Climatic variability associated with increased monsoonal activity during the summer months in the tropical north and seasonal cycles in the temperate south leads to variations in water temperature (e.g. Figure MAR4), rainfall patterns (affecting ocean salinity), surface winds, oceanic currents and tidal regimes, which can influence the degree of vertical mixing through the water column (Feng et al. 2003, Ridgway & Condie 2004, Ridgway 2007, Redondo-Rodriguez et al. 2012, Ceccarelli et al. 2013).
These seasonal cycles in ocean physical processes have been relatively stable on evolutionary timescales, and species within the marine environment have evolved in response. Ocean primary productivity demonstrates variation in response to seasonal cycles in ocean processes (Tilburg et al. 2002, Thompson et al. 2015b). This is reflected in secondary producers and higher-order marine organisms that have also evolved to synchronise biological processes such as breeding or migration with these cycles (e.g. Stevens & Lyle 1989, McPherson 1991, Heithaus 2001, Gill 2002, Patterson et al. 2008). Any change in the duration or intensity of seasonal cycles on timescales shorter than that in which organisms in the marine environment can adapt may lead to mismatches in biological processes, resulting in deleterious impacts on marine populations. Understanding how natural variability drives processes within the marine environment, and how extremes in this variability affect marine organisms and processes is therefore essential to both quantifying and understanding the impacts associated with anthropogenically driven climate change.
Current projections of climate suggest that changes to seasonal cycles are occurring and will continue to occur, but with considerable variability across Australia. Climate zones within the marine environment have moved polewards, resulting in shifts in associated seasonality (Lough 2008). Extremes associated with the summer months will become more prevalent (Reisinger et al. 2014), particularly when coupled with ENSO (see Interannual and subdecadal variability). Modelled simulations of the climate under the Intergovernmental Panel on Climate Change (IPCC) emissions scenarios identify that seasonal monsoon and cyclone systems are likely to intensify (Christensen et al. 2013). How these will affect seasonal cycles in marine ecosystems is not yet clear, but shifts in the onset of seasonal migrations and breeding have been observed elsewhere (e.g. Dufour et al. 2010, Asch 2015).