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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.

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Pearn P. Niiler, Dong-Kyu Lee, and Alex Warn-Varnas

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No abstract available.

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Pearn P. Niiler, William J. Schmitz, and Dong-Kyu Lee

Abstract

Three new deep hydrographic sections taken in July 1980, May 1981 and May 1982, between 29–41°N along 152°E across the high eddy-energy area of the Kuroshio Extension are used to compute the relative geostrophic transport as a function of depth. The mean eastward geostrophic transport relative to the bottom through this section is 57×106 m3 s−1 (≡57 Sv). These sections were occupied across an array of ten moorings deployed from mid-1980 to mid-1982. Using the 22-month average directly measured currents at 1200 and 4000 m depth for reference levels in the least-square sense, the absolute transport during the 1980–82 period is estimated to be 31±16 (×108 m3 s−1) to the west. This lower bound on the uncertainty of the net transport estimate is based on the uncertainty of the measured mean currents. At 55°W, we use July 1976 and July 1977 deep hydrographic sections to compute the bottom-relative geostrophic transport across the high eddy-energy area of the Gulf Stream. Between 42 and 32°N, it is 32 Sv−1 to the east. Using 24-month average directly measured currents at 4000 m for a reference level, the absolute transport during the 1976–77 period is estimated to be 47±36 Sv to the west. Climatological, wind-driven Sverdrup mass divergence requires a net eastward transport of 17 Sv in the Pacific and 10 Sv in the Atlantic. Thus, the net westward circulations must be maintained by eddy or bottom or thermohaline interaction processes.

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Maria K. Flatau, Lynne Talley, and Pearn P. Niiler

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Changes in surface circulation in the subpolar North Atlantic are documented for the recent interannual switch in the North Atlantic Oscillation (NAO) index from positive values in the early 1990s to negative values in 1995/96. Data from Lagrangian drifters, which were deployed in the North Atlantic from 1992 to 1998, were used to compute the mean and varying surface currents. NCEP winds were used to calculate the Ekman component, allowing isolation of the geostrophic currents. The mean Ekman velocities are considerably smaller than the mean total velocities that resemble historical analyses. The northeastward flow of the North Atlantic Current is organized into three strong cores associated with topography: along the eastern boundary in Rockall Trough, in the Iceland Basin (the subpolar front), and on the western flank of the Reykjanes Ridge (Irminger Current). The last is isolated in this Eulerian mean from the rest of the North Atlantic Current by a region of weak velocities on the east side of the Reykjanes Ridge.

The drifter results during the two different NAO periods are compared with geostrophic flow changes calculated from the NASA/Pathfinder monthly gridded sea surface height (SSH) variability products and the Advanced Very High Resolution Radiometer (AVHRR) SST data. During the positive NAO years the northeastward flow in the North Atlantic Current appeared stronger and the circulation in the cyclonic gyre in the Irminger Basin became more intense. This was consistent with the geostrophic velocities calculated from altimetry data and surface temperature changes from AVHRR SST data, which show that during the positive NAO years, with stronger westerlies, the subpolar front was sharper and located farther east. SST gradients intensified in the North Atlantic Current, Irminger Basin, and east of the Shetland Islands during the positive NAO phase, associated with stronger currents. SST differences between positive and negative NAO years were consistent with changes in air–sea heat flux and the eastward shift of the subpolar front. SST advection, as diagnosed from the drifters, likely acted to reduce the SST differences.

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W. Timothy Liu, Xiaosu Xie, and Pearn P. Niiler

Abstract

Many years of high-resolution measurements by a number of space-based sensors and from Lagrangian drifters became available recently and are used to examine the persistent atmospheric imprints of the semipermanent meanders of the Agulhas Extension Current (AEC), where strong surface current and temperature gradients are found. The sea surface temperature (SST) measured by the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and the chlorophyll concentration measured by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) support the identification of the meanders and related ocean circulation by the drifters. The collocation of high and low magnitudes of equivalent neutral wind (ENW) measured by Quick Scatterometer (QuikSCAT), which is uniquely related to surface stress by definition, illustrates not only the stability dependence of turbulent mixing but also the unique stress measuring capability of the scatterometer. The observed rotation of ENW in opposition to the rotation of the surface current clearly demonstrates that the scatterometer measures stress rather than winds. The clear differences between the distributions of wind and stress and the possible inadequacy of turbulent parameterization affirm the need of surface stress vector measurements, which were not available before the scatterometers. The opposite sign of the stress vorticity to current vorticity implies that the atmosphere spins down the current rotation through momentum transport. Coincident high SST and ENW over the southern extension of the meander enhance evaporation and latent heat flux, which cools the ocean. The atmosphere is found to provide negative feedback to ocean current and temperature gradients. Distribution of ENW convergence implies ascending motion on the downwind side of local SST maxima and descending air on the upwind side and acceleration of surface wind stress over warm water (deceleration over cool water); the convection may escalate the contrast of ENW over warm and cool water set up by the dependence of turbulent mixing on stability; this relation exerts a positive feedback to the ENW–SST relation. The temperature sounding measured by the Atmospheric Infrared Sounder (AIRS) is consistent with the spatial coherence between the cloud-top temperature provided by the International Satellite Cloud Climatology Project (ISCCP) and SST. Thus ocean mesoscale SST anomalies associated with the persistent meanders may have a long-term effect well above the midlatitude atmospheric boundary layer, an observation not addressed in the past.

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W. Timothy Liu, Wenqing Tang, and Pearn P. Niiler

Abstract

The distribution of water vapor in the atmosphere affects climate change through radiative balance and surface evaporation. The variabilities of atmospheric humidity profile over oceans from daily to interannual time scales were examined using nine years of daily and semidaily radiosonde soundings at island stations extending from the Arctic to the South Pacific. The relative humidity profiles were found to have considerable temporal and geographic variabilities, contrary to the prevalent assumption. Principal component analysis on the profiles of specific humidity were used to examine the applicability of a relation between the surface-level humidity and the integrated water vapor; this relation has been used to estimate large-scale evaporation from satellite data. The first principal component was found to correlate almost perfectly with the integrated water vapor. The fractional variance represented by this mode increases with increasing period. It reaches approximately 90% at two weeks and decreases sharply, below one week, down to approximately 60% at the daily period. At low frequencies, the integrated water vapor appeared to be an adequate estimator of the humidity profile and the surface-level humidity. At periods shorter than a week, more than one independent estimator is needed. High-frequency surface humidity can be estimated if additional information on the vertical structure of the humidity profile is available or if the integrated water vapor in the boundary layer, instead of the entire atmospheric column, can be measured accurately by spaceborne sensors.

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Maria Flatau, Piotr J. Flatau, Patricia Phoebus, and Pearn P. Niiler

Abstract

Existing theories of the Madden–Julian oscillation neglect the feedback between the modification of sea surface temperature by the convection and development of a convective cluster itself. The authors show that the convection-generated SST gradient plays an important role in cluster propagation and development. The relative importance of radiative and evaporative fluxes in SST regulation is also discussed. Various Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment and Central Equatorial Pacific Experiment observation platforms are used to estimate the effects of equatorial convection on SST changes during March 1993. The data include drifting buoys and TAO-buoy array measurements, combined with the Navy Operational Global Atmospheric Prediction System analyzed surface wind fields and Geostationary Meteorological Satellite cloud-top temperatures. It is shown that during the equatorial convection episode SST is decreasing under and to the west of the convective heat source due to the large wind velocities and solar flux reduction. To the east of the source, in the convergence region of a Kelvin wave, low wind speeds and high insolation cause the SST to increase.

The data are used to formulate an empirical relationship between wind speed and the 24-h SST change on the equator. Although formulated in terms of wind speed, this relationship implicitly includes radiative effects. This equation is then used in a global circulation model to examine the effect of SST feedback on the behavior of equatorial convection. A series of experiments is performed using an R15 general circulation model of the “aquaplanet” with a zonally symmetric SST distribution. In the case with fixed SSTs, equatorial wind fluctuations have the character of waves propagating around the globe with a phase speed of about 20 m s−1. When the effect of SST modification is included, the fluctuations slow down and become more organized. In addition, a 40–60-day peak appears in the spectral analysis of equatorial precipitation.

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Jerome Cuny, Peter B. Rhines, Pearn P. Niiler, and Sheldon Bacon

Abstract

The general circulation of the Labrador Sea is studied with a dataset of 53 surface drifters drogued at 15 m and several hydrographic sections done in May 1997. Surface drifters indicate three distinct speed regimes: fast boundary currents, a slower crossover from Greenland to Labrador, and a slow, eddy-dominated flow in the basin interior. Mean Eulerian velocity maps show several recirculation cells located offshore of the main currents, in addition to the cyclonic circulation of the Labrador Sea. Above the northern slope of the basin, the surface drifters have two preferential paths: one between the 1000-m and 2000-m isobaths and the other close to the 3000-m isobath. The vertical shear estimated from CTD data supports the presence of two distinct currents around the basin. One current, more baroclinic, flows between the 1000-m and 2000-m isobaths. The other one, more barotropic, flows above the lower continental slope. The Irminger Sea Water carried by the boundary currents is altered as it travels around the basin. Profiling Autonomous Lagrangian Circulation Explorer (PALACE) floats that followed approximately the Irminger Sea Water in the Labrador Sea show signs of isopycnal mixing between the interior and the boundary current in summer–fall and convection across the path of the Irminger Sea Water in winter–spring.

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Luca R. Centurioni, Pearn P. Niiler, and Dong-Kyu Lee

Abstract

Velocity observations near the surface made with Argos satellite-tracked drifters between 1989 and 2002 provide evidence of seasonal currents entering the South China Sea from the Philippine Sea through the Luzon Strait. The drifters cross the strait and reach the interior of the South China Sea only between October and January, with ensemble mean speeds of 0.7 ± 0.4 m s−1 and daily mean westward speeds that can exceed 1.65 m s−1. The majority of the drifters that continued to reside in the South China Sea made the entry within a westward current system located at ∼20°N that crossed the prevailing northward Kuroshio path. In other seasons, the drifters looped across the strait within the Kuroshio and exited along the south coast of Taiwan. During one intrusion event, satellite altimeters indicated that, directly west of the strait, anticyclonic and cyclonic eddies resided, respectively, north and south of the entering drifter track. The surface currents measured by the crossing drifters were much larger than the Ekman currents that would be produced by an 8–10 m s−1 northeast monsoon, suggesting that a deeper westward current system, as seen in historical watermass analyses, was present.

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Niklas Schneider, Emanuele Di Lorenzo, and Pearn P. Niiler

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Hydrographic observations southwestward of the Southern California Bight in the period 1937–99 show that temperature and salinity variations have very different interannual variability. Temperature varies within and above the thermocline and is correlated with climate indices of El Niño, the Pacific decadal oscillation, and local upwelling. Salinity variability is largest in the surface layers of the offshore salinity minimum and is characterized by decadal-time-scale changes. The salinity anomalies are independent of temperature, of heave of the pycnocline, and of the climate indices. Calculations demonstrate that long-shore anomalous geostrophic advection of the mean salinity gradient accumulates along the mean southward trajectory along the California Current and produces the observed salinity variations. The flow anomalies for this advective process are independent of large-scale climate indices. It is hypothesized that low-frequency variability of the California Current system results from unresolved, small-scale atmospheric forcing or from the ocean mesoscale upstream of the Southern California Bight.

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