Mid-Cretaceous ocean circulation in the Kiel Climate Model

Reconstructing the climate of mid-Cretaceous is a special challenge for climate modelling. High atmospheric C02 concentrations and a vastly different geography seem to have caused a global temperature increase. At the same time the geologic record provides ambiguous data about the strength and spatia.l distribution of this warming. In the following, 4 experiments with the new, fully coupled "Kiel Climate Model" (KCM) are realised to gain new insights into the mid-Cretaceous climatic conditions and to to investigate their influence on the ocean circulation. Furthermore, the changed boundary conditions (C02 =1200 ppmv, geography) and their respective impacts are examined independently. The experiments show a significant global warming in near surface temperatures of approximately 9°C and of about 6°C for sea surface temperatures, leading to ice free polar caps in the annual mean. Additionally they indicate that greatly increased trace gas concentrations are needed to reproduce distinct warming for tropical and subtropical regions. Changes in geography provide mainly high latitude warming. The ocean surface circulation is dominated by wind-driven gyre circulation comparable to present clay. A predominant westward flow through the Tethys Sea and a largely decreased southern circumpolar current are simulated. Changes in model geography lead to a clear decline in upper ocean salinities and a strong increase with depth for the deeper ocean. The warm, fresh surface layer prevents a deep mixing in the water column and there is no global meridional overturning circulation observable. As a result the model shows a strengthening in vertical ocean stratification for mid-Cretaceous contrarily to prior studies