Past East Coast Lows

A long-term history of East Coast Lows (ECLs) and their impacts on the NSW coastal environment has been established, and coastal hazards due to extreme ECL storm waves determined as part of two projects under the ESCCI-ECL program. The study of past ECLs gives us important insight into the behaviour and impacts of recent and future ECLs, and provides vital information for coastal planning and natural ecosystem assessments.

Research Results

There is more than one type of ECL and this matters in terms of impacts

We can now provide more detailed information on ECL types and their impacts to improve planning and risk assessment.

There are several types of ECLs, each different in terms of where and when they develop, the path they follow, and where and how they impact the coast. ECLs from the tropics or sub-tropics (such as easterly trough lows) are more common and have greater impacts along the north coast of NSW while ECLs that form outside of the tropics (such as southern secondary lows) have a greater influence along the south coast.

ECLs are most frequent and destructive during late autumn and early winter. Many of these storms are either subtropical easterly trough lows which form off the coast in the subtropics and move southwards, or southern secondary lows that typically form south of the continent and move northwards, often following the passage of a cold front.

Clustering of storms may be more important than individual storms

The most significant impacts from ECLs occur when clusters of storms happen in the same season. The most intense ECLs tend to coincide with these seasonal storm clusters.

Seasons with a high frequency of storms often occur when large-scale ocean-atmosphere circulation patterns favour the clustering of ECLs. The main factor determining whether this clustering occurs appears to be at which latitude blocking high pressure systems are positioned in the Tasman Sea. During these stormy seasons ECLs follow similar tracks and are of a similar type.

ECL prediction may be improved by linking to regional climate drivers

Understanding the large-scale ocean-atmospheric circulation patterns or regional climate drivers that lead to high frequency storm seasons may help to improve ECL forecasts.

From season to season the probability of ECLs occurring increases under certain combinations of regional climate drivers. Easterly trough lows are more frequent during neutral or positive phases of the El Niño-Southern Oscillation (ENSO), when there are cool sea surface temperatures in the tropical Indian Ocean, and with neutral to positive Southern Annular Mode (SAM). Southern secondary lows are more frequent when there are warmer waters in the eastern Indian Ocean, warmer waters in the western Pacific, and high-latitude blocking systems in the Tasman Sea. Because there is some predictability in regional climate drivers, these relationships may be of considerable value in improving ECL forecasts.

Past periods of very stormy weather should be considered in risk assessments

ECL activity in the 1950s to 1970s was higher than has been experienced over 1980 to 2015 period. This means that using satellite data from recent decades alone will not capture the full range of ECL variability that can occur. Risk analysis for coastal planning purposes should therefore consider past periods which experienced significantly higher and more persistent ECL storm activity than the 20th century.

Looking back over the paleo-climate record of the last 1200 years, we see that the period from 1600-1900 was much stormier than now. During these centuries there were many more decades of high and persistent storm activity than we see today. This increased storminess occurred when there was more persistent La Niña-like conditions in the Pacific Ocean combined with warmer waters in the northeast Indian Ocean and negative to neutral phases of the southern annular mode.

Storm erosion risk analysis should consider past extreme storm years and decades

ECLs have been with us for a long time and are a significant driver of the shape of our coastline. Storm erosion risk based on the past about 35 years of shoreline data underestimates the potential cumulative storm erosion hazard caused by the clustering of ECL events. Past extreme storm years have been identified when a number of ECL events occurred in the same year, and in some cases stormy years persisted for a number of consecutive years. Such periods were found to significantly affect the shape of the NSW coastline. The storm erosion hazard is most underestimated for the northern half of beaches on the NSW central and north coasts, and the centre third of beaches on the NSW south coast.

Future storm erosion hazard and risk analysis for the NSW coast should consider the storminess of the 1600-1900 period, and use information from the extreme storm years of 1956, 1967,1974, 2007 and potentially 2011 and 2014. This information is critical for coastal planning for urban settlement, public infrastructure and many natural ecosystems, in particular for identifying and managing storm erosion risks.

ESCCI- ECL program project results

Understanding the long-term natural variability of ECLs by using paleoclimate information

Innovative methods were used to reconstruct a long-term history of ECLs and how these storms impacted the coastal environment. This informs our understanding of the probability of large storms and the clustering of such storms, with the findings assisting ECL risk analysis.

ECL frequencies at event and seasonal resolution have been established for the past 140-years from meteorological reanalysis data. Seasonal ECL frequency over the 1955-2012 period – determined by automated detection and tracking algorithms – is highly correlated to regional ocean-atmosphere circulation patterns. Correlation patterns were used to develop a statistical downscaling approach and estimate autumn-winter ECL frequency from seasonal reanalysis data for 1871 to 2012.

ECL occurrences over the past 1200-years have been established at decadal to multi-decadal resolution. The Paleoclimate Reanalysis (Paleo-R) developed used a set of multi proxy climate variables including oxygen isotopes and tree rings from both sides of the Tasman Sea, coral cores from the eastern Pacific, ice cores from Antarctica and speleothems or cave deposits (e.g. stalagmites/stalactites) from caves in New South Wales.

The investigation of ECL activity across multiple timescales has developed a comprehensive picture of ECL variability and how regional climate drivers affect the occurrence of ECLs. It has supported the identification of different ECL types and given us an understanding of where and when the various types of ECLs develop, and the typical path they follow. Investigation over the 20th century shows storm activity in the 1950-1970s was higher than has been experienced over 1980-2015, meaning analysis based on satellite data alone does not capture the full range of ECL variability.

In the context of the past millennium, ECL activity during the 20th century is close to average. The 1600-1900 period saw more decades of significantly higher and more persistent storm activity than the 20thcentury due to the persistent La Niña-like mean state of the Pacific Ocean coupled to the negative phase (warm north-east Indian Ocean sea surface temperatures) of the Indian Ocean Dipole. Understanding the regional ocean-atmosphere circulation patterns conducive to high frequency storm seasons may contribute to improving ECL forecasts.

References

Browning S (2014). The Drivers Of Multidecadal Climate Variability In The Extratropics Over The Late Holocene. Unpublished MQ University PhD Thesis. 1–357.

Browning SA, Goodwin ID (2013). Large-Scale Influences on the Evolution of Winter Subtropical Maritime Cyclones Affecting Australia’s East Coast. Monthly Weather Review 141:2416–2431. doi: 10.1175/MWR-D-12-00312.1

Browning SA, Goodwin ID (2015). Large scale drivers of Australian East Coast Cyclones since 1871. Submitted to Australian Meteorological and Oceanographic Journal

Browning SA, Goodwin ID (2015). The Paleoclimate Reanalysis Project—Phase 1. In Prep for Climate of the Past

Browning SA, Goodwin ID (2015). Long-term natural variability of east coast lows over the past millennium, Paper presented at the Australian Meteorological and Oceanographic Society National Conference, 15-17 July 2015, Brisbane.

Regional coastal and estuarine impacts of extreme ECLs

This project aimed to determine the drivers of storm wave climate along the NSW coast, and to establish whether the modern benchmark storms from the 1950s to 1970s adequately describe the physical coastal risk to extreme storms or clusters of storms. It also sought to define regionally distinct impacts for the NSW coastal zone for a range of storm intensities as recorded in the geohistorical archive, and determine the ‘ultimate storm erosion limit’ for the NSW coast.

An understanding of the drivers and characteristics of ECL storm waves, their propagation patterns and their coastal impacts over a 500 year period has been developed.  Instrumental wave data was statistically clustered to each of the ECL storm types identified in Project 3. This clustered parametric wave data on storm waves along the NSW coast and Southeast Australian Shelf (SEAS) was then used to hindcast storm wave climate for each of the bi-decadal periods over the past 500 years.

In parallel to these paleo-wave climate studies NSW coastal LiDAR data was used to construct a coastal Digital Elevation Model (DEM) as the basis for identifying paleo-storm impacts on the coast. New and existing field observations on the coastal geology and geomorphology were used to ground truth the DEM-based mapping of the ultimate storm scarp position, and estuarine inlet configuration. The fieldwork techniques included GPS topographic survey, Ground Penetration Radar (GPR) sub-surface stratigraphy, and geochronology using Optically Stimulated Luminescence (OSL) dating of deposited dune and strandplain sand.

The NSW coastal geology and geomorphology gives us information on periods of higher wave power and frequent extreme storm waves during the 1600 – 1900 period. During the 1600 to 1820 period ECL types with easterly to east-southeasterly moving storm waves prevailed over multiple decades, resulting in the coastal planform being realigned with an anticlockwise rotation of the coastal geometry. The storm wave climate produced high rates of onshore sand supply, building high foredunes and resulting in dune transgression in land. Such periods were similar to the storm characteristics of the 1950s to 1970s. There were only a few decades during the entire 1600 to 1820 period (25% of the time) when the coast was able to recover.

During the 1820 to 1900 period there were frequent ECL types with southeasterly moving storm waves. This resulted in a clockwise rotation of the coastal planform, with foredune growth towards the sea extending the strandplain. Such periods of storm recovery produced high rates of alongshore sand supply. The 1900-1950 period similarly experience storm recovery conditions with slanting southeasterly storm waves, alongshore sand transport and clockwise shoreline rotation.

During the 1600-1820 period the estuarine inlets were wide mouthed and persistently open from frequent fluvial flood discharge. Post the early 1800’s the inlets were frequently closed and choked by high rates of sand transport.

The coastal geology and geomorphology indicates that the NSW north and central coasts were most impacted by storm events between 1600-1750, and from 1750 -1900 the south coast was most impacted, as the storm centres shifted pole ward.

References

Goodwin I.D., Burke A., Mortlock T., Freeman R., and Browning S.A. (2015). Technical Report of the Eastern Seaboard Climate Change Initiative of East Coast Lows (ESCCI-ECLs) Project 4: Coastal System Response to Extreme East Coast Low Clusters in the Geohistorical Archive, Report prepared by the Marine Climate Risk Group, Climate Futures at Macquarie University and the Department of Environmental Sciences for the Office of Environment and Heritage..

Mortlock, T.R., and Goodwin, I.D. (2015). Directional wave climate and power variability in The Tasman Sea. Continental Shelf Research 98, 36-53, http://dx.doi.org/10.1016/j.csr.2015.02.007

Goodwin, I.D., Browning, S., Mortlock, T.R., Shand, T. (submitted August 2015). Tropical-Extratropical origin storm wave types and their contrasting directional power distribution on the inner shelf. Submitted to Journal of Geophysical Research - Oceans.

Goodwin, I.D., and Browning, S.A. (in preparation August 2015). The mid‐Millennium climate shift and the onset of extreme maritime storms and coastal change in the south‐west Pacific. To be submitted to Nature Climate Change.

Goodwin, I.D., Browning, S.A., Mortlock, T.R. and Burke, A. (in preparation August 2015). Extreme storm wave climate impacts on the southeastern Australian coast and shoreface during 1600-1900 CE. To be submitted to Geology.

Mortlock, T.R. and Goodwin, I.D. (in preparation August 2015). Storm wave types and their impact on cross-shoreface sediment transport. To be submitted to Journal of Geophysical Research‐Oceans.

Goodwin I.D., Browning S. and Mortlock T. (2014). Eastern Australian Coastal behavior in response to extreme storm climate between 1600-1900 AD, determined from a coupled climate reconstruction and costal morphodynamic approach, American Geophysical Union (AGU) Fall Meeting, San Francisco, 15-19 December 2014 (https://agu.confex.com/agu/fm14/meetingapp.cgi#Paper/19510).