In all these studies, two main perspectives were used calculated using the zonally-averaged meridional veloc- ity v to examine the local meridional circulation. Some stud- p ies analyzed latitude-longitude maps of the vertical mass 2πa cos θ (θ; p; t) = v(θ; p0; t)dp0; (1) flux in the middle-troposphere or the divergent flow at g ˆ the upper troposphere. While these variables are use- 0 ful for examination of the general non-rotational circu- where a is Earth’s radius, g is the gravitational acceler- lation they do not directly show the meridional circula- ation, θ is latitude, and p is pressure. The time average tion. Other studies analyzed latitude-pressure maps of of this circulation is shown in Fig. 1a. The two classical the meridional circulation itself hence it was necessary to Hadley cells can be seen between the equator and lati- average the circulation over a longitudinal sector, making tudes ±30◦. Taking the average of the stream function a full global view impossible. Another limitation of past between 400−600 hPa, where the stream function reaches studies was the methodology used for investigating the its maximum [Nguyen et al., 2013], the time dependency seasonal and interannual variability. The seasonal vari- is revealed (Fig. 1b), with the Hadley circulation alter- ability was commonly defined as an average over either nating seasonally between the hemispheres. However, specific months or a period of 3 months for each season, the circulation, as defined by Eq. 1, cannot account for with the presumption that all seasons are equal in their any zonal asymmetry in the meridional circulation. The time span. When examining the interannual variability, longitudinally-dependent meridional circulation can be the modes of variability of the Hadley circulation were calculated via the separation of the 3D wind velocity vec- usually defined based on other modes of variability in the tor into a meridional component and a zonal one. Here tropical region, for example a state of El-Niño or a state we follow the notation of Hu et al. [2017]. First, the of La-Niña . This makes the variability of the Hadley divergence of the wind is calculated circulation a mare reflection of the other modes of vari- −! D = r · V; (2) ability. Therefore, there is a need to define the seasonal −! cycle and modes of interannual variability of the Hadley where V = (u; v) is the full wind vector. Then, a poten- circulation with a more objective method. tial function χ is calculated via Here we use an efficient representation of the r2χ = D: (3) longitudinally-dependent meridional circulation, one that better illustrates the longitudinal-dependence of the This equation can be solved either via decomposition into circulation. We then study the seasonality and interan- spherical harmonics, or by formulating the problem as a nual variability of this field using clustering analysis, al- set of finite difference linear equations and inverting the lowing an objective determination of the most important Laplacian. The methods are equivalent and in this study modes of variability with our restriction of the analysis the latter is used. The potential function is then used to to seasonal or ENSO-related modes. Next, we investi- calculate the divergent wind gate the connection between the longitudinally depen- −! rχ = V ; (4) dent meridional circulation and other variables of the div −! tropical climate system, such as the sea surface temper- where V div = (udiv; vdiv). The zonal (meridional) com- ature, precipitation and air temperature. ponent of the divergent wind is associated with east-west The manuscript is organized as follows: in section 2 (north-south) closed oriented circulations, such as the we present the method by which the local meridional Walker (Hadley) cell. Therefore, the divergent wind can circulation is calculated and the method for clustering be used, similar to Eq. 1, to calculate the longitudinally its seasonal and interannual variability. The results for dependent meridional circulation the seasonal cycle analysis are discussed in section 3.1, p 2πa cos θ and those for the interannual variability in section 3.2. (φ, θ; p; t) = v (φ, θ; p0; t)dp0: (5) g ˆ div We conclude in section 4. 0 2 a b 25 12 10 Seasonal cycle included 200 10 1.5 400 8 600 6 Month P (hPa) 1 4 800 2 1000 30S 0 30N 30S 0 30N Maximal diameter 0.5 Latitude Latitude c 1 2 3 4 5 6 7 8 9 10 1024 30N Seasonal cycle excluded 0 2 Latitude 30S 1.5 60E 120E 180 120W 60W 1 Longitude Maximal diameter 0.5 -30 -24 -18 -12 -6 0 6 12 18 24 30 1010 1 2 3 4 5 6 7 8 9 10 k Figure 1: The climatological meridional circulation.
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