Did an Open Panama Isthmus Correspond to an Invasion of Pacific Water into the Atlantic?

Abstract

Recent general circulation simulations suggest that, prior to the closure of the Panama Isthmus (the narrow strip of land connecting North and South America), low-salinity Pacific Ocean water invaded the Atlantic Ocean via the gap between North and South America. According to this scenario, the invasion decreased the Atlantic Ocean salinity to the point at which North Atlantic Deep Water (NADW) formation was impossible and, consequently, there was probably no ?conveyer belt.? In line with this scenario, it has been suggested that the closure of the isthmus led to an increased salinity in the Atlantic that, in turn, led to the present-day NADW formation and the familiar meridional overturning cell (MOC). Using simple dynamical principles, analytical modeling, process-oriented numerical experiments, and modern-day wind stress, it is shown that, in the absence of NADW formation, one would expect a westward flow from the Atlantic to the Pacific Ocean through an open Panama Isthmus. This contradicts the suggestion made by the earlier numerical models that imply an eastward flow through the ?Panama Gateway.? An analogous present-day situation (for a system without deep-water formation) is that of the Indonesian Throughflow, which brings Pacific water to the Indian Ocean rather than the other way around; that is, it is also a westward flow rather than an eastward flow. ?Island rule? calculations clearly show that the direction of the flow in both situations is determined by the wind field to the east of the gaps. The authors show that exceptionally strong vertical mixing in the Atlantic (as compared with the Pacific) or another means of warm-water removal from the upper layer in the Atlantic (e.g., NADW or strong cooling) could reverse the direction of the flow through the open isthmus. This is most likely what happened in the earlier numerical simulation, which must have invoked (explicitly or implicitly) large quantities of upper-water removal even without NADW formation. On this basis it is suggested that if low-salinity Pacific water did, in fact, invade the Atlantic Ocean prior to the closure of the Panama Isthmus, then this invasion took place via the Bering Strait rather than through the open Panama Isthmus. It is also suggested that, if there were 20 Sv (Sv ≡ 106 m3 s?1) of NADW formation today and the Panama Isthmus were to be suddenly open today, then Pacific water would indeed invade the Atlantic via the Panama Gateway. In turn, this would either collapse the existing NADW formation rate or reduce it to about 10 Sv, which can be maintained even with an open isthmus. In both cases the final outcome is a westward flow in the open isthmus.