Spatial Patterns of the Tropical Meridional Circulation: Drivers and Teleconnections

Abstract

The Hadley circulation is a key element of the climate system (Hartmann, 1994), responsible for the energy and moisture transport from the equatorial region to the subtropics (e.g., Trenberth & Stepaniak, 2003). The circulation is commonly defined as the zonally averaged 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 elements involved in the Hadley circulation, such as the strength of the Inter Tropical Convergence Zone (ITCZ) and the location of the subtropical jets that mark the edges of the Hadley circulation, led to the need to calculate the contributions to the Hadley circulation at different longitudes. A method for calculating localized 2-D circulations from the 3-D wind field was first introduced by Keyser et al. (1989). Decomposing the wind field into a rotational and divergent components (Helmholtz decomposition), the longitudinally dependent circulation can be derived from the divergent part of the flow. This method was implemented in several studies for the analysis of the meridional and zonal circulations in specific longitudinal sectors. It was first used to define the horizontal velocity potential and divergent wind in the upper troposphere, in which both the meridional circulation and the zonal circulation are manifested. This definition enabled the investigation of the global monsoon system and its relation to the two circulations on seasonal to decadal time scales (Tanaka et al., 2004;Trenberth et al., 2000). The Helmholtz decomposition was also examined in comparison to a more general 3-D decomposition of global atmospheric circulation (Hu et al., 2017).Motivated by indications that the extent and strength of the Hadley circulation exhibit natural variability (e.g., Simpson, 2018), and might change in the coming century (e.g., Chemke & Polvani, 2019;Grise et al., 2019;Held & Soden, 2006), Helmholtz decomposition was also used to examine the natural variability and decadal change of the longitudinally dependent meridional circulation (LMC) extent, either for specific sectors of the world (Nguyen et al., 2018), or directly as function of longitude (Staten et al., 2019). More recently, Raiter et al. (2020) used the same method, combined with a Lagrangian tracking of air parcels, to examine in detail the mean tropical

Propagation pathways of Indo-Pacific rainfall extremes are modulated by Pacific sea surface temperatures

“…In order to assess the relative contributions of the mass fluxes in the troposphere to the pair of meridional and zonal overturning circulations we use the method by 56 , 91 – 93 . A Helmholtz decomposition is applied to the global wind field V = ( u , v ), to separate the divergent component from the rotational component as .…”

Changes in the tropical upper tropospheric zonal momentum balance due to global warming

“…In fact, contrary to the zonally symmetric view of the Hadley circulation, Galanti et al (2022) show that the meridional overturning circulation organizes in clusters with variability on interannual timescales (see also, Sun et al, 2019) and Raiter et al (2020) present a large-scale conveyor belt view of the tropical circulation. Further, the long-term trends of these longitudinally localized meridional overturning circulations exhibit large regional differences (Schwendike et al, 2014).…”

“…There has been increasing attention recently on the longitudinally limited portions that make up the zonal mean circulation (Raiter et al, 2020;Hoskins and Yang, 2023). In particular, it has been noted that these longitudinally restricted circulations exhibit variability on interannual timescales (Zhang and Wang, 2013;Sun et al, 2019;Galanti et al, 2022), and may respond differently to greenhouse gas-induced climate change (Staten et al, 2019). These localized north-south overturning circulations are made up of the divergent component of the meridional wind (Zhang and Wang, 2013;Schwendike et al, 2014); the rotational contribution goes to zero when integrated over a latitude circle because it arises from a zonal gradient of the horizontal streamfunction (Equation 2).…”

The Hadley circulation in a changing climate

“…In fact, the local MSF $$\psi (\lambda, \phi, p)$$ can be computed for each longitude $$\lambda$$ using the meridional component of the divergent flow $$v_d(\lambda, \phi,p)$$ and ensuring mass conservation: $$$$\begin{equation} \psi (\lambda,\phi,p)=\frac{1}{g} \int _{0}^{p} v_d(\lambda,\phi,p^{\prime })dp^{\prime }, \end{equation}$$$$which provides a view of the HC whose strength and edges vary continuously with longitude (e.g., Refs. 13, 57–62). Specifically, $$\psi (\lambda, \phi, p)$$ represents a meridional mass flux density, which can be integrated zonally to provide the meridional mass transport across any specified sector.…”