Abrolhos Weather

Abrolhos Islands Weather 

Abrolhos Weather conditions
The Abrolhos are first in the path of any weather moving in from the west or north-west and offer little protection from the wind. Weather conditions can be highly changeable and cold and warm fronts may reach the islands 24 or more hours ahead of weather bureau forecasts for the mainland. The islands are unaffected by mainland sea breezes. Be prepared for a prevailing westerly or south-westerly swell, which may reach several meters in unprotected waters.

More favorable weather conditions for boating usually occur between February and June, and in September/October.

Currents
The 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, however, sometimes passing well out to sea, and sometimes passing directly through the island chain, bathing it in warm tropical water;  Although the direction of the Leeuwin Current is predominantly southerly, Shark Bay and the Abrolhos together act as a topographical trigger for the forming of eddies,  so the Abrolhos can experience currents from any direction, even when the Leeuwin Current is flowing strongly.

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, localized 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.

Temperature and salinity
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 °C (2 °F); in winter it is about half that. There is also an annual cycle, with sea temperature varying by a little less than 4 °C (7 °F) over the year, peaking at nearly 24 °C (75 °F) in March, and falling to around 20 °C (68 °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 °C (64 °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.

A similar annual pattern occurs in salinity. There is a clear seasonal variation, with values ranging from a summer high of around 35.7 ppt, to a winter low of around 35.4 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. Temperatures can also vary substantially from year to year. Annual mean temperatures vary by as much as 1 °C (2 °F); with cooler years usually cooler throughout the year. There is evidence that annual mean temperatures are related to El Niño-Southern Oscillation events.

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 °F) in summer to almost zero in winter, and differences in salinity are very small even when the Leeuwin Current is at its strongest.

Sea levels
As with the rest of southwestern Australia, tides at the Abrolhos are small and irregular. There is little tidal data available for the islands, but what there are accords very closely with the data for Geraldton. Geraldton tides follow a diurnal pattern, with maximum tidal ranges of around 1 meter (3 ft.).

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. There is no published information on wave heights in the Abrolhos. In the open ocean, waves are typically a little over two meters high all year round. Nearer the mainland, they are usually less than 1.2 meters, with a calmer period in March and April, and another in October and November.

Climate
As of 2007, the Australian Bureau of Meteorology has not published climatic data for the Abrolhos Islands. However, an automatic weather station has been installed on North Island since 1990, and hourly measures of precipitation, air temperature, wind speed, wind direction, relative humidity and atmospheric pressure have been publicly available since then. Based on this and other data, researchers have put together a picture of the Abrolhos' climate. The islands have a Mediterranean climate, with warm dry summers and cooler, wet winters. Mean temperatures range from 9.3 °C to 19.5 °C in July and from 19.1 °C to 32.4 °C in February. These temperatures have a substantially smaller range than on the mainland: the summer temperature is typically a degree cooler, while winter temperatures are a good deal warmer. This is due to the influence of the ocean, and to the Leeuwin Current.

86% of the rain falls between April and September; on average there are 89 rain days, resulting in 469 millimeters of rain. The wettest month is June, when over 100 millimeters typically falls. In contrast, only about 70 millimeters can be expected to fall between October and March.

It is nearly always windy. During summer a high pressure ridge lies to the south, causing persistent winds from the southeast or southwest, at speeds exceeding 17 knots (31 km/h) almost half the time. During autumn and winter, the ridge moves northwards, increasing the atmospheric pressure over the islands, resulting in highly variable winds. Winter tends to produce both the strongest gales and the most frequent periods of calm. In addition to these winds, there is daily pattern of land breezes in the morning, followed by the onset of south-westerly sea breezes in the afternoon. This pattern is caused by temperature differences between the land and the ocean, and is not as strong in the Abrolhos as on the mainland, but is present nonetheless.

Three classes of storm have been identified for the region. Brief squalls may occur between December and April. A tropical cyclone occurs in the area about once in three years, between January and April; these may generate extremely high wind speeds that are potentially destructive. During winter, extra-tropical cyclones sometimes pass south of Geraldton, generating winter winds with gusts of up to 35 meters per second, the wind direction from the northwest initially, then gradually moving around to southerly.

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