Seasonal and Interannual Variations of the Energy Flux Equator and ITCZ. Part I: Zonally Averaged ITCZ Position

Abstract

In the zonal mean, the ITCZ lies at the foot of the ascending branch of the tropical mean meridional circulation, close to where the near-surface meridional mass flux vanishes. The ITCZ also lies near the energy flux equator (EFE), where the column-integrated meridional energy flux vanishes. This latter observation makes it possible to relate the ITCZ position to the energy balance, specifically the atmospheric net energy input near the equator and the cross-equatorial energy flux. Here the validity of the resulting relations between the ITCZ position and energetic quantities is examined with reanalysis data for the years 1979-2014. In the reanalysis data, the EFE and ITCZ position indeed covary on time scales of seasons and longer. Consistent with theory, the ITCZ position is proportional to the cross-equatorial atmospheric energy flux and inversely proportional to atmospheric net energy input at the equator. Variations of the cross-equatorial energy flux dominate seasonal variations of the ITCZ position. By contrast, variations of the equatorial net energy input, driven by ocean energy uptake variations, dominate interannual variations of the ITCZ position (e.g., those associated with ENSO).

Seasonal and Interannual Variations of the Energy Flux Equator and ITCZ. Part II: Zonally Varying Shifts of the ITCZ

“…Using reanalysis data, we have recently shown that the zonal-mean EFE and ITCZ position covary on seasonal and interannual time scales and that seasonal and interannual variations of the ITCZ position can be quantitatively related to variations of the AET and its derivatives (Adam et al 2016). In the present paper, we examine the relation between the zonally varying EFE and ITCZ and extend the theoretical framework developed for zonal-mean variations Schneider 2014, 2016;Schneider et al 2014;Adam et al 2016) to regional variations.…”

“…Usually, an ITCZ and EFE north of the equator imply southward AET across the equator, away from the ITCZ, and vice versa. Indeed, the zonal-mean position of the ITCZ was found to be closely related to cross-equatorial AET in observations (e.g., Donohoe et al 2013Donohoe et al , 2014Adam et al 2016) and in climate models of varying complexity (e.g., Chiang and Bitz 2005;Broccoli et al 2006;Yoshimori and Broccoli 2008;Kang et al 2008Kang et al , 2009Donohoe et al 2013Donohoe et al , 2014.…”

“…1), where near-surface meridional mass fluxes vanish. Since the Hadley circulation transports energy away from the ITCZ and dominates the tropical energy transport, column-integrated energy fluxes also vanish and diverge near the ITCZ, forming an atmospheric energy flux equator (EFE) (e.g., Neelin and Held 1987;Broccoli et al 2006;Kang et al 2008Kang et al , 2009Donohoe et al 2013Donohoe et al , 2014Adam et al 2016).…”

“…For example, seasonal meridional migrations of marine ITCZs are small relative to their continental (monsoonal) counterparts (Fig. 1); interannual variations of the ITCZ associated with ENSO are most pronounced over the Pacific but are relatively small elsewhere (e.g., Dai and Wigley 2000;Adam et al 2016); and the eastern Pacific ITCZ bifurcates to a double ITCZ that straddles the equator during boreal spring, but such bifurcations do not usually occur elsewhere (Fig. 1, bottom;Zhang 2001;Gu et al 2005).…”

Extratropical forcing and tropical rainfall distribution: energetics framework and ocean Ekman advection

“…[49][50][51] Thus, expanding the zonal-mean energetics framework to two horizontal dimensions is required for predictions of regional rainfall changes in the tropics, say by accounting for the zonal component of divergent AET. 46,52 The application of the energetics framework is further complicated by ocean dynamical feedback. When the ocean circulation is allowed to respond (i.e., O is no longer fixed in Eq.…”