Modes of variability as simulated by a global climate model

• Main Author: Timmermann, A.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Hamburg 1999
• Subjects: Curl; Pacific; North Atlantic; North Atlantic oscillation
• Online Access: http://hdl.handle.net/21.11116/0000-0005-D56E-0; http://hdl.handle.net/21.11116/0000-0005-D570-C

We discussed in this thesis an orchestra of climate modes with characteristic time scales of about 5, 16 and 35 years (see Fig.6.3). Interannual variability in the Pacific is ENSO-like. This quasi-oscillatory mode can be understood as a coupled air-sea mode. We discussed the origin of ENSO variance modu- lations, which occur on time scales of. 22 and 35 years. In particular the 35 year ENSO modulation arises from an interaction between ENSO and a 35 year Northern Hemi- spheric climate mode. Interdecadal wind stress curl anomalies in the subtropical Pacific initiate Rossby lryaves, which are associated also with subsurface temperature anomalies. These waves enter the equatorial region 8-12 years later. The subsequent changes in the mean equatorial temperature structure are considered to be important for the simulated interdecadal changes of ENSO variability. However, it was shown that the simulated ENSO cycle in the ECHAM3/LSG model is rather insensitive to global warming, whereas it is revealed from the scenario A integration performed with the more realistic CGCM ECHAM4/OPYC3 that global warming will increase the ENSO variability (Timmermann et al. 1998). This discrepancy in the sensitivities of these two climate models can be ex- plained in terms of the mean thermocline structure, which in the ECHAM3/LSG model is highly diffusive, whereas in the ECHAM4/OPYC3 simulation it is relatively sharp. In the North Atlantic, two climate modes are identified with time scales of 14-18 and 30-40 years, which can be understood in terms of the coupled air-sea mode framework. The wind-driven North Atlantic gyre system provides the memory of the decadal North Atlantic mode. The North Atlantic Oscillation is the atmospheric agent in this coupled air-sea mode, serving on the one hand as a stabilizer of oceanic temperature anomalies and on the other hand as an initiator of the temperature tendency which causes the phase reversal after some delay. The dynamics can be condensed within a feedback loop, which is shown in Fig.4.11. ...