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Abstract On decadal time scales, Indian Ocean sea surface temperatures (SSTs) exhibit coherent basin‐wide changes, but their origins are not well understood. Here we analyze observations and model simulations from Coupled Model Intercomparison Project Phase 6 and Community Earth System Model Version 1 to quantify the roles of external forcing and internal climate variability in causing Indian Ocean decadal SST variations. Results show that both external forcing and internal variability since 1920 have contributed to the observed decadal variations in linearly detrended Indian Ocean SSTs, and they exhibit an out‐of‐phase relationship since the 1950s. The internally‐generated variations arise from remote influences from the tropical Pacific and possible contributions from internal local processes, while the influence from the Atlantic Multidecadal Oscillation is opposite to that of the Interdecadal Pacific Oscillation. Decadal SST changes caused by nonlinear variations in greenhouse gases and aerosols are roughly out‐of‐phase with the internal variability, thus dampening observed SST variations since the 1950s.
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Observational analysis of decadal and long-term hydroclimate drivers in the Mediterranean region: role of the ocean–atmosphere system and anthropogenic forcing
Using observations and reanalysis, we develop a robust statistical approach based on canonical correlation analysis (CCA) to explore the leading drivers of decadal and longer-term Mediterranean hydroclimate variability during the historical, half-year wet season. Accordingly, a series of CCA analyses are conducted with combined, multi-component large-scale drivers of Mediterranean precipitation and surface air temperatures. The results highlight the decadal-scale North Atlantic Oscillation (NAO) as the leading driver of hydroclimate variations across the Mediterranean basin. Markedly, the decadal variability of Atlantic-Mediterranean sea surface temperatures (SST), whose influence on the Mediterranean climate has so far been proposed as limited to the summer months, is found to enhance the NAO-induced hydroclimate response during the winter half-year season. As for the long-term, century scale trends, anthropogenic forcing, expressed in terms of the global SST warming (GW) signal, is robustly associated with basin-wide increase in surface air temperatures. Our analyses provide more detailed information than has heretofore been presented on the sub-seasonal evolution and spatial dependence of the large-scale climate variability in the Mediterranean region, separating the effects of natural variability and anthropogenic forcing, with the latter linked to a long-term drying of the region due to GW-induced local poleward shift of the subtropical dry zone. The physical understanding of these mechanisms is essential in order to improve model simulations and predic- tion of the decadal and longer hydroclimatic evolution in the Mediterranean area, which can help in developing adaptation strategies to mitigate the effect of climate variability and change on the vulnerable regional population.
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- Award ID(s):
- 1734760
- PAR ID:
- 10312001
- Date Published:
- Journal Name:
- Climate Dynamics
- ISSN:
- 0930-7575
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Decadal variability in the North Atlantic Ocean impacts regional and global climate, yet changes in internal decadal variability under anthropogenic radiative forcing remain largely unexplored. Here we use the Community Earth System Model 2 Large Ensemble under historical and the Shared Socioeconomic Pathway 3-7.0 future radiative forcing scenarios and show that the ensemble spread in northern North Atlantic sea surface temperature (SST) more than doubles during the mid-twenty-first century, highlighting an exceptionally wide range of possible climate states. Furthermore, there are strikingly distinct trajectories in these SSTs, arising from differences in the North Atlantic deep convection among ensemble members starting by 2030. We propose that these are stochastically triggered and subsequently amplified by positive feedbacks involving coupled ocean-atmosphere-sea ice interactions. Freshwater forcing associated with global warming seems necessary for activating these feedbacks, accentuating the impact of external forcing on internal variability. Further investigation on seven additional large ensembles affirms the robustness of our findings. By monitoring these mechanisms in real time and extending dynamical model predictions after positive feedbacks activate, we may achieve skillful long-lead North Atlantic decadal predictions that are effective for multiple decades.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s00382-021-05765-1
