[Virtual] GFDL Formal Seminar

Thu, Sep 17, 2020, 2:00 pm to 3:00 pm

Future change of the Atlantic meridional overturning circulation (AMOC) results from both the response of AMOC system to external climate forcing and the feedback processes within the AMOC system. We first focus on salt-advection feedback in AMOC system that is tied to AMOC stability. We notice that AMOC changes are moderate in most climate model projections under increasing greenhouse gas forcing. This inter-model consensus may be an artifact of common model biases that favor a stable AMOC, since observationally based freshwater budget analyses suggest that the AMOC is in an unstable regime susceptible for large changes in response to perturbations. To investigate the impact of AMOC stability bias on future climate projection, we use NCAR CCSM3 and correct the model AMOC stability bias by means of flux adjustment. We find that, via a positive salt-advection feedback, the AMOC in the corrected model collapses 300 years after the atmospheric CO<sub>2</sub>concentration is abruptly doubled from the 1990-level. Compared to uncorrected model, the AMOC collapse brings about large, dramatically different climate response such as a prominent cooling over the northern North Atlantic and neighboring areas. We further find that, besides anthropogenic warming and factors such as Greenland ice sheet melting, the ongoing decline of Arctic sea ice can also induce a remarkable weakening of the AMOC on multi-decadal and longer timescales. Due to sea ice loss, anomalous warm water accumulates in the Arctic and spreads to the North Atlantic. At the same time, freshwater that accumulates from seasonal sea ice melting over most of the upper Arctic Ocean also spreads southward, reaching as far as south of Iceland. These warm and fresh anomalies reduce upper ocean density and suppress oceanic deep convection, and therefore lead to a slowdown of the AMOC. Such AMOC change can greatly mediate the global impacts of Arctic sea ice decline. During the first two decades when atmospheric processes dominate, sea ice decline induces a “bipolar seesaw” pattern in surface temperature with warming in the Northern and cooling in the Southern Hemisphere, leading to a northward displacement of the Intertropical Convergence Zone (ITCZ) and an expansion of Antarctic sea ice. In contrast, on multi-decadal and longer timescales, the AMOC slowdown mediates direct sea ice impacts and nearly reverses the original response pattern outside the Arctic. The Southern Hemisphere warms, a Warming Hole emerges in the North Atlantic, the ITCZ shifts southward, and Antarctic sea ice contracts.

Location: 
Virtual
Speaker(s): 

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