Hn-Hz

HNLC

An abbreviation for high nutrient-low chlorophyll regions. These are mainly the waters of the subarctic Pacific, the Southern Ocean around Antarctica, and the equatorial Pacific. These are regions in which the biological pump is inefficient, i.e. the phytoplankton standing stocks are not large enough to assimilate the N and P in the surface waters to deplete them at any time through the year. See iron hypothesis.

HNODC

The Hellenic National Oceanographic Data Centre is located at the Institute of Oceanography at the National Centre for Marine Research (NCMR) in Athens, Greece. It was established in 1986 as part of the NCMR and operates as a national agency responsible for processing, archiving and distributing marine data. HNODC is also responsible for the coordination of IODE in Greece.

[http://hnodc.ncmr.gr/]

HOME

Acronym for Hawaii Ocean Mixing Experiment, an effort to observe and model mixing along the Hawaiian Ridge, a salubrious location for such an investigation since the topography is steep and energy available in the form of tides that strike the Ridge at nearly normal incidence.

[http://chowder.ucsd.edu/home/]

HOPE

Abbreviation for a primitive equation ocean circulation model developed and used at the DKRZ.

Hopen-Bjornoya Current

See Pfirman et al. (1994).

HOT

Acronym for Hawaii Ocean Time-series, a program for making repeated observations of the hydrography, chemistry and biology at a station north of Hawaii since October 1988. The objective of this research is to provide a comprehensive description of the ocean at a site representative of the central North Pacific Ocean. See Karl and Lukas (1996).

[http://hahana.soest.hawaii.edu/hot/hot_jgofs.htmlhttp://hahana.soest.hawaii.edu/hot/hot_jgofs.html

Hough functions

The eigenfunctions of a linearized form of the governing equations of motion on a sphere, i.e. Laplace's tidal equations, as first discovered by Hough (1898). Each Hough mode is a function of latitude and longitude and has three components: (1) a zonal (eastward) wind component, (2) a meridional (northward) wind component, and (3) a geopotential component. A distinct horizontal scale and frequency is associated with each mode, and the modes are orthogonal over the sphere in the continuous case. They are sometimes divided into two classes: (1) low-frequency Rossby-Hough modes that tend to satisfy the geostrophic relation and (2) higher frequency Hough modes that correspond to inertia-gravity waves. They can also be thought of as a generalized Fourier series in which the basis functions are the normal modes of a resting atmosphere. See Daley (1991).

Hoxton, Walter

See Peterson et al. (1996).

horse latitudes

The belts of variable, light winds and fine weather associated with the subtropical anticyclones The name originated with the historical sailing practice of throwing the horses being transported to America or the West Indies overboard when these latitudes were reached and the light winds caused the voyage to be overly extended.

HRP

Abbreviation for High Resolution Profiler.

HRTWN

Abbreviation for Hawaii Regional Tsunami Warning Network.

HSSTD

Abbreviation for Historical Sea Surface Temperature Dataset.

Huanghai Sea

See Yellow Sea.

Hudson Bay

A large inland Arctic sea exceeding 1 million square kilometers in area, connected to the Arctic Ocean at its northern end through Foxe Basin. It can be characterized as shallow, with a mean depth of less than 150 m. The details of the circulation and water masses are not well known, although the water properties depend mainly on exchanges with Foxe Basin and Hudson Strait and the large freshwater input from both rivers surrounding the bay and sea ice melt in the spring and summer. A cyclonic circulation is maintained by the inflow/outflow forcing at the northern end, with the wind and buoyancy driven circulations serving to enhance the cyclone. There is also strong steering of the flow by the bathymetry.

According to Ingram and Prinsenberg (1998):

The open-water circulation of Hudson Bay is primarily driven by wind forcing and buoyancy input. In the summer, northerly winds dominant north of about 60$^\circ$N, with northwesterly winds in the southern half of the bay. The freshwater runoff generates and estuarine-type circulation in the bay as a whole, such that the upper-layer low-salinity flux to Hudson Strait is balanced by an inward flux of higher-salinity ocean water in the lower layer. The incoming higher-salinity and colder water enters at the northern boundary of Hudson Bay and through estuarine-like processes modifies the fresher surface waters. Typical mean surface flow values are 0.04 m s$^{-1}$, equivalent to a two-year period to complete a trajectory around Hudson Bay.

Ingram and Prinsenberg (1998).

[http://www.mar.dfo-mpo.gc.ca/science/ocean/seaice/publications.htm]

Humboldt Current

See Peru Current.

humidity mixing ratio

The ratio of the mass of water vapor in a sample of moist air to the mass of dry air with which it is associated.

Hunter Channel

One of two gaps in the Rio Grande Rise that allow the export of Antarctic Bottom Water (AABW) from the Argentine Basin into the Brazil Basin (the other being the Vema Channel). It is located near 28$^\circ$W with a sill depth of about 4200 m. The mean transport estimated from moored current meters (during the Deep Basin Experiment) was 2.92 ($\pm$1.24) $\times\,{10^6}\,{m^3}{s^{-1}}$, with the the transport equatorward 89% of the time. See Zenk et al. (1999).

HURL

Acronym for Hawaii Underwater Research Laboratory.

hurricane

More later. See Gray (1979).

HYD93

Abbreviation for the Hydrographic Surveys Data Exchange Format for bathymetric soundings and hydrographic features as specifed by the HYD93 Task Group in a meeting at the NGDC in December, 1993. The HYD93 digital format is intended to be used for the transmission of data to and from a data center and may be useful for the exchange of data between marine institutions. This format was based on the earlier MGD77 format for marine geophysical data. The details of the format are available in documents at the HYD93 FTP site.

hydrochlorofluorocarbon (HCFC)

A class of chemicals being used to replace CFCs since they deplete stratospheric ozone to a much lesser extent than CFCs. These have ODPs ranging from 0.01 to 0.1. The production of these chemicals is being gradually phased out, with those with the highest ODPs going first. These will be replaced by HFCs or some other types of chemicals.

hydrofluorocarbon (HFC)

A class of chemicals being used to replace CFCs. They do not contain chlorine or bromine and therefore do not deplete ozone in the stratosphere. These all have an ODP value of zero but can have high GWP values.

hydrodynamic modulation

A type of modulation of backscattered ripple waves measured by SAR that arises through variations in the energy of the Bragg backscattering ripples causes by hydrodynamic interactions betwen the short ripple waves and the longer waves. See Komen et al. (1996).

hydrographic theorem

An expression, developed using the continuity equation, for water budget estimates in two-layer channels with different salinity values in each layer. It was first developed by Knudsen. See Dietrich (1963).

hydrography

The study of the physical features of water bodies like oceans and lakes (in analogy to geography being the study of the physical features on land). Oceanic features of interest include the location and spatial extent of water masses as identified by their characteristic properties such as salinity, temperature and micronutrient concentrations. Early systematic attempts at applying hydrography to the oceans were the core layer method and the isopycnal method in the 1920s and 1930s by Wust, Iselin, Montgomery, Defant and others, and variants of these methods are still used today to provide a first-order general classification of the waters of the world ocean. Much care, however, should be taken when attempting to use the results of these mostly static classification methods to understand the dynamical aspects of the ocean (although the latter is much more closely related to dynamical fields). This is best exemplified by the classic apothegm "the hydrographer's ocean is much smoother than the dynamicist's ocean".

hydrological cycle

The importance of the oceanic component of the hydrologic cycle is evidenced by estimates indicating that 86% of global evaporation and 78% of precipitation occur over the ocean. The global water reservoirs are the ocean, the land, and the atmosphere, holding 1,400,000,000 km$^3$, 59,000,000 km$^3$, and 16 km$^3$ of water, respectively. About 13.5 Sv evaporates from the ocean (2.2 Sv for land) while 12.2 Sv are precipitated (3.5 Sv for land).

The major regional features of the cycle include rainfall dominating over evaporation in the tropics within the ITCZ, with the subtropics characterized by an excess of precipitation except for the SPCZ. Precipitation again dominates in subpolar latitudes, with data near the poles being too sparse to form any generalizations. The ocean must of course compensate for these latitudinal differences, moving water into evaporation zones and away from regions where precipitation dominates. This transport accounts for a significant percentage of the total poleward heat transport on the planet.

Interbasin differences include the Atlantic being saltier than the Pacific due to the dominance of evaporation in the former and precipitation in the latter. The difference is thought to be maintained by water vapor transport across Central America and the lack thereof into the Atlantic from the east. These differences in surface water fluxes lead to interbasin transports in the ocean, although these are known even less accurately than the precipitation and evaporation patterns. See Schmitt (1995).

hydrology

More later.

hydrometeor

Any condensed water particle in the atmosphere of size much larger than individual water molecules, e.g. fog, cloud, some hazes, rain and snow.

hydrosphere

This consists of all water in the liquid phase distributed on the Earth, including the oceans, interior seas, lakes, rivers, and subterranean waters.

hydrostatic approximation

This approximation assumes that the vertical pressure gradient is almost balanced by the forcing due to buoyancy excess. Then the vertical acceleration remains as a much smaller term and can be omitted. A more stringent argument is given by Jones and Marshall (1993):

If, in standard notation,

$${ {Dw} \over {Dt} }\,+\,g\,+\,{1\over\rho}{{\partial p}\over{\partial z}} \,=\,0$$ (8)

is the vertical momentum equation with the height $z$ as the vertical coordinate and $w\,=\,Dz/Dt$, then in the hydrostatic approximation the term $Dw/Dt$ is omitted. But the condition for the validity of the hydrostatic approximation is much more stringest than $Dw/Dt\,\ll\,g$, the acceleration due to gravity, because almost all of $g$ is balanced by the inert hydrostatic pressure gradient associated with the resting reference state.

More helpfully, we can isolate the hydrostatically balanced and dynamically inactive pressure gradient by writing the Boussinesq form of the previous equation as

$${{Dw}\over{Dt}}\,+\,g{{\rho '}\over{\rho_0}}\,+\, {1\over{\rho_0}}{{\partial p'}\over{\partial z}}\,=\,0$$ (9)

where the primes denote a deviation from the hydrostatically balanced reference state and $\rho_0$ is a standard (constant) value of density. Now it can be clearly seen that the condition for the neglect of $Dw/Dt$ is that it should be much smaller than $g'\,=\,g(\rho '/{\rho_0})$ rather than $g$. Let us now try and estimate typical scales for which the hydrostatic approximation is valid, according to this more stringent condition.

Consider a convective event (in an unstratified ocean) which has a characteristic horizontal scale $l$ and vertical scale $H$ with horizontal and vertical velocity scales $u$ and $w$, respectively. The time scale of a particle of fluid moving through the convective system is of order $l/u$ and a typical $w$ will be

$$w\,\sim\,{{uH}\over{l}}.$$ (10)

So

$${{Dw}\over{Dt}}\,\sim\,{ {{u^2}H} \over {l^2} }$$ (11)

and hence $Dw/Dt\,\,,\,g'$ if

$${ {{u^2}H} \over {g'{l^2}} }\,\ll\,1.$$ (12)

If this last condition is not satisfied, then the full vertical momentum equation must be used.

See Jones and Marshall (1993).

hydrostatic equation

An equation relating the vertical pressure gradient to the vertical distribution of density in a fluid (atmosphere or ocean) at rest. It is given by

$${{\partial {p_h}}\over{\partial z}}\,=\,-g\rho$$

where $p_h$ is the static pressure, $z$ the vertical coodinate, g the gravitational acceleration, and $\rho$ the density.

hydrostatic pressure

See static pressure.

hypolimnion

The layer of water below the thermocline in a fresh water lake, as opposed to the epilimnion.

hypoxia

See Kamykowski and Zentara (1990).

hypsographic curve

See hypsometric curve.

hypsometric curve

A plot of the percentage of elevation and depth distribution on the continents and oceans, i.e. the representation of the statistical distribution of elevations over the entire planet. Such a curve was first prepared by Krümmel in 1897, with a more modern version prepared by Kossinna in the 1920s and 1930s.

hypsometric equation

In meteorology, a relation stating that the thickness of an atmospheric layer bounded by two isobaric surfaces is proportional to the mean temperature of that layer and the pressure change across it, i.e.

$${z_2}\,-\,{z_1}\,=\,a\,\overline{T_v}\,\ln \left({{P_1}\over{P_2}}\right)$$

where $\overline{T_v}$ is the average virtual temperature between heights $z_1$ and $z_2$, $a$ is equal to ${R_d}/g\,=\,29.3\,m/K$ (where $R_d$ is the gas constant for dry air), and $P_1$ and $P_2$ are the pressures at the two levels. See Salby (1992) and Stull (1995).

hypsometry

The study of the elevation and depth distribution on the continents and oceans.

hysteresis

More later.

C