摘要:

Residual sea surface height anomaly fields are obtained over the North Atlantic Basin from TOPEX/POSEIDON altimeter data. Evidence of westward propagating anomalies characterized as first‐mode baroclinic Rossby waves is presented. The phase speeds are obtained from the analysis of the autocorrelation matrices of the residual surface height anomaly in three dimensions, on a 5°×5° grid. Periods are calculated by least squares fitting a sinusoidal function to the time series at each grid point. Zonal and meridional wavelengths are obtained from the phase speed and period and are least squares fit to the data as a consistency check. Zonally averaged westward phase speeds vary between 1 and 27 km/d, between 50° and 5°N, increasing southward. Phase speeds are directed to the southwest north of approximately 25°N and to the northwest south of this latitude. The sea surface height signal is divided into two spectral bands, the first with periods between 280 and 450 days and the second with periods between 120 and 280 days. For the first band the average of the periods taken over the whole basin is 370 days. In the second band a signal of period between 6 and 7 months is evident also throughout the basin. The zonally averaged zonal wavelengths associated with the first band are in the range of 400–4600 km between 50° and 5°N and for the second band are in the range 270–2500 km between 45° and 5°N. Longer wavelengths are found close to the southern boundaries. Shorter wavelengths are found in the northern portion of the basin. Comparison with results from numerical models suggests that the baroclinic waves observed at midlatitudes are generated at the eastern boundary by fluctuations of the wind stress curl. Waves observed south of 20°N support the hypothesis of dir

ISSN:0148-0227    

DOI:10.1029/96JC03349

年代:1997

数据来源: WILEY

摘要:

Moored observations of near‐inertial currents and buoyancy fluctuations from the Frontal Air Sea Interaction Experiment (FASINEX) may be understood as nearinertial internal waves reflecting off sheared background currents. The FASINEX moored array measured upper ocean currents, temperature, and conductivity between January and June 1986 in a region of the Sargasso Sea heavily populated by strong upper ocean fronts. The background jets associated with these features have typical cross‐front scales of 20km, maximum currents of 0.5 m s−1, and relative vorticities of ±0.3ƒ. In the vicinity of fronts, observed near‐inertial currents have short horizontal scales, show no clear horizontal phase structure, and trace out elongated current ellipses with elevated across‐front over along‐front variance. During these periods, inertial band‐pass buoyancy fluctuations often correlate with major‐axis currents. WKB ray‐tracing models of propagating, monochromatic plane waves exhibit horizontal phase propagation and correlations between minor‐axis currents and buoyancy and thus cannot explain these results. Models of near‐inertial internal waves reflected or trapped by a barotropic frontal jet produce mode‐like structures with short horizontal scales, little horizontal phase propagation, and anisotropic current ellipses aligned primarily cross front. Model buoyancy may correlate with one or both current ellipse components, depending on the incidence angle of the wave and the degree to which it reflects. Completely reflected waves with horizontal scales similar to those of the front and near‐normal incidence angle produce current ellipses and correlation patterns consi

ISSN:0148-0227    

DOI:10.1029/96JC03209

年代:1997

数据来源: WILEY

摘要:

By focusing on time sequences of basin‐average and global‐average upper ocean temperature (i.e., from 40°S to 60°N) we find temperatures responding to changing solar irradiance in three separate frequency bands with periods of>100 years, 18–25 years, and 9–13 years. Moreover, we find them in two different data sets, that is, surface marine weather observations from 1990 to 1991 and bathythermograph (BT) upper ocean temperature profiles from 1955 to 1994. Band‐passing basin‐average temperature records find each frequency component in phase across the Indian, Pacific, and Atlantic Oceans, yielding global‐average records with maximum amplitudes of 0.04°±0.01°K and 0.07°±0.01°K on decadal and interdecadal scales, respectively. These achieve maximum correlation with solar irradiance records (i.e., with maximum amplitude 0.5 W m−2at the top of the atmosphere) at phase lags ranging from 30° to 50°. From the BT data set, solar signals in global‐average temperature penetrate to 80–160 m, confined to the upper layer above the main pycnocline. Operating a global‐average heat budget for the upper ocean yields sea surface temperature responses of 0.01°–0.03°K and 0.02°–0.05°K on decadal and interdecadal scales, respectively, from the 0.1 W m−2penetration of solar irradiance to the sea surface. Since this is of the same order as that observed (i.e., 0.04°–0.07°K), we can infer that anomalous heat from changing solar irradia

ISSN:0148-0227    

DOI:10.1029/96JC03549

年代:1997

数据来源: WILEY

摘要:

Between 1966 and 1995, subsurface temperature data have been collected in the western North Atlantic Ocean using expendable bathythermographs. Data coverage is sparse in both time and space, but evidence for decadal variability in the upper 400 m of the water column is found. The data were averaged by month onto a 2° of latitude by 4° of longitude grid. Thirty‐one quadrangles in the region bounded by 17°N and 43°N and 78°W and 66°W have sufficient data to provide consistent results. Anomaly time series at 0, 100, 200, 300, and 400 m were estimated by subtracting a mean monthly climatology. The individual records were detrended and filtered to highlight the longer‐period signals. The analysis resulted in 25‐year records (1969–1993) for study. Within the thermocline of the subtropical gyre and the Gulf Stream at 100 and 200 m, periods of predominately positive temperature anomaly end in 1971, 1982, and 1990, while periods of negative anomaly end in 1976 and 1985. Only the events ending in 1971, 1976, and 1990 are in the majority of the records at 300 and 400 m. Most of the events also appear in the sea surface temperature (SST) records but are somewhat masked by significant noise at the surface. Meridional‐vertical temperature sections through the subtropical gyre show that transitions from negative to positive anomaly events are characterized by a deepening of the isotherms throughout the section and transitions from positive to negative events by a rising of the isotherms. Significant lateral migration of the axis of the Gulf Stream, although possibly masked by the 2° averaging, is not necessary to explain either type of event. The transitions in the SST and 100‐m temperature time series occur at essentially the same time as the transitions in an index of the North Atlantic Oscillation (NAO) that has also been detrended (i.e., 1971, 1976, 1980, 1984, 1988). The 1971, 1976, and 1988 NAO events are also observed at 300 and 400 m as described earlier. Periods of positive subsurface temperature anomaly are coincidental with periods of positive NAO index, and periods of negative subsurface temperature anomaly are coincidental with periods of negative NAO index. Thus earlier results showing connections between the NAO and western Atlantic SST at decadal timescales are now extended to at least 400 m in the water column. Trends were computed from the individual 25‐year records. The trends at all depths are predominately negative north of 38°N and positive south of 38°N. Inferences from the horizontal distribution of the trends and results from earlier studies suggest that the 1969–1993 period may be a phase of a 30‐ to 50‐year signal observed in the northern Atlantic since

ISSN:0148-0227    

DOI:10.1029/96JC03544

年代:1997

数据来源: WILEY

摘要:

The Shannon entropy is a measure of the degree of intricacy contained in any graphablen‐dimensional realization of an observable quantity. We use the Shannon entropy to measure the intricacy of density overturns in the oceanic thermocline and find that the Shannon entropy is related to the Thorpe scaleLT[Thorpe, 1977] and the Ozmidov scaleLO[Ozmidov, 1965]. We find that (1) small Shannon entropy corresponds to small values ofROT(≡LO/LT), while large Shannon entropy is associated with large values ofROT; (2) density spectra are typically more steep than inertial subrange spectra when both Shannon entropy andROTare small, whereas the spectral slope tends to be flatter than inertial subrange spectra when both Shannon entropy andROTare large; (3) the Grashof number is very large (O(1010)) when Shannon entropy is small, indicating that these patches are extremely density unstable; (4) spectral bandwidth is much larger for patches with small Shannon entropy than for those with large entropy, indicating that large‐scale, or “bulk” Reynolds number is large when entropy is small. We discuss the hypothesis that the degree of intricacy, and hence the Shannon entropy, increases with increasing time in a turbulent overturn and is observed to decrease only when the resolution limits of the measuring system are exceeded. On the basis of these arguments we suggest that some classes of overturns are created with Thorpe scale larger than the Ozmidov scale. In these overturns the kinetic energy dissipation rate (ε) is small during the initial growth of the overturn. Later, a small‐scale structure develops, and a more complex, higher‐order flow evolves. This behavior is discussed and compared with gridgenerated laboratory turbulence, in which initially small, energetic, rapidly growing boundary layers detached from the grid and advect downstream, forcingROTto be largest adjacent to the grid and thereafter decrease as a result

ISSN:0148-0227    

DOI:10.1029/96JC03605

年代:1997

数据来源: WILEY

摘要:

An array of six current meter moorings was deployed in the Samoan Passage (10°S, 170°W) from September 1992 to February 1994 with the goal of determining the northward abyssal transport. The 17‐month mean transport beneath 4000 m was 6.0 Sv (106m3s−1). The time series were low passed at a period of 100 hours to remove near‐inertial and tidal variability, and the resulting transport had a standard deviation of 1.5 Sv, a minimum of 1.1 Sv, and a maximum of 10.7 Sv. The dominant low‐frequency variability in transport and area‐averaged temperature was at a period of 30 days, where the temperature led the transport as would be expected if the heat balance were advective. The temporal and spatial variability suggested that the mean transport is known to within a 1 standard deviation error of 0.5 Sv. The along‐passage flow was intensified to the west and toward the bottom as might be expected in a rotating geostrophic system. Three pathways through the sill region were identified on the basis of bathymetry and current direction. The flow was strongly controlled by topography as indicated by the flow's orientation and directional steadiness. The 30‐day variability in transport might be described as a resonance of the northwa

ISSN:0148-0227    

DOI:10.1029/96JC03286

年代:1997

数据来源: WILEY

摘要:

A fine resolution numerical model of the Southern Ocean (the Fine Resolution Antarctic Model (FRAM)) has been used to investigate the way in which heat is supplied to the South Atlantic. The heat budget in the model is compared with other estimates and is found to be broadly realistic. The temperature structure in the Atlantic, and therefore the meridional heat transport, depend heavily on the input of heat from the Indian Ocean via the Agulhas Retroflection region. FRAM is compared with three models which do not exhibit a significant input of heat from the Indian Ocean. These models also have a lower equatorward heat transport in the South Atlantic. Horizontal resolution affects the amount of Agulhas transfer with coarser resolution leading to lower heat transport in the Atlantic, a result which has implications for ocean models used in climate simulations.

ISSN:0148-0227    

DOI:10.1029/96JC03207

年代:1997

数据来源: WILEY

摘要:

One week after Tropical Cyclone Ofa passed near Samoa, near‐inertial oscillations of almost 1 m s−1in amplitude were observed with a shipboard acoustic Doppler current profiler (ADCP). The velocity was nearly uniform with depth in the upper 80 m. The strongest currents in this layer were found about 300 km from Ofa's track, farther than expected from previous observations and models. Below 80 m was a layer 50–100 m thick in which the shear exceeded 0.01 s−1. Strong near‐inertial oscillations, with currents over 0.6 m s−1from 130 to 200 m, extended through the shear layer to at least 300 m, the limit of the ADCP measurements. The phase of the oscillations propagated upward, consistent with downward propagation of energy. A simple slab model driven by an idealized wind field approximately matches the observed amplitude and phase of the oscillations in the surface layer within 150 km of Ofa's track, but not the stronger current 300 km fro

ISSN:0148-0227    

DOI:10.1029/96JC03497

年代:1997

数据来源: WILEY

摘要:

The net integrated energy flux into a train of internal waves is evaluated in a two‐layer model. The nonzero value for this integral results from the difference in the stratification between the initial and final state, similar to the energy supply to a surface bore. We apply this expression to waves measured byWesson and Gregg[1988] in the Strait of Gibraltar and to waves measured byFarmer and Smith[1980] in Knight Inlet. We find the energy supply to be important to the energetics, but the data do not allow a definitive test of the conjecture that the primary energy balance is between this supply and dissipation. We contrast our conjecture to the solitary‐wave considerations ofBogucki and Garrett[19

ISSN:0148-0227    

DOI:10.1029/96JC03558

年代:1997

数据来源: WILEY

摘要:

We present a new model to compute turbulent surface heat and momentum fluxes over leads in the Arctic sea ice. The momentum roughness length uses a sea state parameterization which is fully consistent with the surface turbulent flux parameterization. The flux parameterization accounts for the fetch limitation of the airflow over a lead. The surface roughness length for heat is determined from an application of surface renewal theory to the air‐sea interface. The modeled fluxes are compared with in situ observations of lead fluxes. We also compare our model results with a bulk flux algorithm which has been commonly used to evaluate surface heat fluxes from leads. We perform sensitivity studies to examine the role of the surface renewal timescale, the importance of the cool skin, and impact of wave age dependence on the momentum roughness length. We have computed integral heat fluxes as a function of lead width/fetch for various atmospheric states to determine the magnitude of heat flux in a mesoscale model grid in which a lead is presen

ISSN:0148-0227    

DOI:10.1029/96JC03606

年代:1997

数据来源: WILEY