Linear effects of the nontraditional Coriolis terms on ITCZ-forced large-scale flow

TL;DR

This study investigates the linear effects of nontraditional Coriolis terms on large-scale ITCZ-forced flow using a simplified model, highlighting biases caused by neglecting these terms and proposing a scaling measure for their inclusion.

Contribution

The paper introduces a scaling measure for nontraditional Coriolis terms and demonstrates their impact on large-scale flow simulations, emphasizing the importance of including NCTs in models.

Findings

Omitting NCTs causes a significant westerly wind bias in the ITCZ region.

The wind bias ratio increases with narrower or more equatorially located ITCZs.

Scaling the NCT to traditional Coriolis term ratio helps assess their importance in models.

Abstract

This paper promotes a measure to validate the hydrostatic approximation via scaling the nontraditional Coriolis term (NCT) in the zonal momentum equation. To demonstrate the scaling, this study simulates large-scale flow forced by a prescribed heat source mimicking the intertropical convergence zone (ITCZ) using a linearized forced-dissipative model. The model solves two similar equations between which the only difference is inclusion of NCTs. The equations are derived using the following approximations: anelastic, equatorial beta-plane, linearized, zonally symmetric, steady, and a constant dissipation coefficient. The large-scale flows simulated with and without NCTs are compared in terms of the meridional-vertical circulation, the zonal wind, and the potential temperature. Both results appear like the Hadley circulation. With the model parameters controlled, the results without NCTs minus with NCTs are linear biases due to omitting NCTs. The most prominent bias is a westerly wind bias in the ITCZ heating region that emerges because omitting NCTs prevents the associated westward acceleration when heating-induced vertical motion is present. The zonal wind bias divided by the zonal wind with NCTs is 0.120 $\pm$ 0.007 in terms of the westerly maximum and 0.0452 $\pm$ 0.0005 in terms of the root mean square (RMS) when the prescribed ITCZ mimics the observed ITCZ in May over the East Pacific. These normalized measures of the zonal wind bias increase with a narrower ITCZ or an ITCZ closer to the equator because of a weaker subtropical jet stream given the same vertical heating profile. This difference can be traced by a nondimensional parameter scaling the ratio of the NCT to the traditional Coriolis term. The scaling encourages restoring NCTs into global models.