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

Abstract

Intense tropical rainfall occurs in a narrow belt near the equator, called the inter-tropical convergence zone (ITCZ). In the past decade, the atmospheric energy budget has been used to explain changes in the zonal-mean ITCZ position. The energetics framework provides a mechanism for extratropics-to-tropics teleconnections, which have been postulated from paleoclimate records. In atmosphere models coupled with a motionless slab ocean, the ITCZ shifts toward the warmed hemisphere in order for the Hadley circulation to transport energy toward the colder hemisphere. However, recent studies using fully coupled models show that tropical rainfall can be rather insensitive to extratropical forcing when ocean dynamics is included. Here, we explore the effect of meridional Ekman heat advection while neglecting the upwelling effect on the ITCZ response to prescribed extratropical thermal forcing. The tropical component of Ekman advection is a negative feedback that partially compensates the prescribed forcing, whereas the extratropical component is a positive feedback that amplifies the prescribed forcing. Overall, the tropical negative feedback dominates over the extratropical positive feedback. Thus, including Ekman advection reduces the need for atmospheric energy transport, dampening the ITCZ response. We propose to build a hierarchy of ocean models to systematically explore the full dynamical response of the coupled climate system.

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

“…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]. Here, we provide a broader perspective of the response of the ITCZ to climate change.…”

“…Michael P. Byrne michael.byrne@imperial.ac.uk 1 Space and Atmospheric Physics Group, Imperial College London, London SW7 2AZ, UK 2 National Center for Atmospheric Research, Boulder, CO, USA 3 Texas A&M University, College Station, TX, USA air and intense precipitation (Fig. 1a).…”

“…Precipitation in the ITCZ is driven by moisture convergence associated with the northerly and southerly trade winds that collide near the equator. The ITCZ accounts for 32% of global precipitation [1] and shapes climate and society in the tropics; any response of the ITCZ to climate change will have implications for tropical regions. It has also been suggested that, through its influence on the global radiation budget, the ITCZ could influence global temperature and precipitation and their responses to climate change [2][3][4][5].…”

“…The negligible future change predicted by the median climate model is consistent with small changes between preindustrial control and abrupt 4 × CO 2 simulations [26] and a relatively constant annual-mean ITCZ location over recent decades [27] (Figure S1a,d). It has been suggested that interhemispheric radiative forcing asymmetries associated with declining Northern Hemisphere scattering aerosol 1 The models analyzed here are ACCESS1-0, ACCESS1-3, BCC-CSM1-1, BCC-CSM1-1-M, BNU-ESM, CESM1-BGC, CESM1-CAM5, CMCC-CESM, CMCC-CM, CMCC-CMS, CNRM-CM5, CSIRO-Mk3-6-0, CanESM2, EC-EARTH, FGOALS-g2, FIO-ESM, GFDL-CM3, GFDL-ESM2G, GFDL-ESM2M, HadGEM2-AO, HadGEM2-ES, INMCM4, IPSL-CM5A-MR, IPSL-CM5B-LR, MIROC-ESM, MIROC-ESM-CHEM, MIROC5, MPI-ESM-LR, MPI-ESM-MR, MRI-CGCM3, NorESM1-M, and NorESM1-ME. concentrations over the twenty-first century may drive a northward shift of the ITCZ [16,85].…”

“…The energetic theory for zonal-mean ITCZ location has been comprehensively reviewed in four recent articles [1,[24][25][26]. Here, we summarize this body of work, briefly discuss a distinct but complementary dynamical theory for ITCZ location, and outline some outstanding questions.…”