Monitoring the meridional overturning circulation in the North Atlantic: A model-based array design study

A monitoring system for the meridional overturning circulation (MOC) is deployed into an eddy-permitting numerical model (FLAME) at three different latitudes in the North Atlantic Ocean. The MOC is estimated by adding contributions related to Ekman transports to those associated with the zonally integrated vertical velocity shear. Ekman transports are inferred from surface wind stress, whereas the velocity shear is derived from continuous density observations, principally near the eastern and western boundaries, employing thermal wind balance. The objective is to test the method and array setups for possible real observation in the ocean at the chosen latitudes and to guide similar tests at different latitudes. Different mooring placements are tested, ranging from a minimal setup to the theoretical maximum number of measurements. A relatively small number of vertical density profiles (about 10, the exact number depending on the latitude) can achieve a reconstruction of the MOC similar to one achieved by any larger number of profiles. However, the main characteristics of the MOC can only be reproduced at latitudes where bottom velocities are small, here at 26N and 36N. For high bottom velocities, in FLAME at 53N, the array fails to reproduce the strength and variability of the MOC because the depth-averaged flow cannot be reconstructed accurately. In FLAME, knowledge of the complete bottom velocity field could substitute for the knowledge of the depth-averaged velocity field

Impact of observed North Atlantic multidecadal variations to European summer climate: a linear baroclinic response to surface heating

The observed prominent multidecadal variations in the central to eastern (C--E) European summer temperature are closely related to the Atlantic multidecadal variability (AMV). Using the Twentieth Century Reanalysis project version 2 data for the period of 1930--2012, we present a mechanism by which the multidecadal variations in the C--E European summer temperature are associated to a linear baroclinic atmospheric response to the AMV-related surface heat flux. Our results suggest that over the north-western Atlantic, the positive heat flux anomaly triggers a surface baroclinic pressure response to diabatic heating with a negative surface pressure anomaly to the east of the heat source. Further downstream, this response induces an east-west wave-like pressure anomaly. The east-west wave-like response in the sea level pressure structure, to which we refer as North-Atlantic-European East West (NEW) mode, is independent of the summer North Atlantic Oscillation and is the principal mode of variations during summer over the Euro-Atlantic region at multidecadal time scales. The NEW mode causes warming of the C--E European region by creating an atmospheric blocking-like situation. Our findings also suggest that this NEW mode is responsible for the multidecadal variations in precipitation over the British Isles and north-western Europe

Skillful decadal prediction of German Bight storm activity

We evaluate the prediction skill of the Max Planck Institute Earth System Model (MPI-ESM) decadal hindcast system for German Bight storm activity (GBSA) on a multiannual to decadal scale. We define GBSA every year via the most extreme 3-hourly geostrophic wind speeds, which are derived from mean sea-level pressure (MSLP) data. Our 64-member ensemble of annually initialized hindcast simulations spans the time period 1960–2018. For this period, we compare deterministically and probabilistically predicted winter MSLP anomalies and annual GBSA with a lead time of up to 10 years against observations. The model produces poor deterministic predictions of GBSA and winter MSLP anomalies for individual years but fair predictions for longer averaging periods. A similar but smaller skill difference between short and long averaging periods also emerges for probabilistic predictions of high storm activity. At long averaging periods (longer than 5 years), the model is more skillful than persistence- and climatology-based predictions. For short aggregation periods (4 years and less), probabilistic predictions are more skillful than persistence but insignificantly differ from climatological predictions. We therefore conclude that, for the German Bight, probabilistic decadal predictions (based on a large ensemble) of high storm activity are skillful for averaging periods longer than 5 years. Notably, a differentiation between low, moderate, and high storm activity is necessary to expose this skill

ENSO hindcast skill in the DWD - MPI-M - UHH seasonal prediction system

KlimawandelWe present an assessment of the El Niño Southern Oscillation (ENSO) hindcast skill in the DWD - MPI-M - UHH seasonal prediction system based on the earth system model MPI-ESM. The system is initialised from re-analysis in the atmospheric, oceanic and sea-ice component of the model. We use a hindcast ensemble with semi-annual start dates between 1981 and 2014 (10 member ensembles started every May and November for 6 months each). We find hindcast skill for Niño 3.4 sea surface temperatures up to 6 months ahead. Hindcast skill is higher for November start dates than for May start dates. In addition to the Niño 3.4 Index, we also assess hindcast skill for Niño3, the West Pacific Warm Water Volume and the zonal wind variability. In particular we focus on the difference in the hindcast skill in the May start dates for the 1997/98 and the 2014 November conditions - though for these two periods overall similar conditions were observed, the subsequent development with a strong El Niño in 1997/98 and a very weak El Niño in 2014 differed considerably

Jahreszeitenvorhersagen mit dem DWD - MPI-M - UHH Vorhesagesystem

KlimawandelWir stellen erste Ergebnisse des präoperationellen Systems für Jahreszeitenvorhersagen vor. Dieses System wurde gemeinsam vom Deutschen Wetterdienst, dem MPI-M und der Universität Hamburg entwickelt und beruht auf dem MPI-ESM-LR in einer CMIP5 nahen Version. Es soll als deutscher Beitrag zum internationalen Multi-Modell Ensemble EUROSIP beitragen. Jahreszeitenvorhersagen werden als probabilistische Trendprognosen erstellt. Es ist deshalb erforderlich, für jeden Vorhersagezeitraum eine Klimatologie im Ensemble zur Verfügung zu haben, sogenannte Hindcasts. Die Erzeugung der Anfangsbedingungen der Hindcasts und Forecasts geschieht durch Datenassimilation, hier durch Nudging wichtiger Variablen in Atmosphäre, Ozean und Meereis. Das Ensemble wird durch die Technik des Breedings und der Variation eines Physikparameters generiert (Baehr et al, 2014). Diskutiert werden Qualitätsmetriken (skill scores) des Systems. Vorgestellt werden auch Prozeßstudien zur Vorhersagefähigkeit des Klima-Vorhersagesystem im Hinblick auf verschiedene relevante saisonale Ereignisse

Initialization and Ensemble Generation for Decadal Climate Predictions: A Comparison of Different Methods

Five initialization and ensemble generation methods are investigated with respect to their impact on the prediction skill of the German decadal prediction system “Mittelfristige Klimaprognose” (MiKlip). Among the tested methods, three tackle aspects of model‐consistent initialization using the ensemble Kalman filter, the filtered anomaly initialization, and the initialization method by partially coupled spin‐up (MODINI). The remaining two methods alter the ensemble generation: the ensemble dispersion filter corrects each ensemble member with the ensemble mean during model integration. And the bred vectors perturb the climate state using the fastest growing modes. The new methods are compared against the latest MiKlip system in the low‐resolution configuration (Preop‐LR), which uses lagging the climate state by a few days for ensemble generation and nudging toward ocean and atmosphere reanalyses for initialization. Results show that the tested methods provide an added value for the prediction skill as compared to Preop‐LR in that they improve prediction skill over the eastern and central Pacific and different regions in the North Atlantic Ocean. In this respect, the ensemble Kalman filter and filtered anomaly initialization show the most distinct improvements over Preop‐LR for surface temperatures and upper ocean heat content, followed by the bred vectors, the ensemble dispersion filter, and MODINI. However, no single method exists that is superior to the others with respect to all metrics considered. In particular, all methods affect the Atlantic Meridional Overturning Circulation in different ways, both with respect to the basin‐wide long‐term mean and variability and with respect to the temporal evolution at the 26° N latitude

Improved Predictions for the Arctic and connections to the Midlatitudes

Several Blue-Action partners attended the 2017 Arctic Circle event http://www.arcticcircle.org/ in Reykjavik, Iceland, October 13-15. These are the sessions we have been contributing to: http://www.blue-action.eu/index.php?id=4011 More information about the event and full programme: http://www.arcticcircle.org/ Keywords: Blue-Action, Blue Growth, Improved Predictions, Arctic, northern hemisphere, weather, climate, Climate Services, business strategy, Climate service innovation, climate-resilience, Arctic business, H2020, climate actio

Seasonal Predictability over Europe Arising from El Nino and Stratospheric Variability in the MPI-ESM Seasonal Prediction System

Predictability on seasonal time scales over the North Atlantic–Europe region is assessed using a seasonal prediction system based on an initialized version of the Max Planck Institute Earth System Model (MPI-ESM). For this region, two of the dominant predictors on seasonal time scales are El Niño–Southern Oscillation (ENSO) and sudden stratospheric warming (SSW) events. Multiple studies have shown a potential for improved North Atlantic predictability for either predictor. Their respective influences are however difficult to disentangle, since the stratosphere is itself impacted by ENSO. Both El Niño and SSW events correspond to a negative signature of the North Atlantic Oscillation (NAO), which has a major influence on European weather. This study explores the impact on Europe by separating the stratospheric pathway of the El Niño teleconnection. In the seasonal prediction system, the evolution of El Niño events is well captured for lead times of up to 6 months, and stratospheric variability is reproduced with a realistic frequency of SSW events. The model reproduces the El Niño teleconnection through the stratosphere, involving a deepened Aleutian low connected to a warm anomaly in the northern winter stratosphere. The stratospheric anomaly signal then propagates downward into the troposphere through the winter season. Predictability of 500-hPa geopotential height over Europe at lead times of up to 4 months is shown to be increased only for El Niño events that exhibit SSW events, and it is shown that the characteristic negative NAO signal is only obtained for winters also containing major SSW events for both the model and the reanalysis data