Historical climate variation in the northern Coral Sea : using the wind power density vector as an indicator

Proh, David L. (1995). Historical climate variation in the northern Coral Sea : using the wind power density vector as an indicator Master's Thesis, School of Engineering, The University of Queensland.

       
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Author Proh, David L.
Thesis Title Historical climate variation in the northern Coral Sea : using the wind power density vector as an indicator
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 1995
Thesis type Master's Thesis
Supervisor Dr Michael Gourlay
Total pages 419
Language eng
Formatted abstract The primary hydrodynamic influence on the formation and stability of the coral cays of the Great Barrier Reef is wave action caused by wind and modulated by the tide. Wind power density, proportional to wind velocity cubed, is the best measure of the ability of the wind to generate waves. Surface wind records from Thursday Island, Willis Island and Low Isles are used to calculate wind power density vectors on various time scales. These vectors are used as indicators of climate variation in the region.

Previous research indicates that the most dominant component of the interannual climate variation is the quasi-biennial Southern Oscillation. Weak and strong years in the annual climate cycle are identified in this thesis from indices of the Southern Oscillation. The extremes of the weak and strong years are the ENSO (El Nino - Southern Oscillation) and anti-ENSO events responsible for drought and flooding in Australia.

Wind velocity and direction recorded each morning are used to calculate wind power density vectors and other parameters which describe the distribution of wind power density and wind direction on monthly, seasonal and annual time scales. This analysis clearly shows the dominance of the north-west monsoon at Thursday Island and of tropical cyclones at Willis Island and Low Isles during summer. This dominance extends over monthly, seasonal and even annual time scales. A comparison of weak and strong years shows that monsoonal westerlies at Thursday Island are stronger and more persistent in strong years on monthly and seasonal time scales. The trade winds are weaker at Thursday Island and Willis Island during ENSO events and tend to come from a more south-easterly direction at Willis Island.

East-west and north-south wind power density anomalies are calculated by subtracting the station averages of these components from the components of the wind power density vectors determined from the morning wind analysis. This analysis highlights periods varying from 2 to 11 years when easterly or westerly wind power density anomalies persist on an annual time scale. Significant easterly [westerly] anomalies occurred at Thursday Island when the Southern Oscillation Index was mainly negative [positive].

Three hourly winds at Thursday Island and Willis Island are analysed for the year November 1983 to October 1984 to investigate diurnal variations of the wind power density vector and the suitability of the morning wind data to describe climate variations on monthly, seasonal and annual time scales. The analysis shows that the morning winds provide a good estimate of the daily mean wind power density vector. However, estimates of the wind power density vectors on monthly time scales at both stations are significantly affected by individual storm events.

Tropical cyclone tracks in the eastern Australian region are analysed for the period July 1959 to June 1991 to determine any differences in the incidence and persistence of tropical cyclones in the Coral Sea between weak and strong years. During strong years tropical cyclones occur more frequently, cyclone tracks extend further south, and the number of cyclone days for the season increases. The incidence of tropical cyclones at Willis Island increased significantly in strong years. The incidence of tropical cyclones did not vary significantly between strong and weak years at Thursday Island, Low Isles and Raine Island.

Storm events which would generate waves more than 4 metres high at sea are extracted from three hourly wind data for Thursday Island and Willis Island. Storm numbers increased in strong years when compared to weak years. Periods when monsoonal [winter] storms dominate at Thursday Island were identified as periods when westerly [easterly] wind power density anomalies persisted on an annual time scale. Periods when tropical cyclones were present [absent] at Willis Island were identified as periods when westerly [easterly] wind power density anomalies persisted on an annual time scale.

In conclusion, periods when long term and short term recession of the shoreline at Raine Island (200 kilometres south-east of Thursday Island) may have occurred are identified from the results for Thursday Island on the assumption that climate variations at these two sites are similar. Tropical cyclones which have historically affected Raine Island need to be considered as well.

Keyword Winds -- Coral Sea.
Climatic changes -- Coral Sea.

 
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