ALBERT

Notes: Abstract  Distinct periods of warmth have been identified in instrumental records for New Zealand and the surrounding southwest Pacific over the past 120 years. Whether this warming is due to natural climate variability or the effects of increasing greenhouse gases is difficult to determine given the limited length of instrumental record. Longer records derived from tree rings can help reduce uncertainties in detection of possible causes of climatic change, although relatively few such records have been developed for the Southern Hemisphere. In this work, we describe five temperature-sensitive tree-ring width chronologies for New Zealand which place the recent warming trend into a long-term (pre-anthropogenic) context. Included are three pink pine (Halocarpus biformis) chronologies, two for Stewart Island and one for the North Island of New Zealand. Two silver pine (Lagarostrobus colensoi) series, one each from the North and South Islands, are updated from previous work. The length of record ranges from AD 1700 for Putara, North Island to AD 1400 for Ahaura, South Island. The pink and silver pine are different species from those used previously to reconstruct temperatures for New Zealand. All five chronologies are positively and significantly correlated with warm-season (November-April) individual station temperature records, a New Zealand-wide surface air temperature index and gridded land/marine temperatures for New Zealand and vicinity. The highest 20 and 40-year growth periods in all five tree-ring series coincide with the New Zealand temperature increase after 1950. An exception is found for the 40-year interval at Ahaura, the least temperature-sensitive of the five sites. A t-test comparison indicates that these recent growth intervals are significantly higher (0.01 to 0.0001 level) than any of those prior to the twentieth century for three of the five sites, dating as far back as AD 1500. The results suggest that the recent warming has been distinctive, although not clearly unprecedented, relative to temperature conditions inferred from tree-ring records of prior centuries.

Notes: Abstract We describe an improved tree-ring reconstruction of mean warm-season (November–April) temperatures for Tasmania from Huon pine. This record extends back to 1600 BC and is based on a tree-ring chronology that was processed to retain as much low-frequency variance as possible. The resulting reconstruction explains 46.6% of the variance and verifies significantly when compared to withheld instrumental data. Cross-spectral analysis of actual and estimated temperatures over the 1886–1991 common period indicates that most of the unexplained variance is at periods 〈 12 years in length. At periods 〉 12 years, the squared coherency ranges between 0.6–0.8, and the cross-spectral gain indicates that the amplitude of the reconstruction is a nearly unbiased estimate of the true temperature amplitude. Therefore, this reconstruction should be especially useful for studying multi-decadal temperature variability in the Tasmanian sector of the Southern Hemisphere over the past 3592 years. To this end, we examined the time evolution of low-frequency temperature amplitude fluctuations and found evidence for a 35% amplitude reduction after AD 100 that persisted until about AD 1900. Since that time, the low-frequency temperature amplitude has systematically increased. We also show how this reconstruction is related to large-scale sea surface temperatures (SST) in the Indian Ocean and eastward to the dateline. Pointwise correlations between the Tasmanian record and SSTs reveal a relationship that extends across the southern Indian Ocean and towards the Arabian Sea. This pattern is largely determined by inter-decadal temperature variability, with correlations in this 〉 10-year bandwidth commonly exceeding 0.6 over most of the southern Indian and southwestern Pacific sectors. A rotated empirical orthogonal function analysis reveals that the pattern of pointwise correlations found between the temperature reconstruction and SSTs is largely explained by the linear combination of three orthogonal modes of SST variability.

Notes: Abstract.  A tree-ring chronology network recently developed from the subantarctic forests provides an opportunity to study long-term climatic variability at higher latitudes in the Southern Hemisphere. Fifty long (1911–1985), homogeneous records of monthly mean sea-level pressure (MSLP) from the southern latitudes (15–65 °S) were intercorrelated on a seasonal basis to establish the most consistent, long-term Trans-Polar teleconnections during this century. Variations in summer MSLP between the South America-Antarctic Peninsula and the New Zealand sectors of the Southern Ocean are significantly correlated in a negative sense (r=−0.53, P〈0.001). Climatically sensitive chronologies from Tierra del Fuego (54–55°) and New Zealand (39–47°) were used to develop verifiable reconstructions of summer (November to February) MSLP for both sectors of the Southern Ocean. These reconstructions, which explain between 37 and 43% of the instrumentally recorded pressure variance, indicate that inverse trends in MSLP from diametrically opposite sides of Antarctica have prevailed during the past two centuries. However, the strength of this relationship varies over time. Differences in normalized MSLP between the New Zealand and the South America-Antarctic Peninsula sectors were used to develop a Summer Trans-Polar Index (STPI), which represents an index of sea-level pressure wavenumber one in the Southern Hemisphere higher latitudes. Tree-ring based reconstructions of STPI show significant differences in large-scale atmospheric circulation between the nineteenth and the twentieth centuries. Predominantly-negative STPI values during the nineteenth century are consistent with more cyclonic activity and lower summer temperatures in the New Zealand sector during the 1800s. In contrast, cyclonic activity appears to have been stronger in the mid-twentieth than previously for the South American sector of the Southern Ocean. Recent variations in MSLP in both regions are seen as part of the long-term dynamics of the atmosphere connecting opposite sides of Antarctica. A detailed analysis of the MSLP and STPI reconstructions in the time and frequency domains indicates that much of the interannual variability is principally confined to frequency bands with a period around 3.3–3.6 y. Cross spectral analysis between the STPI reconstruction and the Southern Oscillation Index suggests that teleconnections between the tropical ocean and extra-tropical MSLP variations may be influencing climate fluctuations at southern latitudes.

Notes: Abstract The limited length and spatial coverage of instrumental climate data for many areas of the Southern Hemisphere impedes the study of atmosphere-ocean dynamics prior to the past century. Such analyses are important for understanding interannual to decadal variation of the Southern Hemisphere circulation and whether recent changes are related to anthropogenic effects rather than natural variability. We use a middle- to high-latitude tree-ring width data set (from Tasmania, New Zealand and Tierra del Fuego) to reconstruct sea-level pressure (SLP) variability spanning the Tasman Sea and vicinity since ad 1740. The variables reconstructed are austral summer (November–March) SLP for Hobart, Tasmania (43°S, 147°E) and the Chatham Islands, New Zealand (44°S, 177°E), as well as a meridional circulation index (Hobart-Chatham Islands index) which measures the pressure gradient between these two stations. The three reconstructions are well verified statistically and capture between 40 and 48% of the variance in the SLP data. The instrumental and estimated SLP show similar spatial patterns of correlation with the sea surface temperature (SST) field for the Pacific. Statistically significant (above 95% level) 3–3.5 year spectral peaks are identified in the three reconstructions using multitaper spectral analysis, and a significant 4–5 year peak is found in both the Chatham Islands and Hobart-Chatham Islands SLP reconstructions. These two modes are within the bandwidth of the El Nino-Southern Oscillation. Although very speculative, they may also correspond to a proposed Antarctic circumpolar wave of SLP, SST, wind and sea-ice extent, believed to play a key role in atmosphere-ocean circulation for the Southern Hemisphere.

Notes: Abstract Eight tree-ring chronologies from coastal sites along the Gulf of Alaska (GOA) are used to develop a 227-year (1762–1988) reconstruction of spring/summer (March–September) coastal land temperatures for the region. This reconstruction explains 35% of the variance in the instrumental temperature data. The tree-ring records and reconstruction reflect the documented 1976 transition from cold to warm conditions in the North Pacific and are consistent with regional temperature compilations. Three of the eight ring-width series, from elevational timberline sites where trees are particularly stressed by temperature, extend back to A.D. 1600 and are used to identify additional occurrences of such transitions. The first principal component (PC) scores of these three longer records are positively correlated with spring (March–May) land and sea surface temperatures for the GOA region and are used to reconstruct land surface temperatures. Decadal-scale fluctuations in the reconstructions show agreement with decade-long changes in the intensity of the Aleutian Low pressure cell over the past century, suggesting that the tree-ring data may provide an index of past circulation changes for the northeast Pacific. Blackman-Tukey spectral analyses of both reconstructions indicate significant power at 7–11 years, with additional peaks at 3 years for the spring/summer reconstruction and 19 years for the longer spring temperature series. The modes of variation at about 3 and 7 years may correspond to those associated with the El Niño-Southern Oscillation bandwidth, whereas the 19-year term may relate to a proposed 20-year cycle of North Pacific circulation. The spring temperature series shows generally increased growth over the past century, coinciding with warmer spring temperatures in south coastal Alaska over this interval. Comparison with the entire spring series suggests that the recent warming exceeds temperature levels of prior centuries, extending back to A.D. 1600.