Ocean-atmosphere interactions on decadal timescales

• Main Author: Venzke, S.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Hamburg 1999
• Subjects: Curl; Pacific; North Atlantic; Sea ice
• Online Access: http://hdl.handle.net/21.11116/0000-0005-D571-B; http://hdl.handle.net/21.11116/0000-0005-D573-9

In this thesis, different processes that might contribute to the generation of decadal climate variability were investigated using general circulation models (GCMs) of the atmosphere and the ocean. First, the sensitivity of the atmospheric circulation to decadal changes in the underlying sea surface temperatures (SSTs) was esti- mated from an ensemble of six integrations of the Hadley Centre atmospheric GCM HadAMl, all forced by observed SSTs and sea-ice extents for the period 1949-93. Using a novel approach to estimate the 'true' SST-forced atmospheric response in the presence of spatially correlated internal atmospheric variability, the decadal at- mospheric variability was studied over the North Atlantic and North Pacific regions. After filtering out the atmospheric circulation changes associated with the El Niño - Southern Oscillation (ENSO) phenomenon, the dominant mode of forced variability over the North Atlantic exhibits a meridional dipole in the mean sea level pressure (MSLP) field and is related to a tripole in the anomalous North Atlantic SSTs. Over large parts of the North Atlantic region, however, the atmospheric response is not consistent enough to provide feedbacks to the underlying ocean that could cause self-sustained decadal oscillations. Over the North Pacific the atmospheric response is dominated by ENSO. In addition to the ENSO-related response an independent decadal atmospheric signal was detected. It consistently involves iarge-scaie wind stress curl anomalies over the North Pacific region. The effect of such wind stress curl anomalies on the ocean was studied in the second part of this thesis using the Hamburg Ocean Primitive Equation model (HOPE). It is shown how the adjust- ment of the North Pacific gyre circulation to large-scale wind stress curl anomalies determines the decadal timescale and how it may be exploited for predictions of decadal upper-ocean temperature changes in the central North Pacific. The HOPE model was also used to investigate a mechanism for the generation of ...