Ronald J. Lynn

On the influence of the Norwegian-Greenland and Weddell seas upon the bottom waters of the Indian and Pacific oceans*

The bottom waters of the North Pacific and North Indian oceans have temperature and salinity distributions that suggest origins from the extreme waters of the Norwegian-Greenland and Weddell seas. We attempt to trace these waters from their sources to the abyssal Pacific and Indian oceans by examining distributions of temperature and salinity along a stratum defined by density parameters. We assume that the major flow and mixing will take place along such surfaces, though the results make plain that vertical mixing is also important. The density stratum we have chosen to examine extends from the sea surface in the Norwegian-Greenland Sea and from near the surface in the Weddell Sea to depth of about 3500 m in the central oceans and below 4000 m in the North Pacific. The cold and saline water of the Norwegian-Greenland Sea is traced along the density stratum through the Denmark Strait, where vertical mixing raises both temperature and salinity to their maximum values in the central North Atlantic. From there the temperature and salinity decrease monotonically southward toward the Weddell Sea, partly by lateral mixing with the cold, low-salinity waters on this stratum where it lies near the sea surface in the Weddell Sea, and partly by vertical mixing with the underlying Antarctic Bottom Water. From the southern South Atlantic the high values of temperature and salinity (the stratum now lies close to the vertical maximum in salinity) extend eastward with the Antarctic Circumpolar Current into the Indian and Pacific oceans, with monotonically decreasing temperature and salinity as further vertical mixing erodes the maximum in salinity, until the salinity maximum is found at the bottom in the North Pacific Ocean. The stratum we have defined terminates at abyssal depths in the northern Indian and Pacific oceans; since water must rise somewhere to balance the sinking in regions of bottom-water formation, there must be upward flow across the stratum elsewhere. The tremendous areal extent of the salinity maximum, however, suggests that the upward flow through the stratum must be minimal except in the North Indian and North Pacific oceans, where stability is shown to be very low at the depth of the stratum.

Physical-biological coupling in the California Current during the 1997-99 El Nino-La Nina cycle

The rapid transition from strong El Nino to strong La Nina conditions in the equatorial Pacific in 1998 was accompanied by considerable environmental variability in the southern California Current System (CCS). The evolution of this transition in the CCS is investigated based on hydrographic and biological data collected on 25 cruises over a 45-month period (February 1996 to October 1999). The El Nino period was characterized by high steric heights and a deep nutricline in the coastal regions, which effectively limited biological production. This was followed by a period of increased cross-shore dynamic height gradients, a significant shoaling of the nutricline, and a dramatic rebound in primary and secondary production. The observed physical and biological response in the CCS is remarkably similar to that observed in the tropical Pacific, but with a lag of several months.

Characteristics and circulation of deep and abyssal waters

Abstract The deep and abyssal potential temperature and salinity of the major areas of the world ocean have been re-examined in the hope that recent data may extend the conventional concepts of the formation and circulation of the deeper waters. In addition to these properties, which have been considered before, the potential density ( δ ϑ ) has been included. Potential denstity, in which density is calculated from the in situ temperature and salinity values moved adiabatically to sea-surface pressure, has been useful in defining mixing surfaces for shallow water. A peculiarity of this quantity in the western Atlantic Ocean is that it has a maximum above the bottom (though the column is stable) between about 40°N and 35°S. These high values originate from the Greenland-Norwegian Sea area and represent the lower North Atlantic Deep Water. This distribution suggests that δ ϑ surfaces at great depth cannot be interpreted as surfaces of mixing or flow but that δ ϑ can be interpreted as a conservative quantity whose extreme values can represent areas of water-mass formation and paths of flow. The lower North Atlantic Deep Water is a very good example of such a water mass: it can be traced by the maximum in δ ϑ to 35°S at least. Beyond there the δ ϑ maximum is either too weak to be followed or disappears entirely; some other quantity must be used if it is to be followed farther. If we wish to follow deeper water we can calculate density using a deeper reference pressure, for example 4000 dbars. This new quantity increases monotonically downward at great depth. An isopycnal of this quantity can be chosen to represent a mixing surface for the lower North Atlantic Deep Water and to trace this water much farther south.

Dynamic morphology of gas hydrate on a methane bubble in water: Observations and new insights for hydrate film models

Predicting the fate of subsea hydrocarbon gases escaping into seawater is complicated by potential formation of hydrate on rising bubbles that can enhance their survival in the water column, allowing gas to reach shallower depths and the atmosphere. The precise nature and influence of hydrate coatings on bubble hydrodynamics and dissolution is largely unknown. Here we present high-definition, experimental observations of complex surficial mechanisms governing methane bubble hydrate formation and dissociation during transit of a simulated oceanic water column that reveal a temporal progression of deep-sea controlling mechanisms. Synergistic feedbacks between bubble hydrodynamics, hydrate morphology, and coverage characteristics were discovered. Morphological changes on the bubble surface appear analogous to macroscale, sea ice processes, presenting new mechanistic insights. An inverse linear relationship between hydrate coverage and bubble dissolution rate is indicated. Understanding and incorporating these phenomena into bubble and bubble plume models will be necessary to accurately predict global greenhouse gas budgets for warming ocean scenarios and hydrocarbon transport from anthropogenic or natural deep-sea eruptions.

Thermal Property Measurements of Methane Hydrate Using a Transient Plane Source Technique

Knowledge of the thermal properties of gas hydrates and sediments containing gas hydrates is essential for assessing their commercial potential for natural gas recovery and their possible factors in sea-floor stability and climate change. Unlike phase equilibrium properties of hydrates, little information is available on their thermal properties. A major experimental challenge in thermal property measurement is determining the composition of the sample being measured. This chapter describes work being performed at the National Energy Technology Laboratory to develop a means to reliably measure the thermal properties of hydrate and hydrate-containing samples, while facilitating characterization of the sample with minimal decomposition or disturbance. A transient plane source (TPS) technique for simultaneously determining thermal conductivity and thermal diffusivity has been adapted for use at high pressure for this purpose. The TPS element is mounted inside a specially designed cup assembly that not only holds and contains the sample, but can also serve as a sample compaction device. The cup assembly is contained inside a high-pressure vessel that not only facilitates measurements at in-situ conditions, but can also be used to form hydrate or hydrate-containing samples in contact with the TPS element. The part of the cup containing the TPS element can simply be pulled away from the hydrate sample to permit subsequent characterization of the part of the sample that was measured. The formation of uncompacted methane hydrate in the cup and measurement of its thermal properties are described. The recovery of the sample and characterization by Raman spectroscopy are also presented.