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Multidecadal and NA0 Related Variability in a Numerical Model of the North Atlantic Circulation

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Multidecadal and NA0 Related Variability in a Numerical Model of the North Atlantic Circulation Multidecadal and NA0 related variability in a numerical model of the North Atlantic circulation Multidekadische und NA0 bezogene Variabilitäin einem numerischen Modell des Nordatlantiks Jennifer P. Brauch Ber. Polarforsch. Meeresforsch. 478 (2004) ISSN 1618 - 3193 Jennifer P. Brauch UVic Climate Modelling Research Group PO Box 3055, Victoria, BC, V8W 3P6, Canada http://climate.uvic.ca/ [email protected] Die vorliegende Arbeit ist die inhaltlich unverändert Fassung einer Dis- sertation, die 2003 im Fachbereich Physik/Elektrotechnik der Universitä Bremen vorgelegt wurde. Sie ist in elektronischer Form erhältlic unter http://elib.suub.uni-brernen.de/. Contents Zusammenfassung iii Abstract V 1 Introduction 1 2 Background 5 2.1 Main Characteristics of the Arctic and North Atlantic Ocean .... 5 2.1.1 Bathymetry ............................ 5 2.1.2 Major currents .......................... 7 2.1.3 Hydrography ........................... 8 2.1.4 Seaice ............................... 11 2.1.5 Convection ............................ 12 2.2 Variability ................................. 13 2.2.1 NA0 ................................ 13 2.2.2 Variability in the Arctic Mediterranean ............ 17 2.2.3 GSA ................................19 2.2.4 Oscillations in ocean models .................. 20 3 Model description 3.1 Ocean model ................................ 3.1.1 Equations ............................. 3.1.2 Setup ................................ 3.2 Sea Ice model ............................... 3.2.1 Equations ............................. 3.2.2 Setup ................................ 3.3 Data exchange ............................... 4 Atmospheric forcing 40 4.1 OMIPdata .................................40 4.2 NAO+ and NAO- composites .....................41 4.3 Discussion .................................45 5 Spin up 47 5.1 Hydrography ............................... 47 5.2 Circulation ................................. 52 5.3 Heat transport ............................... 57 5.4 Ice distribution .............................. 59 5.5 Discussion ................................. 61 6 Sensitivity experiments 65 6.1 Design ................................... 66 6.2 Results ................................... 67 6.2.1 Response of the ice fields .................... 67 6.2.2 Response of the ocean fields ................... 70 6.3 Discussion ................................. 79 6.4 Conclusions ................................ 81 7 Internal variability 83 7.1 First look at the oscillation ........................ 84 7.2 Role of the Labrador Sea ......................... 91 7.3 Discussion ................................. 96 7.4 Conclusions ................................ 99 8 Final conclusions and outlook 101 Bibliography 105 Acknowledgements 121 Zusammenfassung Das Verständider natürlicheVariabilitä des Nordatlantischen und Arktischen Ozeans ist noch immer unzureichend. Besonders auf Zeitskalen von zwischen- jährlichebis multidekadischer Variabilitätauf welchen die Verfügbarkeider Daten limitiert ist, verbleiben noch viele offene Fragen. Diese Studie beschäftig sich mit der Reaktion eines numerischen Modells des Nordatlantischen und Ark- tischen Ozeans auf Anderungen des Oberflächenantriebund versucht das Ver- ständnider internen Variabilitädes Modells und darübehinaus des realen Ozean-Meereissystems zu verbessern. Das Ozeanmodell basiert auf dem MOM-2 Kode, welches mit einem dynamisch-thermodynamischen Meereismodell mit einer viskos-plastischen Rheologie gekoppelt ist. Der Hauptmode der atmosphärische Variabilitäübe dem Nordatlantik ist die Nordatiantische Oszillation (NAO). Zwei Integrationen übe200 Jahre mit dem gekoppelten Ozean-Meereismodell werden mit atmosphärische Antrieb durchgefŸhrt welcher einerseits mit positiver und andererseits mit negativer NA0 assoziiert ist. Diese Integrationen werden mit einem Kontrollauf übe den gleichen Zeitraum verglichen, der mit einem klimatologischen Antrieb gerechnet wurde. Alle drei Experimente zeigen einen voneinander abweichen- den langfristigen Trend, der in der Zunahme des Maximums der meridionalen Umwälzbewegun (Overturning) ersichtlich ist. Der Grad der internen Variabil- itäunterscheidet sich auch deutlich zwischen den Experimenten. Der Kontrollauf mit dem klimatologischen Antrieb entwickelt nach circa 80 Jahren eine Oszillation mit einer Periodizitävon 40 Jahren. Der Mechanis- mus umfaßden Transport von warmen, salzreichen Oberflächenanomaliesub- tropischen Ursprungs, die in das Gebiet des Subpolarwirbels advehiert wer- den, wodurch sich in diesem Gebiet die Konvektion und somit auch die Tiefen- wasserbildung verstärktDas Zusammenspiel des tiefen westlichen Randstroms (DWBC) und des Golfstroms ist entscheidend fŸ das Aufrechterhalten der Os- zillation. Die Integration mit atmosphärische Antrieb, der mit einer positiven NA0 Phase assoziiert wird, ist der Ausgangspunkt, um die Reaktion des Ozean- Meereis-Systems auf ein langfristiges Umschalten auf NAO- Antrieb zu un- tersuchen. Die Meereisausdehnung reagiert instantan auf das Umschalten, wohingegen sich die Meereisdicke langsamer an den neuen Antrieb anpaßtDer Nordatlantische Ozean durchläufeine schnelle, barotrope Anpassungsphase, die mit einem kurzfristigen Anstieg der meridionalen Umwälzbewegun und des nordwärtigeWärmetransport bei 48ON verbunden ist. Die langfristige An- passung zieht eine Abnahme des nordwärtigeWärmetransports eine Abnahme der Stärk des Subpolaren und Subtropischen Wirbel und eine Abnahme der meridionalen Umwälzbewegunnach sich. Ein Experiment, welches nur füein Jahr mit dem NAO- Antrieb gerechnet wird und anschließenwieder mit NAO+ Antrieb, verschiebt das Konvektions- iii gebiet in der Labrador See nach Nordwesten, was sich als neuer Gleichgewicht- szustand des Ozeans herausstellt. Abstract The natural variability of the North Atlantic and Arctic Ocean is still not com- pletely understood. Especially On interannual to multidecadal time scales, where the amount of observations is limited, many Open questions remain. The objec- tive of this study is therefore to investigate the reaction of a numerical model of the North Atlantic and Arctic Ocean to changes in the atmospheric surface forc- ing and to improve the understanding of the intemal variability of the model and furthermore the real ocean-sea ice system . The ocean model is based on the MOM-2 code, which is coupled to a dynamic-thermodynamic sea ice model with a viscous-plastic rheology. A major mode of atmospheric variability over the North Atlantic Ocean is the North Atlantic Oscillation (NAO). Two 200 year integrations of the coupled ocean-sea ice model with surface boundary conditions which are related to peri- ods of positive and negative NA0 are compared to an Integration with climato- logical atmospheric forcing over the Same number of years. All three experiments are characterised by a different long term trend in the time series of the maximum of the meridional overtuming which approximately lasts for the fast hundred years of Integration. Also, the degree of intemal variability differs between the experiments. The experiment with climatological forcing reveals a self-sustaining oscillation. The period of the oscillation is about 40 years. The mechanism is the advection of saline and warm subtropical surface anomalies in the region of the subpolar gyre, where the deep water formation is enhanced due to enhanced convection. The interplay between the Deep Western Boundary Current (DWBC) and Gulf Stream is important to sustain the oscillation. A switch from a long time NAO+ state to an enduring NAO- situation is also investigated in this study. The sea ice extent increases quickly after this switch, whereas the changes in sea ice volume adapt slower to the changed forcing, be- cause of the integral effect of the ocean mixed layer and ice thickness itself. The ocean adjusts with a fast barotropic circulation anomaly, accompanied by an en- hancement of meridional overtuming and northward heat transport at 48ON. The slow response is a substantial decrease of the northward heat transport, which is cmsed by a reduction of the strength of subpolar and subtropical gyres and a decrease of the meridional overtuming. One year of NAO- forcing shifts the area of convection in the Labrador Sea to a different position, which tums out to be a new state of equilibrium for the ocean. What in water did Bloom, waterlover, drawer of water, watercmier, re- turning to the range, admire? Its universality; its democratic equality and constancy to its nature in seeking its own level; its vastness in the ocean of Mercator's projection; 1. .I the restlessness of its waves and surface particles visiting in turn all points of its seaboard; the independence of its units; the variability of states of sea; its hydrostatic quiescence in calm; its hydrokinetic turgidity in neap and spring tides; its subsidence after devastation; its sterility in the circumpolar icecaps, arctic and antarctic; its climatic and commercial significance; its preponderance of 3 to 1 over the dry land of the globe; [. .I its vehicular ramifications in continental lakecontained streams and confluent oceanflowing rivers with their tributaries and transoceanic currents, gulfstream, noith and south equatorial Courses; its violence in seaquakes, waterspouts, Artesian wells, eruptions, torrents, eddies, freshets, Spates, groundswells, watersheds, waterpartings, geysers, cataracts, whirlpools, maelstroms, inundations, deluges, cloudbursts; its vast ckcumterrestrial ahorizontal
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