Abstract
The Atlantic Meridional Overturning Circulation (AMOC) is a main driver for predictability at decadal time scales, but has been largely ignored in the context of seasonal forecasts. Here, we show compelling evidence that AMOC initialization can have a direct and strong impact on seasonal forecasts. Winter reforecasts with SEAS5, the current operational seasonal forecasting system by the European Centre for Medium-Range Weather Forecasts, exhibit errors of sea-surface temperature (SST) in the western part of the North Atlantic Subpolar Gyre that are strongly correlated with decadal variations in the AMOC initial conditions. In the early reforecast period 1981–1996, too warm SST coincide with an overly strong AMOC transporting excessive heat into the region. In the ocean reanalyses providing the forecast initial conditions, excessive heat transport is balanced by additional surface cooling from relaxing towards observed SST, and therefore the fit to observations is acceptable. However, the additional surface cooling contributes to enhanced deep convection and strengthens the AMOC, thereby establishing a feedback loop. In the forecasts, where the SST relaxation is absent, the balance is disrupted, and fast growth of SST errors ensues. The warm SST bias has a strong local impact on surface air temperature, mean sea-level pressure, and precipitation patterns, but remote impact is small. In the late reforecast period 2001–2016, neither the SST in the western North Atlantic nor the AMOC show large biases. The non-stationarity of the bias prevents an effective forecast calibration and causes an apparent loss of skill in the affected region. The case presented here demonstrates the importance of correctly initializing slowly varying aspects of the Earth System such as the AMOC in order to improve forecasts on seasonal and shorter time scales.
Highlights
The general importance of ocean processes for seasonal forecasts has long been acknowledged, but slow processes with time scales of years or even decades are often assumed not to be of relevance
We demonstrate that the sea-surface temperature (SST) relaxation and warm bias are tightly coupled to the strength of the Atlantic Meridional Overturning Circulation (AMOC) in the reanalyses that provide the forecast initial conditions
We have discussed low frequency variations of winter SST errors over a region north-east of the Grand Banks of Newfoundland that occur in reforecasts with SEAS5
Summary
The general importance of ocean processes for seasonal forecasts has long been acknowledged, but slow processes with time scales of years or even decades are often assumed not to be of relevance. Because of the long time scale of its thermohaline component, the mean AMOC in seasonal forecasts is extremely close to the AMOC in the initial conditions, whereas other aspects in the forecasts can change more quickly. This can lead to imbalances in the forecast. Balanced initialization of slow components of the Earth system is important if a long history of reforecasts is to be used for calibrating a seasonal forecasting system and assessing its skill.
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