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Title: Diurnal Variability of the Tropical Upper Ocean and its Climate Impacts
Author: Bernie, Daniel
ISNI:       0000 0001 3461 6143
Current Institution: University of Reading
Date of Award: 2006
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Forced by variations in solar radiation due to the rotation of the earth, the diurnal cycle is a fundamental mode of variability of the climate system. Several previous studies have examined the diurnal cycle of the atmosphere, but no study has ever been made of the impact of the diurnal cycle of the upper ocean on the mean state and internal variability of the tropical climate. In this thesis the thermodynamical impacts of the diurnal cycle in the western Pacific warm pool are first examined in a simple I-D vertical mixing model. The results from this study are then used to guide the development of a high vertical resqIution Ocean General Circulation Model (OGCM) . configuration that is able to properly resolve the diurnal cycle in the upper ocean globally for the first time. The modified OGCM is then extensively validated against satellite data and used to assess the impact of the diurnal cycle on the dynamical behaviour of the upper ocean and to confirm the applicability of the results from the I-D study in a fully dynamical ocean. A fully coupled ocean atmosphere general circulation model is then developed that is able to revolve the diurnal cycle of ocean-atmosphere coupling and ocean mixed layer variability. This is used to assess the impact of the thermodynamical and dynamical effects of the diurnal cycle on the tropical climate over a range of temporal and spatial scales. Assessment of SST variability in the western Pacific warm pool during the TOGA-COARE period, using the I-D mixed layer'model, showed that in this region the intraseasonal SST response to the Madden-Julian Oscillation (MJO) is increased 40% by the diurnal variability SST. The implication of this is that the diurnal cycle is an important part of the ocean-atmosphere coupling involved in the MJO. Forced OGCM simulations confirm that the rectification of the SST response to the MJO holds for a larger spatial domain while coupled GeM simulations show that this implied increase in strength of ocean-atmosphere coupling leads to a stronger, more coherent MJO with better propagation characteristics compared to observations. The diurnal cycle is shown to increase the strength ofPacific sub-tropical cells by around 10%. This respresents a stronger dynamical response of the upper ocean to atmospheric forcing by altering the momentum balance of the Ekman layer via diurnal cycling. This dynamical impact, combined with the thermodynamical effects of the diurnal cycle, is identified as being involved in a coupled positive feedback mechanism with chang~s in the phase of the seasonal cycle in the eastern Pacific. It produces a larger amplitude annual cycle, though there is little impact on the mean SST in this region. These results all support the conclusion that the diurnal cycle is a central part of the dynamical and thermodynamical interaction between the ocean and the atmosphere. These results have numerous implications for the configuration of CGCM such as ocean vertical resolution, coupling frequency, and a variety of physical parameterisations which are detailed and discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available