| description abstract | A ten-year integration of the Held-Suarez climate model with simplified continents and prescribed, but seasonally varying, ocean temperatures produces mean climatic states that are qualitatively similar to observed seasonal climatology. However, the model's temperature gradients, zonal winds and interannual variability are of lower magnitude than observed. Differences between these seasonal control climates and those obtained from other decadal integrations with fixed temperature anomalies superimposed on the ocean at different latitudes indicate that the seasonal response of the model atmosphere is a function of anomaly position. Thus, although there are increases in upward motion and precipitation in the vicinity of all ocean temperature anomalies, these changes are of considerably greater magnitude for the equatorial and subtropical anomalies than for the midlatitude anomaly. The vertical motion forced by the equatorial ocean temperature anomaly is that of a Walker-type circulation, with overturning to the east and west of the anomalous heating in all seasons. Local changes in 750-250 mb thickness are much less sensitive to anomaly position: annual thickness increases in the vicinity of all the anomalies are roughly the same. However, there are large seasonal variations in the responses of the 750?250 mb thickness to each ocean temperature anomaly. In the case of the midlatitude anomaly, the maximum local and downstream increases in thickness occur in summer, while a much weaker response characterizes spring and winter. Local and remote changes in thickness forced by the subtropical anomaly exhibit the greatest seasonal variation, with large and extensive thickness departures in summer and autumn, but comparatively small differences in spring and winter. Significant changes in thickness associated with the equatorial anomaly are confined mainly to the tropics in spring and summer, but in autumn and winter there is substantial interaction with extratropical latitudes in the Southern and Northern Hemisphere, respectively. In winter an intensification of the Northern Hemisphere subtropical jet stream also results from the anomalous equatorial heating. If the geographically extensive changes in thickness associated with the equatorial and subtropical anomalies in certain seasons are the result of the propagation of waves forced by the heat anomalies, these features appear to contradict the predictions of the linear theory of critical levels. Alternative explanations for these phenomena are proposed, whose merits are examined in Part II of this paper. | |
