On the Southern Hemisphere stratospheric response to ENSO and its impacts on tropospheric circulation

As the leading mode of Pacific variability, the El Niño-Southern Oscillation (ENSO) causes vast and wide-spread climatic impacts, including in the stratosphere. Following discovery of a stratospheric pathway of ENSO to the Northern Hemisphere surface, here we aim to investigate if there is a substantial Southern Hemisphere (SH) stratospheric pathway in relation to austral winter ENSO events. Large stratospheric anomalies connected to ENSO occur on average at high SH latitudes as early as August, peaking at around 10 hPa. An overall colder austral spring Antarctic stratosphere is generally associated with the warm phase of the ENSO cycle, and vice versa. This behavior is robust among reanalysis and six separate model ensembles encompassing two different model frameworks. A stratospheric pathway is identified by separating ENSO events that exhibit a stratospheric anomaly from those that don’t and comparing to stratospheric extremes that occur during neutral-ENSO years. The tropospheric eddy-driven jet response to the stratospheric ENSO pathway is the most robust in the spring following a La Niña, but extends into summer, and is more zonally-symmetric compared to the tropospheric ENSO teleconnection. The magnitude of the stratospheric pathway is weaker compared to the tropospheric pathway and therefore when it is present, has a secondary role. For context, the magnitude is approximately half that of the eddy-driven jet modulation due to austral spring ozone depletion in the model simulations. This work establishes that the stratospheric circulation acts as an intermediary in coupling ENSO variability to variations in the austral spring and summer tropospheric circulation.

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>

Soft Periodic Convolutional Recurrent Network for Spatiotemporal Climate Forecast

Many machine-learning applications and methods are emerging to solve problems associated with spatiotemporal climate forecasting; however, a prediction algorithm that considers only short-range sequential information may not be adequate to deal with periodic patterns such as seasonality. In this paper, we adopt a Periodic Convolutional Recurrent Network (Periodic-CRN) model to employ the periodicity component in our proposals of the periodic representation dictionary (PRD). Phase shifts and non-stationarity of periodicity are the key components in the model to support. Specifically, we propose a Soft Periodic-CRN (SP-CRN) with three proposals of utilizing periodicity components: nearby-time (PRD-1), periodic-depth (PRD-2), and periodic-depth differencing (PRD-3) representation to improve climate forecasting accuracy. We experimented on geopotential height at 300 hPa (ZH300) and sea surface temperature (SST) datasets of ERA-Interim. The results showed the superiority of PRD-1 plus or minus one month of a prior cycle to capture the phase shift. In addition, PRD-3 considered only the depth of one differencing periodic cycle (i.e., the previous year) can significantly improve the prediction accuracy of ZH300 and SST. The mixed method of PRD-1, and PRD-3 (SP-CRN-1+3) showed a competitive or slight improvement over their base models. By adding the metadata component to indicate the month with one-hot encoding to SP-CRN-1+3, the prediction result was a drastic improvement. The results showed that the proposed method could learn four years of periodicity from the data, which may relate to the El Niño–Southern Oscillation (ENSO) cycle.

Influence of the 2015–2016 El Niño on the record-breaking mangrove dieback along northern Australia coast

This study investigates the underlying climate processes behind the largest recorded mangrove dieback event along the Gulf of Carpentaria coast in northern Australia in late 2015. Using satellite-derived fractional canopy cover (FCC), variation of the mangrove canopies during recent decades are studied, including a severe dieback during 2015–2016. The relationship between mangrove FCC and climate conditions is examined with a focus on the possible role of the 2015–2016 El Niño in altering favorable conditions sustaining the mangroves. The mangrove FCC is shown to be coherent with the low-frequency component of sea level height (SLH) variation related to the El Niño Southern Oscillation (ENSO) cycle in the equatorial Pacific. The SLH drop associated with the 2015–2016 El Niño is identified to be the crucial factor leading to the dieback event. A stronger SLH drop occurred during austral autumn and winter, when the SLH anomalies were about 12% stronger than the previous very strong El Niño events. The persistent SLH drop occurred in the dry season of the year when SLH was seasonally at its lowest, so potentially exposed the mangroves to unprecedented hostile conditions. The influence of other key climate factors is also discussed, and a multiple linear regression model is developed to understand the combined role of the important climate variables on the mangrove FCC variation.

Mechanisms linking multi-year La Niña with preceding strong El Niño

El Niño-Southern Oscillation (ENSO), characterized by anomalous sea surface temperature in the central-eastern equatorial Pacific, is a dominant interannual variability, impacting worldwide weather and socioeconomics. The ENSO cycle contains irregularity, in which La Niña often persists for more than two years, called “multi-year La Niña”. Observational records show that multi-year La Niña tends to accompany strong El Niño in the preceding year, but their physical linkage remains unclear. Here we show using reanalysis data that a strong El Niño excites atmospheric conditions that favor the generation of multi-year La Niña in subsequent years. Easterly wind anomalies along the northern off-equatorial Pacific during the decay phase of the strong El Niño are found crucial as they act to discharge ocean heat content (OHC) via an anomalous northward Ekman transport. The negative OHC anomaly is large enough to be restored by a single La Niña and, therefore, causes another La Niña to occur in the second year. Furthermore, analyses of the Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models support the abovementioned mechanisms and indicate that the occurrence frequencies of multi-year La Niña and strong El Niño are highly correlated.

The Influence of 2015-16 El Niño On the Record-Breaking Mangrove Dieback Along Northern Australia Coast

This study investigates the underlying climate processes behind the largest recorded mangrove dieback event along the Gulf of Carpentaria coast in northern Australia in late 2015. Capitalizing on the satellite observation-based mangrove green-fraction dataset, variation of the mangroves during recent decades are studied, including their dieback during 2015. The relationship between mangrove greenness and the climate conditions is examined using available observations and by exploring the possible role of the mega 2015-16 El Niño in altering the favorable conditions for the mangroves. The mangrove greenness is shown to be coherent with the low-frequency component of sea-level height variation related to the El Niño southern oscillation (ENSO) cycle in the equatorial Pacific. The sea-level drop associated with the 2015-16 El Niño is identified to be the crucial factor leading to the dieback event. A stronger sea-level drop occurred during austral autumn and winter, when the anomalies were more than 12% greater than the previous very strong El Niño events. The persistent drop in sea-level height occurred in the dry season of the year when sea-level was seasonally at its lowest, so potentially exposed the mangroves to unprecedented hostile conditions. The influence of other key climate factors is also discussed, and a multiple linear regression model is developed to understand the combined role of the important climate variables on the mangrove greenness variation.

Tropical Indo-Pacific SST influences on vegetation variability in eastern Africa

Mechanisms by which tropical Pacific and Indian Ocean sea surface temperatures (SST) influence vegetation in eastern Africa have not been fully explored. Here, we use a suite of idealized Earth system model simulations to elucidate the governing processes for eastern African interannual vegetation changes. Our analysis focuses on Tanzania. In the absence of ENSO-induced sea surface temperature anomalies in the Tropical Indian Ocean (TIO), El Niño causes during its peak phase negative precipitation anomalies over Tanzania due to a weakening of the tropical-wide Walker circulation and anomalous descending motion over the Indian Ocean and southeastern Africa. Resulting drought conditions increase the occurrence of wildfires, which leads to a marked decrease in vegetation cover. Subsequent wetter La Niña conditions in boreal winter reverse the phase in vegetation anomalies, causing a gradual 1-year-long recovery phase. The 2-year-long vegetation decline in Tanzania during an ENSO cycle can be explained as a double-integration of the local rainfall anomalies, which originate from the seasonally-modulated ENSO Pacific-SST forcing (Combination mode). In the presence of interannual TIO SST forcing, the southeast African precipitation and vegetation responses to ENSO are muted due to Indian Ocean warming and the resulting anomalous upward motion in the atmosphere.