Technical note: Two types of absolute dynamic ocean topography
Abstract
Two types of marine geoid exist with the first type
being the average level of sea surface height (SSH) if the
water is at rest (classical definition), and the second type
being satellite-determined with the condition that the water is usually not at rest. The differences between the two
are exclusion (inclusion) of the gravity anomaly and nonmeasurable (measurable) in the first (second) type. The associated absolute dynamic ocean topography (referred to as
DOT), i.e., SSH minus marine geoid, correspondingly also
has two types. Horizontal gradients of the first type of DOT
represent the absolute surface geostrophic currents due to
water being at rest on the first type of marine geoid. Horizontal gradients of the second type of DOT represent the
surface geostrophic currents relative to flow on the second
type of marine geoid. Difference between the two is quantitatively identified in this technical note through comparison between the first type of DOT and the mean second type
of DOT (MDOT). The first type of DOT is determined by
a physical principle that the geostrophic balance takes the
minimum energy state. Based on that, a new elliptic equation is derived for the first type of DOT. The continuation of
geoid from land to ocean leads to an inhomogeneous Dirichlet boundary condition with the boundary values taking the
satellite-observed second type of MDOT. This well-posed elliptic equation is integrated numerically on 1◦ grids for the
world oceans with the forcing function computed from the
World Ocean Atlas (T , S) fields and the sea-floor topography obtained from the ETOPO5 model of NOAA. Between
the first type of DOT and the second type of MDOT, the
relative root-mean square (RRMS) difference (versus RMS
of the first type of DOT) is 38.6 % and the RMS difference
in the horizontal gradients (versus RMS of the horizontal
gradient of the first type of DOT) is near 100 %. The standard deviation of horizontal gradients is nearly twice larger
for the second type (satellite-determined marine geoid with
gravity anomaly) than for the first type (geostrophic balance
without gravity anomaly). Such a difference needs further attention from oceanographic and geodetic communities, especially the oceanographic representation of the horizontal
gradients of the second type of MDOT (not the absolute surface geostrophic currents).
Description
The article of record as published may be found at http://dx.doi.org/10.5194/os-14-947-2018
Rights
This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.Collections
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