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L. R. M. Maas et al.
virtual excursions much in excess of those realistically possible. These affect also
the estimates of (internal) tides, due to tidal currents amplifying this blow-down—
an unwanted effect—or due to trapping of internal tides within eddies, which we
have not been able to unravel here. Accordingly, we simply eliminated blow-down
events. In future work, one could compute actual instrument displacements using
observed currents along the moorings, combined with a knowledge on the shape of
the mooring line, the weights of instruments and cable, and the buoy’s shape, size
and buoyancy [11].
The reason to nevertheless study mid water-column pressure is the presence of
both coherent as well as incoherent (i.e. intermittent) internal tides [23]. Intermittency is due to the varying paths taken by internal tides from generation sites (near
shelf edges and other bottom irregularities, at critically sloping bathymetry) to measurement locations. These paths vary due to slow changes in density and velocity
fields, associated with low-frequency motions.
Performing harmonic analysis (T_TIDE) over consecutive overlapping intervals
we obtained multiple estimates of harmonic amplitudes and phases (MT_TIDE).
The coherent surface and internal tidal constants are independent of the measurement period and are thus represented by the average of MT_TIDE. The variable part
defines the incoherent internal tide. This allows separation of the coherent and incoherent tides (determined by their robustness and variance, respectively) in a manner
that may be more reliable than the error band provided by T_TIDE from a single
(year long) time series [28].
Both the coherent as well as incoherent tides are fairly strong, especially when
the equivalent surface displacement is reconverted into isopycnal displacement. The
latter can attain excursions of tens to hundreds of meters. The phase variation of
the coherent and incoherent internal tides along a mooring line is usually weak. The
amplitude variations are stronger. Horizontal variations are, obviously much larger,
clearly displaying an internal tidal length scale that is much shorter than that of the
corresponding surface tide.
Acknowledgements This work is dedicated to Eugene Morozov on the occasion of his seventieth birthday. The authors are thankful to Gary Egbert, Lana Erofeeva and Oregon State University
for providing the altimetry-derived barotropic tides and to the Royal Netherlands Institute for Sea
Research for providing the pressure data. Pressure data were collected in the INATEX program,
‘INdian-ATlantic EXchange in present and past climate’, funded by the Netherlands Organization
for Scientific Research (NWO), section Earth and Life Sciences (ALW), through its ZKO Grant
839.08.43. The authors acknowledge Commonwealth Scientific and Industrial Research Organisation (CSIRO) for making available the Atlas of Regional Seas 2009 (CARS2009).
References
1. Alford, M. H. (2003). Redistribution of energy available for ocean mixing by long-range propagation of internal waves. Nature, 423, 159–162.
L. R. M. Maas et al.
virtual excursions much in excess of those realistically possible. These affect also
the estimates of (internal) tides, due to tidal currents amplifying this blow-down—
an unwanted effect—or due to trapping of internal tides within eddies, which we
have not been able to unravel here. Accordingly, we simply eliminated blow-down
events. In future work, one could compute actual instrument displacements using
observed currents along the moorings, combined with a knowledge on the shape of
the mooring line, the weights of instruments and cable, and the buoy’s shape, size
and buoyancy [11].
The reason to nevertheless study mid water-column pressure is the presence of
both coherent as well as incoherent (i.e. intermittent) internal tides [23]. Intermittency is due to the varying paths taken by internal tides from generation sites (near
shelf edges and other bottom irregularities, at critically sloping bathymetry) to measurement locations. These paths vary due to slow changes in density and velocity
fields, associated with low-frequency motions.
Performing harmonic analysis (T_TIDE) over consecutive overlapping intervals
we obtained multiple estimates of harmonic amplitudes and phases (MT_TIDE).
The coherent surface and internal tidal constants are independent of the measurement period and are thus represented by the average of MT_TIDE. The variable part
defines the incoherent internal tide. This allows separation of the coherent and incoherent tides (determined by their robustness and variance, respectively) in a manner
that may be more reliable than the error band provided by T_TIDE from a single
(year long) time series [28].
Both the coherent as well as incoherent tides are fairly strong, especially when
the equivalent surface displacement is reconverted into isopycnal displacement. The
latter can attain excursions of tens to hundreds of meters. The phase variation of
the coherent and incoherent internal tides along a mooring line is usually weak. The
amplitude variations are stronger. Horizontal variations are, obviously much larger,
clearly displaying an internal tidal length scale that is much shorter than that of the
corresponding surface tide.
Acknowledgements This work is dedicated to Eugene Morozov on the occasion of his seventieth birthday. The authors are thankful to Gary Egbert, Lana Erofeeva and Oregon State University
for providing the altimetry-derived barotropic tides and to the Royal Netherlands Institute for Sea
Research for providing the pressure data. Pressure data were collected in the INATEX program,
‘INdian-ATlantic EXchange in present and past climate’, funded by the Netherlands Organization
for Scientific Research (NWO), section Earth and Life Sciences (ALW), through its ZKO Grant
839.08.43. The authors acknowledge Commonwealth Scientific and Industrial Research Organisation (CSIRO) for making available the Atlas of Regional Seas 2009 (CARS2009).
References
1. Alford, M. H. (2003). Redistribution of energy available for ocean mixing by long-range propagation of internal waves. Nature, 423, 159–162.