Trends and variability in climate extremes in the Western Pacific
Creators: name: MCGREE, Simon
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 370108
subject: Meteorology
classificationCode: 370201
subject: Climate change processes
classificationCode: 490399
subject: Numerical and computational mathematics not elsewhere classified

description: This research is focused on examining interannual and long-term change in precipitation and temperature extremes in the western Pacific. The Pacific Islands have been identified as being among the most vulnerable to climate change and climate extremes with changes in climate extremes likely to impact many sectors. In addition, smallness renders island the countries at risk of high proportionate losses and despite the known impacts, relatively little is known about changes in climate extremes in the region.

In Chapter 2, precipitation records for 21 countries and territories in the western Pacific for the period 1951 to 2010 were examined to identify trends in drought occurrence, duration and magnitude. The strength of the relationship between the main climate drivers of variability in the Pacific, El Niño-Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO) and Pacific Decadal Oscillation (PDO) and precipitation were also examined. Station scale drought trends were largely positive but the majority were statistically non-significant with the significant trends mainly in the subtropics. Spatially, trend patterns were largely heterogeneous. A significant relationship between the oceanic component of ENSO and precipitation was confirmed for a large part of the Pacific Islands and east Australia with a strong lagged relationship in the year after the El Niño onset at locations southwest of the South Pacific Convergence Zone (SPCZ) and north of the Intertropical Convergence Zone (ITCZ). Similarly, a strong relationship was found with IPO and PDO at most locations. Drought was found to be longer and more severe southwest of the SPCZ and north of the ITCZ during the positive phase of the IPO and PDO. 

In Chapter 3, trends in mean and extreme annual and seasonal temperature and precipitation over the 1951-2015 period were calculated for 57 stations in 20 western Pacific Island countries and territories. The extremes indices are those of the World Meteorological Organization Expert Team on Sector-specific Climate Indices. The purpose of expert team and indices is to promote the use of globally consistent climate indices to highlight variability and trends in climate extremes which are of particular interest to socio-economic sectors, and to help characterise the climate sensitivity of various sectors. Prior to the calculation of the monthly means and indices, the data underwent quality control and homogeneity assessment. A rise in mean temperature occurred at most stations, in all seasons and in both halves of the study period. The temperature indices also showed strong warming, which for the majority of stations was strongest in December to February and weakest in June to August. The absolute and percentile-based indices show the greatest warming at the upper end of distribution. While changes in precipitation were less consistent and trends generally weak at most locations, declines in both total and extreme precipitation were found in Southwest French Polynesia and the Southern subtropics. There was a decrease in moderate to high intensity precipitation events, especially those experienced over multiple days, in Southwest French Polynesia from December to February. Strong drying trends have also been identified in the low to moderate extreme indices in the June to August and September to November periods. These negative trends contributed to an increase in the magnitude of meteorological drought in both subregions. The relationship between total and extreme precipitation and Pacific basin sea surface temperatures were investigated with a focus on the influence of the ENSO. A strong relationship between ENSO and total precipitation is substantiated and similar relationships for the threshold extreme indices established. The percentile-based and absolute extreme indices are influenced by ENSO to a lesser extent and in some cases the influence is marginal.

The primary objective of Chapter 4 was determining the magnitude of the influence of mean and extreme climate on agricultural productivity in the western Pacific using sugarcane and sugar yield in western Fiji as a case study. Sugarcane is one of Fiji's largest commercial agricultural crops and greater than 80% of the raw sugar produced is exported. Few sugar producing countries are as dependant on the contribution of sugar to the export market as Fiji. There has been a statistically significant decline in sugar yield since 1975. The proportion of sugar extracted from sugarcane has also declined as shown by a positive trend in the tonnes cane to tonnes sugar ratio. The role of climate in these changes was investigated by first using principal component analysis then stepwise regression to predict sugarcane and sugar yield. 'Mild drought conditions', an increase in the diurnal temperature range and cool conditions during the ripening and maturation period are favourable for sugar yield. The impact of future warmer, wetter and drier conditions on sugar yield was also examined, in the absence of adaptive measures. Results show declines in sugar yield with an increase in mean and extreme temperature. Results also show an increase in the number of rain days in March offsets the increase in temperature suggesting that an increase the number of rain days in the late growing season in a future climate may counter the influence of higher temperatures. Irrigation in the late growing season may be an option to increase yields and/or adapt to a warmer climate.