Sensitivity of the thermohaline circulation to tropical and high latitude freshwater forcing during the last glacial‐interglacial cycle

Abstract

Recent studies indicate that the tropical freshwater budget of the Atlantic is modulated by changes in El Niño‐Southern Oscillation (ENSO). If anomalies of the Atlantic freshwater balance persist on the order of decades, these might have a large influence on the North Atlantic thermohaline circulation (THC). Here we present a sensitivity study in which we use a model scenario for ENSO behavior during the last 120 kyr to force a simplified model of the THC with corresponding freshwater exchange between the tropical Atlantic and Pacific. If the steady state strength of the North Atlantic THC is similar to the present‐day, its response to the forcing is of the order of 1–3 Sv (1 Sv = 106 m3 s−1). No mode changes of the THC are simulated for reasonable values of the coupling constant between freshwater exchange and ENSO. If, on the other hand, the steady state overturning is significantly weaker, a collapse of the THC occurs due to forcing from the tropics. The modeled THC variation due to tropical freshwater perturbations are compared to those resulting from middle‐ and high‐latitude freshwater forcing due to long‐term growth and decay of Northern Hemisphere ice sheets. It is found that both forcings are of similar amplitude except during the deglaciation when high latitude forcing dominates. A possible out‐of‐phase relationship between deep water formation in the North Pacific and North Atlantic is also explored.