ALBERT

Description: The variability of the Atlantic meridional overturning circulation (AMOC) and its governing processes during the Last Glacial Maximum (LGM) is investigated in the Kiel Climate Model. Under LGM conditions, multidecadal AMOC variability is mainly forced by the surface heat flux variability linked to the East Atlantic pattern (EAP). In contrast, the multidecadal AMOC variability under preindustrial conditions is mainly driven by the surface heat flux variability associated with the North Atlantic Oscillation. Stand-alone atmosphere model experiments show that relative to preindustrial conditions, the change in AMOC forcing under LGM conditions is tightly linked to the differences in topography.

Description: Stable oxygen isotope records from central Greenland suggest disproportionally large long‐term surface air temperature (SAT) variability during the Last Glacial Maximum (LGM) relative to preindustrial times. Large perturbations in mean atmospheric circulation and its variability forced by extensive Northern Hemisphere ice sheet coverage have been suggested as cause for the enhanced Greenland SAT variability. Here, we assess the factors driving Greenland SAT variability during the LGM by means of dedicated climate model simulations and find remote forcing from the Pacific of critical importance. Atmospheric teleconnections from the Interdecadal Pacific Oscillation (IPO), a multidecadal oscillation of sea surface temperature in the Pacific Ocean, strongly intensify under LGM conditions, driving enhanced surface wind variability over Greenland, which in turn amplifies SAT variability by anomalous atmospheric heat transport. A major role of the IPO in forcing Greenland SAT variability also is supported by a number of models from the Paleoclimate Modeling Intercomparison Project Phase III.

Description: Plain Language Summary: Stable oxygen isotope records, a proxy for the local surface air temperature (SAT), from central Greenland indicate disproportionally large reductions in the multidecadal variability from the Last Ice Age (Last Glacial Maximum, LGM; about 21,000 years before present) to modern times. A climate model simulates the changes in multidecadal Greenland SAT variability as inferred from the proxy data. The enhanced variability during the LGM is largely remotely driven by the Interdecadal Pacific Oscillation (IPO), a multidecadal oscillation of sea surface temperature (SST) in the Pacific Ocean. Atmospheric teleconnections from the IPO strongly intensify under glacial conditions, driving enhanced surface wind variability over Greenland and through atmospheric heat transport the SAT variability.

Description: Key Points: Oxygen isotope records and climate modeling show large reductions in Greenland surface temperature variability from the LGM to modern times Atmospheric teleconnections from the Interdecadal Pacific Oscillation intensify under glacial conditions Greenland surface temperature is forced by atmospheric heat transport and sea ice linked to the Interdecadal Pacific Oscillation

Description: Dommenget (2010) found that El Niño-like variability, termed Slab Ocean El Niño, can exist in the absence of ocean dynamics and is driven by the interaction of the atmospheric surface heat fluxes and the heat content of the upper ocean. Further, Dommenget et al. (2014) report the Slab Ocean El Niño is not an artefact of the ECHAM5-AGCM coupled to a slab ocean model. In fact, atmospheric feedbacks crucial to the Slab Ocean El Niño can also be found in many state-of-the-art coupled climate models participating in CMIP3 and CMIP5, so that ENSO in many CMIP models can be understood as a mixed recharge oscillator/Slab Ocean El Niño mode. Here we show further analysis of the Slab Ocean El Niño atmospheric feedbacks in coupled models. The BCCR_CM2.0 climate model from the CMIP3 data base, which has a very large equatorial cold bias, has an El Niño that is purely driven by Slab Ocean El Niño atmospheric feedbacks and is used as an example to describe Slab Ocean El Niño atmospheric feedbacks in a coupled model. In the BCCR_CM2.0, the ENSO-related variability in the 20°C isotherm (Z20), a measure of upper ocean heat content, is decoupled from the first mode of the seasonal cycle-related variability, while the two are coupled in observations, with ENSO being phase-locked to the seasonal cycle. Further analysis of the seasonal cycle in Z20 using SODA Ocean Reanalysis reveals two different regimes in the seasonal cycle along the equator: The first regime, to which ENSO is phase-locked, extends over the west and central equatorial Pacific and is driven by subsurface ocean dynamics. The second regime, extending in observations only over the cold tongue region, is driven by the seasonal cycle at the sea surface and is shifted by roughly six months relative to the first regime. In a series of experiments with the Kiel Climate Model (KCM) with different mean states due to tuning in the convection parameters, we can show that the strength of the equatorial cold bias and the coupling strength between the seasonal cycle of Z20 and ENSO are anti-correlated, i.e. a strong equatorial cold bias suppresses recharge oscillator dynamics and enhances Slab Ocean El Niño atmospheric feedbacks, supporting the results from the BCCR_CM2.0. This can be explained as with a stronger cold bias the second regime of the seasonal cycle in Z20, which extends in observations only over the small cold tongue region, expands westward and becomes more important, so that it decouples ENSO from the seasonal cycle in Z20. This has implications for some major characteristics of the ENSO like the propagation of SST anomalies, the phase locking of SST to the seasonal cycle, or the nonlinearity of ENSO. Dommenget, D., 2010: The slab ocean El Niño. Geophys. Res. Lett., 37, L20701, doi:10.1029/2010GL044888. ——, S. Haase, T. Bayr, and C. Frauen, 2014: Analysis of the Slab Ocean El Niño atmospheric feedbacks in observed and simulated ENSO dynamics. Clim. Dyn., doi:10.1007/s00382-014-2057-0.

Description: Die Weltmeere sind die Grundlage unserer Existenz und unsere wichtigste Ressource. Und sie sind der Ursprung allen Lebens auf der Erde, eine faszinierende und viel- fach noch völlig unbekannte Welt, deren Zerstörung seit Jahren ungeahnte Ausmaße erreicht hat. Mojib Latif, der renommierte Klima- und Meeresforscher geht in seinem Buch existenziellen Fragen nach: Welche Rolle spielen die Ozeane beim Klimawandel? Welche Konsequenzen folgen aus der Versauerung der Ozeane? Wohin führt die Verschmutzung durch Erdöl? Wie ist die Rolle des Plastikmülls einzuschätzen? Und wie die Belastung durch Radioaktivität? Mojib Latif präsentiert ein eindringliches Plädoyer für die Erhaltung unserer Lebensgrundlage, dem man sich kaum entziehen kann.

Description: While the Earth's surface has considerably warmed over the past two decades, the tropical Pacific has featured a cooling of sea surface temperatures in its eastern and central parts, which went along with an unprecedented strengthening of the equatorial trade winds, the surface component of the Pacific Walker Circulation (PWC). Previous studies show that this decadal trend in the trade winds is generally beyond the range of decadal trends simulated by climate models when forced by historical radiative forcing. There is still a debate on the origin of and the potential role that internal variability may have played in the recent decadal surface wind trend. Using a number of long control (unforced) integrations of global climate models and several observational data sets, we address the question as to whether the recent decadal to multidecadal trends are robustly classified as an unusual event or the persistent response to external forcing. The observed trends in the tropical Pacific surface climate are still within the range of the long-term internal variability spanned by the models but represent an extreme realization of this variability. Thus, the recent observed decadal trends in the tropical Pacific, though highly unusual, could be of natural origin. We note that the long-term trends in the selected PWC indices exhibit a large observational uncertainty, even hindering definitive statements about the sign of the trends.

Description: We present a detailed analysis of the ENSO atmospheric feedbacks in a perturbed atmospheric physics ensemble with the Kiel Climate Model (KCM) and for the CMIP5 data base. We further untangle the interaction between perturbed physics and the mean state differences in the KCM ensemble by conducting additional atmospheric only simulations. The results show that the atmospheric part of the amplifying Bjerknes Feedback (the zonal wind feedback) and the net heat flux damping feedback are strongly, linearly linked with each other via the mean state sea surface temperature (SST) and perturbed model physics play only a minor role. In observations, strong wind and heat flux feedbacks are caused by a convective response in the Niño4 region during ENSO events, resulting from an eastward shift of the raising branch of the Walker Circulation during El Niño (vice versa for La Niña). Coupled General Circulation Models (CGCM), with an equatorial SST cold bias in the Niño4 region and accompanied La Niña-like mean state, yield a too westward raising branch of the Walker Circulation (by up to 30 ◦ ) and hence only a weak convective response, explaining the too weak wind and heat flux feedback. Thus the position of Walker Circulation determines the strength of the wind and heat flux feedback and explains the compensating error between these two feedbacks, seen in KCM and many CGCM of the CMIP5 data base. Furthermore, improved atmospheric feedbacks lead to a substantial improvement of important ENSO properties as phase locking of ENSO to the annual cycle and asymmetry between El Niño and La Niña. In order to successfully represent atmospheric ENSO dynamics in CGCM a correct mean state of the Walker Circulation is important and this serves as an explanation for the too diverse simulated ENSO in current CGCM.

Description: Ohne das Meer gäbe es kein Leben auf unserem Planeten. Es regelt weitgehend das Klima, gibt uns Nahrung und liefert Energie. Darüber hinaus ist es ein wichtiger Verkehrsweg, ein Erholungsraum und ein Quell ästhetischen Vergnügens. Aber das Meer steht unter Stress, denn das alte Prinzip von der „Freiheit der Meere“ hat zu Überfischung, Artenverlust und einer immensen Verschmutzung der Ozeane geführt. Deshalb muss der Umgang mit dem Meer auf nachhaltige und gerechte Grundlagen gestellt werden. Der Meeresatlas 2017 liefert dazu die Daten, Fakten und Zusammenhänge. Er zeigt in zahlreichen Beiträgen und über 50 Grafiken, in welch schlechtem Zustand sich die Weltmeere befinden, warum das so ist und was man tun muss, um die Situation der Ozeane zu verbessern.