ABSTRACT: The opening and closing of ocean gateways has played a major role in shaping global climate by altering oceanic and atmospheric circulation. This study uses planktonic foraminifer assemblage-base sea surface temperature (SST) estimates (Modern Analog Technique) to examine the eastern tropical Pacific (ETP) and subtropical Northwest Atlantic (NWA) during the early stages of shoaling of the CentralAmerican Seaway (CAS). In the subtropical NWA(DSDP 103 and ODP 1006), the 5.2 to 5.1 Ma interval is characterized by an increase in SST and sea surface salinity, indicating a strengthening of the Florida Current (FC) andGulf Stream (GS). Sea surface temperature in the ETP WarmPool (DSDP Site 84) remained relatively stable between 6.9 and 5.1Ma, duringwhich El NiÃ±o-like conditions persisted. A slight cooling is observed after this interval (with synchronous warming in the NWA), followed by the onset of major cooling at ~3.2Ma, both of which are preceded by a shallowing of the thermocline. Stepwise cooling is attributed to enhanced Atlantic meridional overturn cicrulation (AMOC), which caused a shoaling of the main tropical thermocline, thereby strengthening the Walker Circulation and weakening the Pacific North Equatorial Counter Current. Antithetical changes in surface current strength in the NWAand ETP suggest a southward migration of the Intertropical Convergence Zone ~4.3 Ma.During the mid-Pliocene, SST in the Panama Basin was ~0.8°C cooler than today, while the subtropicalNWA was only ~1.1°C warmer. This corroborates evidence for reduced meridional SST gradients during the mid-Pliocene as well as the hypothesis that more vigorous ocean circulation - particularly in the NWA - wascritical during this period. The timing of SST changes in the ETP and NWA(~5.1 Ma) suggest that the termination of permanent El NiÃ±o and enhanced AMOC did not contribute significantly to the onset of major Northern Hemisphere glaciation (NHG), as both of these events occurwell before the beginning of the glacial cycles. However, these processesmay have contributed to the development of the small ice sheets of the late Miocene and early Pliocene, but were most likely only preconditioning factors for the onset of major NHG. However, changes in SST and relative thermocline position suggest that high-latitude processes and global cooling may have influenced thermal structure in the ETP. The SST estimates provided indicate that even in its early stages, the shoaling of the CAS had significant implications for low-latitude ocean circulation and thermal structure, as well as for some of the most significant global climate events of the late Neogene.