Directional Waves in the Nearshore Coastal Region of Perth ...

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Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean TanDue to the normal breakdown of the high pressure belt into anticyclonic cells, the prevailingair circulation over the southwest region is anticlockwise. Along the north side of theanticyclones, the winds gradually become easterlies (or trade winds) and along the southedge, variable winds come in from the west (Gentilli 1971). Therefore, the seasonal shift ofthe high pressure belt accounts for a dominant onshore (i.e. westerly) component in winterand an offshore (easterly) component in summer along the south-west coast of Australia(Gentilli 1972). The anticyclonic high pressure cells progress eastward with a periodicity offive to six days (Gentilli 1971).Mid-latitude depressionsThe anticyclonic system is periodically disrupted by storm events associated with mid-latitudedepressions. During summer, mid-latitude depressions are usually located far too south ofAustralia to have any direct influence on the local climate. However, during the wintermonths, when the high pressure belt is displaced towards the north, the low pressure cellsassociated with the mid-latitude depressions travel northward. According to Gentilli (1971),the storms associated with mid-latitude depressions occur most frequently in July with anaverage of three cyclonic centres skirting the south-western coast of WA.Tropical cyclonesTropical cyclones are intense low pressure systems that form over the tropical waters ofnorthwest of WA, and the paths of these storms greatly influence the coastal wind conditions.The tropical cyclone season occur between November and May with most cyclonic activityoccurring in January or February (Rose 2001). In most cases, if the cyclone travels to thesouth of Australia, it loses most of its characteristics and becomes indistinguishable from anyfrontal cyclonic depression. However, a few cyclones have reached the southern shore ofAustralia without any apparent loss of intensity (Gentilli 1971), and it has been estimated thattropical cyclones directly affect the Perth metropolitan coastline once in every ten years(Lemm 1996).Literature Review 19
Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean Tan2.4.3 Regimes of Sea breeze and Storm events in PerthSea breezesThe sea breeze system is a diurnal atmospheric circulation due to the differential heating ofthe land and sea surfaces causing a pressure gradient, which leads to streams of air flowingonshore or offshore until balance is restored (BOM 1993). Sea breezes are a distinctive andwell established feature of the local climate in the south-west of Australia, and are given localnames such as the Fremantle Doctor, Albany Doctor, and Esperance Doctor “from the nameof the coastal locality near which they cross the shore on their welcome way inland.” (Gentilli1971 p.111). A typical sea breeze cycle usually consists of offshore winds in the generaleasterly direction in the morning, followed by the sudden arrival of south to southwesterly(alongshore to cross-shore) winds towards mid-afternoon. The sea breeze is present more than60% of the time during the summer months, and the mean sea breeze velocity at the coastlineis about 8 ms -1 (Masselink & Pattiaratchi 2001). Wind speeds during the summer sea breezeoften exceed 15ms -1 with maximum values of 20 ms -1 (Pattiaratchi et al 1997).In contrast to the ‘typical’ sea breeze system which blows perpendicular to the shoreline, thesea breeze along the west coast of Australia blows parallel to the shoreline in a southerlydirection (Pattiaratchi et al 1997). The reason for the shore-parallel sea breeze system can beattributed to the interaction between the sea breeze system and the synoptic weather patterns.During summer, the Australian continent is generally under the influence of easterly air flow,and due to the heating of the air flow across the continent, a low pressure trough is formedparallel to the coast of Western Australia (i.e. in a north-south direction) (Pattiaratchi et al1997). The location of the trough affects the intensity and direction of the sea breeze. Thesynoptic pressure gradient acts in a northeasterly direction when the low pressure trough islocated inland from the west coast (Pattiaratchi et al 1997). If we consider the sea breezesystem on the West Australian coast as simply due to the differential heating of land and seasurfaces and the Coriolis force, the sea breeze would flow in a southwesterly direction. Hencethe combination of the southwesterly air flow of the sea breeze pressure system and thesoutheasterly air flow of the synoptic pressure results in a southerly sea breeze. When thelocation of the trough is such that the synoptic pressure gradient induces southerly winds, thesea breeze enhances the southerly winds (Pattiaratchi et al 1997). Therefore the summersynoptic patterns in this region heighten the sea breeze system, and very strong sea breezesoccur during the summer months. The deepest sea breezes at Perth occur in November to20 Literature Review
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