Regional energy budget control of the intertropical convergence zone and application to mid-Holocene rainfall

Abstract

Shifts in the latitude of the intertropical convergence zone-a region of intense tropical rainfall-have often been explained through changes in the atmospheric energy budget, specifically through theories that tie rainfall to meridional energy fluxes. These quantitative theories can explain shifts in the zonal mean, but often have limited relevance for regional climate shifts, such as a period of enhanced precipitation over Saharan Africa during the mid Holocene. Here we present a theory for regional tropical rainfall shifts that utilizes both zonal and meridional energy fluxes. We first identify a qualitative link between zonal and meridional energy fluxes and rainfall variations associated with the seasonal cycle and the El Niño/Southern Oscillation. We then develop a quantitative theory based on these fluxes that relates atmospheric energy transport to tropical rainfall. When applied to the orbital configuration of the mid Holocene, our theory predicts continental rainfall shifts over Africa and Southeast Asia that are consistent with complex model simulations. However, the predicted rainfall over the Sahara is not sufficient to sustain vegetation at a level seen in the palaeo record, which instead requires an additional large energy source such as that due to reductions in Saharan surface albedo. We thus conclude that additional feedbacks, such as those involving changes in vegetation or soil type, are required to explain changes in rainfall over Africa during the mid Holocene.

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

“…The framework, however, faces multiple challenges. For a more complete theory, an account of the following factors is necessary: (1) a theory for TGMS, which measures the efficiency with which the HC transports energy, 16 (2) a prediction of cloud response based on observed relationships between cloud radiative effect and circulation, 44 (3) an expansion to include the zonal dimension to quantitatively predict the regional features, which has recently been attempted by taking into account the zonal component of divergent AET, 46 and (4) the effect of ocean dynamics. Among the aforementioned limitations, we address the issue of partitioning between atmosphere and ocean energy transports.…”

“…For example, the zonal-mean ITCZ position shifts only by about 1°in latitude between El Niño and La Niña composites, while the east-west shifts are 22°in longitude. 46 Also, during the early Holocene, compared to today, the zonal-mean ITCZ shift is estimated to be less than 1°latitude, while paleoclimate records infer that the regional ITCZ shifts reach up to 5°latitude. 47 Although some aspects of regional features, such as the abrupt onset of monsoon are captured by the zonal-mean dynamics, 48 planetary-scale waves are important.…”

“…[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.…”

Response of the Intertropical Convergence Zone to Climate Change: Location, Width, and Strength

“…We choose to focus on the zonal-mean ITCZ as a starting point for understanding the more complex behavior of the zonally varying ITCZ. For example, the well-established energetic theory for ITCZ location was largely developed in a zonal-mean framework [13,14,19], and later the mechanistic insights gained from this simplified setup have inspired a more comprehensive theory that takes into account the zonal structure [22,74]. Our lack of basic understanding of the width and strength of the ITCZ implies that there is still much to learn from the zonal-mean perspective before we proceed to tackling the more complex problem of understanding longitudinal variations.…”

“…An energetic theory has been developed over the past two decades to diagnose how processes in Earth's climate-including radiation asymmetries between hemispheres, atmosphere-ocean coupling, and energy input to the tropical atmosphere-control the zonal-mean ITCZ location [10][11][12][13][14][15][16][17][18][19][20][21]. This theory has recently been extended to understand variations of the ITCZ location with longitude [22,23]. Four recent review articles have comprehensively synthesized this body of research on the ITCZ location [1,[24][25][26].…”