Editing Houtman Abrolhos (section)

==Oceanography==

===Bathymetry===
The Houtman Abrolhos lies in the Indian Ocean about 60 kilometres (40&nbsp;mi) off the coast of Western Australia, near the edge of Australia's [[continental shelf]]. At less than 50 metres (160&nbsp;ft) deep, the shelf is quite shallow. It is also fairly level, the depth increasing to the west at a modest gradient of about 1.3 metres per kilometre (7&nbsp;ft/mi). About five kilometres (3&nbsp;mi) to the west of the Houtman Abrolhos lies the [[shelf break]], beyond which the seabed falls away much more steeply, averaging around 50 metres per kilometre (260&nbsp;ft/mi).<ref name = "Pearce 1997">{{cite book | first = AF | last = Pearce | year = 1997 | chapter = The Leeuwin Current and the Houtman Abrolhos Islands, Western Australia | editor = Wells, FE | title = The Marine Flora and Fauna of the Houtman Abrolhos Islands, Western Australia | volume = 1 | pages = 11–46 | publisher = Western Australian Museum | location = Perth}}</ref>

The islands of each island group arise from a single [[carbonate platform]], so the waters within an island group are mostly shallow. The channels between groups are {{convert|40|to|50|m}} deep, so no impediment to the exchange of offshore and inshore waters.<ref name="Pearce 1997"/>

===Currents===
The Houtman Abrolhos lies almost directly in the path of the [[Leeuwin Current]], which draws warm, low-saline water of tropical origin southwards along the coast of Western Australia. This current flows all year round, but is strongest during [[southern hemisphere]] winter months. In general the Leeuwin Current runs along the shelf break, and thus passes close by the western edge of the Abrolhos. It meanders, sometimes passing well out to sea and sometimes directly through the island chain, bathing it in warm tropical water;<ref name="Pearce 1997"/> Although the direction of the Leeuwin Current is predominantly southerly, [[Shark Bay, Western Australia|Shark Bay]] and the Houtman Abrolhos together act as a [[topography|topographical]] trigger for the forming of [[eddy (fluid dynamics)|eddies]],<ref name="Waite 2007">{{cite journal | first1 = A. M. | last1 = Waite | year = 2007 | title = The Leeuwin Current and its eddies: An introductory overview | journal = Deep-Sea Research Part II | volume = 54 | pages = 789–796 | doi = 10.1016/j.dsr2.2006.12.008 | last2 = Thompson | first2 = P | last3 = Pesant | first3 = S | last4 = Feng | first4 = M | last5 = Beckley | first5 = L | last6 = Domingues | first6 = C | last7 = Gaughan | first7 = D | last8 = Hanson | first8 = C | last9 = Holl | first9 = C | last10 = Koslow | issue = 8–10 | first10 = T. | last11 = Meuleners | first11 = M. | last12 = Montoya | first12 = J.P. | last13 = Moore | first13 = T. | last14 = Muhling | first14 = B.A. | last15 = Paterson | first15 = H. | last16 = Rennie | first16 = S. | last17 = Strzelecki | first17 = J. | last18 = Twomey | first18 = L.| bibcode = 2007DSRII..54..789W |display-authors=2}}</ref> so the Abrolhos can experience currents from any direction, even when the Leeuwin Current is flowing strongly.<ref name="Pearce 1997"/>

Unlike most other major ocean currents, there is no large-scale [[coastal upwelling]] associated with the Leeuwin Current. There is limited evidence for some sporadic, localised upwelling in the vicinity of the Abrolhos, but if so it appears to have little effect on the extremely low levels of nutrients in the water.<ref name="Pearce 1997"/>

===Temperature and salinity===
[[File:Houtman Abrolhos sea temperature and salinity.png|thumb|Mean monthly temperature versus [[salinity]] for the Houtman Abrolhos and the adjacent coast in 1994, showing the Houtman Abrolhos to be much less variable.]]

Sea temperature at the islands varies according to a diurnal cycle, with the water at its coldest between six and eight in the morning, and at its warmest between three and four in the afternoon. In summer the daily temperature range is around 1&nbsp;°C (2&nbsp;°F); in winter it is about half that.<ref name="Pearce 1997"/>

There is also an annual cycle, with sea temperature varying by a little less than 4&nbsp;°C (7&nbsp;°F) over the year, peaking at nearly 24&nbsp;°C (75&nbsp;°F) in March, and falling to around 20&nbsp;°C (68&nbsp;°F) in September. This variability is much less than in nearby coastal waters, which reaches a similar peak in summer but drops as low as 18&nbsp;°C (64&nbsp;°F) in winter. The relatively low variability in sea temperatures at the Abrolhos is largely attributable to the Leeuwin Current, which bathes the islands in warm tropical water during winter months.<ref name="Pearce 1997"/>

A similar annual pattern occurs in [[salinity]]. There is a clear seasonal variation, with values ranging from a summer high of around 35.7 [[parts per trillion|ppt]], to a winter low of around 35.4 ppt. As with water temperatures, the variability in salinity is much smaller than in coastal waters, where summer salinity reaches 36.4 ppt. This difference is partly due to the low-salinity waters of the Leeuwin Current, but there are a number of other factors involved, including high evaporation of coastal waters in summer.<ref name="Pearce 1997"/>

Temperatures can also vary substantially from year to year. Annual mean temperatures vary by as much as 1&nbsp;°C (2&nbsp;°F); with cooler years usually cooler throughout the year. There is evidence that annual mean temperatures are related to [[El Niño-Southern Oscillation]] events.<ref name="Pearce 1997"/>

The [[water column]] is generally well-mixed, with no evidence of a significant [[halocline]] or [[thermocline]]. Mean differences in water temperature between sea surface and sea bed range from only half a degree (Celsius; 1&nbsp;°F) in summer to almost zero in winter, and differences in salinity are very small even when the Leeuwin Current is at its strongest.<ref name="Pearce 1997"/>

===Sea levels===
As with the rest of southwestern Australia, [[tide]]s at the Houtman Abrolhos are small and irregular. There is little tidal data available for the islands, but what there is accords very closely with the data for Geraldton. Geraldton tides follow a diurnal pattern, with maximum tidal ranges of around 1 metre (3&nbsp;ft).<ref name="Pearce 1997"/>

Mean sea levels at Geraldton show seasonal fluctuations, being higher in winter when the Leeuwin Current is at its peak. There are also variations from year to year, which are strongly associated with the [[El Niño-Southern Oscillation]] (ENSO) cycle. Apparently, ENSO events induce a weaker Leeuwin Current, which results in lower sea levels.<ref name="Pearce 1997"/>

There is no published information on wave heights in the Abrolhos. In the open ocean, waves are typically a little over two metres high all year round. Nearer the mainland, they are usually less than {{convert|1.2|m}}, with a calmer period in March and April, and another in October and November.<ref name="Pearce 1997"/>