La-Lm

Labrador Basin

A ocean basin situated between Labrador, Greenland and Newfoundland. It underlies the Labrador Sea as well as most of the Irminger Sea.

Labrador Current

A current that flows southward over the continental shelves and slopes of Labrador and Newfoundland from Hudson Strait at 60$^\circ$N to the Tail of the Grand Banks of Newfoundland at 43$^\circ$N. It was first described by Smith et al. (1937) as a continuation of the Baffin Island Current, which transports the cold and relatively low salinity waters flowing out of Baffin Bay, and the warmer and more saline waters of a branch of the West Greenland Current. It appears as two branches at Hamilton Bank on the southern Labrador Shelf, a small inshore stream carrying about 15% of the transport and the main stream over the upper continental slope carrying about 85%.

Measurements indicate that the Labrador Current carries a transport (relative to 1500 db) of 3.8$\pm$0.9 Sv, 85% of which occurs in a 50 km wide jet over the shelf break between the 400 and 1200 m isobaths. An additional barotropic component has been identified over the 1000 m isobath. It has a maximum monthly speed of about 0.09 m s$^{-1}$, which adds about 7.2 Sv to the transport estimate.

Another southward flow has been identified over the 2500 m isobath from current meter observations, seaward of the traditional Labrador Current. The southward flow is over the full water depth, with a current half-width of order 70 km. This flow is weakest in summer and strongest in winter, as opposed to the baroclinic Current being strongest in late summer and weakest in winter. It is speculated that the traditional Labrador Current over the upper slope is primary buoyancy driven, while the deeper current centered at the 2500 m isobath is part of the North Atlantic subpolar gyre. See Lazier and Wright (1993) and Han and Tang (1999).

Labrador Sea

A part of the north Atlantic recognized as a separate body of water for hydrographic purposes although not officially recognized as such. The southern boundary is a line from the southern tip of Greenland to Cape St. Charles on the coast of Labrador and the northern boundary the 66$^\circ$ N latitude line that joins Greenland and Baffin Island north of the Arctic Circle.

The circulation features include the West Greenland Current and the Labrador Current. The Labrador Sea is part of the pathway through which the low salinity outflows of the Arctic Ocean move southward and downward toward denser surfaces. This low salinity can follow two pathways: mixing across fronts and subsequent incorporation into the subpolar and eventually the subtropical gyre circulation; or the water can move offshore into the Labrador and Irminger Seas and be mixed down the following winter as Irminger Sea Water or LSW.

Observations indicate considerable fluctuations of salinity in the upper waters on both a seasonal and interannual basis, which renders it difficult to create an average picture of the hydrography from historical data (of which there is much in this region). Climatologically, this is a region where the first effects of atmospheric warming induced by CO2 or other greenhouse gases might be seen. Milder winters would lead to less overturning and accumulation of low salinity waters in the upper layers, an affect which would be exacerbated by increased glacial and polar ice melt as well as increased high latitude precipitation. This could very well turn off the production of LSW with possible global consequences. See Tomczak and Godfrey (1994) and U. S. Science Steering Committee for WOCE (1986).

Labrador Sea Convection Experiment

An experiment whose purpose is to improve the understanding of the convective process and therefore the fidelity of parameteric representations used in large-scale models by a combination of field observations, laboratory studies, theory and modeling. The goals of the experiment are to investigate:

• the characteristic scales and properties of convective plumes;
• how plume parameters depend on the forcing and local environment;
• the fluxes of mass, heat and salt from the ensemble of plumes;
• how convection is related to its mesoscale environment;
• the relative importance of lateral (eddy) versus vertical (convective) flux of heat and salt inside/outside the convective region;
• what controls the volume and T/S properties of created water masses;
• how the convected water mass is accomodated into the general circulation; and
• the mean and seasonal variation in the gyre circulation.

The field work for this experiment includes floats and drifters, basin-wide hydrographic surveys, moored instruments to measure the usual variables, and aircraft observations for detailed measurements of the marine boundary layer.

[http://www.ldeo.columbia.edu/~visbeck/labsea/]

Labrador Sea Water (LSW)

A water mass which forms in the Labrador Sea via deep convection in the winter months in a strong cyclonic circulation gyre. It is a large volume of nearly homogeneous water with temperatures ranging from 3.0 to 3.6$^\circ$ C, salinities from 34.86 to 34.96, and consistently high dissolved oxygen content (above 275 $\mu$mol). It is formed by the final modification of Subpolar Mode Water (SPMW) in the Labrador Sea and contributes to the formation and modification of various types of North Atlantic deep waters. Observations indicate considerable interannual variable in the formation process, with no water being formed at all in an estimated 4 out of 10 years. This variability leads to significant interannual variations in LSW properties.

LSW mixes with ABW and eventually becomes part of NADW, although the abovementioned variability is not wholly passed on to NADW. The ABW/LSW mixture recirculates around the Labrador Sea an estimated 2 to 3 times over 12 to 18 months, a process which serves to smooth out interannual fluctuations and results in NADW receiving water with about half the original variation in magnitude of LSW. See Clarke and Gascard (1983), Talley and McCartney (1982a) Tomczak and Godfrey (1994) and Paillet et al. (1998).

lacustrine

Of or pertaining to a lake or lakes, or of plants and animals growing in or inhabiting lakes.

L-ADCP

Abbreviation for Line-Acoustic Dopper Current Profiler.

Lafondstables

Lafond's tables

A set of tables compiled by E. C. Lafond for the purpose of correcting reversing thermometers and computing dynamic height anomalies. These were published by the U.S. Navy Hydrographic Office as H. O. Pub. No. 617.

Lagrangian velocity

That velocity that would be measured by tracking a dyed particle in a fluid. Compare to Eulerian velocity and Stokes velocity.

Lake Baikal

See Kipfer et al. (2000).

Lake Ontario

See Pickett (1977).

Lakshadweep Sea

See Shenoi et al. (1999).

Lamb wave

To be completed.

LAMP

Acronym for Long-Term Autonomous Microstructure Profiler, an instrument designed to autonomously collect microstructure profiles over deployments of several months duration. It consists of an autonomously profiling ALACE float outfitted with two microtemperature probes and a hard disk for data recording. The prototype was deployed in the eastern subtropical North Atlantic in the area of the NATRE study. See Sherman and Davis (1995).

Langmuir circulation

A marine boundary layer phenomena in which wind shear tends to organize convective perturbations into adjacent, counterrotating roll vortices. The surface signature of this Langmuir circulation is wind-rows, i.e. parallel bands of foam, seaweed and slick water that have undoubtedly been familiar to sailors since long ago. The first systematic observations were performed by Langmuir (1938) after whom the phenomenon was eventually named. He studied it in the surface layers of Lake George, New York using sophisticated tracers like leaves, corks, and submerged umbrellas buoyed with light bulbs.

Langmuir circulations tend to carry water with near surface properties down to below the surface convergence zones, with plankton, seaweeds, surface detritus, and dissolved gases tending to be concentrated in the sinking currents. Conversely, water with different temperature and salinity properties and which is also usually richer in nutrients is brought toward the surface. See Langmuir (1938), Leibovich (1983), Weller and Price (1988) and Kraus and Businger (1994).

La Perouse Strait

See Okhotsk Sea.

Laplace's tidal equations

The equations developed by Laplace in 1775-1776 for his dynamic theory of tides. They were obtained from the continuum equations of momentum and mass conservation written in rotating coordinates for a fluid shell surrounding a nearly spherical planet. Assumptions central to their derivation were a perfect homogeneous fluid, small disturbances relative to a state of uniform rotation, a spherical earth, a geocentric gravitational field uniform horizontally and in time, a rigid ocean bottom, and a shallow ocean where both the horizontal component of the Coriolis acceleration and the vertical component of particle acceleration are neglected. The equations are

$${{\partial u}\over{\partial t}}\,-\,2\Omega\sin\theta v\,=\, -{{\partial}\over{\partial\phi}}(\zeta\,-\,\Gamma /g)/a \cos\theta$$

$${{\partial v}\over{\partial t}}\,+\,2\Omega\sin\theta u\,=\, -{{\partial}\over{\partial\theta}}(\zeta\,-\,\Gamma /g)/a$$

$${{\partial\zeta}\over{\partial t}}\,+\,{1 \over {a\cos\theta}} \l... ...rtial\phi}}(uD)\,+\, {\partial\over{\partial\theta}}(vD\cos\theta)\right]\,=\,0$$

where $(\phi,\theta)$ are longitude and latitude with corresponding velocity components $(u,v)$, $\zeta$ the ocean surface elevation, $\Gamma$ the tide-generating potential, $D$ the variable depth of the ocean, $a$ the earth's radius, $g$ the gravitational acceleration at the earth's surface, and $\Omega$ the earth's angular rate of rotation.

These equations were eventually found to not be uniformly valid, a problem that was eventually fixed by relaxing the assumption of homogeneous fluid to allow stratification and developing a similar equation set called the long wave equations. Both sets are used to investigate such long ocean waves as Rossby waves, Poincare waves, Kelvin waves and the like, although these are more commonly and easily studied using an equation set written in cartesian coordinates on a beta plane or an f-plane. For example, the LTEs simplified for long waves in a uniformly rotating flat-bottomed ocean, i.e. an f-plane, are

$${{\partial u}\over{\partial t}}\,-\,{f_0}v\,=\,-g{{\partial\zeta}\over {\partial x}}$$

$${{\partial v}\over{\partial t}}\,+\,{f_0}u\,=\,-g{{\partial\zeta}\over {\partial y}}$$

$${{\partial\zeta}\over{\partial t}}\,+\,{{\partial(uD)}\over{\partial x}}\,+\,{{\partial(vD)}\over{\partial(vD)}}$$

where $f_0$, equal to $2\Omega$, is the Coriolis parameter. See Lamb (1932), sect. 213-221 and Hendershott (1981).

LAPS

Acronym for Large Aggregate Profiling System.

lapse rate

The rate of decrease of temperature with height. This can be either an environmental or a process lapse rate. An environmental lapse rate is a static measure of the state of the environment, e.g. finding the rate of temperature decrease by measuring the vertical temperature profile in some way. A process lapse rate, on the other hand, gives the temperature associated with some action or process, e.g. a rising or sinking air parcel.

Laptev Sea

One of the seas found on the Siberian shelf in the Arctic Mediterranean Sea. It sits between the Kara Sea to the west, the East Siberian Sea to the east, and adjoins the Arctic Ocean proper to the north. The western boundary begins at the Arctic Cape (Komsomolets Island), proceeds along the eastern shores of the Northern Land, through the Red Army Strait, along the eastern shore of October Revolution Island, to the Anuchin Cape and through Shokal'sky Strait to Bol'shevik Island, along the coast to the Vaigach Cape, along the eastern boundary of Vil'kitsky Strait to the head of Khatanga Bay. The northern boundary starts at the Arktichesky Cape, and passes through the crossing point of the meridian of the northern tip of Lotel'ny Island (139$^\circ$E) and the edge of the continental shelf (79$^\circ$ N, 139$^\circ$ 00' E). The eastern boundary begins at the western side of Sannikov Straita, passes around the western shores of the Large and Small Syakhovsky Islands, and then along the western extent of Dmitry Laptev Strait. The southern boundary is the continental shore from Svyatoy Nos Cape to the head of Khatanga Bay.

The width along parallel 75$^\circ$ N is 717 km, and the length from Cheluskin Cape to Dimitry Laptev Strait is 1126 km. The area is around 540,000 km$^2$ with about 66% being less than 100 m deep. The southern and southeastern areas, comprising 45% of the total, have water depths ranging from 10 to 50 m. Depths in excess of 2000 m are found in the northern parts. An incline starting at 100 m and ending at 3000 m divides the sea into the northern and southern parts along the parallel of the Vil'kitsky Strait. This has also been called the Nordenskjold Sea.

Zenkevitch (1963) gives the history of the exploration of the Laptev Sea to 1955:

Nordenskjöld's expedition on the Vega (1878-1879) marked the beginning of the exploration of the fauna and flora of the Laptev Sea, which was continued by the Russian expeditions of Toll on the Zarya (1900-1903) and Vilkitsky on hte Taimyr and Vaigach (1913). In the Soviet era the Norwegian expeditions on the ship Mod (1918-1920 and 1921-1924), and the Soviet expeditions of Khmisnikov (1926) and of Yu. Tchirikhin (1927) on the icebreakers Lithke (1934) and Sadko (1937) have worked in the Laptev Sea.

The runoff into the Laptev Sea from the five major rivers - the Khatanga, Anabar, Lena, Olenek and Yana) is about 767 km$^3$ yr$^{-1}$. This runoff, shelf water exchange with the Arctic Ocean, and perennial ice cover are the main factors determining the water masses. In the upper Laptev Sea, Arctic surface water (-1.80$^\circ$ C, 32.00) and Laptev Sea surface water (-1.40$^\circ$ C, 22.00-25.00) Sea surface water dominate and show only weak seasonality. Warm Atlantic water (2.25$^\circ$ C, 34.98) is spread in the north in the deep water troughs under the Arctic surface water. Below 800-1000 m is cold bottom water with a temperature of 0.4-0.9$^\circ$ C and a salinity of 34.90-34.95. This water is formed by the sinking of cooled water from the continental slopes. The temperature and salinity fields show large gradients between the mixing zones of river and sea water and the uniform thermohaline structure in the north. The thermohaline structure is relatively homogenous in the winter due to a decrease in runoff, increase in ice cover, and decrease in convection.

The currents are influenced by the position and intensity of the Icelandic Low and the Arctic High, water exchange with the Arctic Basin and with the Kara and East Siberian Seas, and by river freshwater input. A well-defined cyclonic circulation exists in the surface layer of the Laptev Sea. A current from west to east along the mainland coast is intensified by the Lena Current. As it approaches the New Siberian Islands, it is directed to the northwest and then the north in the form of the New Siberian Current, finally merging with the Trans-Arctic Current moving from east to west. Near the Northern Land the waters in the southern part of the Trans-Arctic Current are generally headed southward and, in the form of the East-Taimyrskoye Current, reach the coast eastern flow to complete the cyclonic gyre. Part of the New Siberian current moves eastward through the Sannikov Strait into the East Siberian Sea.

The current velocities are small, usually not exceeding 10 cm/s. During the times when the Icelandic Low dominates the Arctic High, there is a well-developed transport of water masses from south to north, and the center of the cyclonic gyre shifts toward the Northern Land. A wind-driven gyre exists in the southeastern region. It is cyclonic when western atmospheric transports prevail over the eastern Laptev Sea, and anticyclonic when eastern transports dominate.

The Laptev Sea is covered by ice of different thickness and age categories from October to May. Formation begins in late September, with extensive fast ice up to 2 m thick formed in winter in water depths of less than 20-25 m. A significant area of polynyas and young ice is preserved to the north of this fast ice zone. The width of the zone varies from 10 to several hundred kilometers, with the ice edge changing its position in summer according to wind and currents. See Zenkevitch (1963), Fairbridge (1966) and Pavlov (1998).

[http://www.nadn.navy.mil/Oceanography/courses/SO426/atlas_summer/html/intro/intro_lp.htm]

Large Aggregate Profiling System (LAPS)

A system for counting the number and size distribution of particles in sea water ranging from 250 microns to several millimeters in size. LAPS consists of a video camera synchronized with a strobe light which flashes at predetermined intervals as the instrument is lowered through the water column. The light is focused so it illuminates only a narrow slab of water where the video camera records images of the large particles in the water. The video images are analyzed using software which counts the number of particles and determines their maximum and minimum dimensions if they are not circular, and this data is used to estimate a volume and mass of the imaged particles. LAPS is an integral part of the Particle and Optics Profiling System (POPS).

large eddy simulation

A compromise between explicitly simulating everything down to the smallest scales of motion via direct simulation and simulating only the mean flow via Reynolds averaging when modeling turbulent flows. This is accomplished by first applying a local spatial filter to the equations of motion. Then the large-scale turbulent motions are explicitly simulated while a turbulence model represents or parameterizes the influence of the unresolved small-scale motions that the filter has separated. The scale of the filter operation is usually within or close to the inertial subrange of 3-D turbulence, where it is expected that the resolved motions will describe the energy-producing mechanisms and associated fluxes of heat and momentum and the unresolved subgrid motions the dissipation of the resolved turbulence energy. This technique originated in meteorology and weather prediction where the aim is to predict the evolution of large-scale eddies over their lifetimes, i.e. the weather, while parametering the effects of not well understood processes like moist convection and radiative transfer. See Mason (1994).

LARVE

A component of the RIDGE initiative whose goal is to investigate larval dispersal and gene flow in vent environments and evaluate the potential role of these processes in generating and maintaining biogeographic patterns along mid-ocean ridges and across ocean basins.

[http://ridge.oce.orst.edu/larve/larve.html]

latent heat

The quantity of heat absorbed or emitted, without change of temperature, during a change of state (from solid to liquid or from liquid to gas) of a unit mass of a material. It is a hidden heat (i.e. it can't be sensed by humans) that doesn't occur until phase changes ocur. An example is the evaporation of liquid water cloud droplets cooling the air by removing heat and storing it as latent heat. Phase changes that cool the air are vaporization (liquid to vapor), melting (solid ice to liquid) and sublimation (solid to vapor), while phase changes in the opposite direction that warm the air are condensation (vapor to liquid), fusion (liquid to solid) and deposition (vapor to ice). The latent heat is $2.5\,\times\,{{10}^{6}}\,{{\text{J/kg}}^{-1}}$ for condensation or evaporation, $3.34\,\times\,{{10}^{5}}\,{{\text{J/kg}}^{-1}}$ for fusion or melting, and $2.83\,\times\,{{10}^{6}}\,{{\text{J/kg}}^{-1}}$ for deposition or sublimation, with the sign depending on the direction of the change.

latent heat flux

The exchange of heat between a moisture-containing surface and atmosphere resulting mainly from the evaporation at the surface and the later condensation within the atmosphere. This is an indirect transfer of heat associated with the phase transitions of water, between liquid and vapor at the surface and later between vapor and liquid or solid phases. See Peixoto and Oort (1992).

LATEX

Acronym for Louisiana-Texas Experiment, sponsored by the MMS of the Department of the Interior. LATEX was a six-year project with the principal objective of identifying the key dynamical processes governing the circulation, transport, and cross-shelf mixing of the waters on the Texas-Louisiana shelf. The program had three components:

• LATEX A - shelf circulation and transport processes (administered by Texas A&M University);
• LATEX B - Mississippi River plum hydrography (administered by Louisiana State University); and
• LATEX C - Gulf of Mexico eddy circulation (administered by Science Applications International Corporation).

[http://ocean.tamu.edu/LATEX/welcome.html]

Laurasia

The name given to a hypothetical northern hemisphere supercontinent consisting of North America, Europe, and Asia north of Himalayas prior to breaking up into its separate components. It was formed in the early Mesozoic by the rifting of Pangaea along the line of the North Atlantic Ocean and the Tethys Sea. The southern hemisphere analogue was called Gondwanaland and both comprised a hypothetical single supercontinent called Pangaea before their splitting up.

Laurentia

The Precambrian craton of central eastern Canada. It forms the ancient core of Canada, the remainder having been accreted via orogeny.

layer coordinates

In numerical modeling, a system of vertical coordinates where an arbitrary number of layers are specified in which some fluid property (e.g. density) remains constant, i.e. an independent variable. The dependent variable is the vertical extent of each layer. Pressure coordinates are an example of layer coordinates. Compare to level coordinates.

layer method

See core layer method.

layering

In physical oceanography, this is a consequence of the double diffusion phenomena. If a layer of colder, fresher water overlies a layer of warmer, saltier water, the differences in molecular diffusivities between salt and heat will cause the water just above/below the interface to become lighter/heavier than that above/below it and thus it will tend to rise/sink. The phenomenon, called layering, can lead to fairly homogeneous layers separated by thinner regions with large gradients.

LCD

Abbreviation for low cost drifter.

LDEO

Abbreviation for Lamont Doherty Earth Observatory, a research division of Columbia University dedicated to understanding how planet Earth works. See the LDEO Web site.

lead

An opening in pack ice which forms as the result of a local divergence in its drift. These occur at weak points in the ice and are characteristically long, narrow channels meters to hundreds of meters wide and kilometers to tens of kilometers long. They have no fixed location, although their occurrence may be predictable by region or season, and the are more prevalent in regions of thinner ice and marginal ice zones. Leads typically cover at least 1% of the total ice area, with the average distance between them varying from 5 km in the marginal ice zone to 275 in the central Arctic pack ice. See SMith et al. (1990).

LEADEX

Acronym for Lead Experiment, a 1992 experiment to study the effect of leads (cracks in the ice) on the polar ocean and atmosphere. See Persson et al. (1997).

[http://nsidc.org//NASA/GUIDE/docs/campaign_documents/arctic_leads_ARI_campaign_document.gd.html]
[http://www.cmdl.noaa.gov/star/agasp2.html]

leddies

Baroclinic eddies composed of LIW as hypothesized by Millot (1999). These are posited to form as the LIW becomes unsable as it passes the open southwest corner of the Sardinian slope. They propagate into the interior of the Algerian Basin and contribute to the occurrence of recent LIW there.

[http://www.com.univ-mrs.fr/LOB/ELISA/]

Leeuwin Current

The eastern boundary current along the west coast of Australia. This differs from other eastern boundary currents in that an abnormally large pressure gradient overrides the usual scenario of equatorward winds producing surface upwelling, an equatorward surface flow, and a poleward undercurrent. The Leeuwin Current flows strongly poleward against equatorward winds due to a difference in dynamic height of 0.5 m along the coast. This difference is related the the throughflow from the Pacific to the Indian Ocean through the Australasian Mediterranean Sea which serves to maintain the same steric height on either side of the throughflow. The same height cannot be maintained by the colder waters off southwest Australia and thus the lower steric height. The result is a self-perpetuating process where the southward flow of warm water leads to surface cooling which keeps the steric height lower which leads to southward flow, etc.

The annual mean transport of the Leeuwin Current has been estimated at 5 Sv with average current velocities ranging from 0.1-0.2 m/s, although its intensity and southward extent vary seasonally. It is strongest in May when the countering wind is weakest with speeds up to 1.5 m/s. The strong fronts on both sides of the current tend to produce eddies during this period. See Godfrey and Weaver (1991), Holloway (1995) and Batteen and Huang (1998).

length scales

A thorough investigation of the length scale relationships in the steady linear theory of rotating stratified fluids can be found in Blumsack (1973).

LEPOR

Acronym for Long-Term and Expanded Program of Oceanic Exploration and Research, an IOC project.

LES

In the modeling of fluid turbulence, this is an abbreviation for Large Eddy Simulation, a technique in which eddies above a certain size are directly simulated and the effects of those of smaller size are parameterized in terms of some variable at the larger scale.

levanter

An easterly wind in the Straits of Gibraltar, most frequent from July to October and in March. It is usually associated with high pressure over western Europe and low pressure to the southwest of Gibraltar over the Atlantic or to the south over Morocco.

Levantine Intermediate Water (LIW)

The saltiest water mass that forms in the eastern Mediterranan Sea. It is exclusively formed in several areas of the Levantine Basin and the southern Aegean Sea in February and March under the influence of dry and cold continental air masses. Mesocale processes play an important role in the formation and spreading of LIW. After its formation, it forms a layer identified by a salinity maximum between the overlying Modified Atlantic Water (MAW) and the underlying Eastern Mediterranean Deep Water (EMDW). The core depth fluctuates from 50 to 600 m. The LIW flows eastwards and westwards, with part of the portion formed in the Levantine Basin entering the Aegean through the eastern straits of the Cretan Arc with core temperature and salinity values at the sill depths of 14.5$\circ$C and 38.9. Some of this water is ventilated and transformed by convective processes to become a slightly denser intermediate water mass called Cretan Intermediate Water (CIW).

While the characteristics of the source waters generated at different regions vary (14.70-16.95$^\circ$C, 38.85-39.15$^\circ$C), mixing results in homogeneity and the loss of heat and salt as the LIW spreads westward. The slow spreading and long renewal time give LIW as relatively low oxygen content of less than 4.5 ml/l. The winter properties of LIW as it leaves the Levantine Sea are 15 $^\circ \le \theta \le$ 16$^\circ$ C, 39.0 $\le S \le$39.2, and 28.9 $\le {\sigma_\theta} \le$ 29.0. In the Ionian Sea the properties are transformed into 14 $^\circ \le \theta \le$ 15$^\circ$ C, $S \ge$38.8, and 29.0 $\le {\sigma_\theta} \le$ 29.1. See Perkins and Pistek (1990), Ö (1993), Stratford and Williams (1997), Stergiou et al. (1997) and Theocharis et al. (1999).

Levantine Sea

See Ö (1993) and Theocharis et al. (1999).

Levantine Surface Water (LSW)

A warm and saline water mass occupying most of the surface layer of the Eastern Mediterranean Sea during the warm part of the year. It is the product of intense evaporation, with the salinity reaching maximum values of around 39.5 by the end of the summer, especially in the northwest Levantine and southeast Aegean Seas around Rhodos. See Balopoulos et al. (1999).

leveche

A hot, dry, southernly wind which blows on the southeast coast of Spain in front of an advancing depression. It frequently carries much dust and sand, with its approach being signaled by a strip of brownish cloud on the southern horizon. This corresponds to the scirocco of North Africa and is named after the direction from which it blows. It is also known as the solano.

level coordinates

In numerical modeling, a vertical coordinate system in which a arbitrary number of height or depth levels are specified at which the changing values of the various dependent variables are calculated. Thus the level heights or depths are independent variables. Compare to layer coordinates.

level of known motion

See reference level.

level of no motion

A reference level at which it is assumed the horizontal velocities are practically zero. See Wunsch and Grant (1982), Killworth (1983) and Olbers and Willebrand (1984).

level surface

See geopotential surface.

LEWEX

Acronym for Labrador Extreme Wave Height Experiment. See Beal (1991).

lexicographer

A writer or compiler of a dictionary or a glossary, i.e. a (relatively) harmless drudge.

LGM

Abbreviation for Large Scale Geostrophic model, an ocean circulation model developed at MPI. It is based on the primitive equations and designed specifically to model global scale geostrophic circulation. The nonlinear advection of momentum is neglected and fast gravity waves are strongly damped via an implicit time integration scheme that uses a time step of 30 days. An upstream advection scheme us used for salinity and temperature transport and vertical convective mixing is applied when the stratification becomes unstable. The Arakawa E-grid is used for horizontal discretization and a small horizontal diffusion specified to alleviate the inherent mode-splitting problems of this grid scheme. Sea ice is computed from a heat balance and by advection by ocean currents with a simplified viscous rheology.

LHPR

Abbreviation for Longhurst-Hardy Plankton Recorder. See Dunn et al. (1993).

libeccio

A strong, squally southwesterly wind in the central Mediterranean, most common in winter.

LIE

Acronym for Line Islands Experiment, a GARP project.

light compensation depth

An ocean depth above which the intensity of photosynthetic assimiliation is greater than losses by respiration, and below which the reverse situation takes place where phytoplankton can exist only due to the organic matter that has been previously formed, i.e. due to conversion to heterotrophic nutrition. See Kagan (1995).

Ligurian Sea

One of the seas that comprise the western basin of the Mediterranean Sea. It is located to the north of Corsica between the Balearic Sea to the west and the Tyrrhenian Sea to the east. See Fairbridge (1966) and Astraldi et al. (1990).

Liman Current

A cold current that flows from the Okhotsk Sea into the Japan Sea. Part of this current continues south along the western coast and eventually becomes the North Korea Current.

Lincoln Sea

A part of the Arctic Mediterranean Sea located on the Greenland-Canadian-Alaskan shelf. It is situated northeastward of Ellesmere Island and northwest of northern Greenland. See Newton and Sotirin (1997).

linear

Characteristic of a system (the climate, etc.) wherein the output is always proportional to the input. As such, the output from a linear system can be predicted from knowledge of the input. An equivalent definition of a linear system is a system that satisifies the superposition principle, i.e. the combined effect on the output of two separate and distinct inputs can be found simply by combining, or superposing, their individual effects.

LISST

Acronym for Laser In-Situ Scattering and Transmissometer, an instrument manufactored by Sequoia Instruments which measures the in-situ size distribution of particles from 5 to 250 microns in diameter in water with sediment loads exceeding 200 $\mu$/l based on the principle of laser diffraction. It is usually attached to the POPS system and together they generate diagrams of the size distribution of particles and small aggregates in the water column without disturbing them via collection in water bottles. Particle aggregates in water bottles tend to fall apart before they can be measured, making the measurements of optical properties and settling dynamics obtained that way incorrect.

The LISST measures particle sizes by projecting a laser beam towards a series of concentric ring-shaped detectors. The angle at which the laser light is scattered depends on the particle size, with the smaller/larger particles scattering light toward the outer/inner detector rings. The distribution of particle sizes is proportional to the volume scattering function which is inverted to obtain the particle concentration and size spectrum. The LISST/POPS system converts this information into graphs showing the abundance of particles of each size. The original LISST-100 model measures particle size spectra, while an advanced model called the LISST-ST also measures in-situ settling velocity distributions. It does this by trapping a water sample and performing a series of measurements of the type described above over time. See the Sequoia Instruments Web site.

Little Climatic Optimum

A period of time lasting from AD 750 to 1200 when the climate of Europe and North America was clement, even as far north as Greenland and Iceland. See Lamb (1985), p. 435-449.

Little Ice Age

A return to colder climatic conditions beginning in about 1450 and ending around 1890. This was an era of moderate, renewed glaciation that followed the warmest known part of the Holocene. It was marked by the advance of valley glaciers in the Alps, Alaska, Swedish Lapland and New Zealand far beyond their present limits as well as by snow on the high mountains of Ethiopia where it is now unknown. The evidence points to two main cold stages, each lasting about a century, during the seventeenth and nineteenth centuries. Regional timings of the cold periods differed and thus some doubts have been raised as to the global nature of this phenomenon. See Grove (1988) for a book-length treatment of this period.

LIW

Abbreviation for Levantine Intermediate Water.

LIWEX

Acronym for Levantine Intermediate Water Experiment, which took place during winter-spring 1995 in the Levantine Basin to study the formation and spreading of Levantine Intermediate Water (LIW).

[http://earth.esa.int/symposia/data/santoleri2/]

LLWI

Abbreviation for lower low water interval.