The relationship between inter-annual and intra-annual variability in atmospheric angular momentum in a GCM with climatological SSTs

Abstract

The internally generated interannual variability in absolute angular momentum is examined with a 100-year run of a GFDL GCM. Interannual forcing of the atmosphere by the ocean is suppressed as the GCM's sea surface temperatures are constrained to undergo the same annual cycle each year. The interannual variation in the model's globally integrated and annually averaged absolute angular momentum (GAAM) is found to be a moderately large fraction of the changes in the GAAM observed in the atmosphere. The meridional structure of the absolute angular momentum is examined with an EOF analysis. Three modes are found; a tropical mode that represents a strengthening and weakening of the tropical easterlies, and two extratropical dipole modes, one in either hemisphere, that represent latitudinal shifts of the jet maxima. It is shown that the interannual variability of all three modes can be interpreted as resulting from climate noise. The absolute angular momentum budget of all three modes is examined. It is found that the tropical mode depends upon a complex three way balance between the friction and mountain torques and the absolute angular momentum flux convergence, whereas the two midlatitude modes are primarily determined by the absolute angular momentum flux convergence. Fluctuations of the tropical mode are shown to be related to the GCM's Madden–Julian Oscillation, and the midlatitude dipole modes are found to show characteristics in common with the zonal index observed in the atmosphere. DOI: 10.1034/j.1600-0870.1995.t01-1-00006.x