North Pacific Climate Response to Freshwater Forcing in the Subarctic North Atlantic: Oceanic and Atmospheric Pathways

Abstract

Sudden changes of the Atlantic meridional overturning circulation (AMOC) are believed to have caused large, abrupt climate changes over many parts of the globe during the last glacial and deglacial period. This study investigates the mechanisms by which a large freshwater input to the subarctic North Atlantic and an attendant rapid weakening of the AMOC influence North Pacific climate by analyzing four different ocean–atmosphere coupled general circulation models (GCMs) under present-day or preindustrial boundary conditions. When the coupled GCMs are forced with a 1-Sv (Sv ≡ 106 m3 s−1) freshwater flux anomaly in the subarctic North Atlantic, the AMOC nearly shuts down and the North Atlantic cools significantly. The South Atlantic warms slightly, shifting the Atlantic intertropical convergence zone southward. In addition to this Atlantic ocean–atmosphere response, all of the models exhibit cooling of the North Pacific, especially along the oceanic frontal zone, consistent with paleoclimate reconstructions. The models also show deepening of the wintertime Aleutian low. Detailed analysis of one coupled GCM identifies both oceanic and atmospheric pathways from the Atlantic to the North Pacific. The oceanic teleconnection contributes a large part of the North Pacific cooling: the freshwater input to the North Atlantic raises sea level in the Arctic Ocean and reverses the Bering Strait throughflow, transporting colder, fresher water from the Arctic Ocean into the North Pacific. When the Bering Strait is closed, the cooling is greatly reduced, while the Aleutian low response is enhanced. Tropical SST anomalies in both the Atlantic and Pacific are found to be important for the equivalent barotropic response of the Aleutian low during boreal winter. The atmospheric bridge from the tropical North Atlantic is particularly important and quite sensitive to the mean state, which is poorly simulated in many coupled GCMs. The enhanced Aleutian low, in turn, cools the North Pacific by increasing surface heat fluxes and southward Ekman transport. The closure of the Bering Strait during the last glacial period suggests that the atmospheric bridge from the tropics and air–sea interaction in the North Pacific played a crucial role in the AMOC–North Pacific teleconnection.

Climate impacts of recent multidecadal changes in Atlantic Ocean Sea Surface Temperature: a multimodel comparison

“…3). This result is consistent with a recent study by Okumura et al (2008) which demonstrates that rapid changes in the Atlantic MOC, leading to a cooling of the North Atlantic Ocean, can result in a deepening of the Aleutian Low.…”

Persistent millennial-scale link between Greenland climate and northern Pacific Oxygen Minimum Zone under interglacial conditions

“…This suggests the existence of a small but significant ice cap over Northern Hemisphere high latitude continents (Bonelli et al, 2009;Ganopolski et al, 2010). During MIS5, DO (Johnsen et al, 2001) and HE-like events (massive iceberg discharge in the North Atlantic) (McManus et al, 1994;Eynaud et al, 2000;Oppo et al, 2006) occurred together with rapid high amplitude sea level changes (Grant et al, 2012).…”

“…4). During cold events simulated by fresh water perturbation under pre-industrial conditions, low temperatures in the northern Pacific enhance the Aleutian Low (Mikolajewicz et al, 1997;Okumura et al, 2009). Cold temperatures over the North American continent favour the establishment of anticyclonic circulation (Kutzbach and Wright Jr., 1985;Romanova et al, 2006).…”

“…Considering that Heinrichlike events probably occurred during the MIS5 (Fig. 5) (McManus et al, 1994;Eynaud et al, 2000;Oppo et al, 2006), changes in the ventilation and/or the nutrient inventory at Baja California cannot be ruled out. Numerical simulations evaluated the relative impact of productivity and ventilation on oxygen level in the Pacific during a simulated HE.…”

Pulses of enhanced North Pacific Intermediate Water ventilation from the Okhotsk Sea and Bering Sea during the last deglaciation

“…Several studies with general circulation models (GCMs) point to fundamental changes in deep Pacific hydrography and circulation during the last deglaciation (Mikolajewicz et al, 1997;Schmittner et al, 2007;Okumura et al, 2009;Okazaki et al, 2010;Chikamoto et al, 2012;Menviel et al, 2012). A few model simulations show an onset of deep-water formation to a depth of ∼ 2500-3000 m in the northwest…”