In this project (Kimberley Marine Research Project 2.2.7), historical data and numerical models have been used to identify the climate sensitivity of the Kimberley coast (Western Australia) to interannual and decadal climate variability in the Pacific and Indian Ocean over the past several decades, especially on the variability of ocean temperature, precipitation and salinity, sea level, and shelf current associated with El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole, and Pacific Decadal Oscillation. These analyses have provided the background understanding of the marine environment off the whole northwest shelf including the Kimberley coast and Scott Reef, to improve the predictability of climate-driven environmental variability, especially extreme events such as marine heatwaves. We have also utilised ocean downscaling models to project future climate change impacts on the marine environment such as the ocean temperature and internal wave characteristics. Key scientific findings from this project include:
Precipitation variability of the Kimberley coast are highly sensitive to ENSO variability in the Pacific. High precipitations tend to occur during La Niña and low precipitations tend to occur during El Niño, due to ENSO modulation of the strength of the Australian monsoon and atmospheric convection activities. El Niño induces weaker Australian monsoon and less cloud coverage in the region;
Coastal sea level variability of the Kimberley coast is also influenced by ENSO, through the ocean waveguides. High sea levels during La Niña and low sea levels during El Niño are influenced by ENSO-related trade wind and sea level anomalies in the western Pacific, transmitted through the Indonesian Seas. High sea level anomalies off the Kimberley coast during La Niña often accompany stronger Indonesian throughflow transport. Shelf circulation is also influenced by the Pacific ENSO – the Holloway Current is stronger under the La Niña condition;
Due to the decadal strengthening trend of trade winds in the Pacific toward a La Niña-like condition over the past two decades, there have been fast rising trends of the coastal sea levels off the Kimberley coast since 1990’s. During the same time period, precipitation rates in the region have also enhanced, influenced by the dominant La Niña condition. The 2015-16 strong El Niño may be the start of a shift of the decadal trends;
Unlike the west coast of Australia, sea surface temperatures off the Kimberley coast are less influenced by ENSO-induced variability of ocean boundary current, but are more influenced by air-sea exchanges. During El Niño, the increased solar radiation (due to less cloud coverage) and weakened Australian monsoon would warm the ocean surface. The co-occurring Indian Ocean Dipole would damp the warming tendency. Thus, the warming off the Kimberley coast can be rather strong under the El Niño without co-occurring Indian Ocean Dipole, such as the 2015-16 summer. Sea surface temperatures off the Kimberley coast have moderate rising trend over the past 30 years, mostly in the austral winter;
From historical data and numerical model analysis, it is identified that the marine heatwaves are more likely to occur in the tropical reefs of Western Australia during El Niño events, such as the Kimberley coast, Scott Reef and Rowley Shoals. The marine heatwaves would be particularly strong during El Niño without co-occurring Indian Ocean Dipole. Intraseasonal variations such as the Madden-Julian Oscillation also affect the strength and timing of the marine heatwaves;
As in observations and model simulations, the Indonesian throughflow waters and the coastal waters off the Kimberley became very fresh during the extended La Niña conditions in 2010-11. The freshening of the Kimberley coastal waters were mostly due to the anomalous precipitation rates in the region. There appeared to be a decadal freshening trend in the region, influenced by decadal trend of the large-scale Pacific climate;
From the 1960s to 1990s, there was a climate trend toward more frequent El Niño events in the Pacific, however, this trend has reversed over the past two decades and there have been more frequent La Niña events in recent years, which have affected the occurrences of extreme climatic events off the Kimberley coast and probably have masked some of the long term climate change signals;
Future climate projections suggest that ocean temperatures will warm by more than 1 degree Celsius (°C ) in the next 50 years off the Kimberley coast. The sea surface temperature rising trends of 0.035-0.04°C per year are much higher than the historical trends in observations. The warming will likely increase the water column stratification on the continental shelf.
The future climate projection using a high-resolution shelf model suggests that internal wave energy on the continental shelf will likely increase, due to surface warming and increased stratification of the shelf; and
The implication of a changing climate would be; as the internal waves are important to bring deep nutrients onto the shelf, increased cross-shelf exchange may occur in the future climate; on the other hand, increased vertical stratification may limit vertical mixing and resuspension processes on the shelf; dispersal process on the shelf is overwhelmed by strong tidal stirring.
