Stationary Eddies and the Zonal Asymmetry of Net Precipitation and Ocean Freshwater Forcing
Transport of water vapor in the atmosphere generates substantial spatial variability of net precipitation (precipitation minus evaporation). Over half of the total spatial variability in annual-mean net precipitation is accounted for by deviations from the zonal mean. Over land, these regional differences determine differences in surface water availability. Over oceans, they account, for example, for the Pacific–Atlantic difference in sea surface salinity, with implications for the deep overturning circulation. This study analyzes the atmospheric-water budget in reanalyses from ERA-Interim and MERRA, to investigate which physical balances lead to zonal variation in net precipitation. It is found that the leading-order contribution is zonal variation in stationary-eddy vertical motion. Transient eddies modify the pattern of zonally anomalous net precipitation by moving moisture from the subtropical and tropical oceans onto land and poleward across the Northern Hemisphere storm tracks. Zonal variation in specific humidity and stationary-eddy horizontal advection play a secondary role. The dynamics leading to net precipitation via vertical motion in stationary eddies can be understood from a lower-tropospheric vorticity budget. The large-scale variations of vertical motion are primarily described by Sverdrup balance and Ekman pumping, with some modification by transient eddies. These results suggest that it is important to understand changes in stationary eddies and their influence on the zonal variation of transient eddy fluxes, in order to understand regional changes in net precipitation. They highlight the relative importance of different atmospheric mechanisms for the freshwater forcing of the North Pacific and North Atlantic.
© 2015 American Meteorological Society. Manuscript received 16 August 2014, in final form 19 February 2015. This research has been supported by NSF Grant AGS-1019211. We thank Ori Adam for his work in developing the data portal GOAT (www.goat-geo.org), which was used to obtain the data for this study. We thank James Rae, Tobias Bischoff, and Momme Hell for useful comments and discussion during the development of this draft.
Our published paper (Wills and Schneider 2015) contains errors in Fig. 9a. The drainage boundaries shown are incorrect in several places, most notably in the Nile River basin. These errors result from a bug in the code used to compute the drainage boundaries and are not inherent to the river topology dataset used (Fekete et al. 2001; Vörösmarty et al. 2000). This bug does not affect the computation of the river runoff fluxes. Therefore, the numerical values in Tables 3 and 4 of Wills and Schneider (2015) are correct as published. All conclusions of Wills and Schneider (2015) are unaffected. We include here an updated Fig. 9.
Published - jcli-d-14-00573.1.pdf
Erratum - jcli-d-17-0524.1.pdf