Marmion Marine Park management Plan - Department of ...

12.0 CLIMATE AND OCEANOGRAPHYThe Park area experiences hot, dry summers and mild, wet winters. Air temperatures are similar tothose in Perth, where mean daily maximum temperatures vary from 30.3•C in summer to 17.6•C inwinter. Mean daily minimum temperatures are 18.6•C in summer and 9.1•C in winter. Rainfall ismoderate (about 880 mm/year) and falls mainly in May through to October. Evaporation in theregion is high (about 1980 mm/year).WindsThe dominant climatic factor is the wind, which generates waves, induces water circulation andtransports sand inland. The wind regime in the area is similar to that at Fremantle (Steedman & Craig,1979, 1983). A diurnal wind variation persists throughout the year, but intensifies during thesummer. In winter, winds are predominantly 'offshore' at night and in the morning, and 'onshore' inthe afternoon. In summer, the sea breeze/land breeze pattern is stronger, and acts additionally to apersistent southerly airstream, so that the resultant wind blows from the southeast at night and in themorning and from the southwest in the afternoon. The passage of low pressure systems in winterbrings northwest winds and gales that back to the west and southwest. The strongest winds blow fromthe southwest.Dissipating tropical cyclones can affect the coast during summer, bringing wind gusts up to 70 knots(35 m/s). Though these cyclonic events are of short duration, associated winds and high energy wavescan have a marked effect on the coast.WavesThe Park area is subject to a prevailing, refracted, long period (8-12 sec) southwest-west swell that iscontinually generated by storms and the "Roaring Forties" in the Indian Ocean. The swell is furtherrefracted, reflected and diffracted as it passes through the reef chains.Superimposed on the swell are locally generated, short-period (4-6 sec) wind waves. During summer,southwest waves are generated by the reinforced sea breeze; during winter, high energy waves aregenerated during northwest and westerly gales. The occasional summer cyclone may also generatewaves from the north, west or south.Depending on the prominence of the adjacent islands and reefs, wave energy reaching the coast mayvary markedly from place to place, with different parts of the coast being subject to swell, waves, swelland waves, or waves with damped swell. The prominent reefs off Mullaloo Point have had a markedinfluence on the swell, with the result that the area behind the reef complex has been the site of majoraccumulation of Holocene sands (see 13.0 Geology and Geomorphology).24