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Impacts of the North Atlantic Decadal variability on European Climate: mechanisms and predictability

Periodic Reporting for period 1 - INADEC (Impacts of the North Atlantic Decadal variability on European Climate: mechanisms and predictability)

Período documentado: 2018-08-01 hasta 2020-07-31

"Will it be raining less in ten years? If so, by how much?” Those are the kind of questions climate scientists attempt to answer more and more accurately. Knowing precisely how the climate of the next decade is going to evolve would provide valuable information for the energy/industry sectors and decision-makers. However, to supply reliable information, several scientific and technical barriers remain to overcome. The project INADEC brings new knowledge that will contribute to open those barriers.

At decadal timescales, the climate variability is mostly driven by oceanic fluctuations. In the North Atlantic basin, slow variations in currents and heat transport translate into a general warming or cooling of the whole North Atlantic that typically last for 20-30 years. Those variations are called Atlantic Multidecadal Variability (AMV). These changes in the ocean conditions impact the atmospheric circulation and subsequently the climate conditions of the climate conditions over continents as well as in other oceanic basins. Regionally, those impacts are superimposed to the global warming signal, either minimizing or amplifying it over several decades. Impacts of the AMV include the links between the North Atlantic ocean warming and drier conditions over the Mediterranean region, Mexico, and south-western United States; cooling in the tropical Pacific; and rainier conditions over the Brazilian Nordeste, the Sahel and South-East Asia. Given the worldwide impacts of the AMV variability, predicting its future evolution has the potential for improving our ability to predict the climate conditions over many regions of the world during the next decades, providing valuable information for the energy/industry sectors and decision-makers.

Numerical climate predictions show that the North Atlantic region is the most predictable region of the world at decadal timescales. This high predictive skill is encouraging for the prospect of getting skilful decadal predictions all over the globe through the North Atlantic teleconnections. To date, however, decadal climate predictions show only limited skill over continents, with no real contribution coming from the prediction of the AMV. Given the observed teleconnections associated with the AMV, this absence of additional skill over continent raises questions.

The INADEC project aimed to evaluate our ability to predict the AMV climate impacts and to identify the current obstacles limiting their predictability. In particular, the project addressed three questions:
Q1) Do the observed teleconnections between the AMV and Europe, Sahel, Americas, South-East Asia, and the tropical Pacific ocean have a physical basis?
Q2) Can the models simulate those teleconnections accurately?
Q3) What are the mechanisms driving the North Atlantic decadal variability and predictability?

Results from the INADEC project revealed that the mean model biases are limiting our capability to predict the climate impacts of the AMV. Moreover, the INADEC project shows that large inter-model disagreements exist in the mechanisms that drove the observed AMV since 1950, raising questions about the ability of the current climate model generation in reproducing such mechanisms. Consequently, this suggests that decadal prediction systems can be substantially improved.
To evaluate whether the current generation of climate models can simulate the observed AMV teleconnections, we performed idealised experiments with climate models. In those experiments, the observed AMV is imposed over climate model’s North Atlantic. Outside of the North Atlantic, models are free to adjust to the imposed observed anomalies. This protocol enables the assessment of models capacity to reproduce the teleconnections associated with the observed AMV.

Those experiments demonstrate that climate models simulate the observed link between a North Atlantic warming and tropical Pacific cooling, rainier conditions over Sahel, as well as warmer and drier conditions during summer over the Mediterranean basin, Mexico, and the south-western United States. Those results confirm the driving role of the North Atlantic ocean in the observed Pacific and continental decadal variations. They also demonstrate that models used for decadal predictions are able to reproduce such North Atlantic impacts. Therefore, it suggests that the current limitation in our decadal prediction skill over those regions is rather coming from an initial shock in the decadal prediction due to incompatibility between the model normal behavior and the observed initial conditions.

Our numerical experiments also reveal the existence of a large inter-model spread regarding the strength of the North Atlantic teleconnections. In particular, they show that, if all models simulate a tropical Pacific cooling in response to a North Atlantic warming, the amplitude of this tropical Pacific cooling differs by a factor of ten across models, highlighting considerable uncertainties in our ability to predict accurately such a response. We demonstrated that these uncertainties are coming from different model mean precipitation biases in the tropical Atlantic.

The INADEC project also revealed a considerable inter-model uncertainty in the wintertime European climate response to the observed North Atlantic decadal variability, with models simulating opposite responses. Such inter-model inconsistency casts doubt on the physical link on this observed teleconnection and/or on the ability of the models to simulate it.

The origin of the North Atlantic decadal prediction skill has been linked to the predictability of the oceanic heat transport anomalies. In particular, the evolution the oceanic heat transport since 1950 is consistent with the idea of a lagged oceanic response to a specific long lasting (~10 years) atmospheric forcing during winter. To evaluate whether climate models are able to reproduce such mechanism, we performed experiments with three climate models in which additional surface heat, fresh water and momentum fluxes, characteristic of this specific atmospheric forcing, are given to the ocean component of the fully coupled models during 10 consecutive winters over the North Atlantic region. Then, the models are let freely adjusting to the atmospheric impulse forcing (i.e. integrated another 20 years after the end of the 10yr long forcing).

Results from those experiments show that for all models the persistent atmospheric forcing leads first to a subpolar North Atlantic cooling of about -0.3ºC followed by a ~0.3ºC warming. Yet, the timing of the response is very different between models, with a maximum warming simulated after 15 to 25 years, depending on the model. Such a disagreement casts doubt on the ability of current climate models to simulate the mechanisms controlling the AMV and its predictability.
The results from the INADEC project reveal the current obstacles that limit our ability to predict the climate impacts of the North Atlantic decadal variability. By exposing those limiting causes, including specific model shortcomings and decadal forecast initialization methods, those results will steer new research towards the critical points to ameliorate in order to improve decadal climate predictions and eventually provide accurate and valuable information for the industry/energy sectors as well as for policy makers.
Impacts of the observed AMV when imposed in numerical climate models