Seasonal and Interannual Variability of Atmospheric Heat Sources and Moisture Sinks as Determined from NCEP–NCAR Reanalysis

Abstract

Using the National Centers for Environmental Predictions (NCEP)?National Center for Atmospheric Research (NCAR) reanalysis, distributions of the heat source Q1 and moisture sink Q2 between 50°N and 50°S are determined for a 15-yr period from 1980 to 1994. Heating mechanisms operating in various parts of the world are examined by comparing the horizontal distributions of the vertically integrated heat source ?Q1? with those of the vertically integrated moisture sink ?Q2? and outgoing longwave radiation (OLR) flux and by comparing the vertical distributions of Q1 with those of Q2. In northern winter, the major heat sources are located (i) in a broad zone connecting the tropical Indian Ocean, Indonesia, and the South Pacific convergence zone (SPCZ); (ii) over the Congo and Amazon Basins; and (iii) off the east coasts of Asia and North America. In northern summer, the major heat sources are over (i) the Bay of Bengal coast, (ii) the western tropical Pacific, and (iii) Central America. Throughout the year, the South Indian Ocean, eastern parts of the North and South Pacific Oceans, and eastern parts of the North and South Atlantic Oceans remain to be heat sinks. The desert regions such as the Sahara are characterized by large sensible heating near the surface and intense radiative cooling aloft. Over the tropical oceans, heat released by condensation with deep cumulus convection provides the major heat source. The radiative cooling and moistening due to evaporation are dominant features over the subtropical oceans where subsidence prevails. Over the Tibetan Plateau, the profiles of Q1 and Q2 show the importance of sensible heating in spring and contributions from the release of latent heat of condensation in summer. Off the east coast of Japan, intense sensible and latent heat fluxes heat and moisten the lower troposphere during winter. Heat sources in various regions exhibit strong interannual variability. A long (4?5 yr) periodicity corresponding to the variations in OLR and sea surface temperature (SST) is dominant in the equatorial eastern and central Pacific Ocean, while a shorter-period oscillation is superimposed upon the long-period variation over the equatorial Indian Ocean. The interannual variations of ?Q1?, OLR, and SST are strongly coupled in the eastern and central equatorial Pacific. However, the coupling between the interannual variations of ?Q1? and OLR with those of SST is weak in the equatorial western Pacific and Indian Ocean, suggesting that factors other than the local SST are also at work in controlling the variations of atmospheric convection in these regions.