Modal Behavior of Hemispheric Sea Ice Covers

Recent papers have described 18-year trends and annual oscillations in the Arctic and Antarctic sea ice extents, areas, and enclosed open water areas based on a newly-formulated 18.2-year ice concentration time series. This time series includes data for the entire Arctic and Antarctic ice covers, as well as for previously defined subregions consisting of 5 sectors in the Antarctic and 9 regions in the Arctic. It was obtained by fine-tuning the sea ice algorithm tie points individually for each of the four sensors used to acquire the data. In this paper, we extend these analyses to an examination of the intrinsic modes of these time series, obtained by means of Empirical Mode Decomposition, with emphasis on periodicities greater than the annual cycle. Quasibiennial and quasiquadrennial oscillations observed with a different technique and reported earlier for the first 8.8 years of this time series were also observed in the present series. However, the intrinsic modes were not monochromatic; they feature frequency as well as amplitude modulation within their respective frequency bands. Modal periods of up to 18 years are observed, with important implications for the trend analyses published earlier. These results are compared with the oscillations in the Length-of-Day and North Atlantic Oscillation parameters similarly determined for the same 18.2-year period

A limited role for unforced internal variability in 20th century warming.

The early twentieth-century warming (EW; 1910–45) and the mid-twentieth-century cooling (MC; 1950–80) have been linked to both internal variability of the climate system and changes in external radiative forcing. The degree to which either of the two factors contributed to EW and MC, or both, is still debated. Using a two-box impulse response model, we demonstrate that multidecadal ocean variability was unlikely to be the driver of observed changes in global mean surface temperature (GMST) after AD 1850. Instead, virtually all (97%–98%) of the global low-frequency variability (.30 years) can be explained by external forcing. We find similarly high percentages of explained variance for interhemispheric and land–ocean temperature evolution. Three key aspects are identified that underpin the conclusion of this new study: inhomogeneous anthropogenic aerosol forcing (AER), biases in the instrumental sea surface temperature (SST) datasets, and inadequate representation of the response to varying forcing factors. Once the spatially heterogeneous nature of AER is accounted for, the MC period is reconcilable with external drivers. SST biases and imprecise forcing responses explain the putative disagreement between models and observations during the EW period. As a consequence, Atlantic multidecadal variability (AMV) is found to be primarily controlled by external forcing too. Future attribution studies should account for these important factors when discriminating between externally forced and internally generated influences on climate. We argue that AMV must not be used as a regressor and suggest a revised AMV index instead [the North Atlantic Variability Index (NAVI)]. Our associated best estimate for the transient climate response (TCR) is 1.57 K (60.70 at the 5%–95% confidence level)

Variability of the contemporary Southern Ocean carbon fluxes and storage

Around half of the ocean’s uptake of anthropogenic carbon from the atmosphere currently takes place in the Southern Ocean. However, the variability of this important carbon sink, as well as the drivers behind this variability, are still debated and it is unclear if the Southern Ocean will remain a carbon sink in the future. Until this PhD project, the development of the Southern Ocean carbon uptake at the air-sea interface was unknown based on observations beyond 2011. Furthermore, the seasonal to interannual variability of dissolved inorganic carbon (DIC) in the interior Southern Ocean had not been analyzed based on observations at regional scale. This dissertation closes these research gaps. In the first part of my dissertation (Appendix A), I investigate the Southern Ocean carbon flux and its drivers until 2016 using an updated observation-based air-sea carbon flux estimate. After a stagnation period in the 1990s, and a reinvigoration in the 2000s, I find that the Southern Ocean carbon uptake weakened again since about 2011. My study reveals that the Southern Annular Mode, the dominant mode of climate variability in the southern high latitudes, is not the driver behind this weakening due to opposing effects that cancel each other out. Instead, regional shifts in surface wind velocity modulate the recent evolution of the carbon uptake in the Southern Ocean. In the second part (Appendix B), I develop a monthly climatology of global mapped interior DIC fields using a neural-network mapping approach. Using this new data product, I describe the seasonal carbon dynamics at global scale, including the phase and amplitude of the surface seasonal cycle, how deep seasonal signals are detectable, and I estimate the net community production. In the third part (Appendix C), I increase the temporal resolution of my new data product to resolve monthly fields from 2004 through 2017. I then re-focus on the Southern Ocean to investigate the interannual variability of DIC in the water column and determine the potential drivers behind this variability. Using this second new data product, I demonstrate that sub-surface DIC is subject to significant decadal fluctuations. These fluctuations extend to at least 500 m and could be linked to changes in the Meridional Overturning Circulation. The methods and the publicly available data products I developed provide an opportunity for further analysis of the global carbon cycle. The findings from my PhD project represent an updated estimate of the carbon uptake and storage in the Southern Ocean and enable an improved description of the processes and drivers of variability. This knowledge forms an essential part of our understanding of the global carbon cycle and can, therefore, contribute to more accurate climate projections, forming an important basis for political decisions aimed at reducing carbon emissionsIm Südpolarmeer findet derzeit etwa die Hälfte der ozeanischen Aufnahme von anthropogenem Kohlenstoff aus der Atmosphäre statt. Über die Variabilität dieser wichtigen Kohlenstoffsenke sowie die Einflussfaktoren dieser Variabilität wird jedoch debattiert, und es ist unklar, ob das Südpolarmeer auch in der Zukunft eine Kohlenstoffsenke bleiben wird. Vor diesem Promotionsprojekt fehlte eine Abschätzung der atmosphärischen Kohlenstoffaufnahme des Südpolarmeers basierend auf Beobachtungsdaten die nach 2011 erhoben wurden. Des Weiteren wurde die saisonale und zwischenjährliche Variabilität des gelösten anorganischen Kohlenstoffs (DIC) im tiefen Südpolarmeer bisher noch nicht anhand von Beobachtungsdaten auf regionaler Ebene analysiert. Diese Dissertation schließt die bestehenden Forschungslücken. Im ersten Teil meiner Dissertation (Anhang A) untersuche ich die ozeanische Kohlenstoffaufnahme aus der Atmosphäre und deren Einflussfaktoren im Südpolarmeer bis 2016 anhand aktualisierter Beobachtungsdaten, die an der Meeresoberfläche erhoben wurden. Nach einer Stagnationsphase in den 1990er Jahren und einem Wiedererstarken in den 2000er Jahren, ermittle ich, dass die Kohlenstoffaufnahme im Südpolarmeer seit ca. 2011 erneut nachgelassen hat. Meine Studie zeigt, dass der Southern Annular Mode, der dominante Modus von Klimaschwankungen in den südlichen hohen Breitengraden, nicht der Einflussfaktor hinter diesem Abschwächen der Senke ist, da sich gegensätzliche Effekte aufheben. Stattdessen kontrollieren regionale Verschiebungen der Oberflächenwindgeschwindigkeit die jüngste Entwicklung der Kohlenstoffsenke im Südpolarmeer. Im zweiten Teil (Anhang B) etabliere ich ein Verfahren, das es erlaubt, mithilfe neuronaler Netzwerke die globale Tiefenverteilung von gelöstem anorganischen Kohlenstoff als monatliche Klimatologie abzubilden. Mit diesem neu entwickelten Datenprodukt beschreibe ich die saisonale DIC-Dynamik auf globaler Ebene. Diese Beschreibung erstreckt sich auf die Phase und Amplitude des saisonalen Zyklus an der Oberfläche und dessen Tiefenausdehnung, sowie eine Abschätzung der Nettoproduktion von organischem Kohlenstoff durch marine Lebensgemeinschaften. Im dritten Teil (Anhang C) erhöhe ich die zeitliche Auflösung dieses Datenprodukts, um auch die zwischenjährlichen Veränderungen der monatlichen DIC-Felder von 2004 bis Ende 2017 aufzulösen. Für die inhaltliche Interpretation der neu generierten Datensätze lege ich den Schwerpunkt erneut auf das Südpolarmeer, um hier die zwischenjährliche Variabilität des gelösten anorganischen Kohlenstoffs in der Wassersäule zu beschreiben und die möglichen Einflussfaktoren für diese Variabilität zu bestimmen. Anhand dieses zweiten neuen Datenprodukts zeige ich, dass der gelöste anorganische Kohlenstoff unterhalb der Meeresoberfläche signifikanten dekadischen Schwankungen unterliegt. Diese Schwankungen erstrecken sich mindestens über die oberen 500 m der Wassersäule und könnten mit Änderungen der meridionalen Umwälzzirkulation verbunden sein. Die von mir entwickelten Methoden und öffentlich zur Verfügung gestellten Datenprodukte eröffnen diverse Möglichkeiten zur weiteren Analyse des globalen Kohlenstoffkreislaufs. Die Ergebnisse meines Promotionsprojekts stellen eine aktualisierte Abschätzung der Kohlenstoffaufnahme und -speicherung im Südpolarmeer dar und ermöglichen eine erheblich verbesserte Beschreibung der beteiligten Prozesse und Einflussfaktoren. Dieses Wissen ist ein wesentlicher Bestandteil unseres Verständnisses des globalen Kohlenstoffkreislaufs und kann somit zu genaueren Klimaprojektionen beitragen. Damit bilden die Befunde auch eine wichtige Grundlage für politische Entscheidungen, die auf die Reduzierung der Kohlenstoffemissionen abziele

A nonlinear method of removing harmonic noise in geophysical data

A nonlinear, adaptive method to remove the harmonic noise that commonly resides in geophysical data is proposed in this study. This filtering method is based on the ensemble empirical mode decomposition algorithm in conjunction with the logarithmic transform. We present a synthetic model study to investigate the capability of signal reconstruction from the decomposed data, and compare the results with those derived from other 2-D adaptive filters. Applications to the real seismic data acquired by using an ocean bottom seismograph and to a shot gather of the ground penetrating radar demonstrate the robustness of this method. Our work proposes a concept that instead of Fourier-based approaches, the harmonic noise removal in geophysical data can be achieved effectively by using an alternative nonlinear adaptive data analysis method, which has been applied extensively in other scientific studies

Comparison of sea-ice freeboard distributions from aircraft data and cryosat-2

The only remote sensing technique capable of obtain- ing sea-ice thickness on basin-scale are satellite altime- ter missions, such as the 2010 launched CryoSat-2. It is equipped with a Ku-Band radar altimeter, which mea- sures the height of the ice surface above the sea level. This method requires highly accurate range measure- ments. During the CryoSat Validation Experiment (Cry- oVEx) 2011 in the Lincoln Sea, Cryosat-2 underpasses were accomplished with two aircraft, which carried an airborne laser-scanner, a radar altimeter and an electro- magnetic induction device for direct sea-ice thickness re- trieval. Both aircraft flew in close formation at the same time of a CryoSat-2 overpass. This is a study about the comparison of the sea-ice freeboard and thickness dis- tribution of airborne validation and CryoSat-2 measure- ments within the multi-year sea-ice region of the Lincoln Sea in spring, with respect to the penetration of the Ku- Band signal into the snow

Cambios interdecadales en las teleconexiones de los océanos con el Sahel: implicaciones en la predictibilildad de la lluvia

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra, Astronomía y Astrofísica I, leída el 09-06-2017The West African Sahel is the transition region between the wet equatorial zone and the dry Sahara desert. Year-to-year, the Sahel alternates an extremely dry season with a strong rainfall regime from July to September. The water resources available during the long dry season depend almost entirely on the intensity of rainfall during the rainy season, also known as the West African Monsoon (WAM). The WAM presents a marked variability at interannual time scales (e.g., Sultan et al. 2003; Sultan and Janicot 2003), being a major topic of study. The severe drought experienced in the Sahel from the 1970s to the 1990s, and the apparent recovery trend in the recent period, also reveals the pronounced interdecadal variability of the WAM (Hulme et al. 2001; Nicholson 2005; Lebel and Ali 2009). The WAM system is primarily determined by the northward shift of the Inter-Tropical convergence Zone (ITCZ) along with a thermal gradient between the Sahara desert to the north and the Guinean Gulf to the south (e.g., Sultan and Janicot 2000; Chiang et al. 2000, 2002; Kushnir et al. 2003; Nicholson 2009). Thus, although land surface processes and internal variability cannot be neglected, the oceanic forcing plays the leading role in the predictability of the WAM (e.g., Folland 1986; Palmer 1986; Fontaine et al. 1998; Skinner et al. 2012; Rodriguez-Fonseca et al. 2015). On the one hand it is presented as the main driver of the decadal variability (e.g., Janicot et al. 2001; Biasutti et al. 2008; Mohino et al. 2011a; Martin et al. 2013). On the other hand, several observational studies address the interannual oceanic teleconnections from the tropical Pacific (Janicot et al. 2001; Rowell 2001; Joly and Voldoire 2009), the tropical Atlantic (Giannini et al. 2003; Polo et al. 2008; Joly and Voldoire 2009; Nnamchi and Li 2011) and the Mediterranean (Rowell 2003; Gaetani et al. 2010; Fontaine et al. 2011a)...Dentro de África Occidental, el Sahel es la región de transición entre la zona húmeda ecuatorial y el desierto del Sahara. Año a año, el Sahel alterna una estación extremadamente seca con un fuerte régimen de lluvias desde Julio a Septiembre. Los recursos hídricos disponibles durante la larga estación seca dependen en su práctica totalidad de la intensidad de las lluvias durante la estación de lluvias, también conocida como el Monzón de África Occidental (WAM). El WAM presenta una marcada variabilidad interannual (p.ej., Sultan et al. 2003; Sultan and Janicot 2003) que constituye el tema principal de esta tesis. La severa sequía experimentada en el Sahel entre las décadas de 1970 y 1990, y la aparente recuperación en el período reciente, también ponen de manifiesto la pronunciada variabilidad interdecadal del WAM (Hulme et al. 2001; Nicholson 2005; Lebel and Ali 2009)...Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEunpu