Abstract
AbstractThis study investigates: (i) how the local meridional (Hadley) and zonal (Walker) circulations change in each phase of the Madden‐Julian Oscillation (MJO); and (ii) the effect of enhanced and suppressed MJO‐related convection on the poleward extent of the local Hadley circulations and, thus, the strengths and positions of the subtropical jets. We examine these effects in ERA‐Interim reanalysis by decomposing the vertical mass flux into zonal and meridional components. We show for the first time, that as the envelope of enhanced convection moves eastwards from Africa to the Central Pacific the local Hadley circulation is enhanced. The regional Walker circulation in the Pacific is strengthened when the envelope of active MJO convection is located over the Maritime Continent and weakened when the region of suppressed convection is located there. In regions of anomalous upper‐level divergence the subtropical jet is enhanced. The core of the subtropical jet over Asia shifts eastwards with the progression of the MJO and shifts farther poleward in regions of anomalous upper‐level divergence linked with enhanced convection. The region of either enhanced or suppressed convection over the Maritime Continent strengthens or weakens the local Hadley circulation, producing disturbances in the subtropical jet. These disturbances then force midlatitude Rossby waves that propagate across the Pacific Ocean in both hemispheres.
Highlights
The definitions of the Hadley and Walker circulations are often based on zonal and meridional averages respectively (e.g., Barry & Carleton, 2001; Hartmann, 1994; L'Heureux et al, 2013; Julian & Chervin, 1978; Power & Smith, 2007; Tokinaga et al, 2012; Trenberth & Stepaniak, 2003; Vecchi et al, 2006)
We seek to answer the following questions: i) How do the local Hadley and Walker circulations change in each phase of the Madden-Julian Oscillation (MJO)? ii) How does the MJO affect the poleward extent of the local Hadley circulations, and the subtropical jets and midlatitude Rossby wave response, both globally and seasonally? We investigate these questions by objectively decomposing the three-dimensional atmosphere into a pair of orthogonal overturning circulations, that is, local Hadley and local Walker circulations (Schwendike et al, 2014)
To address how the propagation of the envelope of enhanced and suppressed convection affects the poleward extent of the local Hadley circulations and the subtropical jets we will analyze how the position and the strength of the subtropical jet changes during different phases of the MJO, and we will quantify changes in the jet over selected regions
Summary
The definitions of the Hadley and Walker circulations are often based on zonal and meridional averages respectively (e.g., Barry & Carleton, 2001; Hartmann, 1994; L'Heureux et al, 2013; Julian & Chervin, 1978; Power & Smith, 2007; Tokinaga et al, 2012; Trenberth & Stepaniak, 2003; Vecchi et al, 2006). In addition to its link with the tropical circulation, MJO convection is known to impact the strength and position of the subtropical jet (e.g., Matthews & Kiladis, 1999; Moore et al, 2010), as well as to generate Rossby wave trains that propagate into the midlatitudes (e.g., Kiladis & Weickmann, 1992; Knutson & Weickmann, 1987; Sardeshmukh & Hoskins, 1988; Matthews et al, 2004; Hendon & Salby, 1994; L'Heureux & Higgins, 2008; Higgins & Mo, 1997; Lukens et al, 2017). Hall et al (2020) have shown that the response to heating in the Indian Ocean is an anticyclone over the Pacific and low-pressure system over northwestern North America They found the opposite pattern occurs when the heating is located in the Western Pacific.
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