Mechanisms of ENSO-forcing of hemispherically symmetric precipitation variability.

• Main Authors: R Seager, N Harnik, W A Robinson, Y Kushnir, M Ting, H.-P Huang, J Velez
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2005
• Subjects: Pacific; aleutian low
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1070.3201; http://ocp.ldeo.columbia.edu/res/div/ocp/pub/seager/qj_pcp.pdf

Summary The patterns of precipitation anomalies forced by the El Niño-Southern Oscillation during Northern Hemisphere winter and spring are remarkably hemispherically symmetric and, in the mid-latitudes, have a prominent zonally symmetric component. Observations of global precipitation variability and the moisture budget within atmospheric reanalyses are examined to argue that the zonally symmetric component is caused by interactions between transient eddies and tropically-forced changes in the subtropical jets. During El Niño events the jets strengthen in each hemisphere and shift equatorward. Changes in the subtropical jet influence the transient eddy momentum fluxes and the eddy-driven mean meridional circulation. During El Niño events eddy-driven ascent in the mid-latitudes of each hemisphere is accompanied by low level convergence and brings increased precipitation. This changes in the transient eddy and stationary eddy moisture fluxes almost exactly cancel each other and, in sum, do not contribute to the zonal mean precipitation anomalies. Propagation of anomalous stationary waves disrupts the zonal symmetry. Flow around the deeper Aleutian Low and the eastward extension of the Pacific jet stream supply the moisture for increased precipitation over the eastern North Pacific and the western seaboard of the United States while transient eddy moisture convergence supplies the moisture for increased precipitation over the southern United States. In each case increased precipitation is fundamentally caused by anomalous ascent forced by anomalous heat and vorticity fluxes.