From an analysis of the International Comprehensive Ocean-Atmosphere Data Set, we were able to confirm salient features of the tropical Atlantic decadal oscillation illustrated in previous works. These features include anti-symmetric behavior of surface air pressure and sea surface temperature with two centers of action residing in the northern and southern tropical Atlantic Oceans. In addition to the recognized features of the oscillation, we found a new aspect: the principal mechanism regulating the oscillation differs between at the north and south centers of action. Decadal components of the relative humidity time series show a maximum value of lag cross-correlation at a lag of -2 years. Such a temporal evolution was irrespective of the centers of action and the reference time series. The two reference series employed are the sunspot numbers and the southerlies at the western equatorial Atlantic. Associated with this, statistically significant phase shifts between the decadal components of variables exist, especially in the northern tropical Atlantic Ocean. For the decadal oscillation, relative humidity change was dominated by specific humidity change over the northern tropical Atlantic and by air temperature change over the southern tropical Atlantic. For the frequency band of the solar cycle, the variation of relative humidity seemed to amplify the earth's response to sun's radiation changes that had previously been considered too small to affect the earth climate. By incorporating our new findings with the known features of the oscillation, we can suggest that the variability of solar radiation may be crucial as an origin, or at least as a regulator, of the oscillation in combination with the climate distribution of clouds and water vapor over the tropical Atlantic.