The linkage between the tropical/extratropical interactions and the stratosphere-troposphere coupling

Using the NCAR/NCEP reanalysis, we here present evidence suggesting that the out-of-phase relationships of temperature anomalies both between the low and high latitudes and between the stratosphere and troposphere are intimately related to the meridional and downward propagation of anomalies of both signs. The temperature anomalies propagate poleward and downward above the tropopause and propagate equatorward below the tropopause. The characteristic time scale for anomalies of one polarity to propagate from the equator to the pole (or the half period of the complete cycle) is about 55 days. The relatively slow meridional propagation helps to explain the well-known seesaw oscillatory pattern between low and high latitudes found in monthly data. The equatorward propagation in the troposphere is synchronized with the poleward propagation of the stratospheric temperature anomalies of the opposite sign in both low and high latitudes, responsible for the out-of-phase relation between the stratospheric and tropospheric temperature anomalies in the polar region. Accompanied with warm (cold) anomalies is a successive levelling (steepening) of isentropic surfaces propagating poleward and downward. The zonal wind anomalies follow the poleward and downward propagating temperature anomalies of the opposite sign.

A global mass circulation paradigm is proposed to qualitatively explain the simultaneous meridional and downward propagation of circulation anomalies that appears responsible for the annular mode variability. When the mass circulation is weaker, the isentropic surfaces in the extratropical stratosphere (troposphere) are steeply (gently) sloped, corresponding to the positive phase of the annular mode. The cold air mass is effectively imprisoned within the polar cap when the mass circulation is weaker, responsible for warm surface temperature anomalies prevailing in the extratropics. Meantime, the weaker mass circulation also implies a temporary reduction of air mass supply over the polar cap, leading to a negative surface pressure anomaly. The warm anomalies brought by the stronger mass circulation cause a lowering of isentropic surfaces in the polar stratosphere, resulting in more gently sloped isentropic surfaces in the extratropical stratosphere. This corresponds to the negative phase of the annular mode in which the meridional temperature gradient in the extratropical stratosphere is weaker accompanied with a weakened polar jet and a falling of the tropopause. The stronger warm air branch of the mass circulation aloft requires a strengthening of the compensating equatorward advancement of the surface air mass, causing massive cold air outbreaks in the extratropics. The more air mass aloft brought by the stronger mass circulation contributes to a rising of the surface pressure over the polar cap till the surface cold air moves out. This explains why the surface pressure anomalies in high latitudes are positive during the negative phase of the annular mode.

Since it takes about 55 days for anomalies of one polarity to propagate from the tropics to the pole, such an intimate linkage between the anomalies in the deep tropics and high latitudes would imply a longer lead time for intra-seasonal climate prediction in the extratropics.