The importance of North Atlantic Ocean transports for seasonal forecasts
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The importance of North Atlantic Ocean transports for seasonal forecasts Steffen Tietsche1 · Magdalena Balmaseda1 · Hao Zuo1 · Christopher Roberts1 · Michael Mayer1 · Laura Ferranti1 Received: 14 November 2019 / Accepted: 6 July 2020 © The Author(s) 2020
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. Keywords Seasonal forecasts · AMOC · ocean reanalysis
1 Introduction 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. However, the slow variations of deep density-driven circulations such as the Atlantic Meridional Overturning Circulation (AMOC) modulate the atmospheric surface climate over large regions of the globe. Here, we show that the initialization of the AMOC can play a crucial role in providing skilful seasonal forecasts for the North Atlantic.
* Steffen Tietsche [email protected] 1
ECMWF, Shinfield Park, Reading RG2 9AX, UK
The AMOC has a major influence on European climate, because it transports vast amounts of heat from the tropics to high latitudes in the North Atlantic. The AMOC varies strongly on decadal or even longer time scales due to changes in water
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