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Chatham Islands Climate Change

Executive Summary

1. Introduction

2. Historical Trends and Patterns in Chatham Islands Climate

3. Expected Temperature and Rainfall Changes

4. Tides, storm surge and the effects of sea level rise

5. Analysis of Heavy Rainfall - Past and Future

6. Discussion and Recommendations

7. References

2. Historical Trends and Patterns in Chatham Islands Climate

Key points:

• During the 20th century, annual mean temperature increased by about 1.0°C at the Chatham Islands, and annual rainfall by about 10%. 1998 was the warmest year on record, as it also was for mainland New Zealand.
• El Niño periods tend to be cooler and drier than normal at the Chatham Islands, and La Niña periods warmer and wetter.

2.1 Trends in Temperature and Rainfall

Climate in the New Zealand region is varying all the time, and that of the Chatham Islands is no exception. This is one of the few parts of New Zealand where climate observations commenced in the 19th century. Temperature and rainfall records began in 1878, with a break during 1915-1938 for temperature and 1915-1917 for rainfall. The records analysed in this report have been adjusted to the current NZ Metservice site, 1 km southwest of Waitangi, which opened in 1956. Earlier sites (overlapping in time) were at: Waitangi township 1878-1915, Whangamarino 1912-1929, and Chatham Island radio 1917-1986.

All temperature records show an increase since the 19th century (Fig. 2.1). Mean temperatures have increased by 1.0°C, maximum temperatures by 1.2°C and minimum temperatures by 0.8°C. The coolest runs of years occurred around 1890 and 1900, with the warmest runs of years in the 1980s and around 2000. 1998 was the warmest individual year. The bottom panel of Figure 2.1 shows that the diurnal temperature range (difference between daily minimum and maximum) has also increased.

Despite temperature increases in the latter part of the 20th century, year-to-year variability is quite large. All seasons show increases in mean temperature (Fig. 2.2). The largest rise occurs in summer (+1.3°C) and the least in spring (0.6°C). However, winter shows the most consistent increasing trend (+1.2°C), with a group of very warm years around 2000.

The rainfall record is virtually complete from 1878 (Fig 2.3). From 1878 to about 1940 the climate of the Chatham Islands was drier, with rainfall only 90% of the 1971-2000 normal. Rainfall was then near this normal for the period 1940 to 1990, but has increased by about 10% since 1990 to make this latest period the wettest on record. Summer and autumn rainfall show more variability than other seasons. Summer rainfall (Fig 2.4) increases to a peak of 120% of normal around 1940 before decreasing, then increasing again by the end of the record (2004). Autumn rainfall was only about 80% of the 1971-2000 normal prior to 1950, then increased to 120% of normal by 1960 and has since decreased slightly to 110% of normal. Winter rainfall (Fig. 2.5) decreased to a minimum of 80% of normal by 1900, and then fluctuated subsequently around normal with a slight increase by the end of the record. Springs were drier prior to 1960, averaging only 90% of normal. Since the 1990s spring rainfall in the Chatham Islands has been 110% of normal.

2.2 Natural Variability

There are two key natural cycles that operate over timescales of years (El Niño-Southern Oscillation, ENSO) and decades (Interdecadal Pacific Oscillation, IPO). Both these natural phenomena are confined largely to the Pacific Ocean, but there is evidence that sea temperature conditions elsewhere, such as in the Indian Ocean, can also affect New Zealand climate at some times of year. ENSO is a tropical Pacific-wide oscillation that affects pressure, winds, sea-surface temperature and precipitation. The El Niño phase produces more southwesterly winds over the New Zealand/Chatham Island area, whilst the La Niña phase results in more northeasterlies.

The status of the ENSO phenomenon is commonly measured by the Southern Oscillation Index (SOI), a measure of how unusual the west-east pressure difference is across the tropical Pacific Ocean. Table 2.1 shows correlations between the SOI and anomalies of temperature and rainfall at the Chatham Islands. Significant positive relationships occur with spring and annual temperature, indicating that La Niña years and springs are warmer than in El Niño. Although there was little association between seasonal values of the SOI and rainfall, higher correlations occurred with annual rainfall. Thus, for the Chatham Islands, La Niña years are wetter and El Niño years drier than average.

The Interdecadal Pacific Oscillation is an "ENSO-like" feature of the climate system that operates on time scales of several decades, and appears to modulate the impacts of interannual ENSO climate variability over New Zealand and the southwest Pacific (Salinger et al., 2001). Three phases of the IPO have identified during the 20th century: a positive phase (1922-1944), a negative phase (1947-1977) and the most recent positive phase (1978-1998). Since 1998, the index used to measure the IPO status has been near zero but otherwise shown little change. Changes between the two phases of the IPO did not give any consistent pattern of change for the Chatham Islands (Table 2.2). Between the first early period (1922-1944) and the second (1947-1977) annual rainfall, and particularly autumn rainfall, increased. The next change to the positive phase in 1978 saw a slight increase in annual rainfall, increases in spring rainfall, but decreases in summer rainfall.

Table 2.2 Percentage change in rainfall between the various phases of the Interdecadal Pacific Oscillation (i) 1922-1944 to 1947-1977 and (ii) 1947-1977 to 1978-1998.