Addu Atoll Research Articles

Excavations on the Coral Mosques of the Maldives

Abstract This report presents the results of an archaeological mission done in the Maldives archipelago located to the south-west of India, in the Indian Ocean. In November 2017, we carried out archaeological excavations and surveys as well as collected oral traditions on two sites, the Fandiyaaru Mosque and Koagannu Cemetery in Hulhumeeddhoo town on Addu Atoll and the Friday Mosque of Fenfushi on Alifu Dhaalu Atoll. Two outcomes were expected from our mission: first, to provide new scientific data on the coral mosques of the Maldives in order to improve the chances of success of nomination of the mosques on the World Heritage List of UNESCO; then to support the conservation project of the Maldivian government and international organisations such as UNESCO and the World Monuments Fund (WMF). One major question during our excavation was the continuity of the settlements from pre-Islamic cultures and influences from Buddhist architecture on local Islamic architecture.

Multiscale Variability of the Atmospheric Boundary Layer during DYNAMO

Abstract Properties of the atmospheric boundary layer (ABL) over the central Indian Ocean are investigated using sounding data obtained during the Dynamics of the MJO (DYNAMO) field campaign in 2011/12. Observations from Gan Island on Addu Atoll, the R/V Revelle, and Malé in the Maldives are used to determine the frequency of well-mixed layers and their mean thermodynamic and wind profiles. Well-mixed boundary layers or mixed layers were observed 68% of the time from the three sites, ranging from ~100-m depth in recovering convective downdraft wakes to ~925 m in undisturbed conditions, with a mean depth of 508 m. At Revelle, the site most representative of the open ocean, the ABL displayed a distinct signal of modulation by the October and November MJOs, with mixed-layer depths gradually increasing through the suppressed phases as the sea surface temperature (SST) increased leading up to the active phases, followed by frequent ABL stabilization and shallow mixed layers in recovering wakes. A distinct diurnal cycle of mixed-layer depths and properties was observed during the MJO suppressed phases in response to a diurnal cycle of the SST under the mostly light-wind, clear-sky conditions. The daytime growth of the mixed layer contributed to an afternoon maximum in cumulus cloud development and rainfall during the suppressed periods by allowing more boundary layer thermals to reach their condensation levels. The variability of the ABL on time scales ranging from convective to diurnal to monthly poses significant challenges for numerical simulations of the MJO and the tropical circulation in general.

Evolution, Properties, and Spatial Variability of MJO Convection near and off the Equator during DYNAMO

Abstract This study investigates the evolution, structure, and spatial variability of Madden–Julian oscillation (MJO) convection observed during the 2011/12 Dynamics of the MJO (DYNAMO) field campaign. Generally, the C-band radars located in the near-equatorial Indian Ocean—Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) on Addu Atoll (Gan) and NASA TOGA on the R/V Roger Revelle (Revelle)—observed similar trends in echo-top heights, stratiform rain fraction, and precipitation feature size across the MJO life cycle. These trends are closely related to changes in mid- to upper-tropospheric moisture, sea surface temperature (SST), zonal wind, and diagnosed vertical air motions. However, the evolution of convection, moisture, and vertical air motion at the R/V Mirai (Mirai), located in the intertropical convergence zone (ITCZ) at 8°S, exhibited a pattern nearly opposite to Gan and Revelle. When the MJO was active over the equator, convection was suppressed around Mirai owing to induced subsidence by the strong upward motion to the north. SST and zonal winds near Mirai were nearly invariant across the MJO life cycle, indicating little influence from the MJO in these fields. Compared to Gan and Revelle, Mirai had a significant amount of precipitation that fell from shallow and isolated convection. There were subtle differences in the evolution and properties of the convection observed between Gan and Revelle. Deep convection occurred slightly earlier at Gan compared to Revelle, consistent with the west-to-east progression of the MJO in the central Indian Ocean. Furthermore, convective deepening was more gradual over Revelle compared to Gan, especially during the October MJO event.

Constructing a Merged Cloud–Precipitation Radar Dataset for Tropical Convective Clouds during the DYNAMO/AMIE Experiment at Addu Atoll

AbstractTo improve understanding of the convective processes key to the Madden–Julian oscillation (MJO) initiation, the Dynamics of the MJO (DYNAMO) and the Atmospheric Radiation Measurement Program (ARM) MJO Investigation Experiment (AMIE) collected 4 months of observations from three radars—the S-band dual-polarization Doppler radar (S-Pol), the C-band Shared Mobile Atmospheric Research and Teaching Radar (SMART-R), and Ka-band ARM zenith radar (KAZR)—along with radiosonde and comprehensive surface meteorological instruments on Addu Atoll, Maldives, in the tropical Indian Ocean. One DYNAMO/AMIE hypothesis suggests that the evolution of shallow and congestus cloud populations is essential to the initiation of the MJO. This study focuses on evaluating the ability of these three radars to document the full spectrum of cloud populations and to construct a merged cloud–precipitation radar dataset that can be used to test this hypothesis. Comparisons between collocated observations from the three radars show that KAZR provides the only reliable estimate of shallow clouds, while S-Pol/SMART-R can reasonably detect congestus within the 30–50-km range in addition to precipitating deep clouds. On the other hand, KAZR underestimates cloud-top heights due to rainfall attenuation in ~34% of the precipitating clouds, and an empirical method to correct KAZR cloud-top height bias is proposed. Finally, a merged KAZR–S-Pol dataset is produced to provide improved cloud-top height estimates, total hydrometeor microphysics, and radiative heating rate retrievals. With this dataset the full spectrum of tropical convective clouds during DYNAMO/AMIE can be reliably constructed and, together with complimentary radiosonde data, it can be used to study the role of shallow and congestus clouds in the initiation of the MJO.

The cloud population and onset of the Madden‐Julian Oscillation over the Indian Ocean during DYNAMO‐AMIE

AbstractVariability of the cloud population in the central equatorial Indian Ocean was observed in the context of the Madden‐Julian Oscillation (MJO) during the Dynamics of the Madden‐Julian Oscillation (DYNAMO) and Atmospheric Radiation Measurement Madden‐Julian Investigation Experiment (AMIE) field campaigns. Radar observations from the polarimetric S‐band radar on Addu Atoll in the Maldives characterize the types of convective and stratiform radar echoes and the heights their 20 dBZ contours reach. To gain insight into the relationship between clouds and humidification of the troposphere leading up to and during an active MJO event, the work relates variability of the observed precipitation structure to that of tropospheric humidity and upper level zonal wind. The variability in stratiform precipitation areas dominates variability in the nature of precipitating convection associated with the MJO. Areal coverage of precipitating radar echo, convective echo top height, and tropospheric humidity above 850 hPa rapidly increase over ~3–7 days near MJO onset. This rate of increase is substantially faster than the 10–20 days needed for buildup of moisture prior to MJO onset as hypothesized by the “discharge‐recharge” hypothesis. Convective echoes become more common during the days prior to MJO onset, and the increased convection occurs before low‐tropospheric moistening. The upper troposphere rapidly moistens as the first widespread stratiform region passes over an area. Thus, clouds likely play a role in tropospheric humidification. Whether increased low‐tropospheric humidity causes vertical growth of convection has not yet been determined.

Chronic small-scale oil pollution in Addu Atoll (Maldives)

Chronic small-scale oil pollution in Addu Atoll (Maldives)

Late Quaternary sea-level highstands in the central and eastern Indian Ocean: A review

The relative sea-level history of several atolls in the central and eastern Indian Ocean, including the Cocos (Keeling) Islands, Chagos Archipelago, and the Maldives–Laccadive Archipelagoes, has been debated for over a century but takes on a particular significance in the face of anticipated climate change. For each of these central and eastern Indian Ocean atolls Pleistocene limestone is encountered at depths of 6–20 m below sea level. On the Cocos (Keeling) Islands this has been dated to Last Interglacial age. Conglomerate platform underlies the reef islands on Cocos within which a sequence of fossil microatolls of massive and branching Porites records a gradual fall of sea level relative to the atoll. In the Maldives, the significance of outcrops of ‘reef rock’ has been vigorously debated without resolving sea-level history. Although in situ Heliopora occurs on the reef flat of Addu Atoll, dated at around 2700 radiocarbon yrs BP, other evidence for higher sea level remains poorly constrained. Conglomerates of a similar age have been described from the Chagos Archipelago, but it has not been unequivocally demonstrated that they formed under conditions of relatively higher sea level. In contrast to reefs further west in the Indian Ocean, each of these atolls has living microatolls of massive Porites that have been constrained in their upward growth by sea level. Interpretation of the upper surface of two such specimens from the Cocos (Keeling) Islands indicates broad fluctuations in the sea surface over the past century; similar microatolls are described from the Maldives implying little change in sea level over recent years. Regardless of minor past fluctuations, most reef islands in the Maldives are particularly low-lying and appear vulnerable to inundation, and extracting a more detailed sea-level history remains an important challenge.

Electricity supplies for Addu Atoll

The authors were appointed by the Industrial Development Unit, Commonwealth Secretariat, London, to assist them in undertaking a study to assess the rehabilitation requirements of power plant and power supplies for the public electricity undertakings of the Maldives Electricity Board in Male, the capital of the Republic of Maldives, and in Addu Atoll. This was one of the projects approved at the Technical Workshop and 5th Meeting of the Commonwealth Regional Consultative Group on Energy held in New Delhi in November 1986. This article briefly describes the situation found by Crown Agents' engineers during a visit in October 1987 and a view forward for Addu Atoll only

The disease spectrum in a Maldivian (Adduan) population

The changing pattern of disease in the population of Addu Atoll, Republic of Maldives, Indian Ocean, is reviewed. Over 30 years the population has nearly trebled with an increasing percentage in those under the age of 15 years. Diarrhoeal diseases and respiratory infections are now the dominant health problems where previously malaria and filariasis were major causes of morbidity. These problems are discussed against a background of changing financial resources and standards of health care.

Bronchial asthma in the southern Maldives.

A whole population survey was performed in Addu Atoll, southern Maldives to establish the prevalence of bronchial asthma. Assessments of atopic eczema, allergic rhinitis, conjunctivitis and intestinal infection were made, and serum specific IgE titres to a number of allergens were measured. Asthma is a common condition in the southern Maldives, and there is no evidence that the IgE produced by intestinal helminths prevents the condition.

Habitats and diet of predatory gastropods at addu atoll, maldives

About 50 species of predatory prosobranch gastropods from 11 families are common in shallow-water, coral-reef habitats at Addu Atoll. The Muricidae are the most diverse and abundant family, with 18 species present attaining population densities up to 31/m 2. Analysis of the food from 542 individuals of 30 species shows that polychaetes, sipunculids. small Crustacea, and other molluscs are the main prey of the gastropods. In the intertidal zone, epifaunal molluscs are the main food, while in the shallow sublittoral zone, polychaetes and sipunculids are the dominant prey.

On a long series of measurements of Indian Ocean equatorial currents near Addu Atoll

Results of nearly 2 years of weekly current and temperature profiles to 300 m in the central equatorial Indian Ocean are discussed. The principal current detected is the eastward jet which appears at both the spring and fall monsoon transitions, driven by strong eastward winds, as noted by Wyrtki ( Science, 181, 262–264, 1973). The jet is almost as strong at the depth of the thermocline as at the surface, and zonal currents at both depths are well correlated with the local wind stress at essentially zero lag. Since climatic atlases ( Nederlands Meteorologisch Instituut, Indische Ocean Oceanografische en Meteorologische Gegevens. Publ. No. 135, Vol. 2, 1952) reveal a period of a month or more at the spring transition during which the winds over the western equatorial ocean response over the central and eastern ocean change to eastward, the importance to future equatorial ocean response studies of obtaining simultaneous wind and current data from a number of longitudes is underscored. A purely local balance of forces fails to account for the observed zonal accelerations; non-local effects, mainly the time-dependent zonal pressure gradient, are important. Following each appearance of the eastward jet is a month or more of westward flow, though the winds show little or no westward stress. A similar effect, due to relaxation of water piled up at the eastern coast by the jet, is found in a time-dependent numerical model of O'Brien and Hurlburt ( Science, 184, 1075–1077, 1974). An eastward equatorial undercurrent beneath a westward surface flow is found in only one of the 2 years, and this is at least consistent with certain differences in the winds of these years.

Reconnaissance of the Indian Ocean equatorial undercurrent near Addu Atoll

In February–March, 1972, eight current and temperature profiles to 300 m were made at stations bracketing Addu Atoll in the Maldives, using the method of Düing and Johnson (1972). The profiles give evidence of the Equatorial Undercurrent, as expected at this time of year, and further show that distortions of the flow due to the atoll occur only near-shore, in agreement with observations and calculations of Hendry and Wunsch (1973). It thus appears that an on-going program of repeated stations, 10–20 nautical miles offshore from Addu, aimed at providing information about long-period variability of the undercurrent, will yield results reasonably representative of open-ocean conditions in this region.

The Conservation of Aldabra

Sixty years ago, on 13 September 1908, the late Sir John Fryer landed on Aldabra, after visiting the neighbouring islands of Astove, Assumption and Cosmoledo, to spend four months carrying out an ecological survey. On 25 April 1910, Fryer described his findings at a meeting of this Society, and his accounts (Fryer 1910, 1911) remain the standard sources of information on these coral islands of the south-west Indian Ocean. At this time there had already been considerable scientific work on Aldabra: by the German geographer Voeltzkow, the ornithologists Abbott and Nicoll, the entomologist Thomasset and the botanist Dupont (Stoddart, 1967). By 1910 Aldabra was one of the better known Indian Ocean reef islands. But in the next sixty years, though scientific visits continued, their results appeared mostly in specialist systematic reports. Commander Cousteau studied the marine life in 1954, an Italian expedition the Lepidoptera in 1953, and a Bristol expedition the reptiles in 1964; but the standard account of the atoll as a whole remained that of Fryer. In the middle 1960*8 it became known that Aldabra, with other Indian Ocean coral islands, was being considered as a possible site for the development of a military airfield by the British Government. Such airfields had been established during the Second World War at Addu Atoll in the Maldive Islands and at CocosKeeling Atoll, both of which have long histories of human disturbance. Studies at Addu Atoll in 1964 demonstrated the scale of ecological change resulting from air? field construction, although in this case no survey had taken place before develop? ment to serve as a base-line for tracing changes (Stoddart, 1966). With the first reports of possible development on Aldabra and other atolls in 1964, it was clear that ecological surveys needed to be carried out before construc? tion work began if the processes of change were to be studied. In most cases, as with Aldabra, the most recent accounts of the ecology of Indian Ocean islands were those of the Percy Sladen Expedition of 1905 and 1908 (Gardiner, 1907-36). The need for new surveys of Aldabra and Diego Garcia Atolls was considered and accepted by the Southern Zone Research Committee ofthe Royal Society in July 1966, after both had been mentioned as possible sites for military development. Soon afterwards it was learnt that an official expedition, including representatives ofthe Defence and other ministries and ofthe B.B.C., was about to leave for Aldabra, and at the Royal Society's request two scientists were attached to it to report on the present ecological status of the atoll. The scientists were the present writer and

The rats of Addu Atoll, Maldive Islands

Summary 1. 1. A population of tree-living Rattus rattus from Addu Atoll is described. Both sexes, but especially the females, have an unusually high tail to head-plus-trunk ratio. 2. 2. It is likely that the population reached Addu from Ceylon well before 1900 and is derived from the form R. r. kandianus Kelaart of that country. R. r. kandianus differs somewhat from the Addu form in its pleage characters but resembles it in possessing a high tail to head-plus-trunk ratio.