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Gary T. Mitchum

Abstract

A method is described for using tide gauge sea levels to monitor time-dependent drift in satellite altimetric measurements of sea surface height. The method depends on a careful assessment of the quality of the tide gauge measurements available for this application and also takes into account the degree of independence between the altimeter minus tide gauge differences in order to construct an optimal drift estimate and an accurate error estimate for it. The method is applied to the TOPEX altimeter measurements, and a recently discovered algorithm error, which resulted in a slow drift in the TOPEX sea surface heights, is exploited to evaluate the success of the tide gauge drift estimation. It is important to note that the tide gauge analysis was done without any prior knowledge of this error. The result is that the tide gauge analysis reproduces the drift due to the algorithm error to within 6 mm rms, which is comparable to the 5–6-mm internal estimate of the uncertainty of the drift analysis. The analysis is then made using the TOPEX data that have been corrected for the algorithm error and shows that the satellite heights are stable to better than 10 mm over the nearly 4 years of data available, although a drift on the order of 2 mm yr−1 remains, the source of which is unknown. This inferred stability is more than adequate for the majority of applications, although questions still exist for more demanding applications, such as the calculation of global sea level change. Limitations of the tide gauge analysis are discussed, along with potential improvements that might be possible in the future.

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Gary T. Mitchum and Roger Lukas

Abstract

The annual cycle of sea level variability is examined for the period 1976–1985 at seven stations that lie along the mere axis (near 7°N)) of the north equatorial countercurrent (NECC) sea level trough in the western Pacific. The technique of complex demodulation is used to describe the year to year modulation of the phase and amplitude of the annual cycle of sea level along this line. This technique reveals differences in the character of the annual cycle that are related to the occurrence of the El Niño/Southern Oscillation (ENSO) phenomenon.

During non-ENSO time periods, the amplitude of the annual cycle averages 4–5 centimeters and increases towards the west at approximately 1 mm per degree of longitude. The annual cycle during thew periods also shows a phase propagation towards the west at about 50 cm/s, which is comparable to the Rossby wave phase speed at this latitude. During the ENSO events, the amplitude of the annual cycle averages 7–8 centimeters and increases towards the west at about 2 mm per degree of longitude. During these times the annual cycle is nearly in phase from the dateline to the coast of the Philippines.

Interannual modulations of the amplitude of the annual cycle of wind in this region are similar to those seen in the sea level field, but no westward phase propagation is observed. A global analysis of the FGGE winds reveals a westward propagating zonal wind component that is obscured at the latitude of the NECC trough by a larger zonally uniform signal. The zonally uniform signal varies with latitude, following the solar heating, and accounts for the zonal mean phase of the annual cycle. The propagating component has a phase speed of about 50 cm/s and has maximum amplitude at 7.5°N. The energy source for this wind signal appears to be in the monsoon region of the Indian Ocean. It is hypothesized that the propagating annual sea level signal observed in the western Pacific results from resonant forcing by this westward propagating feature of the wind field.

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Gary T. Mitchum and Roger Lukas

Abstract

We report an unprecedented description of the latitudinal structure of tropical Pacific Ocean sea level variability for time scales between 3 and 100 days. A plot of calculated sea level variance latitude and frequency is described and exposes a rich variety of phenomena. We find signals due to the fortnightly tide, equatorial waves, zonal current instabilities, and atmospheric forcing.

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Gary T. Mitchum and W. Sturges

Abstract

Three weeks of current-meter, wind and sea-level data off Cedar Key, Florida are analyzed. Currents and sea level are found to be coherent with alongshore wind stress in the “synoptic” band (∼0.05–0.25 cycle per day) and to lag it by approximately half a day. Little coherence is found with cross-shelf wind stress.

At the inshore mooring (22 m depth) currents are nearly barotropic for these winter 1978 data. A linear parameterization of bottom stress in the barotropic alongshore current leads to a bottom friction parameter r of 0.01–0.02 cm s−1 using coastal wind stress. No significant steady alongshore slope is found during this short interval. The dominant momentum balance in the alongshore direction is between wind and bottom stress. The offshore frictional length scale (Csanady, 1978) is estimated to be 75–100 km, which implies a seaward extent to a depth of about 30 m.

At the offshore mooring (44 m depth) there is vertical shear between the currents at 9 and 39 m. The upper cross-shelf components, which is large relative to that at the inshore mooring, is consistent with Ekman transport while the lower record shows a return flow. The u, v velocity components correlate significantly at the offshore mooring and lead to an upper layer ūv̄ gradient on the order of 10−5 cm2 s−2 between the arrays (75 km separation).

The sea-level fluctuations are consistent with a geostrophic balance in the cross-shelf momentum equation with a length scale of 170 km (approximately equal to the shelf width).

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David Roach, Gary T. Mitchum, and Klaus Wyrtki

Abstract

Correlations of monthly mean sea level variations along the Pacific Ocean margin are used to define length scales. In the northeast quadrant a minimum length scale at about 38°N separates two distinct regimes of longer length scale. This transition corresponds to a change from a regime north of 38° that is dominated by local atmospheric forcing to a regime south of 38° that is dominated by remote forcing. In the northwest quadrant a transition occurs near 35°N near the point of the separation of the Kuroshio. A transition at about 30°S in the southwest quadrant is associated with the southernmost extent of the Great Barrier Reef off the coast of Australia. In the southeast quadrant, a transition is observed at 18°S that may be due to a sharp change in the direction of the South American coastline. In general. tropical length scale values of 800 to 3100 km are separated from midlatitude values of 700 to 1300 km by a narrow region in which length scales are as short as 250 km. These results suggest the need for latitude-dependent criteria for the spacing of tide gauges in a global oceanographic sea level system.

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Gary T. Mitchum and Allan J. Clarke

Abstract

Clarke and Van Gorder have recently formulated a model describing the large-scale, low-frequency response of continental shelf waters to synoptic-scale wind stress in terms of a sum of forced waves. The model includes realistic friction and time dependence and provides an efficient method for calculating the response. Evaluation of the model using Wed Florida Shelf data gave the following results.

(i) The model successfully predicts both the coastal sea level and alongshore velocity component.

(ii) The West Florida Shelf coastal pressure field is dominated by the first mode and can be understood as the sum of a forced wave which travels with the southward-propagating wind stress and a free wave generated at the Florida Keys.

(iii) Almost all the wind-induced energy on the Wed Florida Shelf is due to the wind forcing acting on Wed Florida Shelf waters. However, a small but significant energy flux appears to enter the West Florida Shelf from the caucus Florida shelf wave guide.

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Gary T. Mitchum and Allan J. Clarke

Abstract

An analytic barotropic model was used to study the nearshore frictional response to synoptic-scale wind forcing. The results depend only on the region where the surface and bottom Ekman layers interact strongly and the water is consequently well mixed. Thus, the conclusions apply even to shelves where baroclinic effects are not negligible father offshore. The main findings are as follows.

(i) The “blocking” of the surface Ekman flux which drives the shelf flow is a consequence of the interaction of Ekwan layers in the nearshore region. The interaction transfers progressively more of the surface stress directly to bottom stress as the depth decreases. The consequent decrease in Ekman flux toward the shore creates an Ekman flux divergence which drives the interior flow.

(ii) The region over which the shelf is effectively forced is narrow. It extends, approximately, from where the depth divided by the Ekman layer e-folding scale is 0.2 to where it is 2.5.

(iii) Simple equations for pressure and the depth-averaged alongshore velocity component were developed for the region extending from the coast to where the depth h is about three times the Ekamn layer 3-folding scale δ. these equations do not depend on the dynamics outside this region and are easily solved for arbitrary alongshore forcing.

(iv) A “coastal” boundary condition was derived for those models which analyze the offshore region where h⩾3δ. The appropriate condition is that the depth integrated flow perpendicular to the coast should vanish at the model coast h=3δ. A simple formula connecting actual coast and model coast pressure is also given so that predictions at the model coast can be related to the easily obtained coastal tide gauge data.

(v) The equations for pressure and the alongshore velocity component were compared to a dataset on the West Florida Shelf. Although the data were somewhat limited, the comparison showed good agreement.

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Philip R. Thompson, Gary T. Mitchum, Cedric Vonesch, and Jianke Li

Abstract

Interannual to multidecadal variability of winter storminess in the eastern United States was studied using water level measurements from coastal tide gauges. The proximity to the coast of the primary winter storm track in the region allows the use of tide gauges to study temporal modulations in the frequency of these storms. Storms were identified in high-passed, detided sea level anomalies in 20 gauges from all coasts of North America to assess variability in winter storminess along particular storm tracks. The primary result is a significant multidecadal increase in the number of storms affecting the southeastern United States from the early to late twentieth century. The authors propose that this change is due to an increased tendency for the jet stream to meander south over the eastern United States since the 1950s. This mechanism is supported by long-term changes in the large-scale sea level pressure pattern over North America. The nature of the multidecadal change in storm frequency is unclear, because limited tide gauge record lengths prevent distinguishing between a trend and an oscillation.

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Eric S. Johnson, Fabrice Bonjean, Gary S. E. Lagerloef, John T. Gunn, and Gary T. Mitchum

Abstract

Comparisons of OSCAR satellite-derived sea surface currents with in situ data from moored current meters, drifters, and shipboard current profilers indicate that OSCAR presently provides accurate time means of zonal and meridional currents, and in the near-equatorial region reasonably accurate time variability (correlation = 0.5–0.8) of zonal currents at periods as short as 40 days and meridional wavelengths as short as 8°. At latitudes higher than 10° the zonal current correlation remains respectable, but OSCAR amplitudes diminish unrealistically. Variability of meridional currents is poorly reproduced, with severely diminished amplitudes and reduced correlations relative to those for zonal velocity on the equator. OSCAR’s RMS differences from drifter velocities are very similar to those experienced by the ECCO (Estimating the Circulation and Climate of the Ocean) data-assimilating models, but OSCAR generally provides a larger ocean-correlated signal, which enhances its ratio of estimated signal over noise. Several opportunities exist for modest improvements in OSCAR fidelity even with presently available datasets.

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Gary T. Mitchum, David W. Hancock III, George S. Hayne, and Douglas C. Vandemark

Abstract

Data from satellite altimeters are often degraded by the occurrence of unrealistically high radar return cross sections, which indicate a breakdown of the rough surface scattering model used to interpret these measurements in terms of satellite to sea surface height ranges. The TOPEX altimetric data are examined and nearly 200 000 such events during the 7-yr period, 1993–99, inclusive, are identified. The primary purpose of this paper is to make a comprehensive description of where and when these events occur, which is important because many of the communities that make use of the TOPEX data are generally unaware of this phenomenon. It is shown that these events affect almost 6% of the over-ocean TOPEX data, but only approximately 60% of these events are rejected by the recommended TOPEX data flagging. A global description of these events is made, showing that the events are associated with regions of climatologically weak winds (e.g., the summer hemispheres and the western Pacific warm pool region), supporting the existing hypothesis that these events are due to returns from surfaces where centimeter-scale waves are suppressed. The TOPEX results are confirmed with a comparison to anomalous returns from the NASA Scatterometer (NSCAT), and the relationship to very low wind speeds is further examined using the Tropical Ocean Global Atmosphere–Tropical Atmosphere Ocean array (TOGA–TAO) moored buoys. Finally, it is shown that there is some evidence that not all of the events can be accounted for by very low wind speeds. This suggests that future work might exploit the occurrence of these events to study other phenomena, such as surface slicks, that may lead to additional geophysical applications of the altimetric data.

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