The significance of the nontraditional Coriolis terms in tropical large-scale dynamics

The nontraditional Coriolis terms (NCTs) turn eastward motion upward and upward motion westward, and vice versa. NCTs are omitted in most of the current global atmospheric models. However, NCTs are significant in tropical large-scale dynamics in at least three aspects. 1) Using an idealized model that can switch NCTs on or off, omitting NCTs biases large-scale flow response to prescribed diabatic forcing mimicking the intertropical convergence zone (ITCZ). The omission yields a westerly wind bias in the heating region ~ 10% of the westerly jet stream. This bias is due to lack of westward NCT when heating-induced upward motion is present. 2) Comparing a hypsometric equation including the NCT with the traditional hypsometric equation, omitting the NCT biases geopotential height estimation using rawinsonde data. The omission causes geopotential height error of ~ 1 m, which is considerable with respect to geopotential height variability of ~ 10 m associated with the Madden-Julian oscillation (MJO) or convectively coupled equatorial waves (CCEWs). When westerly winds are present, lack of upward NCT erroneously reduces the thickness between two pressure levels; the opposite is true for easterly winds. 3) Including the NCTs into the equatorially confined wave theory, new propagation mechanism is identified. For example, in a moist-neutral troposphere, compressional Rossby waves propagate eastward at a phase speed of 0.24 m s^-1. The eastward propagation is due to vertical density-weighted advection of the meridional planetary vorticity. These results encourage restoring NCTs into the global models for more-accurate simulations of MJO, CCEWs, and ITCZ-forced large-scale flow.