Future changes to the upper ocean Western Boundary Currents across two generations of climate models

Western Boundary Currents (WBCs) are important for the oceanic transport of heat, dissolved gases and nutrients. They can affect regional climate and strongly influence the dispersion and distribution of marine species. Using state-of-the-art climate models from the latest and previous Climate Model Intercomparison Projects, we evaluate upper ocean circulation and examine future projections, focusing on subtropical and low-latitude WBCs. Despite their coarse resolution, climate models successfully reproduce most large-scale circulation features with ensemble mean transports typically within the range of observational uncertainty, although there is often a large spread across the models and some currents are systematically too strong or weak. Despite considerable differences in model structure, resolution and parameterisations, many currents show highly consistent projected changes across the models. For example, the East Australian Current, Brazil Current and Agulhas Current extensions are projected to intensify, while the Gulf Stream, Indonesian Throughflow and Agulhas Current are projected to weaken. Intermodel differences in most future circulation changes can be explained in part by projected changes in the large-scale surface winds. In moving to the latest model generation, despite structural model advancements, we find little systematic improvement in the simulation of ocean transports nor major differences in the projected changes

CMIP5 Intermodel Relationships in the Baseline Southern Ocean Climate System and With Future Projections

This is the final version. Available on open access from Wiley via the DOI in this recordClimate models exhibit a broad range in the simulated properties of the climate system. In the early historical period, the absolute global mean surface air temperature in Coupled Model Intercomparison Project, phase 5 (CMIP5) models spans a range of ~12-15 °C. Other climate variables may be linked to global mean temperature, and so accurate representation of the baseline climate state is crucial for meaningful future climate projections. In CMIP5 baseline climate states, statistically significant intermodel correlations between Southern Ocean surface temperature, outgoing shortwave radiation, cloudiness, the position of the mid-latitude eddy-driven jet, and Antarctic sea ice area are found. The baseline temperature relationships extend to projected future changes in the same set of variables. The tendency for models with initially cooler Southern Ocean to exhibit more global warming, and vice versa for initially warmer models, is linked to baseline Southern Ocean climate system biases. Some of these intermodel correlations arise due to a ‘capacity for change’. For example, models with more sea ice initially have greater capacity to lose sea ice as the planet warms, whereas models with little sea ice initially are constrained in the amount they can lose. Similar constraints apply to Southern Ocean clouds, which are projected to reduce under radiative forcing, and the jet latitude, which is projected to migrate poleward. A first look at emerging data from CMIP6 reveals a shift of the relationship from the Southern Ocean towards the Antarctic region, possibly due to reductions in Southern Ocean biases, such westerly wind representation.Natural Environment Research Council (NERC)Centre for Southern Hemisphere Oceans ResearchAustralian Government National Environmental Science ProgramAustralian Research Council (ARC

Enhanced climate instability in the North Atlantic and southern Europe during the Last Interglacial.

Considerable ambiguity remains over the extent and nature of millennial/centennial-scale climate instability during the Last Interglacial (LIG). Here we analyse marine and terrestrial proxies from a deep-sea sediment sequence on the Portuguese Margin and combine results with an intensively dated Italian speleothem record and climate-model experiments. The strongest expression of climate variability occurred during the transitions into and out of the LIG. Our records also document a series of multi-centennial intra-interglacial arid events in southern Europe, coherent with cold water-mass expansions in the North Atlantic. The spatial and temporal fingerprints of these changes indicate a reorganization of ocean surface circulation, consistent with low-intensity disruptions of the Atlantic meridional overturning circulation (AMOC). The amplitude of this LIG variability is greater than that observed in Holocene records. Episodic Greenland ice melt and runoff as a result of excess warmth may have contributed to AMOC weakening and increased climate instability throughout the LIG

Evaluating ENSO teleconnections using observations and CMIP5 models

Bias correction of global and regional climate models is essential for credible climate change projections. This study examines the bias of the models of the Coupled Model Inter-comparison Project Phase 5 (CMIP5) in their simulation of the spatial pattern of sea surface temperature (SSTs) in different phases of the El Niño Southern Oscillation (ENSO) and their teleconnections—highlighting the strengths and weaknesses of the models in different oceanic sectors. The comparison between the model outputs and the observations focused on the following three features: (i) the typical horseshoe pattern seen in the Pacific Ocean during ENSO events with anomalies in SSTs opposite to the warm/cool tongue, (ii) different signature in the tropical Pacific Ocean from that of the North and tropical Atlantic Ocean, and (iii) spurious signature in the southern hemisphere beyond 45° S. Using these three cases, it was found that the model simulations poorly matched the observations, indicating that more attention is needed on the tropical/extratropical teleconnections associated with ENSO. More importantly, the observed SST coupling between the tropical Pacific Ocean and the Atlantic Ocean is missing in almost all models, and differentiating the models between high/low top did not improve the results. It also found that SSTs in the tropical Pacific Ocean are relatively well simulated when compared with observation. This work has improved our understanding of the simulation of ENSO and its teleconnections in the CMIP5 models and has raised awareness of the bias existing in the models, which requires further attention by climate modellers. © 2018 The Author(s

Sensitivity of South American summer rainfall to tropical Pacific Ocean SST anomalies

A suite of idealised ensemble experiments are used to investigate the sensitivity of the Southern Hemisphere atmospheric circulation to the location of SST anomalies over the tropical Pacific Ocean. Of particular interest is the response of South American rainfall during austral summer. The experiments reveal an approximately opposite response in rainfall over South America when the tropical SST forcing is applied over the western and eastern Pacific Ocean, respectively. The contrasting tropical rainfall conditions are due primarily to the displaced Walker circulation anomalies. Further south, the atmospheric circulation over the South American subtropics is modulated by teleconnection patterns that appear as a wave train. The resulting circulation manifests as an anomalous cyclone over central‐eastern South America which in turn leads to a northward displacement of the westerly moisture transport when the SST forcing is located further west. The opposite pattern occurs when the SST forcing is applied to the eastern equatorial Pacific