Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 19

The Indo‐Australian monsoon is a component of the large‐scale Asian‐Australian monsoon system. The monsoon is associated with the seasonal migration of the ITCZ, which is located 10–15° north of the equator in austral winter and migrates southward over the north of the Australian continent during austral summer, driven by the development of intense heat lows over NW Australia and NW Queensland. North Australia experiences dry low‐level southeast trade winds in winter which undergo a reversal to dominant moist NW winds in summer. The onset of the monsoon usually occurs between late November and early January, but the monsoon season includes periods when southeast winds bring temporarily dry conditions.

The southern limit of the monsoon is defined by areas that receive >85% of the rainfall between November and April. Main rainfall events are associated with cyclones and other disturbances in the monsoon trough. Such conditions do not necessarily translate into high annual rainfall as there is a strong latitudinal gradient of rainfall with large regions along the northern and eastern coastlines having median rainfalls exceeding 1500 mm, but with most of the inland experiencing annual medians from 300 to 600 mm.

The Indo‐Australian wet monsoon season is characterised by excessively wet conditions usually referred to as ‘bursts’ interspersed with relatively dry ‘breaks’; both can sometimes extend for several weeks (Moise et al. 2020). Both Indonesia and Australia are linked in the summer monsoon. These wet and dry periods have a significant impact on recharge of freshwater ecosystems and river discharge on estuarine and marine ecosystems. A ‘burst’ in the Indo‐Australian monsoon is preceded by the development of a well‐defined extratropical wave packet in the Indian Ocean which propagates towards the Australian continent in the few days leading up to the onset of heavy rainfall in the tropics. These extratropical disturbances propagate equatorward over the continent and are accompanied by lower‐tropospheric air mass boundaries which also propagate into low latitudes. Ahead of these boundaries, relatively warm moist air is advected from the surrounding seas. Monsoon bursts are more likely to occur when the active phase of the Madden‐Julian Oscillation (Section 2.6) is in the vicinity of Australia (Duan et al. 2019).