Tropical Pacific climate at the mid-Holocene and the Last Glacial Maximum simulated by a coupled ocean-atmosphere general circulation model

Abstract

Simulations for the mid-Holocene (6000 years before present: 6 ka) and the Last Glacial Maximum (LGM: 21 ka) have been performed by a global ocean-atmosphere coupled general circulation model (GCM). After the initial spin-up periods, both runs were integrated for about 200 years. For 6 ka the model shows an enhanced seasonal variation in surface temperature and a northward shift of the African and the Indian summer monsoon rain area. Overall circulation features in the tropics correspond to a strong Walker circulation state with negative sea surface temperature (SST) and precipitation anomalies in the central Pacific and positive precipitation anomalies over the Indian and Australian monsoon regions. It is noted that there is about a 0.35°C cooling of the global mean SST. In contrast to the 6 ka result, the simulated tropical climate anomaly at 21 ka corresponds to a weak Walker circulation state. The simulated LGM SST decrease is about 2°C over the tropical western Pacific. It is larger (about 5°C) over the Caribbean Sea. This SST anomaly is in broad agreement with the observed proxy data. Interestingly, the model simulates a warmer than present SST over the subtropical Pacific. This may be related to a weaker subtropical anticyclone due to weakened monsoon circulation at the LGM.