Predicting a decadal shift in North Atlantic climate variability using the GFDL forecast system
Rym Msadek, T. L. Delworth, A. Rosati, W. Anderson, G. A. Vecchi, Y.-S. Chang, K. Dixon, R. G. Gudgel, W. Stern, A. Wittenberg, X. Yang, F. Zeng, R. Zhang, and S. Zhang. Journal of Climate. DOI: 10.1175/JCLI-D-13-00476.1.
The goals of this research were to assess the role of AMOC in global climate and identify the predictability of the associated climate impacts. Decadal prediction experiments were conducted as part of CMIP5 using a prototype GFDL-CM2.1 forecast system. The abrupt warming of the North Atlantic subpolar gyre (SPG), observed in the mid-1990s, was used as a case study to evaluate the forecast capabilities of the model, and to better understand the reasons for the observed changes.
Initializing the CM2.1 coupled system results in a skillful prediction of the mid-1990s shift retrospectively, while the shift is not captured by the uninitialized forecasts. All the hindcasts initialized in the early 1990s show a warming of the SPG. However, only the ensemble mean hindcasts initialized in 1995 and 1996 are able to reproduce the observed abrupt warming and the associated decrease and contraction of the SPG.
Examination of the physical mechanisms responsible for the successful retrospective predictions indicates that initializing the ocean is key to predicting the mid-1990s warming. The successful initialized forecasts show an increased Atlantic Meridional Overturning Circulation and North Atlantic current transport, which drive an increased advection of warm saline subtropical waters northward, leading to a westward shift of the subpolar front and, subsequently, a warming and spin down of the SPG.
Significant seasonal climate impacts are predicted as the SPG warms, including a reduced sea-ice concentration over the Arctic, an enhanced warming over the central U.S. during summer and fall, and a northward shift of the mean ITCZ. These climate anomalies are similar to those observed during a warm phase of the Atlantic Multidecadal Oscillation. These results are promising, for future predictions of North Atlantic climate. Building confidence in decadal predictions involves both verifying that the prediction system can capture some predictable element of the climate system, and assessing that the model does it for physically sound reasons.