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Feature

Millennium Coastal Concern

Our beach holidays may be doomed by the weather, but it's the baches at risk from El Nio.

Dr Willem de Lange and Dr Jeremy Gibb

Most people are aware of the variations in our weather caused by the Southern Oscillation, with its extremes of El Niño and La Niña. The variations are part of a cycle known as ENSO that oscillates over periods of 4 to 7 years. What may be less appreciated is that these weather cycles are linked to patterns of coastal erosion around New Zealand.

Research into coastal erosion of the last few decades suggest that at most places around New Zealand the erosion is cyclical. Coastal locations undergo several decades of erosion, followed by similar periods of recovery. In some areas, such as Hokitika, the cycles are linked to pulses of sediment moving along the coast. In other parts of the country, such as the northeast coast between North and East Capes, the cycles seem to be due to sediment moving offshore and onshore. The obvious question then is what drives these cycles?

The pulses of sediment along the West Coast may be attributed to changes in the size and frequency of floods that supply sediment to the beaches. Similarly, cycles along the northeast coast of New Zealand could be weather related. Coastal erosion during storms is affected by the size of the waves, but more importantly by the mean water level and wind direction. More erosion occurs with elevated water levels (storm surges), and onshore winds that drive the surface water shorewards, producing an offshore-directed near-bed flow.

It was already known that for the northeast coast, these conditions occur during La Nia events and the opposite prevails during El Niño. A database of storm events and associated storm surges for Tauranga was compiled using tide gauge records maintained by the Port of Tauranga. This provided detailed information on every event since 1960. Additional information was obtained for severe events earlier this century using newspaper archives and interviews with residents.

The database was used to examine the relationship between storms and the severity of storm surges, with changes in ENSO. Contrary to expectations, the connection was relatively weak, which indicated that some other process might be involved. Fortunately, researchers elsewhere around the Pacific had also been examining long-term cyclic behaviour involving a wide variety of phenomena.

These studies have found subtle changes in worldwide ocean and atmospheric circulation occurring over cycles of 20 to 50 years. These combine to produce major shifts in climatic patterns roughly every 25-35 years. Evidence for these shifts come from a variety of sources:

  • salmon production in the North Pacific
  • monsoon behaviour in South-east Asia
  • frequency of severe hurricanes in the Atlantic Ocean
  • lake sediments in the Andes
  • coral growth patterns in Tahiti
  • tree growth rings on the Kaimai Ranges and Stewart Island

Combining the available information, researchers have determined that the major shifts this century have occurred around 1923, 1948 and 1976.

For New Zealand, the climatic shifts are reflected in a changed frequency and magnitude of ENSO events. The period before 1923 and 1948-1976 was dominated by La Niña events, while 1923-1948 and 1976 to the present have been mainly El Niño conditions. These climatic shifts have affected sea levels around New Zealand, with El Niño conditions suppressing sea level and vice versa. Tide gauge data for the northeast coast show that sea level has not risen significantly since the late 70s, but rose rapidly in the period 1950-1975.

The climatic shifts are also reflected in weather patterns over New Zealand. Climate researchers Jim Salinger and Brett Mullan have recently identified that two main circulation changes have occurred in the New Zealand region around 1950 and 1975. From 1930 to 1950, more south to southwest anomalous flow occurred relative to later years (El Niño dominant). This resulted in lower temperatures over the entire country, wetter conditions in the northeast of the South Island and drier conditions in the north and west of the South Island.

Airflow from the east and northeast increased during the period 1951-1975, producing higher temperatures in all regions (La Nia dominant). The north of the North Island became wetter, and drier conditions prevailed in the southeast of the South Island.

During the current climatic regime (since 1976), more prevalent west to southwest airflows have occurred (El Niño dominant). Annual rainfall decreased in the north of the North Island, and increased in the north, west, south and southeast of the South Island.

Millennium Coastal Concern Figure A (27KB)

The rainfall changes may be sufficient to alter the flood frequency along the West Coast of the South Island, and produce the pulses of sediment associated with erosion cycles there. The changes in airflow over the North Island may be the cause of the erosion cycles on the northeast coast. Applying a new analysis technique, wavelet analysis, to the storm surge database revealed that storm surges at Tauranga varied in frequency and magnitude with the climatic regime shifts between 1960 and 1998. Further, a match was found between severe storm events and the regime shifts between 1870 and 1998. This connection is considerably stronger than with ENSO, suggesting that these interdecadal cycles are a dominant control on coastal erosion.

During El Niño dominated phases (such as 1976-present), storm surges are smaller and less frequent. This is associated with relatively few episodes of coastal erosion, and consequently a build-up of sediments on northeast coast beaches. However, during La Niña dominated phases (such as 1948-1976), storm surges are larger and more frequent. This is associated with frequent coastal erosion events, and a widespread retreat of northeast coast beaches.

Analysis of longer climatic cycle records indicate these variations have been occurring for at least 4,000 years, and is not a consequence of recent global warming. What is more important, it is likely they will continue into the future. Wavelet analysis has been used to predict the next shift, and indicates that it will occur between 2000 and 2006.

We expect that the next shift will see a rapid rise in sea level around New Zealand, and an increased frequency and size of storm surges on the coast between North and East Capes. This will result in more frequent episodes of coastal erosion and a retreat of unprotected sandy coasts.

Unfortunately, the current climate regime has been also associated with a large increase in coastal development all round New Zealand. Thus the impact of future coastal erosion is likely to be more severe than it was during 1950-1976. And the lessons learnt during the previous regime (which saw severe erosion problems at places such as Omaha, Matarangi and many Bay of Plenty beaches) seem to have been forgotten.

Dr Jeremy Gibb is a coastal consultant.
Dr Willem de Lange is at the University of Waikato.