Annual cycle and longitudinal structure of tropical eddy and mean momentum fluxes

TL;DR

This study analyzes the seasonal and longitudinal variations of tropical eddy and mean momentum fluxes in the upper troposphere, revealing regional differences and their impacts on zonal flow dynamics.

Contribution

It provides a detailed regional breakdown of momentum fluxes and identifies the mechanisms driving their seasonal and longitudinal variability.

Findings

Mean convergence acts to decelerate zonal flow across all regions.

Eddy fluxes vary seasonally and regionally, with AWP dominating in summer.

Eddy activity influences local overturning cells, especially in CP-WA.

Abstract

The longitudinal structure and annual cycle of mean meridional and eddy momentum fluxes in the tropical upper troposphere are studied. In zonal mean, these two terms oppose each other and peak during the Indian summer monsoon. This zonal mean character arises from a rich longitudinal structure that is revealed by splitting the globe into three zones, namely, the Asia-West Pacific (AWP), central Pacific-West Atlantic (CP-WA) and African sectors. The mean convergence term is cohesive across all three regions, has a single peak in the boreal summer and always acts to decelerate the zonal flow. A Helmholtz decomposition shows that the advection of absolute vorticity by the divergent meridional wind in localized cross-equatorial cells is responsible for the coherent nature of the mean convergence across all sectors. On the other hand, the eddy convergence goes from being small and seasonally invariant in the African region to one with large seasonal maxima (minima) in AWP (CP-WA) sector that accelerate (decelerate) the zonal mean flow. The disparate nature the eddy flux in the AWP and CP-WA regions is in the winter season and is due to the tropical and extratropical origin of waves, respectively. In summer, the AWP region accounts for almost all of the eddy flux convergence. In fact, the leading role of the rotational zonal - divergent meridional component in the zonal mean eddy flux does not hold in individual sectors. Finally, through the year, the CP-WA region is where the local overturning cell is strongly influenced by eddy activity.