Description: We examine the simulated Atlantic Multidecadal Variability (AMV) in a model that includes a correction for a longstanding problem with climate models, namely the misplacement of the North Atlantic Current. The corrected model shows that in the warm AMV phase, heat is lost by the ocean in the northwestern part of the basin and gained by the ocean to the east, suggesting an advective transfer of heat by the mid-latitude westerlies. The basin wide response is consistent with a role for cloud feedback and is in broad agreement with estimates from observations, but is poorly represented in the uncorrected model. The corrected model is then used to show that the ocean/atmosphere heat transfer is influenced by low frequency variability in the overlying atmosphere. We also argue that changing ocean heat transport is an essential feature of our results.

Description: Highlights: • Mid-Atlantic vent mussel populations are contemporarily isolated • Population connectivity can only be maintained in a stepwise manner • Four mussel lineages exist on the Mid-Atlantic Ridge • Recolonization of perturbed vent localities is uncertain Summary: Deep-sea hydrothermal vents are patchily distributed ecosystems inhabited by specialized animal populations that are textbook meta-populations. Many vent-associated species have free-swimming, dispersive larvae that can establish connections between remote populations. However, connectivity patterns among hydrothermal vents are still poorly understood because the deep sea is undersampled, the molecular tools used to date are of limited resolution, and larval dispersal is difficult to measure directly. A better knowledge of connectivity is urgently needed to develop sound environmental management plans for deep-sea mining. Here, we investigated larval dispersal and contemporary connectivity of ecologically important vent mussels (Bathymodiolus spp.) from the Mid-Atlantic Ridge by using high-resolution ocean modeling and population genetic methods. Even when assuming a long pelagic larval duration, our physical model of larval drift suggested that arrival at localities more than 150 km from the source site is unlikely and that dispersal between populations requires intermediate habitats (“phantom” stepping stones). Dispersal patterns showed strong spatiotemporal variability, making predictions of population connectivity challenging. The assumption that mussel populations are only connected via additional stepping stones was supported by contemporary migration rates based on neutral genetic markers. Analyses of population structure confirmed the presence of two southern and two hybridizing northern mussel lineages that exhibited a substantial, though incomplete, genetic differentiation. Our study provides insights into how vent animals can disperse between widely separated vent habitats and shows that recolonization of perturbed vent sites will be subject to chance events, unless connectivity is explicitly considered in the selection of conservation areas.

Description: Common problems in state-of-the-art climate models are a cold sea surface temperature (SST) bias in the equatorial Pacific and an underestimation of the two most important atmospheric feedbacks operating in the El Niño/Southern Oscillation (ENSO): the positive, i.e. amplifying wind-SST feedback and the negative, i.e. damping heat flux-SST feedback (Bellenger et al. 2014). To a large extent, the underestimation of those feedbacks can be explained by the cold equatorial SST bias, which shifts the rising branch of the Pacific Walker Circulation (PWC) too far to the west by up to 30 ◦ , resulting in an erroneous convective response during ENSO events (Bayr et al. 2018a). Based on simulations from the Kiel Climate Model (KCM) and the 5th phase of Coupled Model Intercomparison Project (CMIP5), we investigate how well ENSO dynamics are simulated in case of underestimated ENSO atmospheric feedbacks (EAF), with a special focus on ocean–atmosphere coupling over the equatorial Pacific. We present a new method based on an offline slab ocean SST that gives a quantitative measure about the error compensation between the wind-SST and the heat flux-SST feedbacks. We show by means of this method that ENSO is not predominantly wind-driven in many models, as observed; instead, and in contrast to observations, ENSO is significantly driven by a positive shortwave radiation feedback. Thus, although these models simulate ENSO, which in terms of simple indices is consistent with observations, it originates from very different dynamics. A too weak wind-driven oceanic forcing on the SST is compensated by weaker atmospheric heat flux damping. The latter is mainly caused by a biased shortwave-SST feedback that erroneously is positive in most climate models. In the most biased models, the shortwave-SST feedback contributes to the SST anomaly growth to a similar degree as the ocean circulation (Bayr et al. 2018b). Our results suggest that a broad continuum of ENSO dynamics exists in state-of-the-art climate models and explain why models with less than a half of the observed EAF strength can still exhibit realistic ENSO amplitude.

Description: Western Boundary Currents, such as the Gulf Stream, are regions of vivid air-sea interaction. Mesoscale features of these currents play a fundamental role in global ocean heat transport and exchange with the atmosphere. Related processes and their interactions across scales have gained increasing attention in the last years, since high-resolution, mesoscale-resolving modeling became computationally feasible on climate time scales. Here, we show the impact of explicitly resolving the oceanic mesoscale in the coupled global climate model FOCI on North Atlantic and European climate. For this purpose, we use the ocean nesting capability in FOCI, which facilitates regional ocean grid refinement. We explore and compare pre-industrial simulations each extending over at least 150 years: a reference run without any grid refinement and an experiment with a nest in the North Atlantic. Technically, the regional ocean nest maintains frequent two-way exchange with the global host grid, which in turn is fully coupled to the atmosphere model. The ocean model NEMO has a global resolution of 1/2˚ model with 46 vertical levels and 1/10˚ refinement in the nest region, while the atmosphere model ECHAM6 has a 1.8˚ horizontal resolution (T63) and 95 vertical levels, including the strato- and mesosphere. Within the nest region, the increased resolution leads to a more eddy-rich simulation and an improved mean state. The North Atlantic Current is considerably better represented, which reduces the typical North Atlantic cold bias from -8˚C in the reference run without nest to -2˚C. Beyond local bias correction of the mean state, we will also discuss the impact of explicitly modeling ocean mesoscale dynamics on atmospheric variability on different time scales, such as the North Atlantic Oscillation or the Atlantic Multidecadal Variability.