Water Supply for Tongatapu; Past, Present and Future.

<p>This thesis presents an investigation of the sustainability of the freshwater aquifer (groundwater) at Tongatapu, the main island of Tonga. Water balance modelling is applied to meteorological data to estimate freshwater recharge at a daily resolution for the period 1980-2018. These results demonstrate a very close coupling between recharge and precipitation but also the critical role played by the ENSO cycle in modulating the supply of freshwater on Tongatapu. They also show that previous water balance modelling for the island, conducted at a monthly resolution, has tended to underestimate the rate of recharge by ~8%. Historical groundwater extraction rates for Tongatapu are also calculated by compiling monitoring data from operational pumping stations across the island. This shows that extraction rates have increased progressively over the past 50 years and approximately doubled in the last 10 years, as a consequence of increased demand from agriculture, tourism and population growth. Although the freshwater resource appears to be sustainable overall at current rates of supply and demand, there have been sustained periods of zero recharge, notably during strong El Nino events in winter (the dry season). Climate model projections of future rainfall show that Tonga is situated in a region of great uncertainty, due to shortcomings in our knowledge of how the inability of the models to capture the ENSO cycle will respond to anthropogenic warming, and but moreover, climate models are currently unable to simulate the precise correct positioning of the South Pacific Convergence Zone which strongly influences the amount and seasonal distribution of regional rainfall. Nevertheless, this study also conducted predictive water balance modelling for Tongatapu for the end of the 21st century using the current CMIP5 climate projections for the region under a medium (scenarios RCP4.5) and high (RCP8.5) emissions scenario, in both cases showing substantial reductions in freshwater recharge rates compared to the present. These results raise serious concerns for the future sustainability of Tonga’s freshwater resource, especially if extraction rates continue to increase and salination of the aquifer increases as is highly likely due to sea level rise. Although Tonga can do little to influence the global climate change mitigation effort, this research highlights the importance of addressing currently resolvable infrastructural problems in water supply and reticulation.</p>

Water Supply for Tongatapu; Past, Present and Future.

<p>This thesis presents an investigation of the sustainability of the freshwater aquifer (groundwater) at Tongatapu, the main island of Tonga. Water balance modelling is applied to meteorological data to estimate freshwater recharge at a daily resolution for the period 1980-2018. These results demonstrate a very close coupling between recharge and precipitation but also the critical role played by the ENSO cycle in modulating the supply of freshwater on Tongatapu. They also show that previous water balance modelling for the island, conducted at a monthly resolution, has tended to underestimate the rate of recharge by ~8%. Historical groundwater extraction rates for Tongatapu are also calculated by compiling monitoring data from operational pumping stations across the island. This shows that extraction rates have increased progressively over the past 50 years and approximately doubled in the last 10 years, as a consequence of increased demand from agriculture, tourism and population growth. Although the freshwater resource appears to be sustainable overall at current rates of supply and demand, there have been sustained periods of zero recharge, notably during strong El Nino events in winter (the dry season). Climate model projections of future rainfall show that Tonga is situated in a region of great uncertainty, due to shortcomings in our knowledge of how the inability of the models to capture the ENSO cycle will respond to anthropogenic warming, and but moreover, climate models are currently unable to simulate the precise correct positioning of the South Pacific Convergence Zone which strongly influences the amount and seasonal distribution of regional rainfall. Nevertheless, this study also conducted predictive water balance modelling for Tongatapu for the end of the 21st century using the current CMIP5 climate projections for the region under a medium (scenarios RCP4.5) and high (RCP8.5) emissions scenario, in both cases showing substantial reductions in freshwater recharge rates compared to the present. These results raise serious concerns for the future sustainability of Tonga’s freshwater resource, especially if extraction rates continue to increase and salination of the aquifer increases as is highly likely due to sea level rise. Although Tonga can do little to influence the global climate change mitigation effort, this research highlights the importance of addressing currently resolvable infrastructural problems in water supply and reticulation.</p>

The South Pacific Convergence Zone as Depicted in Reanalysis and Satellite Products

<p>The South Pacific Convergence Zone (SPCZ) is the largest rainfall feature in the Southern Hemisphere, and is a critical component of the climate of Southwest Pacific Island nations. The small size and isolated nature of these islands leaves them vulnerable to short and long term changes in the position of the SPCZ. Its location and strength is strongly modulated by the El Niño-Southern Oscillation (ENSO) cycle and the Inter-decadal Pacific Oscillation (IPO), leading to large inter-annual and decadal variability in rainfall across the Southwest Pacific. Much of the analysis on the SPCZ has been restricted to the modern period, more specifically the “satellite era”, starting in 1979. Here, the representation of the SPCZ in the Twentieth Century Reanalysis (20CR) product, which reconstructs the three-dimensional state of the atmosphere based only on surface observations is discussed. The performance of two versions of the 20CR (versions 2 and 2c) in the satellite era is tested via inter-comparison with other reanalysis and observational satellite products, before using 20CR version 2c (20CRv2c) to perform extended analysis back to the early twentieth century. This study demonstrates that 20CR performs well in the satellite era, and is considered suitable for extended analysis. It is established that extra data added in the SPCZ region between 20CR versions 2 and 2c has improved the representation of the SPCZ during 1908-1958. Well-established relationships between ENSO and the IPO with the SPCZ are shown to be present through the entire 1908-2011 period, although it is suggested that the physical link between the IPO and the SPCZ has changed between the first and second half of the twentieth century. Finally, evidence of a southward trend of the SPCZ over the past century is presented, potentially due to an expansion of the tropics as a result of climate change.</p>

The South Pacific Convergence Zone as Depicted in Reanalysis and Satellite Products

<p>The South Pacific Convergence Zone (SPCZ) is the largest rainfall feature in the Southern Hemisphere, and is a critical component of the climate of Southwest Pacific Island nations. The small size and isolated nature of these islands leaves them vulnerable to short and long term changes in the position of the SPCZ. Its location and strength is strongly modulated by the El Niño-Southern Oscillation (ENSO) cycle and the Inter-decadal Pacific Oscillation (IPO), leading to large inter-annual and decadal variability in rainfall across the Southwest Pacific. Much of the analysis on the SPCZ has been restricted to the modern period, more specifically the “satellite era”, starting in 1979. Here, the representation of the SPCZ in the Twentieth Century Reanalysis (20CR) product, which reconstructs the three-dimensional state of the atmosphere based only on surface observations is discussed. The performance of two versions of the 20CR (versions 2 and 2c) in the satellite era is tested via inter-comparison with other reanalysis and observational satellite products, before using 20CR version 2c (20CRv2c) to perform extended analysis back to the early twentieth century. This study demonstrates that 20CR performs well in the satellite era, and is considered suitable for extended analysis. It is established that extra data added in the SPCZ region between 20CR versions 2 and 2c has improved the representation of the SPCZ during 1908-1958. Well-established relationships between ENSO and the IPO with the SPCZ are shown to be present through the entire 1908-2011 period, although it is suggested that the physical link between the IPO and the SPCZ has changed between the first and second half of the twentieth century. Finally, evidence of a southward trend of the SPCZ over the past century is presented, potentially due to an expansion of the tropics as a result of climate change.</p>

Impacts of Detoured Madden-Julian Oscillations on the South Pacific Convergence Zone

Madden-Julian Oscillations (MJOs) are a major component of tropical intraseasonal variabilities. There are two paths for MJOs across the Maritime Continent; one is a detoured route into the Southern Hemisphere and the other one is around the equator across the Maritime Continent. Here, it is shown that the detoured and non-detoured MJOs have significantly different impacts on the South Pacific convergence zone (SPCZ). The detoured MJOs trigger strong cross-equatorial meridional winds from the Northern Hemisphere into the Southern Hemisphere. The associated meridional moisture and energy transports due to the background states carried by the intraseasonal meridional winds are favorable for reinforcing the SPCZ. In contrast, the influences of non-detoured MJOs on either hemisphere or the meridional transports across the equator are much weaker. The detoured MJOs can extend their impacts to the surrounding regions by shedding Rossby waves. Due to different background vorticity during detoured MJOs in boreal winter, more ray paths of Rossby waves traverse the Maritime Continent connecting the southern Pacific Ocean and the eastern Indian Ocean, but far fewer Rossby wave paths traverse Australia. Further studies on such processes are expected to contribute to a better understanding of extreme climate and natural disasters on the rim of the southern Pacific and Indian Oceans.

Pacific Meridional Modes without Equatorial Pacific Influence

Investigating Pacific Meridional Modes (PMM) without the influence of tropical Pacific variability is technically difficult if based on observations or fully coupled model simulations due to their overlapping spatial structures. To confront this issue, the present study investigates both North (NPMM) and South PMM (SPMM) in terms of their associated atmospheric forcing and response processes based on a mechanically decoupled climate model simulation. In this experiment, the climatological wind stress is prescribed over the tropical Pacific, which effectively removes dynamically coupled tropical Pacific variability (e.g., the El Niño-Southern Oscillation). Interannual NPMM in this experiment is forced not only by the North Pacific Oscillation, but also by a North Pacific tripole (NPT) pattern of atmospheric internal variability, which primarily forces decadal NPMM variability. Interannual and decadal variability of the SPMM is partly forced by the South Pacific Oscillation. In turn, both interannual and decadal NPMM variability can excite atmospheric teleconnections over the Northern Hemisphere extratropics by influencing the meridional displacement of the climatological intertropical convergence zone throughout the whole year. Similarly, both interannual and decadal SPMM variability can also excite atmospheric teleconnections over the Southern Hemisphere extratropics by extending/shrinking the climatological South Pacific convergence zone in all seasons. Our results highlight a new poleward pathway by which both the NPMM and SPMM feed back to the extratropical climate, in addition to the equatorward influence on tropical Pacific variability.

The role of air-sea coupling on the simulation of intraseasonal rainfall variability over the South Pacific in ECHAM5-SIT

&lt;p&gt;&lt;span&gt;We investigate the impact of air-sea coupling on the simulation of the intraseasonal variability of rainfall over the South Pacific using the ECHAM5 atmospheric general circulation model coupled with Snow-Ice-Thermocline (SIT) ocean model. We compare the fully coupled simulation with two uncoupled simulations forced with sea surface temperature (SST) climatology and daily SST from the coupled model. The intraseasonal rainfall variability over the South Pacific Convergence Zone (SPCZ) is reduced by 17% in the uncoupled model forced with SST climatology and increased by 8% in the uncoupled simulation forced with daily SST. The coupled model best simulates the key characteristics of the two intraseasonal rainfall modes of variability in the South Pacific, as identified by an Empirical Orthogonal Function (EOF) analysis. The spatial structure of the two EOF modes in all three simulations is very similar, suggesting these modes are independent of air-sea coupling and primarily generated by the dynamics of the atmosphere. The southeastward propagation of rainfall anomalies associated with two leading rainfall modes in the South Pacific depends upon the eastward propagating &lt;/span&gt;&lt;span&gt;Madden-Julian Oscillation (&lt;/span&gt;&lt;span&gt;MJO&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt; signals over the Indian Ocean and western Pacific. Air-sea interaction seems crucial for such propagation as both eastward and southeastward propagations substantially reduced in the uncoupled model forced with SST climatology. Prescribing daily SST from the coupled model improves the simulation of both eastward and southeastward propagations in the uncoupled model forced with daily SST, showing the role of SST variability on the propagation of the intraseasonal variability, but the periodicity differs from the coupled model. The change in the periodicity is attributed to a weaker SST-rainfall relationship that shifts from SST leading rainfall to a nearly in-phase relationship in the uncoupled model forced with daily SST.&lt;/span&gt;&lt;/p&gt;

Thermodynamic Contribution to Lightning Activity in the Fourth Chimney

&lt;p&gt;Lightning activity over oceans is normally greatly suppressed in comparison with continents.&amp;#160; The most conspicuous region of enhanced lightning activity over open ocean is found in the equatorial Pacific (150 W) in many global lightning climatologies (OTD, LIS, WWLLN, GLD360, RHESSI, Schumann resonance Q-bursts) and is associated with the South Pacific Convergence Zone (SPCZ).&amp;#160; This oceanic lightning anomaly completes the zonal wavenumber-4 structure of continent-based lightning maxima (with nominal 90-degree longitudinal separation between sources), and so is appropriately named &amp;#8220;the fourth chimney&amp;#8221;.&amp;#160; This region is now under continuous surveillance by the Geostationary Lightning Mapper (GLM) on the GOES-17 satellite (at 137 W).&amp;#160; This total lightning activity is compared with Convective Available Potential Energy (CAPE) from ERA-5 reanalysis.&amp;#160; These CAPE values are correlated with values extracted from thermodynamic soundings at proximal stations Atuona, Rikitea and Tahiti. &amp;#160;The shape of the regional climatology of CAPE resembles that of the SPCZ and is oblique to the equator.&amp;#160; The total lightning flash rate is positively correlated with CAPE, and lightning locations are found preferentially in regions of elevated CAPE on individual days.&amp;#160; The diurnal variation of total lightning for January exceeds a factor-of-two and shows a phase at odds with the usual behavior of oceanic lightning near continents.&lt;/p&gt;

Tracking the South Pacific convergence zone variability and recent acidification reconstructed from tropical corals

&lt;p&gt;Massive tropical corals represent one of the most important natural archives of modern climate change. Coral based reconstructions give us the possibility to extend the instrumental oceanographic records and observe hydrographic variability on seasonal to interdecadal scales in tropical oceans. South Pacific convergence zone (SPCZ) variability, Interdecadal Pacific Oscillation (IPO) and El Ni&amp;#241;o-Southern Oscillation (ENSO) events are major drivers of global climate and may exert control on regional CO&lt;sub&gt;2&lt;/sub&gt; absorption, outgassing and pH variability.&lt;/p&gt;&lt;p&gt;&lt;em&gt;Porites&lt;/em&gt; sp. corals from Tonga and Rotuma (Fijian dependency) are being analyzed for multi-proxy (e.g. Sr/Ca, &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O, &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C, &amp;#948;&lt;sup&gt;11&lt;/sup&gt;B, B/Ca) reconstructions of sea surface temperature and salinity (SST, SSS) and carbonate chemistry, on a monthly to annual resolution. Preliminary data of the Rotuma &lt;em&gt;Porites&lt;/em&gt; sp. coral shows &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O has been decreasing by 0.004 &amp;#8240; per year at the end of the 20th century, suggesting freshening and/or warming of the surface water. In the same period, we observe a &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C decrease of 0.017 &amp;#8240; per year in-line with the anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; driven Suess effect. Initial results of the &amp;#948;&lt;sup&gt;11&lt;/sup&gt;B Tonga &lt;em&gt;Porites&lt;/em&gt; sp. show high interannual variability, and a strong trend of decrease of -0.0626 &amp;#8240; per year in the last five decades of the record (1949-2004) suggesting acidification. The results are in agreement with published coral-based reconstructions from the region.&lt;/p&gt;&lt;p&gt;When completed, the new records will facilitate exploring the effects of modern anthropogenic influence on ocean carbonate system and pH variation, and the relationship between them and interannual and decadal-interdecadal climatic fluctuations.&lt;/p&gt;

Evaluating performances of one-year simulation by using 3.5 km mesh global nonhydrostatic model

&lt;p&gt;Recent advancement of supercomputing enables us to conduct a climate simulation by using a global model with horizontal grid spacing of a few kilometers. We may need to tune the model in order to conduct a reliable simulation. In order to test feasibility of a few kilometer climate simulation in near future, we conducted one-year simulation from June 2004 to May 2005 by using Nonhydrostatic Icosahedral Atmospheric Model (NICAM) with horizontal grid spacing of 28 km, 14 km, 7 km, and 3.5 km, and evaluated their simulation performances. In general, global models have shown weak wind speed of tropical cyclones compared to its central sea level pressure due to insufficient horizontal resolution. As expected, the 3.5 km simulation showed improvement of this bias. As for simulated mean state, globally annual mean precipitation tended to be decreased with finer horizontal resolution in NICAM. Compared with observation (Global Precipitation Climatology Project V2.2; 2.71 mm day&lt;sup&gt;-1&lt;/sup&gt;), 7 km and 3.5 km simulations underestimated the global mean precipitation (2.54 mm day&lt;sup&gt;-1&lt;/sup&gt; and 2.67 mm day&lt;sup&gt;-1&lt;/sup&gt;), while 14 km and 28 km simulations overestimated (2.84 mm day&lt;sup&gt;-1&lt;/sup&gt; and 2.78 mm day&lt;sup&gt;-1&lt;/sup&gt;). The 3.5 km simulation showed the best performance for reproducing globally annual mean precipitation. However, the 3.5 simulation showed underestimation of the South Pacific Convergence Zone. In order to conduct a reliable simulation, we need to improve performance of the 3.5 km global model. This demands extensive computing resources. The supercomputer Fugaku will give us extensive computing resources for addressing this issue.&lt;/p&gt;