Stephen P. Murray

Direct observations of seasonal exchange through the Bab el Mandab Strait

The exchange flow between the Red Sea and the Gulf of Aden-Indian Ocean through the Bab el Mandab Strait was measured continuously for 10 months, June 1995–March 1996. ADCP and temperature-salinity chain moorings allow an unprecedented look at the magnitude and seasonal evolution of the inflow layer from the Gulf of Aden, and the high salinity outflow layer from the Red Sea. The timing, structure, and evolution of the summer season mid-depth intrusion of cold, low salinity water into the Red Sea from the Gulf of Aden is measured for the complete intrusion cycle of 1995. We unexpectedly find the deep outflow still strong in June 1995, with speeds of 0.6 m/sec and transport of 0.4 Sv (1 Sv = 106 m³/sec). From July to mid–September, the deep outflow persists but is attenuated to speeds of 0.2 m/sec and transport of 0.05 Sv. The dominant summer feature, the cold low salinity intermediate layer intrusion, persists for 3 months, occupies 70% of the water column in the Strait and carries approximately 1.7 × 1012 m³ of cold nutrient-rich water into the Red Sea. The winter regime begins in mid-September, is fully developed by early November, and continues to the end of our first observation interval in March 1996. Speeds in the lower layer are 0.8–1.0 m/sec and 0.4–0.6 m/sec in the upper layer. At maximum exchange in mid-February, outflow transport reaches 0.7 Sv. Ubiquitous oscillations in current and salinity at synoptic and intraseasonal periods appear closely related to fluctuations in the along-channel wind forcing and perhaps to coastally-trapped waves.

Morphology and dynamic sedimentology of the eastern Nile delta shelf

Coleman, J.M., Roberts, H.H., Murray, S.P. and Salama, M., 1981. Morphology and dynamic sedimentology of the eastern Nile delta shelf. Mar. Geol., 42: 301-326 Some 13,000 km of bathymetry and overlapping side-scan sonar data were collected over a 4450-km2 area of the eastern Nile River shelf. These data, coupled with more detailed surveys of selected areas, including bottom sampling and current measurements, contribute to the understanding of shelf morphology, lithofacies relationships, and sediment transport processes. Four morphologic zones were defined: Zone 1. Smooth-bottomed, flocculated clays and silty clays extending from the shore-line 40 km seaward to the 25-m contour. Zone 2. A sand belt (5-20 km wide) extending eastward from the Damietta mouth and curving toward the coast at the boundary of the study area. Dominant bottom features were migratory bedforms of various dimensions superimposed on linear sand ridges. Zone 3. Smooth clay to silty-clay bottom topography punctuated with distinct algal mounds. The mounds (up to 10 m relief) are composed of living coralline algae, which contribute coarse debris to the surrounding sediments. Zone 4. Mud diapirs and shelf-edge slumps characterize this distal part of the shelf and the upper slope. Sand on the inner shelf is actively migrating and probably does not represent a relict deposit. The Damietta distributary causes a large-scale perturbation in the mean easterly drift along the Egyptian coast. Unusually strong currents, capable of transporting and reworking large volumes of sand, are associated with an eddy trapped behind this feature. The directionality of these currents and the curvature of the eddy axis correspond well to the distribution pattern of sand on the inner shelf.

Low-frequency fluctuations in the Indonesian throughflow through Lombok Strait

A significant component of the Indonesian throughflow, apparently about 25%, passes through the Lombok Strait. Direct observations in 1985 reported a ∼2 Sv annual average, with an annual cycle of amplitude ∼2 Sv. There are also significant fluctuations in this transport in the 0.01–0.1 cpd frequency band. Shallow pressure gauge data (sea level) inside the strait during the current meter observations were of limited use in explaining the large fluctuations in currents. Sea level data at Cilacap, 720 km west (upcoast), however, overlap the current observations for 5 months and correlate exceedingly well (r = 0.87) with the observations inside the strait at these frequencies. These sea level oscillations in the Indian Ocean force fluctuations in the Lombok throughflow that reach 50–70 cm/s, equivalent to 2–3 Sv. Lagged regression analysis indicates Cilacap sea level leads Lombok currents by 1–2 days, suggesting a low-frequency, progressive wave. Simultaneous data in 1989 from four stations extending from the near-equatorial station at Padang at 1°S to Benoa in the Lombok Strait (2000 km downcoast) clearly show the persistent propagation of low-frequency waves of 20- to 40-cm range along this coast. Lagged correlation on station pairs indicates a phase speed consistent with coastally trapped internal Kelvin waves. We speculate that further eastward progression of these waves to the Timor Passage of Ombai Strait will further modulate the throughflow. The forcing of these waves is not yet identified, but it appears likely that intraseasonal oscillations in the equatorial Indian Ocean winds, as demonstrated by Enfield [1987] for the Pacific, are a probable mechanism. Improved wind data quality in 1991 due to the assimilation of satellite data (special sensor microwave/imager) will allow investigation of remote forcing on more recent data sets.

Continuous dynamical modes in straits having arbitrary cross sections, with applications to the Bab al Mandab

The continuous dynamical modes of the exchange flow in the Bab al Mandab are computed in an attempt to assess the hydraulic character of the flow at the sill. First, an extended version of the Taylor‐Goldstein equation for long waves that accounts for cross-channel topographic variations, is developed. A series of calculations using idealized background velocity U(z) and buoyancy frequency N(z) are presented to illustrate the effects of simple topographic cross sections on the internal modes and their speeds. Next, hydrographic and direct velocity measurements from April to November 1996 using moored CTDs and a bottom-mounted ADCP are utilized to construct monthly mean vertical profiles of N 2(z) and at the U(z) sill. An analytical approximation of the true topography across the strait is also constructed. The observed monthly mean profiles are then used to solve for the phase speeds of the first and second internal modes. Additional calculations are carried out using a selection of ‘‘instantaneous’’ (2-h average) profiles measured during extremes of the semidiurnal tide. The results are compared with a three-layer analysis of data from the previous year. Many of the authors’ conclusions follow from an intriguing observation concerning the long-wave phase speeds. Specifically, it was nearly always observed that the calculated speeds c21 and c1 of the two waves belonging to the first internal mode obey c21 , Umin , Umax , c1, where Umin and Umax are the minimum and maximum of the velocity profile. An immediate consequence is that neither wave has a critical level. For monthly mean profiles, each of which have Umin , 0 , Umax, the flow is therefore subcritical (the phase speeds of the two waves have opposite signs). For instantaneous profiles this relationship continues to hold, although the velocity profile can be unidirectional. Thus the flow can be critical ( c21 5 0 and/or c1 5 0) or even supercritical (c21 and c1 have the same sign) with respect to the first mode. Similar findings follow for the second baroclinic mode phase speeds (c22 and c 2). The authors conclude that hydraulically critical flow is an intermittent feature, influenced to a great extent by the tides. It is noted that the phase speed pairs for each mode lie very close to Umin and Umax. As suggested by the analysis of idealized profiles, this behavior is characteristic of flows that are marginally stable, perhaps as a result of prior mixing. This suggestion is supported by Richardson number (Ri) profiles calculated from the monthly mean and instantaneous data. Middepth values of Ri were frequently found to be O(1) and sometimes ,1/4, a result consistent with the presence of mixing over portions of the water column.

Coseismic event of May 15, 1992, Huon Peninsula, Papua New Guinea: Comparison with Quaternary tectonic history

The May 15, 1992, earthquake on the Huon Peninsula, Papua New Guinea, resulted in the uplift and large-scale mortality of intertidal fringing coral reefs. Reduction in the highest level of survival of intertidal massive corals was used as a proxy for assessing the amount of coseismic uplift over 45 km of the Huon coastline. Measurements were gathered from three sites one and three months after the earthquake. Sea-level pressure gauges in place in the northern end of Sialum Lagoon showed not uplift, but subsidence, from the earthquake. Uplift ranged from ∼7 to 13 cm, and subsidence ranged from 8 to 14 cm. The May 15, 1992, earthquake corroborates the coseismic origin of the raised reef terraces of the Huon Peninsula. Greater uplift of the land in the southeast relative to the northwest is consistent with the regional Quaternary uplift pattern. The close proximity of subsidence (shown by sea-level gauges) to uplift (shown by coral mortality) is a manifestation of the abundant fault blocks in the area. Uplift rates of 3.0-5.2 m/ka calculated from the earthquake are only marginally higher than previous estimates based on radiometric age dates and terrace geomorphology. The first directly observed earthquake uplift event on the Huon Peninsula has yielded only centimetre-scale coseismic uplift. Thus, individual Holocene and Pleistocene terraces thought to have been the result of metre-scale displacement from single earthquakes may rather have been due to successive episodes of centimetre-scale uplift on constructional reef platforms. The clustered history of earthquakes on the Huon Peninsula throughout the past 100 years indicates the complexity involved in assessment of seismic risk throughout the world.

Hydraulic Interpretation of Direct Velocity Measurements in the Bab al Mandab

Abstract Acoustic Doppler Current Profiler velocity measurements in the Bab al Mandab during the period June 1995–March 1996 are used to assess the hydraulic character of the exchange flow. The strait is 150 km long and contains two distinct geometrical choke points: the Hanish sill and Perim narrows. The authors use a three-layer approximation of the monthly mean velocity and density structure at the sill and narrows to calculate the phase speeds of the first and second internal, long gravity waves. The first (second) mode is generally characterized by in-phase (out-of-phase) motions of the two interfaces. The calculations take cross-strait topographic variations into consideration by using a piecewise linear representation of the actual bottom. The resulting phase speeds are used to determine whether the flow is subcritical, supercritical, or critical with respect to the first and second modes. Subcritical (supercritical) means that the two wave pairs corresponding to a given mode propagate in opposite ...

Evolution and structure of a coastal squirt off the Mississippi River delta: Northern Gulf of Mexico

In early October 1992, satellite-derived sea surface temperature data revealed a 200 km long and 10- to 30-km-wide stream of cool water flowing toward the southwest from the Mississippi River delta region. Satellite imagery and in situ measurements have enabled a detailed study of the squirts kinematics and subsurface characteristics over a 2-week period. In its early stages, the squirt appeared as a narrow, high-speed (>75 cm/s) jet of water which flowed westward over the Mississippi Canyon, forcing a semi-submersible drilling rig to suspend operations from October 2 to 4. After crossing back onto the shelf, the squirt spread laterally, yielding a mushroom-shaped feature, 75 km wide, which consisted of counter-rotating vortices. Northeasterly wind forcing (averaging 10–15 m/s) and water level setup east of the delta appear to have been the primary mechanisms for evolution of the high-velocity currents. Satellite and in situ measurements demonstrate that the dipole eddy was comprised of a cool, low-salinity, low-density water mass at least 26 m deep in the center and 16 m deep along its margins. This event demonstrates that strong northeasterly winds over the northern Gulf of Mexico can initiate along-shelf and off-shelf flows of cooler coastal waters, contributing significantly to seasonal cooling and freshening of the continental shelf and to shelf/slope exchanges of water. During this event, approximately 100 km3 of inner shelf and river water was transported off the continental shelf, a volume equivalent to 17% of the average annual discharge of the Mississippi and Atchafalaya Rivers.

Characteristics of Circulation in an Indonesian Archipelago Strait from Hydrography, Current Measurements and Modeling Results

The Lombok Strait, a gap in the lower Indonesian Archipelago second in cross sectional area only to the Timor passages, provides a major pathway for the Pacific to Indian throughflow. A global reduced gravity model, corroborated by dynamic height climatology from the Generalized Digital Environmental Model, predicts annual mean sea levels 15-20 cm higher at the Pacific entrance to the Indonesian Seas than in the Indian Ocean south of the archipelago straits. Consistent with this regional pressure gradient, Pacific core layers of the Northern Subtropical Central Water and the North Pacific Intermediate Water are traced southward from the Makassar Strait into the Lombok Strait. Maps of temperature, salinity, and density distributions and sea surface dynamic heights in the Lombok Strait from January, June, and September 1985 also indicate a persistent southward flow of appreciable magnitude. Geostrophic speeds, however, are clearly too large by a factor of two or more. Current meter arrays in the north strait (January 1985 – March 1986) provide direct measurements of southward currents which persist through most of the year and are concentrated in the upper few hundred meters consistent with Wyrtki’s (1987) analysis of the regional pressure gradient. Maximum sustained speeds of over 70 cm/sec occur from July to September with a long period of weak currents from mid-October 1985 through January 1986. Tropical cyclones in the Timor Sea (December-April) force strong northward flow reversals which can persist for ten days. The wind-forced numerical model identifies the strong westward wind stresses in the Timor Sea during the southeast monsoon as the major cause of the annual cycle of current in the Strait.

Physical processes and sedimentation on a broad, shallow bank

An integrated study of the meteorology, physical oceanography, sedimentationand coastal morphology on the broad, shallow Miskito Bank off the eastern coast of Nicaragua has uncovered systematic interrelationships between driving forces. Bank geometry and sedimentologic environments on the Bank. Extremely high rainfall results from an interaction between meteorological processes over the Bank and topographic effects along the coast. Both acoustic and radio sounding of the lower atmosphere have documented the feedback between convective plumes, inversion layers and the incessant rainfall, which brings three times more freshwater and 15 times more sediment down to a unit length of coast than on the U.S. Atlantic shore. The resultant brackish, turbid coastal water moves as a highly organized band of water parallel to the coast. Seaward of this coastal boundary layer, offshore water from the Caribbean Current rides up on the Bank and provides an environment ideal for carbonate production. A zone of fine-grained terrigenous sediment underlying the coastal boundary current merges abruptly into a smooth carbonate plain covering most of the surface of the Bank. These central Bank carbonates are composed primarily of the disintegration products of prolific calcareous green algae. A trend of high relief, luxuriant coral reef growth is aligned along the steep dropoff at the Bank edge, a zone of observed upwelling of cooler and saltier basin water. A threefold southerly increase in wave energy at the shoreline due to the decreasing width of the shallow shelf results in wave-dominated coastal morphologies in the south compared to fluvial domination in the north and a systematic change from straight, linear bars and beaches in the north to rhythmic topography in the south.

Inertial Currents Over the Inner Shelf Near 30°N

Abstract Analysis of two month-long current records, one in February and one in May, from the inner shelf (28.9°N) west of the Mississippi River delta show strong oscillations in the diurnal-inertial frequency band. Lack of correlation of these currents with the predicted or measured tide and strong association with frontal passages suggest that they are wind-induced inertial oscillations. The observed oscillations are well reproduced by a time-dependent wind-driven model including Coriolis acceleration and friction (Pollard and Millard, 1970).