The Signature of the Annular Modes in the Moisture Budget

Abstract

The mid- and high-latitude variations of mass and momentum, variously termed the Arctic and Antarctic oscillations and/or the Northern and Southern Annular Modes also have a signature in the moisture budget. This is investigated, using associated regression pattern analysis, in the control simulation of the Canadian Centre for Climate Modelling and Analysis (CCCma) coupled climate model and in NCEP reanalysis data. The analyzed behavior is similar in the two hemispheres and is characterized, when pressure is low in polar regions, by anomalous westerlies and a thermodynamically indirect meridional cell poleward of midlatitudes, and by anomalous easterlies and a thermodynamically direct meridional cell equatorward of midlatitudes. Moisture transports are associated primarily with advection by the lower branches of these anomalous meridional circulation cells and secondarily with anomalous eddy transports giving, in this phase of the mode, moisture divergence and decreased precipitation at midlatitudes and moisture convergence and increased precipitation at poleward latitudes. The opposite behavior is seen when pressure is high in polar regions. Patterns tend to be more zonally symmetric in the Southern than the Northern Hemisphere in both model results and reanalyses. The meridional circulations and eddy transports that affect the moisture budget at lower levels in the atmosphere extend throughout the troposphere and are those associated also with the momentum budget of the annular modes. The annular modes should be considered as interlinked variations of mass, momentum, and moisture. The amplitudes of the annular modes and associated moisture quantities exhibit an upward trend in both hemispheres in simulations with the CCCma coupled model forced with historical and projected greenhouse gas concentrations and aerosol loadings for the period 1900?2100. It appears, however, that the behavior of the annular modes and their signature in the moisture budget is basically unchanged and is superimposed on a forced climate trend.