Dynamics of ITCZ Width: Ekman Processes, Non-Ekman Processes, and Links to Sea Surface Temperature

Michael P Byrne,Rhidian Thomas

doi:10.1175/jas-d-19-0013.1

Abstract

Abstract The dynamical processes controlling the width of the intertropical convergence zone (ITCZ) are investigated using idealized and CMIP5 simulations. ITCZ width is defined in terms of boundary layer vertical velocity. The tropical boundary layer is approximately in Ekman balance, suggesting that wind stress places a strong constraint on ITCZ width. A scaling based on Ekman balance predicts that ITCZ width is proportional to the wind stress and inversely proportional to its meridional gradient. A toy model of an Ekman boundary layer illustrates the effects of wind stress perturbations on ITCZ width. A westerly wind perturbation widens the ITCZ whereas an easterly perturbation narrows the ITCZ. Multiplying the wind stress by a constant factor does not shift the ITCZ edge, but ITCZ width is sensitive to the latitude of maximum wind stress. Scalings based on Ekman balance cannot fully capture the behavior of ITCZ width across simulations, suggesting that non-Ekman dynamical processes need to be accounted for. An alternative scaling based on the full momentum budget explains variations in ITCZ width and highlights the importance of horizontal and vertical momentum advection. Scalings are also introduced linking ITCZ width to surface temperature. An extension to Lindzen–Nigam theory predicts that ITCZ width scales with the latitude where the Laplacian of SST is zero. The supercriticality theory of Emanuel is also invoked to show that ITCZ width is dynamically linked to boundary layer moist entropy gradients. The results establish a dynamical understanding of ITCZ width that can be applied to interpret persistent ITCZ biases in climate models and the response of tropical precipitation to climate change.

Highlights

The intertropical convergence zone (ITCZ) is a planetary-scale band of low-level mass convergence and intense rainfall (Waliser and Gautier 1993)
A key result of this study is that Ekman processes place a strong constraint on ITCZ width: The boundary layer is approximately in Ekman balance (Fig. 2a) and the Ekman component of vertical velocity is large at the ITCZ edge (Fig. 8)
Scalings for ITCZ width based on Ekman dynamics are useful for understanding the behavior of ITCZ width across simulations (Figs. 3–7)

Summary

Introduction

The intertropical convergence zone (ITCZ) is a planetary-scale band of low-level mass convergence and intense rainfall (Waliser and Gautier 1993). For analysis purposes we specify the boundary layer top as the s 5 0.8 level in the idealized GCM simulations, though note that the model determines the boundary layer height dynamically This is similar to the definition of Byrne and Schneider (2016a) but in their analysis the midtropospheric mass streamfunction is used to define the ITCZ edges. In the boundary layer momentum budget the frictional force provides an eastward acceleration to the zonal wind at the ITCZ edge because the tropical surface winds are easterly (Fig. 2b); there is a transfer of zonal momentum from the surface to the overlying atmosphere This eastward acceleration is largely balanced by westward acceleration associated with equatorward advection of air with relatively low planetary angular momentum by the surface branch of the Hadley cell (the Coriolis term). The vertical advection term in the zonal momentum budget is nonzero at the ITCZ edge and accelerates the easterly zonal wind for the majority of simulations (Fig. 1b)

Scalings for ITCZ width based on Ekman balance

Boundary layer convergence

Scaling for ITCZ width based on the full momentum budget

Findings

Summary