Interconnections between combined Length-of-Day and polar motion parameters concerning joint atmosphere-ocean-hydrosphere modes

• Antragsteller: Professor Dr. Maik Thomas; Professor Dr. Uwe Ulbrich
• Fachliche Zuordnung: Geophysik und Geodäsie
• Förderung: Förderung von 2006 bis 2014
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 5471782

• Erstellungsjahr: 2014

Zusammenfassung der Projektergebnisse

The main goal of the project was the identification of joined atmosphere-ocean modes in modelled and observed Earth rotation time series. To this end, state estimates of the Earth subsystems ocean, atmosphere and land-hydrology were analysed together with their respective angular momentum functions. Model-only simulations, models constrained by observations and observation-only estimates were analysed for combined atmospheric-oceanic modes which impact Earth rotation excitations. Furthermore, to quantify the impact of climate change on Earth rotation excitations, a reanalysis of the recent climate was compared to Special Report on Emissions Scenarios (SRES) simulations of future climates. A coupled atmosphere-ocean excitation mechanism could be identified and described as a combination of the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO) and the Pacific North American (PNA), i.e., three major climate indices. The main coupling mechanism consists of NAO induced meridional jet stream shifts which depend on the phase of the PDO. When the PDO is in a warm phase, a negative NAO will result in significant positive Polar Motion (PM) excitations. Likewise, when the PDO is in a cold phase, a positive NAO will result in strong negative PM excitations. The respective excitations are increased further when PDO and PNA are in the same phase. When NAO and PDO are both positive or both negative no systematic PM excitations arise. These findings could be confirmed throughout all project phases: model-only simulation, atmospheric reanalyses and reanalyses forced models and in the observations. The comparison of the climate scenarios with the estimate of the recent climate showed that Earth Rotation Parameters are sensitive to climatic effects. The atmospheric mass-related angular momentum shows a significant negative trend of 0.04 mas/yr for χ2 and 0.01 mas/yr for χ1 , while the relative AAM trend is negligible. The regions where these changes originate were identified to be the Himalayas and the land masses around the Mediterranean. The detectability of atmospheric climate change signals in PM excitation strongly depends on the signal’s geographic location and spatial distribution. In general, southern hemispheric excitations result from annular wind systems, i.e., are rotational symmetric, and vanish in the global integral of the Polar Motion excitation. Therefore, observable excitation signals are more likely to originate from the northern hemisphere, where the distribution of land masses disturbs annular signals. Substantial differences between the annual excitations of recent reanalyses and simulations of future climates were identified. In the future climate scenarios the annual atmospheric excitation of Polar Motion becomes stronger due to the intensification of the westerlies. In the oceans the mass-related excitation cycle becomes stronger (more negative) and the relative angular momentum cycle becomes weaker (less positive). Both developments combine to a weaker ocean angular momentum anomalie. The main region where these developments originate was identified to be the Southern Ocean. In general, the scenario’s annual atmospheric and oceanic excitation show phase shift delays of one to three months. In future work, the demonstrated impact of specific combinations of NAO, PDO and PNA and their respective phases on Polar Motion excitation may offer a possibility to reconstruct missing and evaluate existing estimates of climate indices with an independent dataset.

Projektbezogene Publikationen (Auswahl)

• Excitation of Polar Motion and Length-of-Day induced by northern hemisphere teleconnection patterns in ECHAM5/OM1, Geodätische Woche, Cologne, Germany, 2010
  Schön, N., Ulbrich, U., Leckebusch, G., Névir, P., Thomas, M.
  
• Impact of the August 2010 Pakistan flood on Length-of-Day, Geodätische Woche, Cologne, Germany, 2010
  Schön, N., Dill, R.
  
• Influence of the North Atlantic Oscillation and the Quasi-Biannual Oscillation on Earth Orientation Parameters, Geophysical Research Abstracts, Vol. 13, EGU2011, Vienna, Austria, 2011
  Schön, N., Ulbrich, U., Leckebusch, G., Névir, P., Thomas, M.
  
• NAO index values estimated from Earth Orientation Parameters, Geodätische Woche, Nuremberg, Germany, 2011
  Schön, N., Leckebusch, G., Névir, P., Thomas, M., Ulbrich, U.
  
• NAO index values estimated from Earth Orientation Parameters, WCRP OpenScience Conference, Denver, CO, USA, 2011
  Schön, N., Ulbrich, U., Leckebusch, G., Névir, P., Thomas, M.
  
• Climate change impact on Polar Motion excitation, Geophysical Research Abstracts, Vol. 14, EGU2012, Vienna, Austria, 2012
  Schön, N., Kniebusch, M., Leckebusch, G., Névir, P., Thomas, M., Ulbrich, U.
  
• Uncertainties in relative atmospheric angular momentum computed from zonal winds in reanalysis data, J. Geophys. Res., Vol. 117, D09101
  Lehmann, E. and Névir, P.
  
• Impact of climate change on oceanic Earth rotation excitation, Geophysical Research Abstracts, Vol. 15, EGU2013, Vienna, Austria, 2013
  Saynisch, J. and Thomas, M.