Climate and system variability

2016

Seasonal variability

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).

Interannual and subdecadal variability

Australia’s marine environment is also influenced by cycles in climate on interannual timescales associated with several natural climate phenomena, such as ENSO, the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM), also known as the Antarctic Oscillation. Of the climate phenomena that occur in the Southern Hemisphere, ENSO and the IOD potentially have the most widespread influence on the Australian marine environment, habitats, communities and species groups, whereas the influence of the SAM is mostly across the central and southern parts of Australia. Variability in the phases of these climate phenomena change rainfall patterns, sea surface temperatures, surface winds and oceanic currents, which can influence the degree of vertical mixing through the water column and the relative location of cyclone events (McBride & Nicholls 1983, Lough 1994, Lau & Nath 2000, Feng et al. 2010). Changes in the timing and magnitude of seasonal variability caused by these climate phenomena (McBride & Nicholls 1983, Mitchell & Wallace 1996, Lau & Nath 2000, Feng et al. 2010) have flow-on impacts on the marine environment and, in particular, species that have adapted to take advantage of seasonal cycles for processes such as breeding or feeding.

Simultaneous fluctuations in the phases of these climatic phenomena, or particularly strong phases of each, can result in extreme changes in ocean processes, contributing to events such as the marine heatwave that occurred in the waters off Western Australia during the summer of 2010–11 (Pearce et al. 2011, Pearce & Feng 2013). This event, driven by the occurrence of a strong La Niña phase of ENSO, superimposed on long-term increasing water temperatures associated with climate change (see Climate change), resulted in widespread bleaching of corals across the North-west Marine Region. It was also associated with fish and invertebrate deaths, extensions and contractions in species distributions, variations in recruitment and growth rates, impacts on trophic relationships, and shifts in community structure, particularly in relation to kelp forests across the North-west and South-west marine regions (Wernberg et al. 2016). Variations in the catch rates of exploited species were also recorded after the event (Pearce et al. 2011, Pearce & Feng 2013).

The strongest El Niño phase of ENSO since 1998 occurred in 2015–16. Similarly to the Western Australian heatwave, this was superimposed on an increasing baseline of ocean temperatures associated with climate change, and resulted in the highest sea surface temperatures across the Great Barrier Reef on record. These extreme temperatures caused extensive coral bleaching and die-off, particularly across northern regions of the Great Barrier Reef and parts of north-western Australia (see Quality of habitats and communities). A marine heatwave was also recorded off eastern Tasmania from December 2015 to May 2016 in association with the same conditions, although impacts of this heatwave on the marine environment are yet to be established.

The ability to predict climate variability in the marine environment beyond the presence of annual cycles is not well developed, and forecasts beyond 1–2 years are highly inaccurate (Kirtman et al. 2013, Evans et al. 2015). Even our current ability to predict the phase of ENSO varies. It is possible to predict the likelihood of an El Niño event, but it is not so easy to predict its timing or strength. The accuracy of prediction declines rapidly as the time to the event increases, because even small perturbations to the system (such as the random occurrence of a tropical cyclone) can lead to a very different ENSO phase later in the year (Jin et al. 2008).

Predictability of ENSO is further complicated by the diversity of ENSO behaviours. No 2 events are alike, and the onset and progression of each event are characterised by unique changes to sea surface temperatures, surface winds and the mixed layer (Singh et al. 2011). Long-term records of ENSO demonstrate high multidecadal variability, with some decades experiencing less variability in ENSO and others experiencing more (Harrison & Chiodi 2015). Although it is almost certain that climate phenomena such as ENSO will continue to occur and be the dominant mode of interannual variability, the influence that climate change might have on these phenomena is unclear (Brown et al. 2013, Christensen et al. 2013, Evans et al. 2015).

Interdecadal variability

Climate cycles also occur on longer timescales, with the Pacific Decadal Oscillation (PDO; Mantua & Hare 2002) the most important phenomenon influencing the Australian marine environment. The PDO has been described as a pattern of climate variability similar to ENSO, with positive phases having similar effects on the climate to El Niño and negative phases similar effects to La Niña, but operating on scales from 15 years to as long as 70 years (Mantua & Hare 2002). This results in initially abrupt transitions to conditions that are then stable across multiple decades (Hilborn et al. 2003). Interactions between phases of the PDO and ENSO can either modulate or strengthen the phases of ENSO (Cai & van Rensch 2012), with flow-on effects on ocean processes and the marine environment (see Interannual and subdecadal variability). Biological responses to the PDO include changes in primary productivity, which are transmitted through the food chain, resulting in changes in productivity of higher trophic levels (Hare & Mantua 2000, Hilborn et al. 2003). A recent negative phase of the PDO has been associated with increased rainfall and La Niña conditions in the Temperate East Marine Region (Cai & van Rensch 2012). However, how the PDO drives variability is less well understood for the Australian marine environment than for other regions of the Pacific Ocean. Climate variability on decadal scales associated with features such as the PDO is not currently predictable (Evans et al. 2015).

Evans K, Bax NJ, Smith DC (2016). Marine environment: Climate and system variability. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/marine-environment/topic/2016/climate-and-system-variability, DOI 10.4226/94/58b657ea7c296