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Ruoying He
and
Robert H. Weisberg

Abstract

The principal semidiurnal (M 2 and S 2) and diurnal (K 1 and O 1) tidal constituents are described on the west Florida continental shelf (WFS) using a combination of in situ measurements and a three-dimensional, primitive equation numerical model. The measurements are of sea level and currents along the coastline and across the shelf, respectively. The model extends from west of the Mississippi River to the Florida Keys with an open boundary arcing between. It is along this open boundary that the regional model is forced by a global tide model. Standard barotropic tidal analyses are performed for both the data and the model, and quantifiable metrics are provided for comparison. Based on these comparisons, the authors present coamplitude and cophase charts for sea level and velocity hodographs for currents. The semidiurnal constituents show marked spatial variability, whereas the diurnal constituents are spatially more uniform. Apalachicola Bay is a demarcation point for the semidiurnal tides that are well developed to the southeast along the WFS but are minimal to the west. The largest semidiurnal tides are in the Florida Big Bend and Florida Bay regions with a relative minimum in between just to the south of Tampa Bay. These spatial distributions may be explained on the basis of local geometry. A Lagrangian Stokes drift, coherently directed toward the northwest, is identified but is of relatively small magnitude when compared with the potential for particle transport by seasonal and synoptic-scale forcing. Bottom stress-induced tidal mixing is examined and estimates are made of the bottom logarithmic layer height by the M 2 tidal currents.

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Robert H. Weisberg
and
Lin Qiao

Abstract

Horizontal divergence and vertical velocity (w) are estimated at 0°, 140°W using an array of five subsurface moored acoustic Doppler current profilers deployed from May 1990 to June 1991 during the Tropical Instability Wave Experiment. The record-length mean flow is divergent within the near-surface region and convergent within the thermocline, with maximum convergence located at the high speed core of the Equatorial Undercurrent (EUC). This pattern of divergence results in upwelling at and above the EUC core (with maximum value of 2.3 × 10−5 m s−1 located at 60-m depth) and downwelling below the core. The relative slopes in the zonal plane between the mean velocity vectors and the isotherms suggest a net diffusive heat flux. Assuming that this occurs vertically, an entrainment velocity parameterization provides an estimate of the “diapycnal vertical velocity” profile that reverses sign at the EUC core depth. Several kinematical and dynamical consistency checks are developed on both the time-dependent and the mean motions to supplement a discussion of errors for the mean w profile. The time-dependent fluctuations in w may be an order of magnitude larger than the mean values, and on synoptic timescales w may be directed either up or down over the entire upper 250-m region sampled.

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Yonggang Liu
and
Robert H. Weisberg

Abstract

The across-shelf structures of the ocean circulation and the associated sea surface height (SSH) variability are examined on the west Florida shelf (WFS) for the 3-yr interval from September 1998 to December 2001. Five sets of characteristic circulation patterns are extracted from 2-day, low-pass-filtered data using the self-organizing map: extreme upwelling and downwelling structures with strong currents, asymmetric upwelling and downwelling structures with moderate currents, and a set of transitional structures with weak currents. The temporal variations of these structures are coherent with the local winds on synoptic weather time scales. On seasonal time scales they are related to both the local winds and the water density variations. The circulation is predominantly upwelling during autumn to spring months (October–April) and downwelling during summer months (June–September). Coastal sea level fluctuations are related to both the dynamical responses of the inner shelf circulation to meteorological forcing and the offshore SSH. On long time scales, the offshore SSH variations appear to dominate, whereas on synoptic weather time scales, the inner shelf wind-driven circulation responses are largest. The across-shelf distribution of SSH is estimated from the velocity, hydrography, wind, and coastal sea level data, and the results are compared with satellite altimetry data, thereby providing a means for calibrating satellite altimetry on the shelf.

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Robert H. Weisberg
and
Wilton Sturges

Abstract

Narragansett Bay is a weakly stratified estuary comprised of three connecting passages of varying depths. The vertical distribution of horizontal velocity was observed in the West Passage using moored current meters. The instantaneous motion was characterized by semi-diurnal tidal currents of amplitude 25–60 cm s−1. These currents exhibited a phase advance with depth (total water depth=12.8 m) ranging with lunar phase from 0–3 h. The net current time series obtained by filtering out motions at tidal and higher frequencies were found to be an order of magnitude less than the instantaneous motion and well correlated to the prevailing 2–10 m s−1 winds. For periodicities of 2–3 days, the coherence between the longitudinal components of wind and net near surface current was as high as 0.8 with the current lagging the wind by about 3 h. The mean near surface speed, obtained by averaging over one month, was 1.2±1.6 cm s−1. The large error bounds were a result of the large variability of the net current time series (and not a result of inadequate sampling). A measure of this variability due to day-to-day changes in weather is given by the root mean square deviation of the net current time series or 2.6 cm s−1. The net transport of water through the West Passage was observed to be seaward or landward over the entire water column for several days duration, with typical wind induced transport fluctuations of &plusmn m2 s−1. Hence, a net communication of water exists between the East and West Passages with water flowing either way in response to the wind. Wind is concluded to be the dominant mechanism driving the net circulation in the West Passage of Narragansett Bay. This is in contrast with the classical views of gravitationally convected net estuarine circulation.

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Chunzai Wang
and
Robert H. Weisberg

Abstract

Equatorially trapped waves of a simplified coupled ocean-atmosphere system are described by applying the formalism for conventional equatorially trapped waves with the assumption that the zonal wind stress and sea surface temperature perturbations are proportional. In this system, inertial-gravity and Rossby-gravity waves are unaffected by coupling whereas Rossby and Kelvin waves are affected, and in the low-frequency limit, these Rossby and Kelvin waves transform to slow westward and eastward propagating wave modes, respectively. The primary modifications by air-sea coupling are a decrease in phase speed and an increase in meridional scale. The properties of these coupled waves are useful in discussing several features, observed and modeled, relative to the evolution of the El Niño-Southern Oscillation.

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Chunzai Wang
,
Robert H. Weisberg
, and
Huijun Yang

Abstract

The thermodynamical process of latent heat flux is added to an analogical delayed oscillator model of the El Niño–Southern Oscillation (ENSO) that mainly considers equatorial ocean dynamics and produces regular, non–phase-locked oscillations. Latent heat flux affects the model sea surface temperature (SST) variations by a positive feedback between the surface wind speed and SST operating through evaporation, which is called the wind speed–evaporation–SST feedback. The wind speed–evaporation–SST feedback in which the atmosphere interacts thermodynamically with the ocean through surface heat flux differs from the conventional zonal wind stress–SST feedback in which the atmsophere interacts dynamically with the ocean through momentum flux.

The combination of equatorial ocean dynamics and thermodynamics produces relatively more realistic model oscillations. When the annual cycle amplitude of the zonal wind in the wind speed–evaporation–SST feedback is gradually increased, the model solution undergoes a transition from periodic to chaotic and then to periodic oscillations for some ranges of the parameters, whereas for other ranges of the parameters the transition goes from periodic to quasiperiodic and then to periodic oscillations. The route to chaos is the intermittency route. Along with such irregularity, the nonlinear interactions between the annual and interannual cycles operating through the wind speed–evaporation–SST feedback also produce a phase-locking of ENSO to the seasonal cycle. The model ENSO onset and peak occur in the boreal winter and spring, respectively, consistent with the observed phase-locking of ENSO in the far eastern Pacific. It is shown that ENSO decadal or interdecadal variability may result from the nonlinear interactions between the annual and interannual cycles in the Tropics.

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Yonggang Liu
,
Robert H. Weisberg
, and
Ruoying He

Abstract

Neural network analyses based on the self-organizing map (SOM) and the growing hierarchical self-organizing map (GHSOM) are used to examine patterns of the sea surface temperature (SST) variability on the West Florida Shelf from time series of daily SST maps from 1998 to 2002. Four characteristic SST patterns are extracted in the first-layer GHSOM array: winter and summer season patterns, and two transitional patterns. Three of them are further expanded in the second layer, yielding more detailed structures in these seasons. The winter pattern is one of low SST, with isotherms aligned approximately along isobaths. The summer pattern is one of high SST distributed in a horizontally uniform manner. The spring transition includes a midshelf cold tongue. Similar analyses performed on SST anomaly data provide further details of these seasonally varying patterns. It is demonstrated that the GHSOM analysis is more effective in extracting the inherent SST patterns than the widely used EOF method. The underlying patterns in a dataset can be visualized in the SOM array in the same form as the original data, while they can only be expressed in anomaly form in the EOF analysis. Some important features, such as asymmetric SST anomaly patterns of winter/summer and cold/warm tongues, can be revealed by the SOM array but cannot be identified in the lowest mode EOF patterns. Also, unlike the EOF or SOM techniques, the hierarchical structure in the input data can be extracted by the GHSOM analysis.

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Aida Alvera-Azcárate
,
Alexander Barth
, and
Robert H. Weisberg

Abstract

The surface circulation of the Caribbean Sea and Gulf of Mexico is studied using 13 years of satellite altimetry data. Variability in the Caribbean Sea is evident over several time scales. At the annual scale, sea surface height (SSH) varies mainly by a seasonal steric effect. Interannually, a longer cycle affects the SSH slope across the current and hence the intensity of the Caribbean Current. This cycle is found to be related to changes in the wind intensity, the wind stress curl, and El Niño–Southern Oscillation. At shorter time scales, eddies and meanders are observed in the Caribbean Current, and their propagation speed is explained by baroclinic instabilities under the combined effect of vertical shear and the β effect. Then the Loop Current (LC) is considered, focusing on the anticyclonic eddies shed by it and the intrusion of the LC into the Gulf of Mexico through time. Twelve of the 21 anticyclonic eddies observed to detach from the LC are shed from July to September, suggesting a seasonality in the timing of these events. Also, a relation is found between the intrusion of the LC into the Gulf of Mexico and the size of the eddies shed from it: larger intrusions trigger smaller eddies. A series of extreme LC intrusions into the Gulf of Mexico, when the LC is observed as far as 92°W, are described. The analyses herein suggest that the frequency of such events has increased in recent years, with only one event occurring in 1993 versus three from 2002 to 2006. Transport through the Straits of Florida appears to decrease during these extreme intrusions.

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Yonggang Liu
,
Robert H. Weisberg
, and
Clifford R. Merz

Abstract

Concurrently operated on the West Florida shelf for the purpose of observing surface currents are three long-range (4.9 MHz) Coastal Ocean Dynamics Applications Radar (CODAR) SeaSonde and two median-range (12.7 MHz) Wellen Radar (WERA) high-frequency (HF) radar systems. These HF radars overlook an array of moored acoustic Doppler current profilers (ADCPs), three of which are presently within the radar footprint. Analyzed herein are 3 months of simultaneous observations. Both the SeaSonde and WERA systems generally agree with the ADCPs to within root-mean-square differences (rmsd) for hourly radial velocity components of 5.1–9.2 and 3.8–6.5 cm s−1 for SeaSonde and WERA, respectively, and within rmsd for 36-h low-pass filtered radial velocity components of 2.8–6.0 and 2.2–4.3 cm s−1 for SeaSonde and WERA, respectively. The bearing offset and tidal and subtidal currents of total velocities are also assessed using the ADCP data. Despite differences in a variety of aspects between the direction-finding CODAR SeaSonde (long range, effective depth of 2.4 m, integration time of 4 h, and idealized antenna patterns) and the beam-forming WERA (median range, effective depth of 0.9 m, and integration time of 1 h), both HF radar systems demonstrated good surface current mapping capability. The differences between the velocities measured with the HF radar and the ADCP are sufficiently small in this low-energy shelf that much of these rmsd values may be accounted for by the expected measurement differences due to the horizontal, vertical, and temporal sampling differences of the ocean current observing systems used.

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Yang Yang
,
Robert H. Weisberg
,
Yonggang Liu
, and
X. San Liang

Abstract

A recently developed tool, the multiscale window transform, along with the theory of canonical energy transfer is used to investigate the roles of multiscale interactions and instabilities in the Gulf of Mexico Loop Current (LC) eddy shedding. A three-scale energetics framework is employed, in which the LC system is reconstructed onto a background flow window, a mesoscale eddy window, and a high-frequency eddy window. The canonical energy transfer between the background flow and the mesoscale windows plays an important role in LC eddy shedding. Barotropic instability contributes to the generation/intensification of the mesoscale eddies over the eastern continental slope of the Campeche Bank. Baroclinic instability favors the growth of the mesoscale eddies that propagate downstream to the northeastern portion of the well-extended LC, eventually causing the shedding by cutting through the neck of the LC. These upper-layer mesoscale eddies lose their kinetic energy back to the background LC through inverse cascade processes in the neck region. The deep eddies obtain energy primarily from the upper layer through vertical pressure work and secondarily from baroclinic instability in the deep layer. In contrast, the canonical energy transfer between the mesoscale and the high-frequency frontal eddy windows accounts for only a small fraction in the mesoscale eddy energy balance, and this generally acts as a damping mechanism for the mesoscale eddies. A budget analysis reveals that the mesoscale eddy energy gained through the instabilities is balanced by horizontal advection, pressure work, and dissipation.

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