The Madden-Julian Oscillation and the Angular Momentum Balance in a Barotropic Ocean Model

Author: Rui M. Ponte and D.S. Gutzler
Date: 
October 16, 1990
Type: 
Journal Article
Venue: 
Journal of Geophysical Research
Citation: 

Ponte, R. M., and D. S. Gutzler (1991), The Madden-Julian Oscillation and the Angular Momentum Balance in a Barotropic Ocean Model, J. Geophys. Res., 96(C1), 835–842, doi:10.1029/90JC02277.

Surface wind stresses associated with the Madden-Julian oscillation transfer angular momentum between the atmosphere and the underlying ocean. The resulting anomalies in oceanic angular momentum (related to either changes in zonal currents or latitudinal redistribution of mass) and the torques coupling the ocean and the solid Earth are examined for simple zonal stress patterns representing idealized 40- to 50-day fluctuations of Pacific winds. Because the contribution of baroclinic variability to the angular momentum budget is in general small when vertically integrated quantities are considered (Ponte, 1990), only the barotropic response is studied. Numerical solutions of the shallow water equations with linear bottom friction are found for a constant depth basin centered on the equator and extending over 140° in longitude. The angular momentum exchanges between the ocean and solid Earth in the model involve the continental torque (i.e., body forces on eastern and western walls due to coastal sea level anomalies) and the bottom friction torque. Solutions yield a dominant balance between the wind and continental torques, with the bottom friction torque being negligible. The amplitude of the total torque acting on the ocean and the corresponding angular momentum fluctuations are very small compared with the amplitude of the applied wind torque. The ocean delivers to the solid Earth, with no significant time delays, the angular momentum anomalies received from the atmosphere. In general, sea level anomalies are nearly uniform along the eastern boundary, and the respective meridionally integrated torque is much larger than the one exerted on the western wall. Solutions also suggest that coastal (barotropic) sea level fluctuations as small as 0.5–1.5 cm are sufficient to cause variations in the length of day comparable to observed values in the 40- to 50-day band.