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The Middle Miocene Climatic Transition (MMCT, approximately 14 Ma) isa key period in Cenozoic cooling and cryospheric expansion. Despite beingwell documented in isotopic record, the causes of the MMCT are still a matterof debate. Among various hypotheses, some authors suggested that it was duethe final closure of the eastern Tethys seaway and subsequent oceaniccirculation reorganisation. The aim of the present study is to quantify theimpact of varying Tethys seaway depths on middle Miocene ocean andclimate, in order to better understand its role in the MMCT.  We present four sensitivity experiments with a fully coupled ocean-atmospheregeneral circulation model. Our results indicate the presence of a warm andsalty water source in the northern Indian Ocean when the eastern Tethys isdeep open (4000 or 1000 m), which corresponds to the Tethyan IndianSaline Water (TISW) described on the basis of isotopic studies. This watersource is absent in the experiments with shallow (250 m) and closedTethys seaway, inducing strong changes in the latitudinal density gradient andultimately the reinforcement of the Antarctic Circumpolar Current (ACC).Moreover, when the Tethys seaway is shallow or closed, there is awestward water flow in the Gibraltar Strait that strengthens the AtlanticMeridional Overturning Circulation (AMOC) compared to the experiments withdeep-open Tethys seaway. Our results therefore suggest that the shoaling andfinal closure of the eastern Tethys seaway played a major role in the oceaniccirculation reorganisation during the middle Miocene.  The results presented here provide new constraints on the timing of theTethys seaway closure and particularly indicate that, prior to14 Ma, a deep-open Tethys seaway should have allowed the formation ofTISW. Moreover, whereas the final closure of this seaway likely played amajor role in the reorganisation of oceanic circulation, we suggest that itwas not the main driver of the global cooling and Antarctica ice-sheetexpansion during the MMCT. Here we propose that the initiation of the MMCTwas caused by an atmospheric <i>p</i>CO<sub>2</sub> drawdown and that the oceanic changesdue to the Tethys seaway closure amplified the response of globalclimate and East Antarctic Ice Sheet

The role of eastern Tethys seaway closure in the Middle Miocene Climatic Transition (ca. 14 Ma)