Propagation of Thermohaline Anomalies and their predictive potential along the Atlantic water pathway

We assess to what extent seven state-of-the-art dynamical prediction systems can retrospectively predict winter sea surface temperature (SST) in the subpolar North Atlantic and the Nordic Seas in the period 1970-2005. We focus on the region where warm water flows poleward, i.e., the Atlantic water pathway to the Arctic, and on interannual-to-decadal time scales. Observational studies demonstrate predictability several years in advance in this region, but we find that SST skill is low with significant skill only at lead time 1-2 years. To better understand why the prediction systems have predictive skill or lack thereof, we assess the skill of the systems to reproduce a spatio-temporal SST pattern based on observations. The physical mechanism underlying this pattern is a propagation of oceanic anomalies from low to high latitudes along the major currents; the North Atlantic Current and the Norwegian Atlantic Current. We find that the prediction systems have difficulties in reproducing this pattern. To identify whether the misrepresentation is due to incorrect model physics, we assess the respective uninitialized historical simulations. These simulations also tend to misrepresent the spatio-temporal SST pattern, indicating that the physical mechanism is not properly simulated. However, the representation of the pattern is slightly degraded in the predictions compared to historical runs, which could be a result of initialization shocks and forecast drift effects. Ways to enhance predictions, could be through improved initialization, and better simulation of poleward circulation of anomalies. This might require model resolutions in which flow over complex bathymetry and physics of mesoscale ocean eddies and their interactions with the atmosphere are resolved.

North Atlantic freshwater events influence European weather in subsequent summers

Amplified Arctic ice loss in recent decades has been linked to increased occurrence of extreme mid-latitude weather. The underlying dynamical mechanisms remain elusive, however. Here, we demonstrate a novel mechanism linking freshwater releases into the North Atlantic with summer weather in Europe. Combining remote sensing, atmospheric reanalyses and model simulations, we show that freshwater events in summer trigger progressively sharper sea surface temperature gradients in subsequent winters, destabilising the overlying atmosphere and inducing a northward shift in the North Atlantic Current. In turn, the jet stream over the North Atlantic is deflected northward in the following summers, leading to warmer and drier weather over Europe. Our results suggest that growing Arctic freshwater fluxes will increase the risk of heat waves and droughts over the coming decades, and could yield enhanced predictability of European summer weather, months to years in advance.

Revealing mechanisms of change in the Atlantic Meridional Overturning Circulation under global heating

<p>The North Atlantic ocean is key to climate through its role in heat transport and storage, but the response of the circulation’s drivers to a changing climate is poorly constrained. The transparent machine learning method Tracking global Heating with Ocean Regimes (THOR) identifies drivers of circulation with minimal input: depth, dynamic sea level and wind stress. Beyond a black box approach, THOR's predictive skill is transparent. A dataset is created with features engineered and labeled by an explicitly interpretable equation transform and k-means application. A multilayer perceptron is then trained, explaining its skill using relevance maps and theory. THOR reveals a weakened circulation with abrupt CO<sub>2</sub> quadrupling, due to a shift in deep water formation areas and locations of the Gulf Stream and Trans Atlantic Current transporting heat northward. If CO<sub>2</sub> is increased 1% yearly, similar but transient patterns emerge. THOR could accelerate model analysis and facilitate process oriented intercomparisons.</p>

Environmental variability off NE Greenland (western Fram Strait) during the past 10,600 years

To reconstruct the climatic and paleoceanographic variability offshore Northeast Greenland during the last ~10 ka with multidecadal resolution, sediment core PS93/025 from the outermost North-East Greenland continental shelf (80.5°N) was studied by a variety of micropaleontological, sedimentological and isotopic methods. High foraminiferal fluxes, together with high proportions of ice-rafted debris and high Ca/Fe ratios, indicate a maximum in bioproductivity until ~8 ka related to a low sea-ice coverage. Sortable silt values, planktic foraminifer associations, and stable isotope data of planktic and benthic foraminifers suggest a strong westward advection of relatively warm Atlantic Water by the Return Atlantic Current during this time, with a noticeable bottom current activity. This advection may have been facilitated by a greater water depth at our site, resulting from postglacial isostatic depression. For the following mid-Holocene interval (ca. 8–5 ka), isotope data, lower foraminiferal fluxes and a shift in grain size maxima point to a lasting but successively decreasing Atlantic Water inflow, a weakening productivity, and a growing sea-ice coverage which is also revealed by the PIIIIP25 index. A final stage in the environmental development was reached at ~5 ka with the establishment of pre-industrial conditions. Low Ca/Fe ratios, low foraminiferal fluxes, low sortable silt values and the sea-ice indicating PIIIIP25 index point to a limited productivity and a weak Atlantic Water inflow by the Return Atlantic Current to our research area, as well as a higher and/or seasonally more extended sea-ice coverage during the Late Holocene. Two intervals with somewhat enhanced Atlantic Water advection around 2.0 and 1.0 ka are indicated by slightly increased foraminiferal fluxes and the reoccurrence of subpolar foraminifers. These intervals may correlate with the Roman Warm Period and the Medieval Climate Anomaly, as defined in the North Atlantic region.