Joint Seminar: AMOC and North Atlantic responses to Sea Ice decline and tropical teleconnections

Two salient features of anthropogenic climate change are the reduced Arctic sea ice cover and the warming of the tropical Indian Ocean (TIO) sea surface temperatures (SST). Since the mid-20th century, the Arctic has been warming and losing sea ice cover at an alarming rate. Nevertheless, several model-based studies find that in response to Arctic sea ice decline, the Atlantic meridional overturning circulation (AMOC) weakens on multi-decadal timescales, reaching a new state, while others studies see only a weak reduction followed by a recovery. To investigate this problem, we conduct a multi-member ensemble experiment, lasting 200 years, wherein Arctic sea ice albedo is reduced by ~23%. After several decades we observe an AMOC slowdown, the intensification in the western Subpolar region leads to AMOC full recovery. To further investigate the sensitivity of AMOC to anthropogenic climate changes, a novel approach of nudging the TIO is explored. The TIO is warming faster than the other tropical oceans. To that end, we conduct ensemble experiments nudging the SST of the TIO by artificially warming or cooling. We find that the warming within the TIO relative to the rest of the tropical oceans contributes to increased Walker circulation, reduced precipitation, and increased wind induced evaporation within the tropical Atlantic. We find a +9.3 Sv per 1°C AMOC sensitivity to relative TIO warming. The TIO teleconnection to the tropical Pacific is additionally explored. Here, we find that warming or cooling the TIO changes the magnitude of the mean equatorial Pacific zonal wind stress proportionally to the imposed forcing. Surprisingly, ENSO variability generally increases in both TIO cooling and warming experiments, relative to the control. TIO warming also results in ENSO strengthening through stronger trade winds, largely associated with the strengthening of the CP-type events, not described by the conventional Bjerknes index. Our results display an interplay of different atmospheric and oceanic processes, from changes in the Walker circulation to the modulation of the AMOC, that control TIO teleconnections to Atlantic and Pacific climate and can be important under the current and future climate scenarios.




15:15 h


Virtual Seminar


Brady Ferster, Institut Pierre Simon Laplace, Paris


Armin Köhl

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