ALBERT

Description: ABSTRACT The focus of this study is the climatological relationship between southwest Pacific Tropical Cyclone (TC) activity and the Madden–Julian Oscillation (MJO) on the interannual time-scale. It investigates TC seasonal characteristics (e.g. cyclogenesis, track morphology, ex-tropical transition, and intensity) as modulated by the phase and intensity of the MJO. The approach is novel as in addition to the focus on the intraseasonal variability of TCs there is also a focus on the interannual variability of TCs even though the MJO is an intraseasonal phenomenon. Links between the MJO, the Southern Annular Mode (SAM) and TC variability are also investigated. Using the MJO phases defined by Wheeler and Hendon ( ), southwest Pacific TC frequency exhibited a statistically significant decrease during MJO phases 2 and 3, and an increase during phases 6 and 7; and during the positive phase of the SAM, an increased frequency of TCs undergoing extratropical transition was observed. In summary, the results show a clear intraseasonal climatological relationship between specific phases of the MJO with respect to a decreased (increased) frequency of TCs during the paired MJO phases 2–3 (6–7); as well as the existence of a statistically significant relationship between the MJO and SAM with respect to the extratropical transition (ETT) of TCs. During positive phases of the SAM, coincident with weak interannual MJO phases 4–5 and 6–7, there are statistically significant greater frequency percentages of TCs undergoing ETT.

Description: ABSTRACT In this study, we focus on possible climatological relationships between southwest Pacific tropical cyclone (TC) activity and the Southern Annular Mode (SAM) and ENSO, at both interannual and synoptic time-scales. The investigation focuses on TC seasonal characteristics (e.g. cyclogenesis, track characteristics, extratropical transition, and intensity) as modulated according to the polarity of the SAM and ENSO. At synoptic time-scales, we also look at possible triggers of TC cyclogenesis in the SAM signal up to 20 days in advance. The physical basis for this relationship is assessed using a number of different gridded data fields. During seasons characterized by positive phases of SAM and both positive and negative Southern Oscillation Index (SOI) values, there is an increased frequency of TCs undergoing extratropical transition near New Zealand. At synoptic time-scales, we found an interesting yet inconclusive relationship between the SAM and tropical cyclogenesis during SOI positive seasons. While statistically significant, there is no clear physical mechanism linking the mid-latitude SAM phenomenon to the genesis of TCs. In summary, the results show a clear interannual climatological relationship between an increased frequency of TCs undergoing extratropical transition near New Zealand and the positive phase of SAM.

Description: ABSTRACT 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.

Description: 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

Description: ABSTRACT Tropical cyclones (TCs) pose a significant risk to the nations and territories of the southwest Pacific (SWP). The spatio-temporal variability of TCs makes it challenging to forecast where and when a TC is likely to develop. Therefore, the aim of this study is to better understand the link between large-scale climatic variability, the environmental conditions required for tropical cyclogenesis (TC genesis), and the spatial variability of TC activity. Three modes of climate variability are investigated: (1) El Niño/Southern Oscillation (ENSO), (2) ENSO Modoki, and (3) the Interdecadal Pacific Oscillation (IPO); along with TC genesis parameters: sea surface temperature; 700 hPa relative humidity; 700 hPa vorticity; and vertical wind shear (difference between 200 hPa and 850 hPa winds). Our findings reaffirm the well-established northeast/southwest modulation of TC activity according to El Niño (EN)/La Niña (LN) using an extended TC dataset (1945–2011). In addition, new insights into how ENSO Modoki and the IPO modulate TC activity according to phase and season are identified. Importantly, we show that depending on phase, the IPO can enhance or alter the spatial modulation of TC genesis during ENSO/ENSO Modoki events, in favour of the northeast/southwest modulations typical of IPO positive/negative events. This is particularly the case during the latter half of the SWP TC season. For example, EN events that occur within IPO positive (negative) epochs result in a shift of TC activity up to 1087 km (1288 km) further east (west) during February to April, compared to the typical location of TC activity during EN events. Importantly, these statistical relationships are also associated with anomalously favourable genesis parameters, providing some insights into the physical mechanisms behind the modulations. The findings of this study provide baseline metrics with which to compare climate model simulations and may also facilitate improved seasonal outlooks and better quantification of TC-related risks for the vulnerable island nations and territories of the SWP.

Description: ABSTRACT 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 Niño-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 550 km 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.

Description: There is overwhelming evidence that the climate system has warmed since the instigation of instrumental meteorological observations. The Fifth Assessment Report of the Intergovernmental Panel on Climate Change concluded that the evidence for warming was unequivocal. However, owing to imperfect measurements and ubiquitous changes in measurement networks and techniques, there remain uncertainties in many of the details of these historical changes. These uncertainties do not call into question the trend or overall magnitude of the changes in the global climate system. Rather, they act to make the picture less clear than it could be, particularly at the local scale where many decisions regarding adaptation choices will be required, both now and in the future. A set of high-quality long-term fiducial reference measurements of essential climate variables will enable future generations to make rigorous assessments of future climate change and variability, providing society with the best possible information to support future decisions. Here we propose that by implementing and maintaining a suitably stable and metrologically well-characterized global land surface climate fiducial reference measurements network, the present-day scientific community can bequeath to future generations a better set of observations. This will aid future adaptation decisions and help us to monitor and quantify the effectiveness of internationally agreed mitigation steps. This article provides the background, rationale, metrological principles, and practical considerations regarding what would be involved in such a network, and outlines the benefits which may accrue. The challenge, of course, is how to convert such a vision to a long-term sustainable capability providing the necessary well-characterized measurement series to the benefit of global science and future generations. Instigating and maintaining a reference network for climate would aid future generations in understanding climate change. Such measurements require strict adherence to metrological best practices and sustained support. This article explores what would be required to make such a network work. Figure is the schematic of the instrumentation at a typical USCRN station in the CONUS. The triplicate configuration of temperature sensors is repeated in the three precipitation gauge weighing mechanisms and in the three sets of soil probes located around each tower.