Gaurav Madan

• Madan, Gaurav; Gjermundsen, Ada; Iversen, Silje Christine & Lacasce, Joseph Henry (2023). The weakening AMOC under extreme climate change. Climate Dynamics. ISSN 0930-7575. 62, s. 1291–1309. doi: 10.1007/s00382-023-06957-7. Fulltekst i vitenarkiv Vis sammendrag

  Changes in the Atlantic Meridional Overturning Circulation (AMOC) in the quadrupled CO₂ experiments conducted under the sixth Coupled Model Intercomparison Project (CMIP6) are examined. Increased CO₂ triggers extensive Arctic warming, causing widespread melting of sea ice. The resulting freshwater spreads southward, frst from the Labrador Sea and then the Nordic Seas, and proceeds along the eastern coast of North America. The freshwater enters the subpolar gyre north of the separated Gulf Stream, the North Atlantic Current. This decreases the density gradient across the current and the current weakens in response, reducing the infow to the deepwater production regions. The AMOC cell weakens in tandem, frst near the North Atlantic Current and then spreading to higher and lower latitudes. This contrasts with the common perception that freshwater caps the convection regions, stifing deepwater production; rather, it is the infow to the subpolar gyre that is suppressed. Changes in surface temperature have a much weaker effect, and there are no consistent changes in local or remote wind forcing among the models. Thus an increase in freshwater discharge, primarily from the Labrador Sea, is the precursor to AMOC weakening in these simulations.

• Rodal, Marie; Krumscheid, Sebastian; Madan, Gaurav; Lacasce, Joseph Henry & Vercauteren, Nikki (2022). Dynamical stability indicator based on autoregressive moving-average models: Critical transitions and the Atlantic meridional overturning circulation. Chaos. ISSN 1054-1500. 32(11). doi: 10.1063/5.0089694. Fulltekst i vitenarkiv Vis sammendrag

  A statistical indicator for dynamic stability, known as the [Formula: see text] indicator, is used to gauge the stability and, hence, detect approaching tipping points of simulation data from a reduced five-box model of the North Atlantic Meridional Overturning Circulation (AMOC) exposed to a time-dependent hosing function. The hosing function simulates the influx of fresh water due to the melting of the Greenland ice sheet and increased precipitation in the North Atlantic. The [Formula: see text] indicator is designed to detect changes in the memory properties of the dynamics and is based on fitting auto-regressive moving-average models in a sliding window approach to time series data. An increase in memory properties is interpreted as a sign of dynamical instability. The performance of the indicator is tested on time series subject to different types of tipping, namely, bifurcation-induced, noise-induced, and rate-induced tipping. The numerical analysis shows that the indicator indeed responds to the different types of induced instabilities. Finally, the indicator is applied to two AMOC time series from a full complexity Earth systems model (CESM2). Compared with the doubling [Formula: see text] scenario, the quadrupling [Formula: see text] scenario results in stronger dynamical instability of the AMOC during its weakening phase.

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