Johanna Baehr

Multiyear Prediction of Monthly Mean Atlantic Meridional Overturning Circulation at 26.5°N

Stable Flow Whole-ocean deep circulation in the Atlantic, the Atlantic Meridional Overturning Circulation (AMOC), transports great quantities of heat from low latitudes to higher ones, which, for example, helps to warm Europes climate. Matei et al. (p. 76) describe a modeling technique that allows AMOC strength to be predicted for up to 4 years in advance and suggests that AMOC should remain stable until at least 2014. The strength of an ocean current that transports heat to Europe can be predicted up to 4 years in advance. Attempts to predict changes in Atlantic Meridional Overturning Circulation (AMOC) have yielded little success to date. Here, we demonstrate predictability for monthly mean AMOC strength at 26.5°N for up to 4 years in advance. This AMOC predictive skill arises predominantly from the basin-wide upper-mid-ocean geostrophic transport, which in turn can be predicted because we have skill in predicting the upper-ocean zonal density difference. Ensemble forecasts initialized between January 2008 and January 2011 indicate a stable AMOC at 26.5°N until at least 2014, despite a brief wind-induced weakening in 2010. Because AMOC influences many aspects of climate, our results establish AMOC as an important potential carrier of climate predictability.

Seasonal predictability over Europe arising from El Niño and stratospheric variability in the MPI-ESM seasonal prediction system

AbstractPredictability 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 Nino–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 Nino 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 Nino teleconnection. In the seasonal prediction system, the evolution of El Nino events is well captured for lead times of up to 6 months, and stratospheric variability is re...

Timely Detection of Changes in the Meridional Overturning Circulation at 26°N in the Atlantic

It is investigated how changes in the North Atlantic meridional overturning circulation (MOC) might be reliably detected within a few decades, using the observations provided by the RAPID-MOC 26°N array. Previously, detectability of MOC changes had been investigated with a univariate MOC time series exhibiting strong internal variability, which would prohibit the detection of MOC changes within a few decades. Here, a modification of K. Hasselmann’s fingerprint technique is used: (simulated) observations are projected onto a time-independent spatial pattern of natural variability to derive a time-dependent detection variable. The fixed spatial pattern of natural variability is derived by regressing the zonal density gradient along 26°N against the strength of the MOC at 26°N within the coupled ECHAM5/Max Planck Institute Ocean Model’s (MPI-OM) control climate simulation. This pattern is confirmed against the observed anomalies found between the 1957 and the 2004 hydrographic occupations of the section. Onto this fixed spatial pattern of natural variability, both the existing hydrographic observations and simulated observations mimicking the RAPID-MOC 26°N array in three realizations of the Intergovernmental Panel on Climate Change (IPCC) scenario A1B are projected. For a random observation error of 0.01 kg m 3 , and only using zonal density gradients between 1700- and 3100-m depth, statistically significant detection occurs with 95% reliability after about 30 yr, in the model and climate change scenario analyzed here. Compared to using a single MOC time series as the detection variable, continuous observations of zonal density gradients reduce the detection time by 50%. For the five hydrographic occupations of the 26°N transect, none of the analyzed depth ranges shows a significant trend between 1957 and 2004, implying that there was no MOC trend over the past 50 yr.

Optimization of an observing system design for the North Atlantic meridional overturning circulation

Abstract Three methods are analyzed for the design of ocean observing systems to monitor the meridional overturning circulation (MOC) in the North Atlantic. Specifically, a continuous monitoring array to monitor the MOC at 1000 m at different latitudes is “deployed” into a numerical model. The authors compare array design methods guided by (i) physical intuition (heuristic array design), (ii) sequential optimization, and (iii) global optimization. The global optimization technique can recover the true global solution for the analyzed array design, while gradient-based optimization would be prone to misconverge. Both global optimization and heuristic array design yield considerably improved results over sequential array design. Global optimization always outperforms the heuristic array design in terms of minimizing the root-mean-square error. However, whether the results are physically meaningful is not guaranteed; the apparent success might merely represent a solution in which misfits compensate for each ...

The prediction of surface temperature in the new seasonal prediction system based on the MPI-ESM coupled climate model

AbstractA seasonal forecast system is presented, based on the global coupled climate model MPI-ESM as used for CMIP5 simulations. We describe the initialisation of the system and analyse its predictive skill for surface temperature. The presented system is initialised in the atmospheric, oceanic, and sea ice component of the model from reanalysis/observations with full field nudging in all three components. For the initialisation of the ensemble, bred vectors with a vertically varying norm are implemented in the ocean component to generate initial perturbations. In a set of ensemble hindcast simulations, starting each May and November between 1982 and 2010, we analyse the predictive skill. Bias-corrected ensemble forecasts for each start date reproduce the observed surface temperature anomalies at 2–4 months lead time, particularly in the tropics. Niño3.4 sea surface temperature anomalies show a small root-mean-square error and predictive skill up to 6 months. Away from the tropics, predictive skill is mostly limited to the ocean, and to regions which are strongly influenced by ENSO teleconnections. In summary, the presented seasonal prediction system based on a coupled climate model shows predictive skill for surface temperature at seasonal time scales comparable to other seasonal prediction systems using different underlying models and initialisation strategies. As the same model underlying our seasonal prediction system—with a different initialisation—is presently also used for decadal predictions, this is an important step towards seamless seasonal-to-decadal climate predictions.

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.

Detection and attribution of climate change signal in ocean wind waves

AbstractSurface waves in the ocean respond to variability and changes of climate. Observations and modeling studies indicate trends in wave height over the past decades. Nevertheless, it is currently impossible to discern whether these trends are the result of climate variability or change. The output of an Earth system model (EC-EARTH) produced within phase 5 of the Coupled Model Intercomparison Project (CMIP5) is used here to force a global Wave Model (WAM) in order to study the response of waves to different climate regimes. A control simulation was run to determine the natural (unforced) model variability. A simplified fingerprint approach was used to calculate positive and negative limits of natural variability for wind speed and significant wave height, which were then compared to different (forced) climate regimes over the historical period (1850–2010) and in the future climate change scenario RCP8.5 (2010–2100). Detectable climate change signals were found in the current decade (2010–20) in the No...

Potential Predictability of the North Atlantic Heat Transport Based on an Oceanic State Estimate

AbstractThis paper investigates the potential predictability of the meridional heat transport (MHT) in the North Atlantic on interannual time scales using hindcast ensembles based on an oceanic data assimilation product. The work analyzes the prognostic potential predictability (PPP), using the ocean synthesis of the German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) as initial conditions and as boundary conditions. The PPP of the MHT varies with latitude: local maxima are apparent within the subpolar and the subtropical gyres, and a minimum is apparent at the boundary between the gyres. This PPP minimum can also be seen in the PPP structure of the Atlantic meridional overturning circulation (AMOC), although it is considerably less pronounced. The decomposition of the MHT shows that within the subpolar gyre, the gyre component of the MHT influences the PPP structure of the MHT. Within the subtropical gyre, the overturning component of the MHT characterizes the...

Human-Induced Climate Change: What is the economic value of information about climate thresholds?

The field of integrated assessment of climate change is undergoing a paradigm shift towards the analysis of potentially abrupt and irreversible climate changes (Alley et al., 2003; Keller et al., 2006b). Early integrated studies broke important new ground in exploring the relationship between the costs and benefits of reducing carbon dioxide (CO2) emissions (e.g., Nordhaus, 1991; Manne and Richels, 1991; or Tol, 1997). These studies project the climate response to anthropogenic CO2 emissions to be relatively smooth and typically conclude that the projected benefits of reducing CO2 emissions would justify only small reductions in CO2 emissions in a cost–benefit framework. The validity of the oftenassumed smooth climate response is, however, questionable, given how the climate system has responded to forcing in the geological past. Before the Anthropocene, the geological time period where humans have started to influence the global biogeochemical cycles considerably (Crutzen, 2002), the predominant responses of the climate system were forced by small changes in solar insolation occurring on timescales of thousands of years (Berger and Loutre, 1991). Yet this slow and smooth forcing apparently triggered abrupt climate changes – a threshold response where the climate system moved between different basins of attraction (Berger, 1990; Clement et al., 2001). Anthropogenic forcing may trigger climate threshold responses in the future (Alley et al., 2003; Keller et al., 2006b). Examples of such threshold responses include (i) a collapse of the North Atlantic meridional overturning circulation (Rahmstorf, 2000; Stommel, 1961), (ii) a disintegration of the West Antarctic Ice Sheet (Mercer, 1978; Oppenheimer, 1998), (iii) abrupt vegetation changes (Claussen et al., 1999; Scheffer et al., 2001), or (iv) changes in properties of the El Nino Southern Oscillation, ENSO (Fedorov and Philander, 2000; Timmermann, 2001). Here we focus on the first two examples: a possible collapse of the North Atlantic meridional overturning circulation and a possible disintegration of the West Antarctic Ice Sheet. Our analysis addresses three main questions. (i) What underlying mechanisms define a climate threshold? (ii) What are the key scientific uncertainties in predicting whether these thresholds may be crossed in the future? (iii) What might be reasonable order-of-magnitude estimates of the expected economic value of reducing these uncertainties? Our analysis suggests that climate strategies designed to reduce the risk of crossing climate thresholds have to be designed in the face of large uncertainties. Some of the key uncertainties (e.g., the sensitivity of the meridional overturning circulation to anthropogenic greenhouse gas emissions) can be reduced by observation systems. We conclude by identifying future research needs.

Simulations of a Line W-based observing system for the Atlantic meridional overturning circulation

In a series of observing system simulations, we test whether the Atlantic meridional overturning circulation (AMOC) can be observed based on the existing Line W deep western boundary array. We simulate a Line W array, which is extended to the surface and to the east to cover the basin to the Bermuda Rise. In the analyzed ocean circulation model ORCA025, such an extended Line W array captures the main characteristics of the western boundary current. Potential trans-basin observing systems for the AMOC are tested by combining the extended Line W array with a mid-ocean transport estimate obtained from thermal wind “measurements” and Ekman transport to the total AMOC (similarly to Hirschi et al., Geophys Res Lett 30(7):1413, 2003). First, we close Line W zonally supplementing the western boundary array with several “moorings” in the basin (Line W-32°N). Second, we supplement the western boundary array with a combination of observations at Bermuda and the eastern part of the RAPID array at 26°N (Line W-B-RAPID). Both, a small number of density profiles across the basin and also only sampling the eastern and western boundary, capture the variability of the AMOC at Line W-32°N and Line W-B-RAPID. In the analyzed model, the AMOC variability at both Line W-32°N and Line W-B-RAPID is dominated by the western boundary current variability. Away from the western boundary, the mid-ocean transport (east of Bermuda) shows no significant relation between the two Line W-based sections and 26°N. Hence, a Line W-based AMOC estimate could yield an estimate of the meridional transport that is independent of the 26°N RAPID estimate. The model-based observing system simulations presented here provide support for the use of Line W as a cornerstone for a trans-basin AMOC observing system.