How the Central American Seaway alters large-scale ocean circulation, climate and marine biogeochemistry
Creators: name: Sentman, Lori
Publisher: name: No Publisher Supplied
Year: 2018

description: The study of past climates using climate models and paleoclimate proxy records is helpful for understanding how the Earth system responds to external natural forcing on time scales longer than the current instrumental records. The Central American Seaway (CAS) was an important ocean gateway connecting the Pacific and Atlantic Oceans until its gradual shoaling and final closure near the end of the Pliocene (5.3-2.6 Ma), when paleoclimate proxy records indicate a major reorganization in large-scale ocean circulation and shifting spatial patterns in global climate and marine biogeochemistry. Climate models are not consistent in reconciling the impact of the seaway on global deep-water circulation, tropical Pacific and Southern Hemisphere physical mean state, and interannual tropical Pacific climate variability, and have not been able to explore the coupled impacts on ocean biogeochemistry or sediment calcium carbonate (CaCO3) long-term burial. For the first time, as far as this author knows, a suite of four idealized experiments, including a very narrow (109 km-wide) single meridional grid point channel, are performed for multi-millennial scale simulations using the Geophysical Fluid Dynamics Laboratory Earth System Model, GFDL-ESM2G, with high ocean spatial resolution to explore the mechanistic role of changing topography – varying only seaway widths and sill depths – associated with the various stages of seaway constriction and shoaling on global ocean circulation, climate and marine biogeochemistry compared to “preindustrial” 1860 climate. Model output is combined with an uncoupled box model to obtain the first sediment CaCO3 (as calcite) long-term burial estimates and atmosphere pCO2 concentrations associated with a very narrow seaway for comparison with proxy records.    Independent of the CAS configuration in GFDL-ESM2G, the open CAS alters ocean physical mean state and deep water properties globally, driven by the direct impacts of the seaway on global mass, heat and salt transports. Net mass transport from the Pacific through the CAS into the Caribbean is 20.5-23.1 Sv with the 2000-m deep seaways, but only 14.1 Sv for the 200-m shallow seaway. The CAS provided a shortcut for southern sourced Pacific water mass transport, warming the South Atlantic and reducing Indonesian Throughflow mass transport by 59-82%. The CAS suppressed Antarctic Bottom Water northward extent, allowing North Atlantic Deep Water to deepen ~500 m and slightly strengthen (~2Sv), in contrast to preindustrial observational estimates and previous studies with an open CAS using climate models. Global mean climate and tropical Pacific interannual variability are sensitive to the presence of the CAS, with the largest sensitivity occurring in the Southern Hemisphere for the relatively wide (1308 km) and 2000-m deep CAS. In response to the global ocean reorganization associated with the CAS opened to various shoaling stages, global mean surface air temperatures warm 0.4-0.7°C with a bipolar, asymmetric response of Northern Hemisphere cooling up to ~2°C in the northwest Pacific and Southern Hemisphere warming up to ~8°C near the Ross Sea, in contrast to global mean cooling in climate models. In the tropical Pacific, opening the CAS leads to a global mean warming 0.4-0.8°C in the top 300 m, increased equatorial sea surface temperature gradient in the central and east Pacific, decreased meridional sea surface temperature asymmetry about the equator at 110°W, and the thermocline deepens 5-11 m. Opening the CAS leads to larger El Niño-Southern Oscillation (ENSO) amplitude with more La Niña or cold events, a weaker annual cycle, and ~3 months earlier development.    Opening the CAS results in stronger ventilation and a reduction in the sequestration efficiency of the biological pump, allowing respired CO2 to escape to the atmosphere via increased ocean CO2 outgassing. The loss of dissolved inorganic carbon increases the deep ocean carbonate (CO32-) leading to a short-term (&lt; 500 kyr) increase in global CaCO3 burial of 0.002 PgC a-1 corresponding to large long-term increases in the global sediment CaCO3 pool (~200 PgC over 105 years) and a net migration of ~150 PgC from the spatial distribution of CaCO3 in the active layer of 10 cm surface sediment, providing the upper limit on the decrease in ocean alkalinity (~300 PgC), in which alkalinity and DIC are removed in a 2:1 ratio. The enhanced burial of CaCO3 leads to an additional release of 237.9 ppmv (506.8 PgC) to the atmosphere from the partitioning of carbon species implying short-term warming of 0.4-1.0 K in the Pliocene with a very narrow CAS. Overall, this paleoclimate application has broad implications for the sensitivity of coupled ocean-atmosphere dynamics and ocean biogeochemistry to changing ocean circulation with far-reaching, long-term climate, ENSO, marine ecosystem, ocean biogeochemical, and atmosphere pCO2 impacts.