Predicting a Decadal Shift in North Atlantic Climate Variability Using the GFDL Forecast System

Abstract

Decadal prediction experiments were conducted as part of phase 5 of the Coupled Model Intercomparison Project (CMIP5) using the GFDL Climate Model, version 2.1 (CM2.1) forecast system. The abrupt warming of the North Atlantic Subpolar Gyre (SPG) that was observed in the mid-1990s is considered as a case study to evaluate forecast capabilities and better understand the reasons for the observed changes. Initializing the CM2.1 coupled system produces high skill in retrospectively predicting the mid-1990s shift, which 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 spindown 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 United States 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, which is encouraging for future predictions of North Atlantic climate.

Full-field initialized decadal predictions with the MPI earth system model: an initial shock in the North Atlantic

“…High correlation can only be found in case of ORAS4-ANOM with ACC = 0.7 , whereas ORAS4-FULL and GECCO2-FULL reveal only moderate correlations with ACC = 0.29 and 0.3, respectively. The considerable rapid warming in the 1990s, which was successfully predicted in the case studies of Robson et al (2012b), Yeager et al (2012), andMsadek et al (2014), is best predicted by the ORAS4-ANOM hindcasts.…”

“…High forecast skill for both SST and OHC in the North Atlantic subpolar gyre (SPG) region arises in part from the correct initialization of the ocean flow Robson et al 2012a, b;Yeager et al 2012;Robson et al 2013Robson et al , 2014Msadek et al 2014). Unfortunately, direct observations of oceanic flow are sparse, therefore estimates of initial flow conditions are not well constrained and vary considerably due to the following reasons: First of all, ocean state estimates or reanalyses (in the following we will use both terms interchangeably) do not provide a coherent picture of the ocean flow (e.g., Karspeck et al 2015); on top of that, it is unlikely that their flow characteristics are well adopted by the prediction system in the assimilation step .…”

The Decadal Climate Prediction Project (DCPP) contribution to CMIP6

“…Some CMIP5 decadal hindcasts successfully predicted this event (Robson et al, 2012;Yeager et al, 2012;Msadek et al, 2014) together with some aspects of associated climate impacts (Robson et al, 2013;Smith et al, 2010). The proposed experiments will investigate in more detail the role of initialization of the Atlantic subpolar gyre.…”

Recent Progress in Understanding and Predicting Atlantic Decadal Climate Variability

“…The subpolar North Atlantic (SPNA; comprising roughly the cyclonic ocean gyre north of about 50°N) is consistently the region with the largest relative improvement in surface temperature skill due to initialization, especially beyond the first few years of predictions [124, 130•, 131, 132, 135]. The improvement in skill is, in part, due to the initialization, and persistence, of substantial low-frequency variability of ocean heat content [136,137]. However, several studies have shown significant improvements in the skill of upper ocean heat content and SST beyond persistence, especially for lead times longer than a few years [131,136].…”