Showing papers on "Climate oscillation published in 1977"

Simulation of the tropical climate of an ice age

Abstract: Numerical time integrations of a general circulation model of the atmosphere are performed with both modern and ice age boundary conditions. It is shown that the climate of continental portions of the tropics in the ice age simulation is much drier than that of the modern climate simulation. According to comparisons of results from the two experiments, tropical continental aridity of the ice age results from stronger surface outflow from (or weaker surface inflow into) continents. The intensification of outflow from (or weakening of inflow into) tropical continental regions results from the fact that in response to ice age boundary conditions, atmospheric temperature is reduced more over continents than over oceans. With the exception of high latitudes, boundary condition differences between the two experiments consist mainly of changes of the prescribed values of sea surface temperature and continental albedo. In order to evaluate the relative contributions of these changes in producing continental tropical aridity in the ice age simulation, a third numerical experiment is time-integrated in which a hybrid combination of ice age sea surface temperatures and modern continental albedo values is prescribed. From intercomparisons be- tween results from this and the previous two experiments it is shown that the effect of increased continental albedo is mainly responsible for the weak Asian monsoon in the ice age simulation. During the past 20 years, mathematical models of the atmo- sphere have improved sufficiently to simulate many of the geographical features of climate. Therefore it is quite reason- able to consider the possibility of using such a model for the study of an ice age climate. This is one of the important reasons why a large group of geologists (i.e., Climap Project Members, 1976) have embarked upon an ambitious project aiming at the reconstruction of conditions at the earth's sur- face (i.e., sea surface temperature, glacial ice, surface albedo, etc.) during the period of the last glacial maximum, which occurred approximately 18,000 yr B.P. This paper discusses a part of the results from a numerical simulation with a global model of the atmosphere in which geological reconstructions made by Climate: Long-Range Investigation Mapping and Prediction (Climap) are used as lower boundary conditions. It is expected that the combined effort of geological reconstruc- tion and mathematical simulation will yield a comprehensive picture of an ice age climate. In addition, by comparing the simulated ice age climate with a simulation of the present climate, one may obtain. an appreciation of the circumstances under which an ice age climate is maintained. Furthermore, such comparisons may yield some insight into how climate (or atmospheric circulation) responds to anomalies in the lower boundary conditions, i.e., anomalies in the distributions of sea surface temperature, surface albedo, and glacial ice. It should be poifited out that this is not the first attempt to simulate an ice age climale. Earlier attempts of a similar nature include works by Alyea (1972), Williams et al. (1974), Saltzman and Venekar (1975), and Gates (1976a, b). In particular, Gates conducted a numerical integration of a global model using Climap boundary conditions which are very similar to those used for the present study. Nevertheless, it was decided to carry out a similar numerical integration using a model devel- oped at the Geophysical Fluid Dynamics Laboratory of NOAA in view of its success in simulating modern distribu- tions of hydrologic quantities (i.e., precipitation and evapora- emphasis of this study upon the analysis of the simulated climate and the hydrologic cycle in the tropics. Earlier literature (e.g., Charlesworth, 1957) suggests that in the tropics the climate of a glacial period is more pluvial than that of an interglacial period. One may speculate that the meridional temperature gradient during a glacial period is larger than that of an interglacial period owing partly to the large reflectivity of the extensive ice sheets in high latitudes. Therefore it may not be unreasonable to expect stronger Had- ley cells and, accordingly, more intense tropical precipitation during a glacial period. However, more recent geological evi- dence (e.g., Williams, 1975) indicates that many parts of tropi- cal continents were more arid during the late Pleistocene, in apparent contrast to the earlier notion of pluvial tropics dur- ing glacial periods. This paper describes the aridity (or lack of aridity) of the tropical climate of the model, evaluates the two contradictory viewpoints in the light of results from the nu- merical experiments, and proceeds to discuss how the lower boundary conditions of an ice'age influence the climate and, in particular, the hydrologic cycle in the tropics. Before this introductory section is concluded, it should be noted that this study does not attempt to simulate the seasonal variation of an ice age climate. Instead, it was decided to limit the scope of this study to the simulation of only one season (July-August) pending completion of the reconstruction of the January-February boundary conditions by the Climap group. Nevertheless, the results from this study appear to be very useful for identifying the important mechanisms which control the aridity of tropical continents. Therefore it was decided to write this paper despite the highly preliminary nature of the results. Future plans include the time integration of a model with ice age boundary conditions for an extended period of more than 1 year. DESCRIPTION OF THE MODEL