Abstract

Atmospherically driven flow in the Providence River (a partially mixed estuary) has been examined using a 51-day velocity record measured 2 m from the bottom. Velocity fluctuations at time scales between the steady-state gravitational convection and the tidal oscillations were large and almost exclusively wind-induced. The mean and variance of the velocity component lying along the channel axis were 11.7 cm s⁻¹ (landward) and 166.9 cm² s⁻². Of this axial current variance 48% resided at subtidal frequencies as compared to 45% associated with semidiurnal tides (the remaining 7% was mostly due to higher tidal harmonies). Over the most energetic portion of the axial current spectrum (periodicities of 4–5 days), 97% of the variance was coherent with the wind velocity component lying along the direction of maximum fetch, with the current lagging the wind by about 4 h. Owing to this extremely high coherence, a linear time-invariant stochastic model reproduced the axial current from the two orthogonal wind velocity components to within an rms error of 2.3 cm s⁻¹. The wind also had a marked effect upon the density field. It is concluded that the effects of wind can permeate the entire water column of a partially mixed estuary arid can be of equal (or greater) importance to the circulation as the tides or gravitational convection.

Abstract

The stability, periodicity, and horizontal structure of equatorial modes in a coupled ocean-atmosphere model, simplified by the assumption that zonal wind stress anomalies are proportional to sea surface temperature anomalies lagged by a zonal phase difference, are examined analytically in an unbounded basin. The gravest coupled Rossby and Kelvin modes coexist with additional westward and eastward slow modes whose phase speeds are smaller than the former. Two of these four modes, one propagating westward and the other eastward, are destabilized in each case depending upon the model parameters. For some particular parameter choices. coupled Rossby and Kelvin modes merge with westward and eastward slow modes, respectively. For other parameters, however. they separate and remain distinct from the slow modes. For all of these modes the primary modifications by coupling relative to uncoupled oceanic equatorial waves are a decrease in phase speed and an increase in meridional scale.

Among the model parameter effects, those of the zonal phase lag between the wind stress and SST anomalies and the coefficients of thermal and mechanical damping are the most interesting. Positive and negative phase lags represent the wind stress anomalies located to the west and east of the SST anomalies, respectively. The frequency of all modes is symmetric about zero phase lag, whereas the growth rate is antisymmetric about zero phase lag relative to the uncoupled damping rate. Wind stress anomalies to the west of SST anomalies favor slow mode growth and coupled Rossby and Kelvin mode decay. Dissipation for the slow modes and the coupled Rossby and Kelvin modes is controlled differently. For the slow modes the dissipation is mainly thermal, whereas for coupled Rossby and Kelvin modes the dissipation is mainly mechanical.

Abstract

A linear perturbation, coupled ocean–atmosphere model is revisited for further insights into the El Niño–Southern Oscillation phenomenon. The model oscillates as a slow, eastward propagating mode interpreted as a divergence mode, whose energetics are controlled by the ocean. Growth requires that the work performed by the wind stress minus the work required to effect the ocean divergence exceeds the loss terms. The intrinsic scale of the atmosphere relative to the basin width is important. For sustainable oscillations, the ocean basin must be large enough so that oppositely directed divergence can develop on opposite sides of the basin. The global aspect of the atmospheric pressure field suggests that continental heating may provide either a direct source affecting adjacent oceans, or a connection between oceans. The important model parameters are the coupling and warming coefficients and the ocean Kelvin wave speed. The importance of the Kelvin wave speed derives from its specification of the background buoyancy state for the ocean. Upon further simplification, an analytical solution gives similar parameter dependence as found numerically and shows that growth requires both large zonal wavelength and a zonal phase lag between the anomalies of wind stress and SST.

Abstract

The annual cycle of sea surface temperature and ocean–atmosphere fluxes on the west Florida shelf is described using in situ measurements and climatology. Seasonal reversals in water temperature tendency occur when the net surface heat flux changes sign in boreal spring and fall. Synoptic-scale variability is also important. Momentum and heat flux variations result in successive water column stratification and destratification events, particularly at shallower depths during spring. Fall is characterized by destratification of the water column and a series of steplike decreases in the temperature. These are in response to both tropical storms and extratropical fronts. Tropical storms are responsible for the largest momentum fluxes, but not necessarily for the largest surface heat fluxes. A one-dimensional analysis of the temperature equation suggests that surface heat flux is primarily responsible for the spring and fall seasonal ocean temperature changes, but that synoptic-scale variability is also controlled by the ocean circulation dynamics. During summer, the situation is reversed and the major influence on water temperature is ocean dynamics, with the heat flux contributing to the synoptic-scale variability. There is also evidence of interannual variability: the wintertime temperatures get increasingly colder from 1998 to 2000, and the greatest stratification and coldest subsurface temperatures occur in 1998. NCEP–NCAR reanalysis fields do not reproduce the high spatial flux variability observed in situ or with satellite measurements. Reconciling these differences and their impacts on the climate variability of this region provides challenges to coupled ocean–atmosphere models and their supporting observing systems.

Abstract

The evolution of the 1997–98 El Niño is described using NCEP SST and OLR data, NCEP–NCAR reanalysis sea level pressure (SLP) fields, and The Florida State University surface wind data. From November 1996 to January 1997, the eastern Pacific is characterized by equatorial cold SST and high SLP anomalies, while the western Pacific is marked by off-equatorial warm SST anomalies and off-equatorial anomalous cyclones. Corresponding to this distribution are high OLR anomalies in the equatorial central Pacific and low OLR anomalies in the off-equatorial far western Pacific. The off-equatorial anomalous cyclones in the western Pacific are associated with a switch in the equatorial wind anomalies over the western Pacific from easterly to westerly. These equatorial westerly anomalies then appear to initiate early SST warmings around the date line in January/February 1997 and around the far eastern Pacific in March 1997. Subsequently, both the westerly wind and warm SST anomalies, along with the low OLR anomalies, grow and progress eastward. The eastward propagating warm SST anomalies merge with the slower westward spreading warm SST anomalies from the far eastern Pacific to form large-scale warming in the equatorial eastern and central Pacific. The anomaly patterns in the eastern and central Pacific continue to develop, reaching their peak values around December 1997. In the western Pacific, the off-equatorial SST anomalies reverse sign from warm to cold. Correspondingly, the off-equatorial SLP anomalies in the western Pacific also switch sign from low to high. These off-equatorial high SLP anomalies initiate equatorial easterly wind anomalies over the far western Pacific. Like the equatorial westerly wind anomalies that initiate the early warming, the equatorial easterly wind anomalies over the far western Pacific appear to have a cooling effect in the east and hence help facilitate the 1997–98 El Niño decay. This paper also compares the 1997–98 El Niño with previous warm events and discusses different ENSO mechanisms relevant to the 1997–98 El Niño.

Abstract

Evidence is presented for the generation of planetary waves by barotropic instability within the cyclonic shear region of the Atlantic Ocean's South Equatorial Current (SEC). Immediately following the springtime intensification of the southeast trade wind, which accelerates the SEC westward, a packet of waves with central periodicity of around 25 days is observed lasting for about three cycles. Independent wavenumber analyses on 1983 and 1984 data give newly identical zonal wavelengths and phase speed estimates of around 1100 km and −50 cm s⁻¹. The waves are anisotropic and spatially inhomogeneous with generation confined primarily to the mixed layer.

An energetics analysis using 1983 data centered upon the equator at 28°W shows a rapid increase in total perturbation energy (TPE) reaching values of 2000 erg cm⁻³ within two weeks. The subsequent decrease in TPE at this location is due primarily to meridional pressure-work divergence. Baroclinic instability is negligible because both the meridional and zonal components are small and cancelling.

Thermodynamically, the waves effect a southward heat transport during the period when the North Equatorial Countercurrent (NECC) is most rapidly gaining heat, suggesting that the waves act to regulate the heat stored in the NECC. Also the Reynolds' heat flux convergence upon the equator appears to halt the upwelling induced cooling and to increase sea surface temperature. In 1983 this convergence was equal to the climatological atmosphere-ocean net heat flux.

Dynamically, the waves decelerate the SEC north of the equator and reduce its shear. This occur simultaneously with a deceleration of the SEC by the basinwide adjustment of the zonal pressure gradient (ZPG). The seasonal modulation of the waves is therefore a consequence of both the ZPG response to seasonally varying wind stress as well as the instability itself since both are stabilizing. The basin size and hence the ZPG adjustment time differences between the Atlantic and Pacific Oceans would thus account for the observed differences in wave season durations between these two oceans.

Abstract

The annual cycle of the upper ocean's vertical velocity component (w) on the equator at 28°W is examined by integrating the continuity equation using current meter data from the Seasonal Response of the Equatorial Atlantic Experiment. The annual cycle consists in part of an intense, but brief (∼1 month), upwelling season beginning with the onset of strong easterly wind stress in boreal spring. This upwelling is followed by weaker downwelling during the summer despite the persistence of strong easterly wind stress. The record-length averaged w profile shows that maximum upwelling (0.6 × 10⁻³ cm s⁻¹) is located slightly above the core of the Equatorial Undercurrent and downwelling is located below the base of the thermocline. The standard deviations are about tenfold the magnitude of the means. Independent evidence supporting these results are that 1) sea surface temperature (SST) is related to w during the springtime changes in easterly wind stress with the observed and computed isotherm displacements in agreement, 2) temperature and w are coherent and in quadrature within the thermocline over a broad range of frequencies exclusive of the instability wave band, 3) during the instability wave season, upwelling is associated with increasing SST and the vertical Reynolds' heat flux is maximum and divergent in the thermocline, and 4) after the instability waves abate, w and easterly wind stress are coherent and out-of-phase. The observed evolution of w differs from that implied by climatology, and these differences are attributed to the ocean's response to rapidly varying winds that are observed in-situ versus slowly varying winds characteristic of climatology.

Abstract

Temperature and velocity time series, obtained by surface moorings during the Seasonal Response of the Equatorial Atlantic Experiment, are used to investigate the role of ocean dynamics upon the annual cycle of equatorial sea surface temperature (SST) and upper ocean heat. The annual cycle in SST is explained by different mechanisms, each operant at different phases of the cycle. The boreal springtime decrease in SST results from upwelling in response to the seasonal intensification of easterly wind stress. This upwelling causes the seasonal formation of the cold tongue along the equator in the central and eastern portions of the basin. An early summer increase in SST is attributed to the meridional convergence of Reynolds' heat flux associated with surface current instability-generated waves. After the instability waves abate, SST and mixed layer depth remain relatively steady from late summer through fall when the advective terms are small and cancelling, suggesting that surface heating is then balanced by a diffusive flux at the base of the mixed layer. SST increases in wintertime following the seasonal relaxation in easterly wind stress, thus, completing its annual cycle. This increase is attributed to the concentration of the surface flux over a mixed layer that is shoaling due to both the basin-wide adjustment of the thermocline and the local reduction in turbulent energy production. Thus, SST variations are found to be most closely controlled by ocean dynamics during those times when the thermocline is adjusting basin-wide to the seasonal changes in wind stress; either directly by large advective fluxes (boreal spring-summer) or indirectly by mediating mixed layer depth (boreal winter). Analyses at 75 m depth show zonal and vertical advection to be important, and within a control volume over the upper 150 m all of the advective terms are important.

Abstract

Using COADS data spanning 1947–1988, we describe the regional nature of the Atlantic Ocean wind-driven circulation between 30°8 and 60°N and its annual and interannual variability. The Sverdrup streamfunction defines the circulation gyres. Our focus is on three central gyres: the Northern Hemisphere anticyclonic subtropical gyre, the cyclonic tropical gyre just north of the equator, and the clockwise equatorial gyre straddling the equator. This rendition of the Sverdrup streamfunction, computed with constant drag coefficient and air density, compares favorably with that from other climatologies. In the Straits of Florida, analyses suggest that differences between the annual cycle in Sverdrup transport and observations may be due to regional winds farther north. In the tropical gyre, the Sverdrup circulation argues against a continuous western boundary current transporting water from the equatorial region into the Caribbean in boreal winter, bringing to question the mechanisms for the known interhemisphere and intergyre exchanges of heat and mass. A conceptual model is proposed involving two stages. First, the western boundary current closing the clockwise equatorial gyre is instrumental in storing heat and mass between the North Equatorial Countercurrent ridge and the North Equatorial Current trough in boreal summer. Transport farther north, across the tropical gyre and into the subtropical gyre, in boreal winter is then accomplished by Ekman transport, as the seasonal change in wind-stress torque deepens the thermocline, thus allowing for vortex stretching and northward Sverdrup transport over the region of warmest waters. Once in the subtropical gyre, the Ekman transport continues to be northward despite the fact that the Sverdrup transport reverses to be southward. Annual and interannual variability is addressed by examining the spectrum of curl and its regional distribution. Outside the tropics and the Sargasso Sea, interannual exceeds annual variability by at least a factor of 1.5. A pentadal analysis in the subtropical gyre indicates that wind-stress curl was not a major factor in the density structure differences reported between 1955–1959 and 1970–1974; hence, these require other explanations.

Abstract

Observed variations in the Atlantic Ocean's equatorial thermocline are compared at four locations with simulations using an analytical reduced-gravity model. The comparison shows the essential features of the seasonal wind-forced thermocline response to be accounted for by a linear superposition of equatorial long waves, evolving basinwide, tending to bring the zonal pressure gradient into balance with the wind stress. A frequency response function is derived whose properties provide a basis for discussing the large scale features of the equatorial Atlantic Ocean's seasonal cycle—for example, its evolution along the equator, the maximum upwelling region observed in the Gulf of Guinea and the secondary upwelling season also observed there. Clarification is also given to the issue of remote versus local forcing for these features.