Search

We objectively identified an optimal number of atmospheric weather regimes, also called synoptic types, within the southwest (SW) Pacific tropical–subtropical domain and examined their potential drivers. Six atmospheric weather regimes in this region are characterized by spatially heterogeneous geopotential height, sea surface temperature, and regional precipitation patterns. The identified weather regimes are phase-locked to the seasonal cycle with a moderate degree of coherency, and some are capable of persisting for weeks or more in extreme cases. Correlations between the SW Pacific weather regimes and global precipitation reanalysis fields indicate a strong connection between regional weather patterns and South Pacific convergence zone (SPCZ) mean position changes and relative intensity of convective loci within the SPCZ. Climate field correlations to SW Pacific weather regimes also show distinct geopotential height and SST signatures across Southern Hemisphere middle and high latitudes and the Indian Ocean basin. Strong statistical significance for portions of those spatial patterns lends support to the assertion of extra-basin teleconnections for SW Pacific weather regimes. There are strong precipitation impacts from SW Pacific weather regime frequency changes and regime persistence on extreme rainfall deficits and/or surpluses for small islands during austral summer. Diagnostic analysis of the spatial correlation fields and each weather regime indicates these weather patterns are connected to eastern equatorial Pacific-styles of El Nino and La Nina and Modoki La Nina. Another regime type appears to be connected to an enhancement of the Hadley–Ferrell circulation, while two other types are influenced by phenomena that arise outside the Pacific basin (Madden–Julian oscillation, Southern Annular Mode, and Pacific South American mode). SW Pacific weather regime investigations in the context of modern climate, palaeoclimatology, and future climate change scenarios can help to surmount spatial-scale mismatches that exist between global models and small Pacific islands, while helping to improve general understanding of island-scale impacts from atmospheric circulation.

The relevant literature on extreme rainfall events in the Pacific remains relatively sparse compared to other regions (e.g. the coterminous United States, Europe, etc.). Moreover, several recent reports on climate in the Pacific mention the paucity of extremes information and often list ‘trends in historical climate’ as a necessary next step. This scientific assessment meets this need by examining historical trends in and drivers of extreme rainfall events across the entire Pacific Basin, inclusive from Alaska southward to Australia, and longitudinally from the Philippines eastward to North America, with an emphasis on island and coastal locations (within 200 km of the coastline). There is evidence of a general decrease in the frequency of annual extreme rainfall events, yet the amount of extreme precipitation contributing to annual and seasonal totals appears to be on the rise. Region-wide, the number of consecutive dry days is increasing for those locations that are already dry, while the number of consecutive wet days is increasing for the already wet locales. The data for extreme rainfall statistics are considered to be relatively high quality for trend detection, while the level of understanding of the physical causes behind extreme rainfall is positively high. Since the ability to analyze the changes in historical rainfall extremes with some confidence is relatively recent, understanding is expected to improve in the future with the advancement of new datasets and ‘climate reanalysis’ projects.

Auckland, New Zealand, is influenced by ex-tropical cyclones (ETCs); however, there is no established climatology for these weather systems at present. In this study, we present a climatology of ETCs for Auckland and investigate whether their spatio-temporal traits are influenced by large-scale climate modes such as the El Ni˜ no-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). We utilize the South Pacific Enhanced Archive for Tropical Cyclone research (SPEArTC) to establish a climatology that covers the high-quality TC data period from 1970 to 2010. ETCs have entered a region within 550km of Auckland city regardless of ENSO or SAM phase, and on average one event per year occurs. Peak ETC season for Auckland is during March, preceded by an increase in activity during February, with a wide-range of meteorological impacts. Regional circulation patterns, including reduced blocking in the southwest Pacific and synoptic type presence, allow ETCs into the Auckland sector of the southwest Pacific while guiding them either to the east or west of the city.

This article examines the influence of El Nino Southern Oscillation (ENSO), Southern Annular Mode (SAM) and Interdecadal Pacific Oscillation (IPO) on the synoptic weather types over New Zealand. The effects of ENSO (indicated by the Southern Oscillation Index, SOI), SAM and IPO on the occurrence of synoptic types are estimated in a holistic framework using the maximum likelihood method via applications of generalized linear models, both annually and by seasons. The average within-class variations in the intensity and air-mass characteristics (as expressed in the Auckland Airport meteorological variables) of synoptic systems were examined for individual types at the annual level. The results show that ENSO, SAM and IPO have significant effects on the probability of occurrence and to lesser degree the intensity and air-mass characteristics of some synoptic types. The effects vary considerably with seasons, synoptic types and phases of SOI, SAM and IPO and it is the confounding effects of different large-scale modes that lead to the observed changes in the type frequencies. The findings, with respect to ENSO and SAM in all cases and for IPO at the seasonal level, are in good agreement with the literature. However, the annual changes in type frequencies associated with the 1976/1977 IPO phase shift do not support the observed changes in the strength of anomalous southwesterly windflows over New Zealand, and the influence of the recent negative IPO phase also appears different from what we expect based on the existing research. It appears that the interaction between IPO and ENSO is important for understanding the observed climatic effects of IPO on New Zealand's weather and climate. This aspect deserves further attention in future studies. The findings from this work have important methodological and practical implications for New Zealand climate research. Copyright © 2012 Royal Meteorological Society

The ecosystems and economies of small island nation states and territories of the tropical southwest Pacific region are widely agreed to be among the most vulnerable to climate variability and weather extremes anywhere in the world. Tropical Cyclones (TCs) are capable of exacerbating existing hazards and those made more severe by climate change (e.g. local sea level rise). In order to properly understand TC impacts in this region, a comprehensive database of TC tracks is required. This work has collated TC best track data from forecast centres around the globe with the aim of producing a unified global best TC track dataset for the historical period. Data from the International Best Tracks for Climate Stewardship (IBTrACS) project information for the southwest Pacific (135°E–120°W, 5°–25°S) is built upon and included in this effort. We document the construction of an enhanced TC database for the southwest Pacific, the quality controls needed to construct the database, and discuss how it has enhanced the chronology of region-wide historical TC activity in light of newly discovered data. We suggest this enhanced dataset can be used in forthcoming climate and weather studies to better characterize the climatology and behaviour of TCs in the southwest Pacific. Copyright © 2011 Royal Meteorological Society

Although many of the main characteristics of the El Nino-Southern Oscillation (ENSO) phenomenon have been established, uncertainties remain concerning its multidecadal- to millennial-scale evolution. Because of the shortness of the instrumental record, we need to resort to proxy-based reconstructions to investigate ENSO's history prior to the mid 19th century, but the available proxy data is limited in both time and space. Here we investigate the potential for ENSO reconstruction from the tree rings of Agathis australis (kauri). Kauri is a long-lived endemic New Zealand conifer and grows in an ENSO teleconnection region not previously represented in ENSO multi-proxy studies. A high quality 423 year kauri regional master chronology (AD 1580–2002) is constructed. Statistical analysis of the period AD 1876–2002 confirms previous findings that kauri tree rings carry a strong regional-scale climate signal and that ENSO is a significant contributor (predominantly via the western pole of the Southern Oscillation). Kauri carries a signal of both ENSO phases, but with a slight El Nino bias. Growth sensitivity is primarily registered through a five-season window, extending from March (prior to growth initiation in September) through to the following May, with strongest relationships across the middle three seasons (June–February). Relationships appear to be stationary. We conclude that kauri has sufficient ENSO event capture skill to make it a useful addition to future multi-proxy ENSO reconstruction efforts. It may also have potential for stand-alone reconstruction of multidecadal- to millennial-scale evolution of ENSO activity, especially ENSO robustness. Copyright © 2007 Royal Meteorological Society