Search Results

You are looking at 1 - 10 of 34 items for

  • Author or Editor: P. P. Niiler x
  • Refine by Access: All Content x
Clear All Modify Search
P. van Meurs
and
P. P. Niiler

Abstract

Data from Argos-tracked mixed layer drifters in fall and winter 1987 (49 drifters) and 1989 (16 drifters) are used to investigate the differences in the large-scale surface velocity and eddy activity in the northeast Pacific. The velocities were corrected for wind-induced slippage and corrected for wind-driven (Ekman) flow by matching an Ekman model to the observed currents. The model, which explains 15%–30% of the variance, indicates that the currents are at 60° to the right of the wind. The magnitude of the currents is 30% of the magnitude of the wind stress. In 1987–88, the geostrophic motion in the region from 46.5° to 48.5°N, 142° to 133°W was characterized by an eastward flow of 0.9 (±0.4) cm s−1 and a northward flow of 0.7 (±0.4) cm s−1. In 1989–90, for the same region, the geostrophic eastward component was 3.8 (±0.5) cm s−1, more than four times as large as in 1987–88, and the northward component was 0.3 (±0.5) cm s−1. In this region ageostrophic contributions to the velocities are small.

In 1987–88 the drifter tracks reveal evidence of the presence of several persistent, warm core mesoscale eddies. In 1989–90 there is no evidence of any significant eddy activity. The mean speed of the drifters in 1987–88 was 7.0 (±0.3) cm s−1 and in 1989–90 was 6.5 (±0.4) cm s−1. So, although the average speed is the same, drifters in 1987–88 take a longer time to travel eastward because of the significant north–south excursions due to the mesoscale eddies. Data from two drifter experiments have shown that the variability of mesoscale eddies can result in large interannual differences in estimates of mean velocity.

Full access
C. J. Koblinsky
and
P. P. Niiler

Abstract

Observations of currents collected at the POLYMODE array III cluster C (16°N, 54°W) are compared with radiosonde winds measured at Barbados (13°N, 62°W) using a linear response analysis. The winds and the currents are coherent throughout the water column (5400 m) over the subinertial frequency range of 0.025 to 0.25 cpd. The coherence is highest between the east-west wind stress and north-south currents over smooth topography. The wind-related ocean currents have a rms of 2.5 cm s−1 at 500 m and 1.1 cm s−1 at 4000 m and account for approximately one-third of the total eddy kinetic energy. The wind-current phase is depth independent and does not vary significantly over the 200 km horizontal scale of the array. The response amplitude is surface intensified and increases with decreasing frequency which is consistent with a deterministic theoretical model. The coherence values and response estimates suggest an off-resonant barotropic response to large-scale wind forcing.

Full access
P. P. Niiler
,
E. Simco
, and
R. Larue

Abstract

A two-layer model of a mid-oceanic thermocline is developed, and a comparison of the depth and temperature of the thermocline in this model is made with a root-mean-square description of the hydrographic structure of the North Atlantic. It is shown that the Sverdrup balance is maintained within the rms errors, and the entire estimated heat flux from the atmosphere is used to produce the observed density changes along the path of persistent flow.

Full access
T. K. Chereskin
,
P. P. Niiler
, and
P. M. Poulain

Abstract

The shape and slip of freely drifting, two-dimensional, flexible weighted drogues tethered to a surface buoy in a specified upper-ocean velocity profile are examined numerically. A simple analytic solution for a drogue in a linear shear flow, in the limit of small deviations from a straight vertical configuration, is used to identify the parameters of the problem and to predict the functional dependence of the slip and shape of the drogue on those parameters. The numerical computations, using a finite elements static equilibrium model, confirm the functional dependence predicted by the analytic solution and estimate the parametric dependences. However, a linear shear is not the “worst case” shear one needs to design for. In optimizing a drogue for linear shear, one can make use of the symmetry of the velocity profile to minimize the slip. The design problem arises from not knowing a priori the shear for which one is designing (especially since a drogue eventually moves far from its deployment site) and from asymmetric shear (i.e., the “worst case” shear is one with a bias). The final computations examine three different drogue configurations in a series of profiles that model the diurnal cycle of the mixed layer (a diurnal jet) overlying a linear shear. The best design is found to be one that maximizes the drogue over the depth interval of interest, while minimizing the drag area of the tether. The drogue length needs to be larger than the depth interval of interest to account for the rise and tilt of the drogue in shear flow, but not so large that it averages too far outside the interval. For the practical cases considered, a drogue length that was twice the averaging interval gave the best results.

Full access
Stephen E. Pazan
and
Peter P. Niiler

Abstract

The authors have quality controlled six global datasets of drifting buoy data, made comparisons of 15-m drogued and undrogued buoy observations, and developed a 2D linear regression model of the difference between drogued and undrogued drifter velocity as a function of wind. The data were acquired from 2334 Surface Velocity Program (SVP) drifters, including 1845 SVP drifters after they lost their drogues; 704 AN/WSQ-6 Navy drifter buoys; and 503 First Global GARP Experiment (FGGE) drifter buoys. Meridional and zonal surface wind velocity components from the global synoptic FNMOC model, the global synoptic ECMWF model, and the global synoptic NCEP model were interpolated to naval AN/WSQ-6, WOCE–TOGA buoy, or FGGE buoy positions and date/times in the datasets. Two-day mean buoy drift velocities and positions were computed: 122 101 SVP drifter mean velocities before they lost their drogues and 58 201 SVP drifter mean velocities after they lost their drogues, 21 799 Navy drifter mean velocities, and 42 338 FGGE drifter mean velocities. A regression analysis was made on selected data in selected 2° lat × 8° long bins: U undrogued = A undrogued + B undrogued W undrogued, U drogued = A drogued + B drogued W drogued, where U undrogued, U drogued was ensemble mean buoy velocity and W was wind velocity, and the real and imaginary parts of these quantities were the zonal and meridional components, respectively. The difference in these complex valued regression coefficients, B difference = B undroguedB drogued measured the linear response to the wind. Navy and SVPL buoy response to the wind was identical and FGGE buoy response was generally the same; the global weighted mean value of |B difference| was 0.0088 ± 0.002. The difference in the complex valued y intercept, A difference = A undroguedA drogued, was nearly always zero within error. The buoy response to wind was also estimated by b = ( U undrogued U drogued)/ W undrogued, where the velocity difference U undrogued U drogued was calculated from ensemble mean buoy velocities and W was ensemble mean wind velocity. The global-weighted mean value of |b| was 0.0097 ± 0.005. The analysis also found that the phase angle of either B difference or b, which measures the velocity difference with respect to the wind, was zero within error and not a function of the surface wind or the Coriolis parameter.

Full access
James W. Stevenson
and
Pearn P. Niiler

Abstract

Heat flux, CTD and current profile data from the Hawaii-to- Tahiti Shuttle Experiment are used to study the upper ocean heat budget in order to better understand the seasonal evolution of sea surface temperature (SST) in the central tropical Pacific Ocean between February 1979 and June 1980. The surface heat flux is estimated using bulk formulas and the standard meteorological data taken aboard ship. Upper ocean heat storage is computed from CTD data in such a way (using temperature vertically averaged between the sea surface and fixed isotherm depths) as to filter internal waves. It is found that the surface heat flux plays a large role in the seasonal evolution of SST. A time-latitude correlation coefficient of 0.70 is found between the surface heat flux and heat storage. The seasonal evolution of the vertically averaged temperature whose time rate of change determines storage is very closely correlated with the seasonal evolution of SST.

At 155°W, there is no evidence for a relation between changes of main thermocline depths and changes in SST. Also, we see no feedback from the ocean to the atmosphere through SST governed heat flux. Horizontal heat advection is estimated from Firing et al. profiling current meter data. The advection of cold water from the east is important in the 15-cruise (16-month) mean but the data are too noisy to estimate the seasonal evolution of heat advection.

Full access
W. Timothy Liu
and
Pearn P. Niiler

Abstract

A simple statistical technique is described to determine monthly mean marine surface-layer humidity, which is essential in the specification of surface latent heat flux, from total water vapor in the atmospheric column measured by space-borne sensors. Good correlation between the two quantities was found in examining the humidity sounding from radiosonde reports of mid-ocean island stations and weather ships. The relation agrees with that obtained from satellite (Seasat) data and ship reports averaged over 2° areas and a 92-day period in the North Atlantic and in the tropical Pacific. The results demonstrate that, by using a local regression in the tropical Pacific, total water vapor can be used to determine monthly mean surface layer humidity to an accuracy of 0.4 g kg−1. With a global regression, determination to an accuracy of 0.8 g kg−1 is possible. These accuracies correspond to approximately 10 and 20 W m−2 in the determination of latent heat flux with the bulk parameterization method, provided that other required parameters are known.

Full access
Pearn P. Niiler
and
Melinda M. Hall

Abstract

A current meter mooring maintained for over three years at 28°N, 152°W, in the eastern North Pacific has yielded velocity and temperature data throughout the water column, with particularly good thermocline resolution The flow is characterized by weak primarily westward mean velocities, with a superimposed eddy field having rms velocities ranging from 10 cm s−1 in the upper thermocline to 3 cm s−1 at 1000 m depth. The eddy energy is divided into two main bands: the low frequency eddies have spatial scales of 250–300 km and periods of 100–200 days, propagate southwestward, and have slightly more zonal than meridional energy. The high frequency eddies also propagate southwestward, have spatial scales of 150–175 km and periods of 40–80 days, and are strongly meridionally oriented. Vertical EOF structure calculated in the frequency domain suggests that the low frequency eddies are more wavelike (linear) in nature than are the high frequency. The entire band appears to derive energy baroclinically from a secularly varying background flow; as a function of time, the eddy heat flux tends to be down the very low frequency varying temperature gradient. Some interesting points of comparison are found with eddies in a three-layer nonlinear model of the eastern North Pacific recently described by Lee.

Full access
Pierre-Marie Poulain
and
Pearn P. Niiler

Abstract

A kinematic description of the surface circulation in the southern California current System is presented using the statistics of the 7–11 month long trajectories of 29 satellite-tracked mixed layer drifters. The drifters were released north of 30°N and traveled southward at an average speed of 3–4 cm s−1 along Baja California through an inhomogeneous field of mesoscale eddies of 15 cm s−1 rms variability. Lagrangian and Eulerian statistics of the variations about this mean southward drift are computed. The drifter ensemble mean Lagrangian decorrelation time scale is 4–5 days and the Lagrangian decorrelation space scale is 40–50 km. The computation of dispersion of single particles about the mean drift shows that the theory of diffusion by homogeneous random motion (Taylor's theory) describes these dispersive motions well. Ensemble mean diffusivities of about 4 × 107 cm2 s−1 are found. On a 200 × 200 km2 spatial average, single-partial diffusivities are found to be proportional to the kinetic energy of the locally inhomogeneous fluctuations. Particle-pair statistics are used to study the relative dispersion of particles. The relative diffusivities depend on the initial separation and on the duration of drift. The results are compared to Richardson's 4/3 power law. The Eulerian spatial and temporal correlation of the velocity field indicates that the eddy field is isotropic for scales less than 200 km. The zero time lag correlation indicates an Eulerian length scale of 80 km. The 25-day lagged correlation function indicates that a 2 cm S−1 northwestward propagation of features exists roughly perpendicular to the mean flow.

Full access
Jeffrey D. Paduan
and
Pearn P. Niiler

Abstract

In October 1987, 49 Lagrangian surface drifters (TRISTAR-II) were released in a 200-km × 200-km square area southeast of Ocean Station Papa as part of the OCEAN STORMS Experiment. The drifters measured temperature at the drogue level and reported their position through ARGOS approximately 11 times per day. Thirty-one of the drifters retained drogues for longer than three months, and data from those instruments are used to describe the evolving fall 1987 pattern of current and temperature structures at 15 m in the area between 46° and 49°N, 142°W and 132°W. Time variable currents were dominated by mesoscale eddies of anticyclonic rotation with horizontal radii of 53–86 km and rotational speeds of 10–20 cm s−1. These eddies persisted for at least 90 days as evidenced by successive drifter trajectories through the eddies. Currents with periods longer than 1 day had a mean to the east of 4.4 cm s−1 and a mean to the north of 0.7 cm s−1. Background eddy kinetic energy levels were 40 cm s−2. Thus, eddy kinetic energy was four times larger than mean kinetic energy. The eastward single particle diffusivity was 1100 m2 s−1 and northward diffusivity was 1600 m2 s−1. The local change of thermal energy at 15-m depth was −2.9 W m−3, while on average, flow advected cold water to the east at a rate of 0.8 W m−3. Therefore, large-scale advective processes accounted for 28% of the thermal energy balance at 15 m. This horizontal heat convergence in the open ocean is comparable in magnitude to that produced by powerful equatorial currents in the eastern Pacific cold tongue.

Full access