Abstract The changes in the sea-ice conditions and sea surface temperatures in the Subarctic Pacific caused by the closure of the Central American Seaway and their influence on the East Asian monsoon are investigated by a series of closed (CE), open (OE) and re-closed (RCE) seaway experiments with a climate model. It is found that a permanent halocline forms in the Subarctic Pacific because of the termination of saline water transport through the seaway in CE and RCE. Efficient cooling by shallow convection in the stratified permanent halocline causes thicker and more extensive sea ice in winter, and leads to colder surface water in summer in the Subarctic Pacific. Colder air, over more extensive sea ice cover in winter and over the colder water in summer, produces higher surface air pressure with anticyclonic wind anomalies in both seasons. Southeasterly and southerly wind anomalies develop around the Japanese archipelago in the East Asian monsoon region and induce warm and humid surface air with increased precipitation over the East Asian continent. These results indicate that the East Asian monsoon is weakened in winter and strengthened in summer as a result of closing the Central American Seaway.

Abstract The rise of the Panama Isthmus was the last step in the closure of the circumtropical seaways. The closure of the Panama Isthmus had fundamental consequences for global ocean circulation, evolution of the tropical ecosystems and potentially influenced the switch to the modern ‘cold house’ climate mode. The Atlantic and Pacific marine ecosystems became gradually separated whereas terrestrial organisms suddenly had the means to migrate between North and South America. Combining high-resolution geochemical proxies for the closure history with data on fossil distributions and genetic data provides independent evidence on the closure history. These datasets provide new boundary conditions for Earth System models to simulate the effects of palaeoceanographic change on global climate and allow exploration of hypotheses for the Northern Hemisphere glaciation.