Nn-Nz

NOAA

Acronym for the National Oceanic and Atmospheric Administration.

NOAMP

Acronym for Nordostatlantisches Monitoring Program, a field program designed to investigate regional flows at great depths, particularly mesocale dispersion and pathways of suspended matter in the deep ocean within the near-bottom layers. The field phase of NOAMP ran from September 1984 to May 1986, and comprised six cruises of the R. V. Meteor. During each leg, 7 to 9 current meter moorings were deployed in the central NOAMP box (47$^\circ$05'N-47$^\circ$35'N, 18$^\circ$45'W-20$^\circ$32'W), with standard measuring heights 10, 30, 70 and 150 meters above the bottom. Short moorings also had instruments at 250 and 750 meters above the bottom, while long moorings had them at 200, 300, 400 and 500 m. A quasi-Lagrangian experiment with 14 3500 m deep drifting SOFAR floats was carried out from May 1985 to June 1986. See Klein and Mittelstaedt (1992).

NOARL

Abbreviation for Naval Ocean and Atmosphere Research Laboratory, the precursor to the NRL. The precursor to NOARL was the Naval Environmental Prediction Research Facility (NEPRF), established in 1971 and functioning as a field activity of the Naval Air Systems Command. In 1989 NEPRF was combined with the Naval Oceanography Research and Development Activity and the Institute for Naval Oceanography - both located at the Stennis Space Center in Mississippi - to form a single naval ocean sciences laboratory called NOARL. In 1992, NOARL was incorporated into NRL.

NOBREX

Acronym for NOrth BRazilian current EXperiment.

NOCN

Abbreviation for NOAA Communications Network, a system created to serve the communications and data quality enhancement needs of the NOAA ocean community.

NODC

Acronym for National Oceanographic Data Center, a centralized facility for providing ocean data/information on a continuing basis in a usable form to a wide user community as established within the framework of the IOC IODE structure. These facilities acquire, process, perform quality control, inventory, archive and disseminate data in accordance with national responsibilities. They are also charged with the responsibility of conducting international data exchange. Member states without an established NODC assign the responsibility of international exchange of data to another agency referred to as a Designated National Agency (DNA). The fundamental responsibility of the NODC/DNA as regards international exchange is to actively seek and acquire data which are exchangeable internationally, process and perform quality control on the data, and submit the data in a timely fashion to the approriate WDC for Oceanography or to a RNODC. See the IODE Web site.

NODC

Abbreviation for National Oceanographic Data Center, a branch of the NESDIS division of the Office of Environmental Information Services of NOAA that develops and maintains a national marine database. The NODC consists of three divisions:

• Ocean Climate Laboratory,
• Coastal Ocean Laboratory,
• NOAA Central Library.

[http://www.nodc.noaa.gov/]

NODDS

Acronym for the Navy Oceanography Data Distribution System, a state-of-the-art methodology that makes environmental products and satellite data available worldwide via the Web. This was developed at the FNMOC for the distribution of their products.

NODS

Acronym for NASA Ocean Data System, the former name of what is now known as PODAAC.

NOIC

Abbreviation for National Oceanographic Instrument Center, established by the Naval Oceanographic Office as a facility to establish criteria and procedures for testing and calibrating certain types of instruments. This office was active in the 1960s and 1970s, but is no longer in existence.

[http://www.lib.noaa.gov/edocs/stratton/chapter6.html]

noise

In geophysical data processing this is most simply defined as any unwanted signal, and given that one person's signal can be another person's noise, this is ultimately a relative term. For example, if a time series is created by taking the temperature at some location every hour for five years, then the daily cycle of temperature that will be seen in such a record is a signal for someone looking for the daily cycle but is noise to someone looking for monthly or seasonal temperature variations.

NOMADS

Acronym for North Sea Model Advection Dispersion Study, a program for the intercomparison of advection-dispersion models for the North West European continental shelf. The objectives were to compare the spatial and temporal coherence of simulation results for a well-defined realistic test and to compare the characteristics of the models by direct point-by-point comparison for an idealized 3-D test case. The project ran for two years starting in February 1995.

[http://www.pol.ac.uk/coin/nomads/]

nonlinear

Said of a system (an electronic circuit, the climate, etc.) in which the output is not strictly proportional to the input. One consequence of this is that small changes in input can lead to very large and unpredictable changes in output.

NORCSEX

Acronym for the Norwegian Continental Shelf Experiment, a pre-launch ERS field investigation carried out during a 25-day period in March 1988 on the continental shelf off the coast of Norway centered at 64$^\circ$ N. The overall goal was to investigate the capability of the ERS 1 type active microwave sensors to measure marine variables such as near-surface wind, waves and ocean surface current and their interaction in weather conditions ranging from moderate to extreme.

The primary objectives of NORCSEX included studies of SAR imaging of surface current features, SAR imaging of ocean surface gravity waves, combined airborne SAR and ship-mounted scatterometer measurements of near-surface wind fields, radar altimeter measurements of sea surface topography, significant wave height, and wind speed, integrated use of SAR and radar altimeter for significant wave height measurements, and comparison and validation of numerical ocean circulation model results to remote sensing and in situ observations. See Johannessen (1991).

Nordenskjold Sea

See Laptev Sea.

Nordic Seas

A term used to collectively refer to the fairly shallow Barents Sea and three deep ocean regions: the Norwegian Sea, the Greenland Sea and the Iceland Sea. The deep parts of the latter three are separated from another by deep submarine ridges. The Nordic Seas are separated from the North Atlantic to the south by the Greenland-Scotland Ridge, and are connected to the Arctic Ocean to the north via the 2200 m deep Fram Strait. The Nordic Seas along with the Arctic Ocean are collectively referred to as the Arctic Mediterranean. See Hansen and Osterhus (2000).

normal modes

A decomposition solution procedure based on the eigenvectors of the linearized dynamical equations, i.e. an inherently linear concept. For example, the equations of large-scale motion in the atmosphere or ocean yield a sum of normal mode solutions for which each has a fixed vertical structure and behaves in the horizontal dimension and in time in the same way as a homogenous fluid with a free surface. Assuming the validity of the assumptions leading to the normal mode solution, the complete solution to the original differential equation is then approximated as a sum of the normal mode solutions. This technique can be applied to either a continuously or discretely stratified ocean model, with the former yielding an infinite set of normal modes and the latter a finite number of modes.

NORPAX

Acronym for the NORth PAcific eXperiment, a shuttle experiment that took place from Feb. 1979 through Jun. 1980. It included 15 approximately monthly cruises on a track running directly south from Hawaii to 4$^\circ$S, east to 153$^\circ$W, north to 12$^\circ$N, east to 150$^\circ$W, and then south to the island of Papeete at around 18$^\circ$S. The ships involved collected CTD data every 1$^\circ$ of latitude or longitude, and occupied profiling current meter stations every 1$^\circ$ between 6$^\circ$S and 10$^\circ$N (with additional half-degree stations with 3$^\circ$ of the equator). Acoustic Doppler current profiles were collected continuously along the ship's track, which was traversed in alternate directions. A set of three vector-averaging current meter moorings were also maintained during the experiment. See Wyrtki et al. (1981).

North African Trough

See CapeVerdeBasin.

North American Basin

A large depression centered around the Bermuda rise at about 85$^\circ$ W and 30$^\circ$ N in the western North Atlantic Ocean. It includes the Sohm Abyssal Plain to the northeast, the Hatteras Abyssal Plain to the west, and the Nares Abyssal Plain (or Nares Deep) to the southeast. Other prominent features in this basin include the Vema Gap, the Blake-Bahama Outer Ridge, and Blake-Bahama Basin and the Puerto Rico Trench. See Fairbridge (1966).

North Atlantic Central Water (NACW)

See Slope Water entry for now. See Poole and Tomczak (1999).

North Atlantic Current (NAC)

A western boundary current (WBC) that flows north along the east side of the Grand Banks in the northwestern Atlantic from 40$^\circ$ to 51$^\circ$ N, where it turns sharply to the east at a location than has come to be known as the Northwest Corner. It is part of the subtropical gyre circulation in the North Atlantic and begins where the Gulf Stream curves north around the Southeast Newfoundland Rise. The path of the NAC is delineated by a well-defined front while it flows north as a WBC, but broadens into a widening band of eastward drift without a sharp or permanent front after it makes its turn at the Grand Banks. Some authors limit the extent of the NAC to point at which it turns east and refer to the more diffuse eastward extension as the subpolar front, while others have it extending further downstream.

The northward flowing front features currents with maximum speeds typically near 1 m s$^{-1}$ in the upper 300 meters and maximum transports more than 40 Sv (decreasing to around 20 Sv when the extension crosses the Mid-Atlantic Ridge). The NAC also meanders greatly, although unlike in the Gulf Stream the meanders appear to be stable, with only one recorded instance of one breaking off to form an eddy. The meanders have been observed to grow, recede, and disappear entirely, but not to propagate other than in the case of the one exception. The meanders appear to be induced by and bound to several major topographic features along the current path, i.e. the Southeast Newfoundland Rise, the Newfoundland Seamounts, and the Flemish Cap. Strong recirculation cells develop on the concave sides of some meanders, with the strongest and most persistent located at the first meander at about 44$^\circ$ W and 42$^\circ$ N. This is hypothesized to be a permanent feature and is called the Mann eddy. The recirculation regions to the east of the NAC can combine to form a narrow, extended recirculation cell with a north-south extent of around 600 km, although isopycnal float data suggest that this occurs only infrequently.

The permanence of the NAC is evidenced by a couple of lines of evidence. The persistence of the Northwest Corner where the NAC turns east is evidenced by its remaining a sharp feature even in hydrographic data data averaged over 50 years. Isopycnal float trajectory data indicate that it acts effectively as a boundary between the gyres on shallow density surfaces since only a very few floats have crossed the current entirely. Thus the NAC functions as an effective barrier between the subtropical and subpolar waters in the Northeast Atlantic.

The NAC extends the WBC regime in the North Atlantic further poleward than is seen in any other ocean, and thus tranports a greater volume of warm water into the polar regions. It is not known exactly why the Gulf Stream continues as a WBC in the form of the NAC rather than heading east after it leaves the coast in a manner similar to the Kuroshio Current. The reasons for this are thought to be some unknown combination of the unique demands of the Atlantic thermohaline circulation, the strong bathymetric control experienced by the NAC, and the wind forcing. See Krauss (1986), Rossby (1996), Kearns and Rossby (1998) and Carr and Rossby (1999).

North Adriatic Deep Water (NADW)

A water mass formed in the north Adriatic Sea. It is formed during strong bora wind events when evaporation can be as high as 15 mm/day (compared to an annual average total of 1000 mm). The very dense deep water formed is characterized by temperatures less than 10$^\circ$C and a relatively low salinity of about 38.3. After formation, the NADW flows southwards along isobaths near the bottom of the Italian shelf. The flow partially sinks into the Jaluka Pit, although the major portion moves further southwards to reach the shelf off Bari, where a canyon intersects the shelf and the water deepens. At this point the NADW plays some role in the formation of Adriatic Bottom Water (ABW). See Artegiani et al. (1993).

North Atlantic Deep Water (NADW)

A water mass that fills the depth range between 1000 and 4000 m in the Atlantic Ocean. It is seen as a layer of relatively high salinity (above 34.9) and oxygen (above 5.5 ml/l) extending southward from the Labrador Sea to the Antarctic Divergence.

NADW originates in the northern North Atlantic in the GIN Sea. The main sources in are the dense overflows on either side of Iceland from intermediate depths in the Nordic Seas, the lower part of the Labrador Sea Water (LSW) layer including both a recirculating and an entraining component, and a recirculating Antarctic Bottom Water (AABW) derivative of southerly origins in the deepest layers of either basin.

The total direct transport of the dense overflows is about 5.6 Sv, about equally divided east and west of Iceland over the various sections of the Greenland-Scotland Ridge. The sill depth in the Denmark Strait to the east of Iceland is 600 m. To the west is is 450 m on the Iceland-Faroe Ridge and 850 m in the Faroe Bank Channel. The most saline of the overflows in through the Faroe Bank Channel which, although it overflows as a relatively fresh source, mixes intensely with overlying warm saline water from the local thermocline to become more saline. This water has been labeled by some as Northeast Atlantic Deep Water (NEADW). The coldest and densest of the overflows is the Denmark Strait Overflow, which has a characteristic salinity minimum. This overflow has been sometimes labeled as Northwest Atlantic Bottom Water.

The AABW component passes into the eastern GIN Sea basin through the Vema Fracture Zone at 11$^\circ$ N at a rate of about 2.0-2.5 Sv. This eventually combines with the overflows east of Iceland to give an estimated 6.6 Sv of flow west through the Charlie Gibbs Fracture Zone. This flows west to combine with the overflow west of Iceland, which has mixed with the LSW to contribute to a total eventual southward flow of NADW east of Newfoundland of about 13 Sv. The recirculation and entrainment processes that increase the 5.6 Sv of overflow water to the 13 Sv of NADW flowing south are the least well known parts of NADW formation. See Dickson and Brown (1994) for the present best summary of NADW formation processes.

The resulting mixture, as it moves south, is often separated into upper, middle and lower NADW. Upper NADW (UNADW) comes from Mediterranean outflow spreading into the Central and North Atlantic at depths of 1000 to 1500 m, and is identified by a salinity maximum. Middle NADW (MNADW) is formed by ocean convection in the Labrador Sea flowing into the Western North Atlantic Basin. Lower NADW (LNADW) is formed by a complex series of mixing flows over the Greenland-Scotland Ridge and thereafter, and comprises the bulk of the totality of NADW. The middle and lower forms of NADW are both identified by oxygen maxima in the subtropics at 2000-3000 m and 3500-4000 m, respectively.

More recent work has identified four different water types within the southward flowing NADW comprising the DWBC. The shallowest part of the upper NADW - called either Shallow Upper NADW (SUNADW) or Upper Labrador Sea Water (ULSW) - has a salinity maximum correlated to elevated concentrations of tritium and chlorofluorocarbons, with the salinity maximum near 1600 m at the equator and deepening to 2500 m at 25$^\circ$S. The LSW below the SUNADW shows a lower CFC signal than the SUNADW in the tropics, and is additionally differentiated there by an oxygen maximum. The LNADW also exhibits internal differences, with an CFC minimum located above a CFC maximum. This has been separated into:

• old LNADW with a CFC minimum originating from the Charlie Gibbs Fracture Zone Water; and
• overflow NADW (OLNADW) with CFC and oxygen maximums originating from the Denmark Strait Overflow Water.

As the thickness of the NADW decreases southwards - with a depth range from about 1200 to 3900 m near the equator and 1700 to 3000 m in the Brazil-Falkland Confluence zone - the various NADW water types merge, with the extrema either broken or merged. See Warren (1981), Dickson and Brown (1994), Smethie Jr. (1999) and Stramma and England (1999).

North Atlantic Drift

The northward limb of the anticyclonic subtropical gyre in the North Atlantic Ocean. It is a northerly extension of the Gulf Stream but, due to a different dynamical regime, is a broader, slower current that carries warm water towards Europe, serving to ameliorate the climate there.

North Atlantic Oscillation (NAO)

An index defined as the normalized winter pressure differential between the Icelandic Low and the Azores High centers of action. Low (high) NAO index values occur when sea level pressure is above (below) average in the vicinity of the subpolar Icelandic Low and below (above) average near the subtropical Azores High pressure cell. See Van Loon (1978) and Rogers (1984).

North Atlantic Subpolar Mode Water

A type of Subpolar Mode Water that forms in the North Atlantic. North Atlantic SPMW is a dense, cool (8-10$^\circ$C) pycnostad spreading northwards past Ireland and turning westwards through the Iceland and Irminger Basins. It follows a cyclonic circulation within which the density gradually increases as temperature decreases to eventually form the coldest, densest pycnostad in the Labrador Sea. See McCartney and Talley (1982).

North Atlantic Subtropical Mode Water

A type of Subtropical Mode Water that forms in the North Atlantic.

North Atlantic Water (NAW)

A water mass transported by the Continental Slope Current from the North Atlantic into the Nordic Seas. It has typical property values in the Rockall Channel region of the Greenland-Scotland Ridge of 9.5-10.5$^\circ$C and 35.35-35.45. This makes it the saltiest and warmest of the waters exchanged over the Ridge. The origins of the high salinity are to date a matter of debate, with some postulating a Mediterranean source, and others conjecturing that the high salinity is acquired by a combination of winter cooling at constant temperature (i.e. sea ice formation and the resultant brine release) for intermediate depths and evaporation and advection from southern areas for near-surface layers. See Hansen and Osterhus (2000) and references therein.

North Brazil Current (NBC)

A current that flows in the western South Atlantic Ocean along the Brazilian coast from about 10 to 3$^\circ$ S along around 35$^\circ$ W. Geostrophic calculations (relative to 1000 m) show a broad (300 km wide), northwestward current transporting about 37 Sv at 5$^\circ$ S. It is concentrated in a subsurface core at 100-200 m depth. It continues as a coherent feature until the subthermocline layers retroflect at between 3 and 5$^\circ$ N to feed the North Equatorial Undercurrent (NEUC) and then the upper layers retroflect at between 5 and 8$^\circ$ N to feed the North Equatorial Countercurrent (NECC).

The NBC originates south of the equator where the South Equatorial Current approaches the coast. Historically, it was thought to be simply the northward flowing part of the bifurcation of the Central South Equatorial Current (CSEC) at near 5$^\circ$ S (with the Brazil Current (BC) the southward flowing part), but recent investigations have shown a more complicated picture. The simple view was prompted by surface current distributions obtained from ship drift and surface drifter trajectories, which turn out to have obscured the overall geostrophic flow patterns.

Geostrophic calculations have shown that the NBC originates just south of 10$^\circ$ 30' S where the convergence of the southern branch of the CSEC with part of the Southern South Equatorial Current results in a transport (relative to 1000 m) of about 21 Sv at near 10$^\circ$ S. It continues north from there and eventually merges with the northern branch of the CSEC just north of 5$^\circ$ S where the transport has increased to the aforementioned 37 Sv.

The NBC is believed to have a pronounced annual cycle where during March-June most of it moves northwestward up the coast of South America to eventually enter the Caribbean Sea via the passages of the Lesser Antilles, while during the rest of the year it separates sharply from the coast at 6-7$^\circ$N and curves back on itself (i.e. retroflects) to feed the North Equatorial Countercurrent (NECC). During the retroflection phase the NBC occasionally curves back upon itself sufficiently to pinch off large anticyclonic current rings, which move northwestward toward the Caribbean.

Immediately following separation the rings are about 400 km in diameter and translate northwestward at 8-15 cm s$^{-1}$. They have a typical sea surface height anomaly of 15 cm at the center, penetration depths from 950 to 1500m, a mean radius of maximum velocity of 100-150 km, a maximum swirl velocity of 80 cm s$^{-1}$, and swirl velocities greater than 20 cm s$^{-1}$ beyond a radius of 200 km. This ring shedding is thought to account for as much as one-third of the net warm water transport across the equatorial-tropical gyre boundary into the North Atlantic to compensate for the southward export of North Atlantic Deep Water (NADW). The annual mass flux from all eddies formed during a year is estimated as 2.6-4.0 Sv, or roughly 20-30% of the total strength of the Atlantic meridional overturning circulation. It is speculated that waves or eddies leaving the equatorial waveguide near the western boundary may translate northward along the coast of South America embedded within the North Brazil Current, and may serve as a catalyst for the shedding of the rings. See da Silveira (1994) and Johns et al. (1998).

North Brazil Undercurrent (NBUC)

A current underlying the North Brazil Current (NBC) off the northeast coast of Brazil. The NBUC shows a subsurface core at about 200 m with velocities of up to 90 cm/s, resulting in large northward transports of more than 22 Sv in the upper 1000 m. See Stramma et al. (1995).

North Cape Current

See Pfirman et al. (1994).

North Equatorial Countercurrent (NECC)

An eastward flow in the Atlantic and Pacific located approximately between 5 and 10$^\circ$ N. It is located between the NEC and the SEC and called a countercurrent because it flows counter to the direction of the easterly trade winds. The NECC is strongest during July and August and weak in the northern winter and spring, and is known to migrate from a northernmost position in the northern summer to a position closest to the equator in the northern winter. Some evidence indicates that during this latter period the NECC is discontinuous and may even vanish in parts of the eastern Pacific. Even so, it is the most well developed of any of the equatorial currents. In the Indian Ocean this and the NEC are seasonally controlled by the monsoon circulation patterns.

According to Richardson et al. (1992), in the Atlantic Ocean ...

... the geostrophic NECC continues to flow eastward throughout the year, fastest in fall and slowest in spring. Drifting buoys and historical ship drifts show that the near-surface Countercurrent reverses each spring even when systematic errors due to windage are taken into account. The seasonally fluctuating winds drive an Ekman surface current that is eastward in fall, adding to the geostrophic current, and westward in spring, countering and overwhelming the geostrophic current. The reversal of the Countercurrent in spring occurs in the near-surface layer and is driven by the Northeast Trades. Thus the near-surface velocity in the Countercurrent is determined by a competition between local wind stress and the larger field of wind stress curl, both of which have large seasonal variations in the tropical Atlantic.

See Leetmaa et al. (1981), Richardson et al. (1992) and McPhaden (1996).

North Equatorial Current (NEC)

A westward flow in the Atlantic and Pacific located north of the NECC past 10$^\circ$ N. In the Indian Ocean this and the NECC are seasonally controlled by the monsoon circulation patterns. See Leetmaa et al. (1981),

North Equatorial Undercurrent (NEUC)

A permanent eastward flowing feature of the equatorial Atlantic circulation whose core is located near 200 m depth a few degrees north of the Equator. A satisfactory dynamical explanation for this is as yet nonexistent. See Cochrane et al. (1979) and Tomczak and Godfrey (1994), p. 260.

North Hawaiian Ridge Current (NHRC)

A current that exists in the mean along the coasts of the Hawaiian Islands. It originates as a northern branch of westward moving interior flow and flows coherently along the islands at an average speed of 0.10-0.15 m s$^{-1}$. It veers westward at the northern tip of the Hawaiian Islands. It exists due to the imbalance between the interior Sverdrup transport and the net southward transport as constrained by the Hawaiian Islands. See Qiu et al. (1997).

North Icelandic Irminger Current

A current containing the westernmost of Atlantic water inflow to the Nordic Seas, flowing northwards through the Denmark Strait between Greenland and Iceland, and then turning eastwards around the northern tip of Iceland. The name indicates an origin in the Irminger Current which, as it flows northward through the Denmark Strait, splits into two branches. One branch turns west and then southwest to run parallel to the East Greenland Current. The other turns northeast and then east to become the North Icelandic Irminger Current. It feeds the North Icelandic shelf area with relatively warm, saline water which rapidly loses its Atlantic character (heat and salt) so that the percentage of Atlantic water is reduced to less than 30% by the northeastern corner of Iceland. See Swift (1986) and Hansen and Osterhus (2000).

North Icelandic Winter Water (NIWW)

A water mass identified and named by (). It is homogenized in winter north of Iceland from a mixture of MNAW from the North Icelandic Irminger Current and water from the near-surface layers of the Iceland Sea plus some coastal water influenced by runoff. The typical properties of NIWW are T = 2-3$^\circ$C and S = 34.85-34.90. See Hansen and Osterhus (2000).

North Korea Current

A current that flows along the western coast in the Japan Sea. It is the southward continuation of part of the Liman Current and ultimately turns east and then northward (at around 38-40$^\circ$ N) to become part of the flow in the Polar Front.

North Pacific Central Mode Water (NPCMW)

A type of mode water found in the subtropical gyre in the North Pacific, and first discussed by Nakamura (1996) and Suga et al. (1997). NPCMW is distributed mainly between 30-40$^\circ$N and 170$^\circ$E-150$^\circ$W as a 9-13$^\circ$C thermostad. It is formed by wintertime deep convection north of its distribution area. See Nakamura (1996), Suga et al. (1997) and Yasuda and Hanawa (1997).

North Pacific Current

The eastward continuation of the Kuroshio and Oyashio Extensions, with which it forms the southern limb of the North Pacific subpolar gyre. This is a broad band of eastward flow around 2000 km wide that, at some not well known location east of the Emperor Seamounts, becomes well distinguished from the two aforementioned narrower and strongly frontal flows that eventually merge into its broader flow. This current eventually turns north and, along with the Alaska Current, forms the eastward limb of the North Pacific subpolar gyre. See Tomczak and Godfrey (1994).

North Pacific Equatorial Water (NPEW)

In physical oceanography, a water mass formed at the boundary between the subtropical gyres via mixing in the Equatorial Countercurrent and the Equatorial Undercurrent. NPEW is a mixture of WNPCW and SPEW. This combination of formation process and ancestral water masses makes NPEW one of the few water masses not formed through air-sea interaction. See Tomczak and Godfrey (1994), p. 166.

North Pacific Intermediate Water (NPIW)

A water mass present mostly in the northern Philippine Sea. NPIW was originally identified as such by Sverdrup et al. (1942). According to Talley (1993), it is identified by S $<$ 34.4 psu and 2.5 $<$ $O_2$ $<$ 3 ml l$^{-1}$ on density surfaces of 26.5-26.8 $\sigma_\theta$, defined as the main salinity minimum in the subtropical North Pacific, and found in the depth range 300-700 m. Other published definitions of NPIW have defined it more generally as:

• the salinity minimum;
• all water in some density interval around the salinity minimum; and
• the isopycnal 26.8 $\sigma_\theta$.

According to Talley (1993):

NPIW occurs throughout the subtropics in the region where potential vorticity suggests wind-driven circulation, thus north of about 20$^\circ$N. It appears to ``leak'' into the subpolar gyre near the eastern boundary and into the Mindanao Current. It is not found consistently in the broad California Current. Throughout most of its domain, NPIW is a vertically smooth salinity minimum with gradual spatial variations in density. Its density shifts from lowest in the northwestern subtropical gyre to highest in the Kuroshio, presumably along a very long path around the gyre. Oxygen is highest and salinity lowest in the northwest as well. Vertical profiles through the NPIW in the northwestern subtropical gyre are markedly different from elsewhere, being strongly intrusive and sometimes having associated temperature minima.

These observations lead to the conclusion that NPIW is ``formed'' as a salinity minimum in the northern part of the mixed water region just east of Hokkaido and the northern coast of Honshu.

$\cdots$

While salinity minima occur at many densities in the North Pacific, and especially in the mixed water region, NPIW as a large-scale salinity minimum because it reflects the bulk, large-scale outcropping properties of a fairly large region of the western subpolar gyre; that is, cold, fresh subpolar waters at densities greater than 26.6 $\sigma_\theta$ are advected into the mixed water region by the Oyashio to meet warmer, more saline subtropical and Tsugaru waters. Cooling of the latter and an increase in salinity of the former produce winter surface waters at all densities between 26.2 $\sigma_\theta$ and 26.6 $\sigma_\theta$, with salinity decreasing with increasing density. The coldest, freshest, densest surface layer then either slides beneath or mixes laterally beneath the saltier surface layers, forming the salinity minimum and erodes from above to produce NPIW. NPIW, thus, is just below the boundary between the densest outcropping subpolar waters of the western Pacific outside the Okhotsk Sea and the part of the water column that is not ventilated outside the Okhotsk Sea.

See Talley (1993), Qu et al. (1999) and You et al. (2000).

North Pacific Subtropical Mode Water (NPSTMW)

A type of mode water found in the subtropical gyre in the North Pacific. The NPSTMW thermostad has a core temperature range of 16$^\circ$-19$^\circ$ C centered at 150$^\circ$-160$^\circ$ E south of the Kuroshio Extension. The formation area is centered just north of 30$^\circ$ N and west of 155$^\circ$ E. The temperature of the layer decreases eastward. Two processes contribute to the thickness of this mode water:

• large winter heat loss due to advection of warm water into the region; and
• cold, dry air blowing off the continents, with the tilt of isopycnals in the Kuroshio Extension and recirculation creating a bowl of warm water between the Extension and the recirculation.

See Suga et al. (1997).

North Pacific Tropical Water (NPTW)

A water mass characterized by high salinity ( $34.75\,<\,S\,<\,35.25$) and high oxygen concentration ($>$4.0 ml l$^{-1}$ on density surfaces around 24.0 $\sigma_\theta$. It is thought to form at about 20$^\circ$N, 140$^\circ$E-160$^\circ$W as the result of excess evaporation. It extends westward from its formation area in the North Equatorial Current (NEC) between 10$^\circ$ and 25$^\circ$N. At the western boundary, part of the NPTW seems to continue southward, coinciding with the Mindanao Current. See Qu et al. (1999).

North Sea

An epicontinental sea occupying the shelf area between the British Isles and Norway, Denmark, Germany, Holland and Belgium. The oceanic boundaries are a line across the Straits of Dover to the south, a line running from the northern tip of Scotland to the Orkney and Shetland Islands and then directly east to the coast of Norway to the north, and the Skagerrak to the east. It covers about 575,000 km$^2$, has an average depth of 94 m, and a volume of 54,000 km$^3$.

According to Rodhe (1998):

The mean currents of the North Sea form a cyclonic circulation. The bulk of the transport in this circulation is concentrated in the northern part of the North Sea and in the region of the Norwegian Trench, with the main outflow along the Norwegian coast. The amount of water leaving along the Norwegian coast is estimated at 1.3-1.8 $\times\,{10^6}\,{m^3}{s^{-1}}$, with most of this outflow compensated for by an inflow along the western slope of the Norwegian Trench. Considerable inflows also take place east of Shetland and between Shetland and the Orkney Islands. Less than 10% enters through the English Channel. Most of the water in the inflows from the northwest is guided eastward to the Norwegian Trench by the topography along the 100 m depth contour. Only a small part flows southward along the coast of Scotland and England.

See Fairbridge (1966), Lee (1970), Otto et al. (1990) and Rodhe (1998).

North Subsurface Countercurrent

An eastward flow in the Pacific Ocean whose core is located near 600 m depth a few degrees north of the Equator. A satisfactory dynamical explanation for this is as yet nonexistent. See Tomczak and Godfrey (1994), p. 128.

North Subtropical Front (NSTF)

The northern boundary of the Subtropical Frontal Zone (STFZ).

North Water

A polynya region occupying a large region in Baffin Bay and Smith Sound situated between Greenland on the east and Ellesmere and Devon Islands on the west. This region is undefined in summer when Baffin Bay is ice free, but well defined during the winter. The northern boundary is an ice arch that blocks the southward flow of ice through Smith Sound. Ice forms continually through the winter in the polynya and drifts southward due to both local northerly winds and southward currents. The southern boundary is more poorly defined, although it does expand southward in spring as the ice melts until it joins the northward expansion of open water associated with the West Greenland Current. The North Water, being a coastal polynya and highly dependent on synoptic meteorological processes, occurs intermittently. See SMith et al. (1990).

Northeast Atlantic Deep Water (NEADW)

A water mass that originates from flow through through passages between Iceland and Scotland. This includes about 1.7 Sv through the Faroe Bank Channel and about 1 Sv over the Greenland-Scotland Ridge. The latter component is about evenly split between the main channel immediately east of Iceland and four lesser channels further east. See Swift (1984) and Dickson and Brown (1994).

Northeastern Atlantic Basin

See West Europe Basin.

Northern South Equatorial Countercurrent (NSEC)

One of three distinct branches into which the South Equatorial Current splits in the western South Atlantic Ocean. See Stramma (1991).

Northern Subsurface Countercurrent

An eastward flowing countercurrent that flows beneath the surface at around 4$^\circ$ N in the North Pacific Ocean. It flows between the eastward flowing North Equatorial Countercurrent (NECC) to the north and the westward flowing South Equatorial Current (SEC) to the south. See Gouriou and Toole (1993).

Northwest Atlantic Bottom Water

A water mass that originates from flow through the Denmark Strait, i.e. the Denmark Strait Overflow. See Swift et al. (1980), Swift (1984) and Dickson and Brown (1994).

Northwest Corner

See North Atlantic Current.

Norwegian Atlantic Current

See Swift (1986) and Pfirman et al. (1994).

Norwegian Coastal Current

See Skagerrak.

Norwegian North Atlantic Water (NNAW)

A water mass formed in the recirculating gyre of the Norwegian Basin of the Norwegian Sea as a mixture of mainly MNAW and NSAIW. See Hansen and Osterhus (2000).

Norwegian Sea

A marginal sea of the North Atlantic Ocean which consists of the waters between the continental shelves of Norway and Spitsbergen to the east and the Mohn Ridge and Jan Mayen Ridge to the west. It adjoins the Barents Sea to the northeast, the Greenland Sea to the northwest, the Iceland Sea to the west, and the North Sea to the southeast. It covers an area of 1,383,000 km$^2$, has a volume of 2,408,000 km$^3$, and a mean depth of 1742 m. The term Norwegian Sea has also been used to collectively refer to the sea described here along with the Greenland Sea and the Iceland Sea. See Hopkins (1991) and Fairbridge (1966).

Norwegian Sea Arctic Intermediate Water (NSAIW)

A water mass whose typical values are -0.5-0.5$^\circ$C and 34.87-34.90. See Hansen and Osterhus (2000).

Norwegian Sea Deep Water (NSDW)

The densest water mass in the Norwegian and Iceland Seas, although it is also found on the periphery of the Greenland Sea. It is characterized by salinities from 34.90 to 34.94 with the 0$^\circ$ C isotherm traditionally used as the upper limit of NSDW. Most is colder than -0.4$^\circ$ C, with typical NSDW -0.5 to -1.1$^\circ$ C and 34.92. The deepest component of NSDW results from a fairly even mixture of GSDW and EBDW. See Swift (1986) and Hansen and Osterhus (2000).

NOS

Acronym for the National Ocean Service, a program office of NOAA that monitors, assesses and forecasts conditions in the coastal and oceanic environment. More information can be found at the NOS Web page.

NOSAMS

Acronym for the National Ocean Sciences Accelerator Mass Spectrometer Facility, established in 1989 at WHOI to serve the ocean sciences community with high-precision C-14 AMS measurements. See the NOSAMS Web site.

Nova Scotian Current

A current located in the Gulf of St. Lawrence. See Han et al. (1999).

Novaya Zemlya Current

See Pfirman et al. (1994).

NOW

Acronym for the International North Water Polynya Study, part of the IAPP. NOW is a program to understand the functioning and importance of the North Water ecosystem.

[http://www.fsg.ulaval.ca/giroq/now/]

NPCMW

Abbreviation for North Pacific Central Mode Water.

NPEW

See North Pacific Equatorial Water.

NPIW

See North Pacific Intermediate Water.

NPO

Abbreviation for North Pacific Oscillation.

NPP

Abbreviation for Net Primary Productivity, the net annual uptake of carbon dioxide by vegetation. It is equal to the GPP minus autotrophic respiration. This is difficult to measure directly in terrestrial ecosystems, not only because it requires a direct measurement of gross photosynthesis, but also because it requires measurement of the respiration of the plant or plant community, including roots. See Woodwell (1995).

NPSTMW

Abbreviation for North Pacific Subtropical Mode Water.

NPTW

Abbreviation for North Pacific Tropical Water.

NPTZ

Abbreviation for North Pacific Transition Zone.

NROSS

Abbreviation for Navy Remote Ocean Sensing Satellite, a planned mission that apparently collapsed under its own financial weight and never got off the ground.

[http://www.pmel.noaa.gov/pubs/outstand/mcph1720/app-c.shtml]

NSANE

Acronym for Near Shore Acoustical Network Experiment, the objective of which was to make comprehensive measurements of the physical processes associated with the surf zone to determine which processes are dominant in determining the acoustic propagation features.

[http://pulson.seos.uvic.ca/nsane/nsane.html]

NSCAT

Acronym for NASA Scatterometer, an instrument that will measure wind speeds and directions over at least 90% of the ice-free global oceans every 2 days under all weather and cloud conditions. It is based on the Seasat scatterometer and uses an array of six stick-like antennas that radiate microwave pulses across the Earth's surface. It will orbit the Earth at an altitude of 800 km in a near-polar, sun-synchronous orbit. NSCAT will fly on the ADEOS mission. See Naderi et al. (1991).

[http://podaac.jpl.nasa.gov/nscat/]

NSCC

Abbreviation for Northern Subsurface Countercurrent.

NSDW

Abbreviation for Norwegian Sea Deep Water.

NSEC

Abbreviation for Northern South Equatorial Current.

NSFE

Abbreviation for Nantucket Shoals Flux Experiment, conducted across the shelf and upper slope south of Nantucket from March 1979 to April 1980 to measure the flow of shelf water from the Georges Bank/Gulf of Maine region into the Middle Atlantic Bight. The principal objectives of NSFE were:

• to measure the alongshelf flux of heat, mass, salt and nutrients over one annual cycle;
• to measure the vertical and cross-shelf structure of the low-frequency current and temperature fluctuations over the upper slope and near the shelf-water/slope-water front; and
• to examine the relationship between the alongshelf volume transport and the cross-shelf pressure fields.

The field experiment contained two principal components: a moored array of current meters and bottom instrumentation deployed at six locations across the shelf and upper slope, and a series of 27 hydrographic surveys made along or near the moored array transect during the experiment. The six-element array of moored instrumentation was deployed along a transect across the continental shelf and upper slope south of Nantucket Island near 70$^\circ$W. It was chosen to start about 10 km south of Davis South Shaols, the shallow southernmost section of Nantucket Shoals, since little alongshelf transport was believed to occur over the shoals. The transport was approximately perpendicular to the local isobaths and cut across the upper slope about 10 km west of Atlantic Canyon and about 10 km east of Alvin Canyon. The six mooring locations were separated horizontally by 16-23 km and in water ranging from 46 m to 810 m deep. See Beardsley et al. (1985).

NSIDC

Abbreviation for National Snow and Ice Data Center, a data and information resource for snow and ice processes, especially for interactions among snow, ice, atmosphere and ocean. This is one of the DAACs.

[http://nsidc.org/index.html]

NSTF

Abbreviation for North Subtropical Front.

NTF

Abbreviation for the Australian National Tidal Facility, which operates the Australian Baseline Sea Level Monitoring Network and is involved in several others.

[http://www.ntf.flinders.edu.au/]

Nuevo Gulf

According to Piccolo (1998), this gulf on the Argentine coast ...

... is a 2500 km$^2$ basin that is 65 km long in its major diameter. It is oriented east-west, although its entrace is at the southeast. Its maximum width is 50 km and its entrance is only 16 km wide. The mean depth is 100 m. The annual hydrography of the Nuevo Gulf shows great changes in the density field. Minimum density values are found in the summer season ($\sigma_t$ = 25.5) comparing with the adjacent shelf. The gulf behaves as a dilution basin where precipitation exceeds evaporation. There are no river flows into the gulf. Therefore, the low density values are produced by reduced evaporation and summer heating. On the other hand, the gulf behaves in winter as a concentration basin with increased density compared with the adjacent shelf. Water exchange between the gulf and the shelf changes seasonally because of the density behavior. There is gulf bottom water flowing into the shelf in winter and a shelf water bottom flow during summer. Current circulation suggests that tides and wind are the main forcing mechanisms.

See Piccolo (1998).

numerical stability

In numerical modeling, a numerical computational scheme is said to be stable if the infinite set of computed solutions of the discrete algebraic equations created by the process of discretization of some original continuum differential equations is always below some uniformly bounded upper-limit as the computational grid spacing is shrunk to zero. There are reasonably efficacious methods for exploring the stabilility of a given linear set of discretized equations, although it is much trickier with nonlinear equations, with the most popular option for the latter being the linearization thereof.

NURC

Abbreviation for National Undersea Research Center. See NURP.

NURP

Acronym for National Undersea Research Program, a NOAA program whose mission is to advance knowledge essential for wise use of the nation's oceanic, coastal, and large lake resources. It supports NOAA and local research needs through a partnership with regional, university-based National Undersea Research Centers (NURCs).

[http://www.ucc.uconn.edu/~wwwnurc/nurp.html]

nutrients

The nutrients used as tracers in physical oceanography are essential dissolved chemicals eaten by plants in the ocean, i.e. phytoplankton. The basic nutrients are carbon, nitrogen and phosphorous, with all three having to be present for plant material to grow. Additionally, calcium and silicon are used as skeleton building materials. Micronutrients are those nutrients used in very small quantities. These are magnesium, iron, vanadium, molybdenum and selenium. Also used in small quantities but of no known value are cadmium and barium. See Barnes (1957).

NWABW

Abbreviation for Northwest Atlantic Bottom Water.

NWLON

Abbreviation for National Water Level Observation Network, a network of water level measurement stations in the U.S. coastal ocean, including the Great Lakes and connecting waterways, and in U.S. Trust Territories and Possessions. This is administered by OLLD through the National Water Level Program. The NWLON consists of about 140 continuously operating stations in U.S. tidal regions, 49 continuously operating stations in the Great Lakes, and about 50 temporary stations operated each year in support of NOS mapping, charting, and hydrography and Great Lakes water resources management.

NWLP

Abbreviation for National Water Level Program, a program being updated with the NGWLMS.

NWT

Abbreviation for northern warm tongue, a tongue of relatively warm water located at the eastern boundary of the WPWP. It is located at around 7 $^\circ$ N. See Ho et al. (1995).

NYBE

Abbreviation for New York Bight Experiment, a joint U.S.-Russian internal wave remote sensing experiment taking place in July 1992 in the New York Bight region near Long Island. It used arrays of aircraft, ships, buoys and satellites from both nations to observe the ocean surface to study how remote sensing can reveal important information on sea surface conditions and air-sea interactions.

[http://www6.etl.noaa.gov/projects/nybe.html]

Nyquist frequency

In sampling theory, this is defined by

$${f_N}\,=\,1/(2\delta t)$$

where $\delta t$ is the sampling interval. It is the maximum frequency that can be detected from data sampled at time spacing $\delta t$. Higher frequencies are subject to aliasing which can cause the spectrum to differ from the true spectrum. See Nyquist theorem. See Peixoto and Oort (1992).

Nyquist theorem

In sampling theory, no information will be lost from a temporal or spatial series of data in the sampling interval is smaller than $1/(2{f_{max}})$, where $f_{max}$ is the maximum frequency present in the series. In other words, the Nyquist frequency must be higher than the maximum frequency present in the series. See Peixoto and Oort (1992).

NZOI

Abbreviation for New Zealand Oceanographic Institute.