The Role of the Divergent Circulation for Large-Scale Eddy Momentum Transport in the Tropics. Part I: Observations

Abstract

This work investigates the role played by the divergent circulation for meridional eddy momentum transport in the tropical atmosphere. It is shown that the eddy momentum flux in the deep tropics arises primarily from correlations between the divergent eddy meridional velocity and the rotational eddy zonal velocity. Consistent with previous studies, this transport is dominated by the stationary wave component, associated with correlations between the zonal structure of the Hadley cell (zonal anomalies in the meridional overturning) and the climatological-mean Rossby gyres. This eddy momentum flux decomposition implies a different mechanism of eddy momentum convergence from the extratropics, associated with upper-level mass convergence (divergence) over sectors with anomalous westerlies (easterlies). By itself, this meridional transport would only increase (decrease) isentropic thickness over regions with anomalous westerly (easterly) zonal flow. The actual momentum mixing is due to vertical (cross isentropic) advection, pointing to the key role of diabatic processes for eddy–mean flow interaction in the tropics.

On the Relationship Between the Madden‐Julian Oscillation and the Hadley and Walker Circulations

“…While we focus here more on the changes in the midlatitudes, it is worth pointing out that there are significant changes in upper‐level wind speed in the tropics. These wind speed anomalies are similar to those discussed in Zurita‐Gotor (2019), who found that the large‐scale eddy momentum transport in the tropics is associated with the correlation between the divergent meridional velocity and the non‐divergent zonal velocity. They also pointed out that the Hadley circulation plays the dominant role in the tropical momentum transport.…”

The Sensitivity of Superrotation to the Latitude of Baroclinic Forcing in a Terrestrial Dry Dynamical Core

“…Consistent with previous work(Zurita-Gotor and Held 2018;Zurita-Gotor 2019), the divergent component (in particular the term) is responsible for the equatorial acceleration (Fig.3, top) while the rotational eddy momentum fluxes only decelerate the tropics. To capture the full impact of the latter, we show this term meridionally integrated over the tropics (| | ≤ 20, Fig.…”

“…where overbars and primes denote zonal averages and deviations therefrom and the and suscripts denote the rotational and divergent velocity contributions to the horizontal velocity ( , ), is vertical vorticity and horizontal divergence. We used a Cartesian expression above for simplicity (see Zurita-Gotor (2019) for the full spherical expressions including cosine factors). The cospectra show that these two eddy contributions to the equatorial momentum balance are due to different waves.…”

The Detailed Dynamics of the Hadley Cell. Part II: December–February

“…However, Hartley and Black (1995) found that in their climate model the momentum fluxes on the equator that are equatorward in the Northern Hemisphere winter were associated with convective outflow rather than Rossby wave propagation and tilted troughs. Also, Zurita-Gotor (2019) found that the near equatorial momentum transfers were dominated by the correlation of the divergent meridional wind and the rotational zonal wind.…”