North West Cape Research Articles

Photographic Capture-Recapture Analysis Reveals a Large Population of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) With Low Site Fidelity off the North West Cape, Western Australia

Little is known about the ecology of Indo-Pacific bottlenose dolphins (Tursiops aduncus) inhabiting the coastal waters of tropical north-western Australia. We used photoidentification data collected between 2013 and 2015, site fidelity indexes and capturerecapture models to estimate the abundance and site fidelity patterns of Indo-Pacific bottlenose dolphins (T. aduncus) inhabiting the coastal waters off the North West Cape (NWC), Western Australia. A standardized site fidelity index (SSFI) indicated low site fidelity (SSFI = 0.019) at the population level to the 130 km2 study area. Agglomerative hierarchical clustering (AHC) of individual re-sight rates classified 58% of individuals as “transients.” Open POPAN modeling estimated a super-population size of 311 (95% CI: 249–373) individuals over the study period. A maximum likelihood transient model which considers both resident and non-resident individuals in a population, estimated a resident population of 141 (95% CI: 121–161) individuals and a super-population of 370 (95% CI: 333–407) individuals. These models indicate that a large population of Indo- Pacific bottlenose dolphins of relatively high density (an average of 2.4–2.8 bottlenose dolphins per km2) inhabits the waters off the NWC. The large number of both resident and non-resident bottlenose dolphins found throughout the coastal waters off the NWC suggest this area, as well as neighboring waters outside our study area, are of high importance to this species.

Stepwise Weakening of the Pliocene Leeuwin Current

AbstractThe Indonesian Throughflow (ITF) operates as an important link in global thermohaline circulation, and ITF variability probably modulated Pliocene climate change. Yet, whether ITF variability accounted for oceanographic change south of Northwest Cape remains controversial. Here, we present a multiproxy oceanographic reconstruction from the Perth Basin and reconstruct the Pliocene history of the Leeuwin Current (LC). We show that the LC was active throughout the Pliocene, albeit with fluctuations in intensity and scope. Three main factors controlled LC strength. First, a tectonic ITF reorganization caused an abrupt and permanent LC reduction at 3.7 Ma. On shorter timescales, eustatic sea level and direct orbital forcing of wind patterns hampered or promoted the LC. At 3.3 Ma, for instance, LC intensity plunged in response to a eustatic ITF restriction. Site U1459 then fell outside the extent of a weakened LC, and the latitudinal sea surface temperature gradient along West Australia doubled its steepness.

Assortative interactions revealed in a fission–fusion society of Australian humpback dolphins

Abstract Understanding individual interactions within a community or population provides valuable insight into its social system, ecology, and, ultimately, resilience against external stimuli. Here, we used photo-identification data, generalized affiliation indices, and social network analyses to investigate dyadic relationships, assortative interactions, and social clustering in the Australian humpback dolphin (Sousa sahulensis). Boat-based surveys were conducted between May 2013 and October 2015 around the North West Cape, Western Australia. Our results indicated a fission–fusion society, characterized by nonrandom dyadic relationships. Assortative interactions were identified both within and between sexes and were higher among members of the same sex, indicating same-sex preferred affiliations and sexual segregation. Assortative interactions by geographic locations were also identified, but with no evidence of distinct social communities or clusters or affiliations based on residency patterns. We noted high residency among females. Models of temporal patterns of association demonstrated variable levels of stability, including stable (preferred companionships) and fluid (casual acquaintances) associations. We also demonstrated some social avoidance. Our results point to greater social complexity than previously recognized for humpback dolphins and, along with knowledge of population size and habitat use, provide the necessary baseline upon which to assess the influence of increasing human activities on this endemic, Vulnerable species.

Новые таксоны Agrostis, Deschampsia, Hyalopoa и Poa (Poaceae) с Дальнего Востока России и из Восточной Сибири

Six new taxa in the family Poaceae are described from the Russian Far East and from East Siberia: Deschampsia komandorensis Prob. (sect. Deschampsia) from Commander Islands (Bering Island, North-West Cape), Agrostis × avatschensis Prob. (A. kudoi Honda × A. mertensii Trin.) from Avachinskii Volcano in Kamchatka, Poa archarensis Prob. (sect. Stenopoa) (revealed chromosome number 2n = 28) from Amur Region and Yakutia, Poa × alexandrae Prob. (sect. Stenopoa) from Magadan Region, Hyalopoa amgunensis Prob. (H. aggr. lanatiflora) from Khabarovsk Territory (Amgun River basin), Poa turgensis Prob. (sect. Stenopoa) from Transbaikalia.

Southbound migration corridor of pygmy blue whales off the northwest coast of Australia based on data from ocean bottom seismographs.

A line array of 14 ocean bottom seismographs was deployed on the Exmouth Plateau northwest of the North West Cape in Western Australia in December 2014-January 2015. Acoustic data collected with this array were used to evaluate the corridor of the southbound migration of pygmy blue whales of the eastern Indian Ocean population. It is found that pygmy blue whales tended to travel southward much further away from the Western Australian coast, at distances of up to 400 km from shore, than that expected from data on their northbound migration. This is an important observation providing additional information on the migration pattern of pygmy blue whales, which is crucial for assessing their population and migration by passive acoustic means.

Conspicuously concealed: revision of the arid clade of the Gehyra variegata (Gekkonidae) group in Western Australia using an integrative molecular and morphological approach, with the description of five cryptic species.

The methods used to detect and describe morphologically cryptic species have advanced in recent years, owing to the integrative nature of molecular and morphological techniques required to elucidate them. Here we integrate recent phylogenomic work that sequenced many genes but few individuals, with new data from mtDNA and morphology from hundreds of gecko specimens of the Gehyra variegata group from the Australian arid zone. To better understand morphological and geographical boundaries among cryptic forms, we generated new sequences from 656 Gehyra individuals, largely assigned to G. variegata group members over a wide area in Western Australia, with especially dense sampling in the Pilbara region, and combined them with 566 Gehyra sequences from GenBank, resulting in a dataset of 1,222 specimens. Results indicated the existence of several cryptic species, from new species with diagnostic morphological characters, to cases when there were no useful characters to discriminate among genetically distinctive species. In addition, the cryptic species often showed counter-intuitive distributions, including broad sympatry among some forms and short range endemism in other cases. Two new species were on long branches in the phylogram and restricted to the northern Pilbara region: most records of the moderately sized G. incognita sp. nov. are near the coast with isolated inland records, whereas the small-bodied saxicoline G. unguiculata sp. nov. is only known from a small area in the extreme north of the Pilbara. Three new species were on shorter branches in the phylogram and allied to G. montium. The moderately sized G. crypta sp. nov. occurs in the western and southern Pilbara and extends south through the Murchison region; this species was distinctive genetically, but with wide overlap of characters with its sister species, G. montium. Accordingly, we provide a table of diagnostic nucleotides for this species as well as for all other species treated here. Two small-bodied species occur in isolated coastal regions: G. capensis sp. nov. is restricted to the North West Cape and G. ocellata sp. nov. occurs on Barrow Island and other neighbouring islands. The latter species showed evidence of introgression with the mtDNA of G. crypta sp. nov., possibly due to recent connectivity with the mainland owing to fluctuating sea levels. However, G. ocellata sp. nov. was more closely related to G. capensis sp. nov. in the phylogenomic data and in morphology. Our study illustrates the benefits of combining phylogenomic data with extensive screens of mtDNA to identify large numbers of individuals to the correct cryptic species. This approach was able to provide sufficient samples with which to assess morphological variation. Furthermore, determination of geographic distributions of the new cryptic species should greatly assist with identification in the field, demonstrating the utility of sampling large numbers of specimens across wide areas.

Evidence for a widely expanded humpback whale calving range along the Western Australian coast

AbstractThe recognized calving grounds of humpback whales (Megaptera novaeangliae) that breed along the Western Australian coast (Breeding Stock D) extend along the Kimberley coast between Camden Sound and Broome (15°–18°S). However, there are reports of neonates further south, suggesting that the calving areas may be poorly defined. During aerial photogrammetric research in 2013 and 2015, we sighted large numbers of humpback whale calves along North West Cape (21°47′–22°43′S). We estimated the minimum relative calf abundance to be 463–603 in 2013 and 557–725 in 2015. We categorized the calves as either neonate or post neonate according to their color and size. The majority of calves sighted in both years (85% in 2013; 94% in 2015) were neonates. Our observations indicate that a minimum of approximately 20% (17.1%–24.3%) of the expected number of calves of this population are born near, or south of, North West Cape. We thus demonstrate that the calving grounds for the Breeding Stock D population extend south from Camden Sound in the Kimberley (15°S) to at least North West Cape (22°43′S), 1,000 km southwest of the currently recognized calving area.

Assessing the Risk of a Canine Rabies Incursion in Northern Australia.

Rabies is a globally distributed virus that causes approximately 60,00 human deaths annually with >99% of cases caused by dog bites. Australia is currently canine rabies free. However, the recent eastward spread of rabies in the Indonesian archipelago has increased the probability of rabies entry into northern Australian communities. In addition, many northern Australian communities have large populations of free-roaming dogs, capable of maintaining rabies should an incursion occur. A risk assessment of rabies entry and transmission into these communities is needed to target control and surveillance measures. Illegal transportation of rabies-infected dogs via boat landings is a high-risk entry pathway and was the focus of the current study. A quantitative, stochastic, risk assessment model was developed to evaluate the risk of rabies entry into north-west Cape York Peninsula, Australia, and rabies introduction to resident dogs in one of the communities via transport of rabies-infected dogs on illegal Indonesian fishing boats. Parameter distributions were derived from expert opinion, literature, and analysis of field studies. The estimated median probability of rabies entry into north-west Cape York Peninsula and into Seisia from individual fishing boats was 1.9 × 10−4/boat and 8.7 × 10−6/boat, respectively. The estimated annual probability that at least one rabies-infected dog enters north-west Cape York Peninsula and into Seisia was 5.5 × 10−3 and 3.5 × 10−4, respectively. The estimated median probability of rabies introduction into Seisia was 4.7 × 10−8/boat, and the estimated annual probability that at least one rabies-infected dog causes rabies transmission in a resident Seisia dog was 8.3 × 10−5. Sensitivity analysis using the Sobol method highlighted some parameters as influential, including but not limited to the prevalence of rabies in Indonesia, the probability of a dog on board an Indonesian fishing boat, and the probability of a Seisia dog being on the beach. Overall, the probabilities of rabies entry into north-west Cape York Peninsula and rabies introduction into Seisia are low. However, the potential devastating consequences of a rabies incursion in this region make this a non-negligible risk.

Lizards in pinstripes: morphological and genomic evidence for two new species of scincid lizards within Ctenotus piankai Storr and C. duricola Storr (Reptilia: Scincidae) in the Australian arid zone

The scincid lizard genus Ctenotus is one of the most species-rich genera of squamate reptiles, but few molecular phylogenetic studies have been undertaken on the group. Here we assess molecular and morphological variation within C. piankai and C. duricola, an arid-adapted pair of nominate species characterized by a pattern of thin pale longitudinal lines on a dark background that occur primarily in the western deserts and Pilbara region of Australia. We sequenced mitochondrial DNA and genome-wide restriction-site associated DNA (RAD) from geographically widespread samples of these lizard taxa, with particularly dense sampling from the Pilbara region. Phylogenetic analyses of the mitochondrial cytochrome B gene and approximately 5,000 nuclear loci identified four highly divergent lineages within the two taxa. The four genetically-defined populations were concordant with geography and are distinguishable based on multiple morphological and color pattern characters, despite appearing superficially similar in appearance. Despite limited mtDNA exchange between two lineages in the Pilbara, we found no evidence for ongoing gene flow across the nuclear genome. For the western desert lineages, there was no evidence of introgression for either mtDNA or nDNA in our data. To resolve the taxonomy of the group, we redescribe C. piankai and C. duricola, and recognize the two divergent lineages as new species: C. rhabdotus sp. nov., from the south-eastern Kimberley, Ord, Victoria River and northern Tanami Desert regions of Western Australia and the Northern Territory, and C. pallasotus sp. nov., from the western Pilbara and North West Cape regions of Western Australia.

Modeling of temporal variation of very low frequency radio waves over long paths as observed from Indian Antarctic stations

AbstractCharacteristics of very low frequency (VLF) signal depends on solar illumination across the propagation path. For a long path, solar zenith angle varies widely over the path and this has a significant influence on the propagation characteristics. To study the effect, Indian Centre for Space Physics participated in the 27th and 35th Scientific Expedition to Antarctica. VLF signals transmitted from the transmitters, namely, VTX (18.2 kHz), Vijayanarayanam, India, and NWC (19.8 kHz), North West Cape, Australia, were recorded simultaneously at Indian permanent stations Maitri and Bharati having respective geographic coordinates 70.75°S, 11.67°E, and 69.4°S, 76.17°E. A very stable diurnal variation of the signal has been obtained from both the stations. We reproduced the signal variations of VLF signal using solar zenith angle model coupled with long wavelength propagation capability (LWPC) code. We divided the whole path into several segments and computed the solar zenith angle (χ) profile. We assumed a linear relationship between the Wait's exponential model parameters effective reflection height ( ), steepness parameter (β), and solar zenith angle. The and β values were later used in the LWPC code to obtain the VLF signal amplitude at a particular time. The same procedure was repeated to obtain the whole day signal. Nature of the whole day signal variation from the theoretical modeling is also found to match with our observation to some extent.

Demographic characteristics of Australian humpback dolphins reveal important habitat toward the southwestern limit of their range

The paucity of information on the recently described Australian humpback dolphin Sousa sahulensis has hindered assessment of its conservation status. Here, we applied capturerecapture models to photo-identification data collected during boat-based surveys between 2013 and 2015 to estimate the abundance, site fidelity and residence patterns of Australian humpback dolphins around the North West Cape (NWC), Western Australia. Using Pollock's closed robust design, abundance estimates varied from 65 to 102 individuals, and POPAN open modelling yielded a super-population size of 129 individuals in the 130 km2 study area. At approximately 1 humpback dolphin per km2, this density is the highest recorded for this species. Temporary emigration was Markovian, suggesting seasonal movement in and out of the study area. Hierarchical clustering showed that 63% of individuals identified exhibited high levels of site fidelity. Analysis of lagged identification rates indicated dolphins use the study area regularly, following a movement model characterised by emigration and re-immigration. These density, site fidelity and residence patterns indicate that the NWC is an important habitat toward the southwestern limit of this species' range. Much of the NWC study area lies within a Marine Protected Area, offering a regulatory framework on which to base the management of human activities with the potential to impact this threatened species. Our methods provide a methodo logical framework to be used in future environmental impact assessments, and our findings represent a baseline from which to develop long-term studies to gain a more complete understanding of Australian humpback dolphin population dynamics.

The 25 April 2015 Nepal Earthquake: Investigation of precursor in VLF subionospheric signal

AbstractWe present a critical analysis of the observations and interpretation of VLF subionospheric measurements related to the main Nepal Gorkha earthquake which occurred on 25 April 2015 (Mw7.8) and its major aftershock on 12 May 2015 (Mw7.3). The VLF narrowband signal used is from North West Cape (NWC) (19.8 kHz) VLF transmitter located in Australia and recorded at Allahabad (latitude 25.41°N, longitude 81.93°E). Allahabad is located very close (~360 km) to these earthquake epicenters. Two widely used analysis, viz., (1) terminator time and (2) nighttime fluctuation techniques, are applied to extract seismic related effects in the NWC narrowband VLF data. The terminator time analysis yields statistically significant shifts of ~45 and ~26 min, respectively, in evening terminator time in the NWC VLF amplitude signal, 1 day before both the earthquakes. The nighttime fluctuation method shows a consistent, statistically significant, increase in three parameters 1 day before the earthquake. The observed terminator time and nighttime fluctuation shifts were associated with these earthquakes only after scrutinizing possible contributions from other potential sources such as solar activity; other earthquakes on the signal path; and meteorological disturbances such as lightning activity, wind speed, and temperature along the transmitter‐receiver great circle path. The VLF subionospheric signal analysis results unambiguously point toward the presence of seismically excited atmospheric gravity waves during these major earthquakes and their important role in providing the coupling between the seismic source region and overlying ionosphere.

North west cape-induced electron precipitation and theoretical simulation**Supported by the China Seismo-Electromagnetic Satellite Mission Ground-Based Verification Project of the Administration of Science, Technology, and Industry for National Defense and Asia-Pacific Space Cooperation Organization Project (APSCO-SP/PM-EARTHQUAKE).

Enhancement of the electron fluxes in the inner radiation belt, which is induced by the powerful North West Cape (NWC) very-low-frequency (VLF) transmitter, have been observed and analyzed by several research groups. However, all of the previous publications have focused on NWC-induced > 100-keV electrons only, based on observations from the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) and the Geostationary Operational Environmental Satellite (GOES) satellites. Here, we present flux enhancements with 30–100-keV electrons related to NWC transmitter for the first time, which were observed by the GOES satellite at night. Similar to the 100–300-keV precipitated-electron behavior, the low energy 30–100-keV electron precipitation is primarily located east of the transmitter. However, the latter does not drift eastward to the same extent as the former, possibly because of the lower electron velocity. The 30–100-keV electrons are distributed in the L = 1.8–2.1 L-shell range, in contrast to the 100–300-keV electrons which are at L = 1.67–1.9. This is consistent with the perspective that the energy of the VLF-wave-induced electron flux enhancement decreases with higher L-shell values. We expand upon the rationality of the simultaneous enhancement of the 30–100- and 100–300-keV electron fluxes through comparison with the cyclotron resonance theory for the quasi-linear wave-particle interaction. In addition, we interpret the asymmetry characteristics of NWC electric power distribution in north and south hemisphere by ray tracing model. Finally, we present considerable discussion and show that good agreement exists between the observation of satellites and theory.

Monsoonal and other climatic influences on radiolarian species abundance over the last 35 ka, as recorded in core FR10/95-GC17, off North West Cape (Western Australia)

Species abundance counts from 46 surface sediment samples have been used to develop transfer functions for the reconstruction of salinity, and dissolved nitrates in the eastern Indian Ocean. Application of the functions to samples from deep-sea core FR10/95-GC17 [22° 07.74′S, 113° 30.11′E] shows that, offshore North West Cape, Western Australia, regional changes in salinity and the availability of dissolved nitrates were more significant in determining radiolarian species distributions over the last 35ka than temperature. In addition, variations in the reconstructed data have been shown to reflect changes in marine currents and terrestrial rainfall over the study period.

Changes in the D region associated with three recent solar eclipses in the South Pacific region

AbstractWe estimate D region changes due to 22 July 2009 total solar eclipse (SE), 13–14 November 2012 total SE, and 9–10 May 2013 annular SE, using VLF navigational transmitters signal observations at Suva, Fiji. The North West Cape (NWC) signal (19.8 kHz) showed an amplitude and phase decrease of 0.70 dB and 23° during November SE and 2.0 dB and 90° during May SE. The modeling using Long Wave Propagation Capability code for NWC‐Suva path during November and May SEs showed an increase in average D region reflection height (H′) and sharpness factor (β) by 0.6 and 0.5 km and 0.012 and 0.015 km−1, respectively. The July total SE for JJI‐Suva path showed an increase in H′ of 1.5 km and a decrease in β of 0.055 km−1. The decrease in the electron density calculated using SE time H′ and β is maximum for July total SE and minimum for May annular SE. The effective recombination coefficient estimated from the decay and recovery of signal phase associated with May annular SE was higher (27%) than normal daytime value 5.0 × 10−7 cm−3 s−1 and varied between 1.47 × 10−6 and 1.15 × 10−7 cm−3 s−1 in the altitude 70 to 80 km. Morlet wavelet analysis of signals amplitude shows strong wave‐like signatures (WLS) associated with three SEs with period ranging 24–66 min, but the intensity and duration of WLS show no clear dependence on SE magnitude and type. Apart from the cooling spot, the eclipse shadow can also generate WLS associated with atmospheric gravity waves.

Multi-Year Observation of Holloway Current along the Shelf Edge of North Western Australia

ABSTRACT Bahmanpour, M.H.; Pattiaratchi, C., Wijeratne, E.M.S., Steinberg, C., and D'Adamo, N., 2016. Multi-year observation of a boundary current along the shelf edge of North Western Australia. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 517–521. Coconut Creek (Florida), ISSN 0749-0208. Analysis of multi-year current meter data across North West shelf of Australia identified various aspects of the mean flow in a macro-tidal environment. Main features of the flow appear to be a continuous alongshore south-westward flow, i.e., Holloway current located along the continental shelf edge in depth 100–200 m. Annually, the current transports ∼ 1 Sv of lower salinity, higher temperature water from the tropical regions to North West Cape. The Holloway current is at its maximum intensity (up to 2 Sv) during autumn/winter (Apr–Jul) when the winds are ei...

Observations of the shelf circulation dynamics along Ningaloo Reef, Western Australia during the austral spring and summer

The dynamics of the three-dimensional circulation on the shelf off Ningaloo Reef in Western Australia were investigated using observations from two field experiments (Nov 2009–Jan 2010 and Sep 2010–Nov 2010). The observations revealed an onshore geostrophic flow of ~0.015ms−1 along the relatively straight Ningaloo Peninsula, coinciding with a substantial poleward increase of the along-shelf flow along this short (∼60km) section of coast. The cross-shelf transport analysis distinguished between two cross-shelf advection scenarios prevalent along this coast: 1) a transient coastal upwelling scenario when the offshore surface Ekman transport was replenished by an onshore transport, supplied mainly from the interior of the water column due to the combined effects of the stratification and steep continental slope; and 2) a downwelling scenario when offshore bottom boundary layer Ekman transport occurred, associated with a deeper pressure-gradient driven flow. A detailed along-shelf momentum balance indicated that there was a southward increase in the along-shelf pressure gradient from the northern limit of the Ningaloo Peninsula (the North West Cape), which explains the increase in the along-shelf flow down the coast. The results indicate that the Ningaloo shelf can be considered as one of the important formation regions of the poleward Leeuwin Current which, while very weak on Australia North West Shelf, is the dominant current feature south of Ningaloo and along the remaining southwestern Australia coast.

Long‐duration geomagnetic storm effects on the D region of the ionosphere: Some case studies using VLF signal

AbstractThe present work investigates the effects of long‐duration geomagnetic storms on VLF signal during ionospheric sunrise time, commonly known as D Layer Preparation Time (DLPT) depth. The VLF signal at 19.8 kHz transmitted from Northwest Cape, Australia, and received at a low‐latitude station, Tripura, India, is used for the present analysis. The data for the analysis are selected from November 2008 to October 2011. In the active period of the geomagnetic storms, the average DLPT depth is found to have a negative correlation coefficient of 0.91 with geomagnetic Ap index. It is also found that with each 10 unit increase of Ap index, the DLPT depth (the day and night asymmetry level) changes by 1.25 dB. The results are supported with modeled International Reference Ionosphere (IRI) electron density data and DLPT depth at 71 km height for the three positions, namely, receiver position, signal hop position, and the transmitter position along the total Great Circle Path. It is found that the receiver position electron density is the main controlling factor for DLPT depth. The correlation between IRI electron density and DLPT depth increases from −0.13 at transmitter position to −0.33 at the first hop position, to −0.46 at the receiver position, respectively. The percentage change of post storm electron density, at 71 km height, is found to increase by more than 100% at the receiver position. The results are discussed on the basis of the electron density changes over the signal propagation path, mainly caused by the geomagnetic storms.

Response of the low‐latitude D region ionosphere to extreme space weather event of 14–16 December 2006

AbstractThe response of the D region low‐latitude ionosphere has been examined for extreme space weather event of 14–16 December 2006 associated with a X1.5 solar flare and an intense geomagnetic storm (Dst = −146 nT) using VLF signals from Northwest Cape, Australia (NWC) (19.8 kHz) and Lualualei, Hawaii (callsign NPM) (21.4 kHz) transmitters monitored at Suva (Geographic Coordinates, 18.10°S, 178.40°E), Fiji. Modeling of flare associated amplitude and phase enhancements of NWC (3.6 dB, 223°) and NPM (5 dB, 153°) using Long‐Wave Propagation Capability code shows reduction in the D region reflection height (H′) by 11.1 km and 9.4 km, and enhancement in ionization gradients described by increases in the exponential sharpness factor (β) by 0.122 and 0.126 km−1, for the NWC and NPM paths, respectively. During the storm the daytime signal strengths of the NWC and NPM signals were reduced by 3.2 dB on 15 and 16 December (for about 46 h) and recovered by 17 December. Modeling for the NWC path shows that storm time values of H′ and β were reduced by 1.2 km and 0.06 km−1, respectively. Morlet wavelet analysis of signal amplitudes shows no clearly strong signatures of gravity wave propagation to low latitudes during the main and recovery phases. The reduction in VLF signal strength is due to increased signal attenuation and absorption by the Earth‐ionosphere waveguide due to storm‐induced D region ionization changes and hence changes in D region parameters. The long duration of the storm effect results from the slow diffusion of changed composition/ionization at D region altitudes compared with higher altitudes in the ionosphere.

Land–sea correlations in the Australian region: post-glacial onset of the monsoon in northwestern Western Australia

Deep-sea core Fr10/95-GC17, collected offshore North West Cape at the western tip of Western Australia, is located beneath the path of the Leeuwin Current. This shallow, warm and low salinity current is an offshoot of the Indonesian Throughflow that transfers water and heat from the West Pacific Ocean into the Indian Ocean. The location is at the edge of the Indo Pacific Warm Pool, the source of large-scale transfer of moisture and heat from the ocean to the atmosphere. For this core, we combine previously published data with new research and use a revised chronology to re-examine the timing of climate change during the last 34,000 years in the tropics of northern Australia. The age model for the core is based on 15 radiocarbon dates complemented by luminescence ages and an oxygen isotope record. This study draws on an extensive range of analyses that have been performed on the core, including micropalaeontology of planktic and benthic foraminifera and coccoliths, stable isotopes analysis of foraminifera and their faunal composition, clay content, sediment composition and pollen analyses. Sea-surface and land temperatures are estimated from the foraminifer faunal analyses and from pollen spectra, respectively. The clay fraction and sediment composition and radiogenic isotopes of that fraction helped identify changes both on land and at sea: changes such as rainfall as shown by river discharge, and oceanic current tracing by neodymium, strontium and lead isotopes obtained from sediments.The most significant finding is that a major threshold was crossed at 13kaBP. Prior to that time, rainfall over NW Western Australia was low as was sea-surface temperature (SST); river discharge to the ocean was also low as a result of the lack of monsoonal activity and finally, ocean alkalinity would have been lower than at present due to the uptake of atmospheric CO2. By 13kaBP, the entire system moved away from glacial period conditions. The Indo-Australian monsoon commenced in and offshore northwestern Western Australia. SST and land temperature increased dramatically and ocean alkalinity changed due to the formation of a “barrier layer” (a low salinity cap), over the Indo Pacific Warm Pool. During the Holocene, river discharge and the land and ocean temperatures did not covary, for example, the highest rainfall did not coincide with the highest SST. Finally, the last 5ka saw a strengthening of ENSO in the region.