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

Abstract

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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Elise A. Ralph, Kenong Bi, Pearn P. Niiler, and Yves du Penhoat

Abstract

During the Tropical Oceans Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE) intensive observing period (IOP), sustained westerly winds were observed between 20 December 1992 and 10 January 1993 in the area between 155°E and 180°. The oceanic response to this event was monitored by 33 Lagrangian mixed layer drifters, six of which were equipped with SEACAT salinity sensors. The drifters were distributed over several hundred kilometers meridionally and over a zonal extent of 2400 km. During the wind event, the drifters accelerated eastward and formed a strong equatorial jet that was relatively independent of longitude. Following the drifters, the water parcels cooled and became more saline; Sea surface temperature (SST) maps suggest that evaporative cooling occurred.

In order to consider the dynamics and thermodynamics of this jet in more detail, wind stress and buoyancy forcing along the track of each individual drifter were constructed from the TOGA COARE European Centre for Medium-Range Weather Forecasts analysis. The mixed layer depth scale and the zonal pressure gradient were calculated from a linear regression between the acceleration and the wind stress. In the meridional direction, the wind stress was smaller and not coherent with the acceleration at any period. During the December wind burst, the entire western equatorial Pacific cooled and a large-scale zonal temperature gradient with cooler water to the west was established west of the date line. Cooled water was advected to the east during this episode.

A 4-yr-long TOGA–Tropical Atmosphere Ocean (TAO) current meter record at 165° E and the historical dataset from 250 drifters in the western Pacific within 3° of the equator, together with temperature gradients computed from the National Meteorological Center (renamed the National Centers for Environmental Prediction) SST analysis along the equator, were used to compute a time ensemble average heat advection. On average, cooled water was advected along the equator eastward from the “warm” pool, and this occurred when equatorial currents were to the east.

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