Kathryn A. Kelly

Western Boundary Currents and Frontal Air–Sea Interaction: Gulf Stream and Kuroshio Extension

Abstract In the Northern Hemisphere midlatitude western boundary current (WBC) systems there is a complex interaction between dynamics and thermodynamics and between atmosphere and ocean. Their potential contribution to the climate system motivated major parallel field programs in both the North Pacific [Kuroshio Extension System Study (KESS)] and the North Atlantic [Climate Variability and Predictability (CLIVAR) Mode Water Dynamics Experiment (CLIMODE)], and preliminary observations and analyses from these programs highlight that complexity. The Gulf Stream (GS) in the North Atlantic and the Kuroshio Extension (KE) in the North Pacific have broad similarities, as subtropical gyre WBCs, but they also have significant differences, which affect the regional air–sea exchange processes and their larger-scale interactions. The 15-yr satellite altimeter data record, which provides a rich source of information, is combined here with the longer historical record from in situ data to describe and compare the curr...

Ocean currents evident in satellite wind data

Satellite-mounted radar scatterometers designed to quantify surface winds over the ocean actually measure the relative motion between the air and the ocean surface. Estimates of the wind stress from conventional surface wind measurements are usually derived neglecting ocean currents. However, when the relative motion is used, the differences in the estimated stress can be as large as 50% near the equator and may even reverse sign during an El Nino. This assertion is supported by the strong relationship between the surface currents measured by the Tropical Atmosphere-Ocean (TAO) array in the Pacific Ocean and the differences between the winds estimated from scatterometer data and those measured by TAO anemometers. The fact that the scatterometer measures relative motion, and not wind alone, makes scatterometer-derived stress a more accurate representation of the boundary condition needed for both atmospheric and oceanic models than stress fields derived neglecting ocean currents.

Upper-Ocean Heat Balance in the Kuroshio Extension Region

Abstract A horizontally two-dimensional mixed-layer model is used to study the upper-ocean heat balance in the Kuroshio Extension region (30°–40°N, 141°–175°E). Horizontal dependency is emphasized because, in addition to vertical entrainment and surface thermal forcing, horizontal advection and eddy diffusion make substantial contributions to changes in the upper-ocean thermal structure in this region. By forcing the model using the wind and heat flux data from ECMWF and the absolute sea surface height data deduced from the Geosat ERM, the mixed-layer depth (hm) and temperature (Tm) changes in the Kuroshio Extension are hindcast for a 2.5-year period (November 1986–April 1989). Both phase and amplitude of the modeled Tm and hm variations agreed well with the climatology. The horizontal thermal patterns also agreed favorably with the available in situ SST observations, but this agreement depended crucially on the inclusion of horizontal advections. Although the annually averaged net heat flux from the atmo...

Variability of the near-surface eddy kinetic energy in the California Current based on altimetric, drifter, and moored current data

Low-pass-filtered velocities obtained from World Ocean Circulation Experiment (WOCE) surface drifters deployed in the California Current off northern California during 1993-1995 have been compared with surface geostrophic velocity estimates made along subtracks of the TOPEX/POSEIDON altimeter and with moored acoustic Doppler current profiler (ADCP) data. To obtain absolute geostrophic velocities, a mean sea surface height (SSH) field was estimated using the mean drifter velocities and historical hydrographic data and was added to the altimetric SSH anomalies. The correlation between collocated drifter and altimetric velocities is 0.73, significant at the 95% level. The component of the drifter velocity which was uncorrelated with the altimetric velocity was correlated with the wind in the Ekman transport sense. Monthly averages of eddy kinetic energy (EKE), estimated using all drifter and altimeter data within the domain (124°-132°W, 33°-40.5°N), show energy levels for the drifters that are about 13% greater than those for the altimeter. Drifter, altimeter, and ADCP measurements all exhibit similar seasonal cycles in EKE, with the altimeter data reaching maximum values of about 0.03 m 2 s -2 in late summer/fall. Wavenumber spectra of the altimeter velocity indicate that the velocity fluctuations were dominated by features with wavelengths of 240-370 km, while the ADCP data suggest that the temporal scales of these fluctuations are at least several months. Between 36° and 40.5°N, the region of monthly maximum EKE migrates westward to about 128°W on a seasonal timescale. This region of maximum EKE coincides with the maximum zonal SSH gradient, with increased EKE associated with increased southward flow. A simple model shows that much of the seasonal cycle of the SSH anomalies can be produced by linear processes forced by the curl of the wind stress, although the model cannot explain the offshore movement of the front.

An Inverse Model for Near-Surface Velocity from Infrared Images

Abstract An inverse model to infer the near-surface velocity from the heat equation was applied to a series of six infrared satellite images from northern California. The inversion used a two-dimensional nondiffusive heat equation with a simple representation of surface heat fluxes and vertical entrainment. The along-isotherm component of the velocity was in the null space of this problem. An overdetermined problem was defined by adding weighted constraints on the energy, divergence and curl of the velocity and representative solutions were chosen from the family of solutions corresponding to a designated misfit level for the heat equation. A series of solutions with an average energy of 250 km2 d−2 and a divergence of 0.36 d−1 compared well with simultaneous Doppler acoustic log (DAL) measurements in regions with strong temperature gradients. The decorrelation time for the solutions was about one day. Horizontal advection accounted for about 40% of the variance of the temporal temperature derivative. The...

Mean flow and variability in the Kuroshio Extension from Geosat altimetry data

Using altimeter data from the Geosat Exact Repeat Mission (ERM), we investigated the mean flow and temporal and spatial variations of the Kuroshio Extension in the region of 140°–180°E and 30°–40°N. Mean surface height profiles were estimated along individual tracks by assuming the velocity profile of the Kuroshio Extension to be Gaussian-shaped and by successively fitting this synthetic currents height profile to the residual height data. Using the mean profiles from ascending and descending tracks, we derived the mean surface height field by an inverse method and obtained the absolute surface height fields for the first 2.5 years of the Geosat ERM. Both the mean and the instantaneous height fields thus derived compared well with the available hydrographic data and the SST patterns from the NOAA satellites. The mean surface height difference across the Kuroshio Extension attains its maximum around 146°E between the two quasi-stationary meanders, and its decrease thereafter is mainly due to large-scale recirculations on the southern side of the Kuroshio Extension. The ratio of the eddy kinetic energy over the mean kinetic energy has a nearly constant value of 1.5–2.0 along the Kuroshio Extension path. Propagation of mesoscale fluctuations in the height fields is generally westward except for the upstream region of the Kuroshio Extension. Effects of deep mean flow and baroclinic shear are found to be important in explaining the observed propagation speeds. In the upstream region of 141°E and 154°E, annual variations in the surface height difference across the Kuroshio Extension(δh) have a September maximum with an average amplitude of 0.2m. For large-scale interannual fluctuations, anomalies in δh are found to be significantly correlated with those of the current axis positions: a larger surface height difference corresponds to a more northerly position of the Kuroshio Extension. The interannual changes in δh are possibly related to the 86/87 ENSO event in the low-latitude Pacific Ocean.

Comparison of velocity estimates from advanced very high resolution radiometer in the Coastal Transition Zone

Two methods of estimating surface velocity vectors from advanced very high resolution radiometer (AVHRR) data were applied to the same set of images and the results were compared with in situ and altimeter measurements. The first method used an automated feature-tracking algorithm and the second method used an inversion of the heat equation. The 11 images were from 3 days in July 1988 during the Coastal Transition Zone field program and the in situ data included acoustic Doppler current profiler (ADCP) vectors and velocities from near-surface drifters. The two methods were comparable in their degree of agreement with the in situ data, yielding velocity magnitudes that were 30–50% less than drifter and ADCP velocities measured at 15–20 m depth, with rms directional differences of about 60°. These differences compared favorably with a baseline difference estimate between ADCP vectors interpolated to drifter locations within a well-sampled region. High correlations between the AVHRR estimates and the coincident Geosat geostrophic velocity profiles suggested that the AVHRR methods adequately resolved the important flow features. The flow field was determined to consist primarily of a meandering southward flowing current, interacting with several eddies, including a strong anticyclonic eddy to the north of the jet. Incorporation of sparse altimeter data into the AVHRR estimates gave a modest improvement in comparisons with in situ data.

Heat Budget in the Gulf Stream Region: The Importance of Heat Storage and Advection

Abstract A simple three-dimensional thermodynamic model is used to study the heat balance in the Gulf Stream region (30°–45°N, 40°–75°W) during the period from November 1992 to December 1999. The model is forced by surface heat fluxes derived from NCEP variables, with geostrophic surface velocity specified from sea surface height measurements from the TOPEX/Poseidon altimeter and Ekman transport specified from NCEP wind stress. The mixed layer temperature and mixed layer depth from the model show good agreement with the observations on seasonal and interannual time scales. Although the annual cycle of the upper-ocean heat content is underestimated, the agreement of the interannual variations in the heat content and the sea surface height are good; both are dominated by the large decrease from 1994 to 1997 and the increase afterward. As expected from previous studies, the surface heat flux dominates the seasonal and interannual variations in the mixed layer temperature. However, interannual variations in t...

Contributions of wind forcing, waves, and surface heating to sea surface height observations in the Pacific Ocean

The dominant processes affecting sea surface height (SSH) variability observed by the TOPEX/Poseidon altimeter vary regionally in the Pacific; baroclinic Rossby waves, equatorially trapped Kelvin waves, steric response to seasonal heating, and the response to wind stress curl forcing are all important. The steric response to surface heating dominates seasonal SSH variability in the subpolar gyre and the eastern subtropical gyre. South of the Kuroshio Extension and south of 20°N in the eastern Pacific, the dominant contribution to SSH is from near-annual period Rossby waves. To quantify the wave energy, observed SSH was assimilated into a kinematic model of westward propagating waves. These waves account for >70% of SSH variance between 10°S and 10°N but only ∼30% between 10°N and 30°N. Although wave energy in the eastern Pacific is correlated with SSH anomalies at the equator, the much larger wave energy in the western Pacific is correlated with wind stress curl, suggesting that the Rossby waves there are locally forced. In addition to these planetary waves, the ocean response to wind forcing via Ekman pumping is observed in several places, specifically in the North Equatorial Current. A quasi-steady topographic Sverdrup balance is detectable over most of the North Pacific at latitudes as low as 10–15°N, as well as in the South Pacific, where it is seen north of 50°S. The decomposition of the SSH signal into propagating waves, an Ekman pumping response, and Sverdrup transport is consistent with the results from an isopycnal numerical model.

Heat Budget in the Kuroshio Extension Region: 1993–99

Abstract Processes responsible for the seasonal and interannual variations of the sea surface temperature as well as of the heat content of the upper ocean (0–400 m) in the Kuroshio Extension region are examined from a 3D advection–diffusion model in finite elements, with an embedded bulk mixed layer. The geostrophic velocity is specified externally from TOPEX/Poseidon altimeter data, and Ekman velocity is specified from NCEP wind stress. The thermal field from the model shows good agreement with observations. While both atmospheric and oceanic processes are required to explain observed nonseasonal SST changes, the interannual heat storage rate is dominated by horizontal advection. In particular, the transition between an elongated and a contracted state of the Kuroshio caused by geostrophic advection has a clear signature in the SST. There is an indication that this process is accompanied by consistent changes in nonseasonal entrainment: when the Kuroshio is in an elongated state and warmer waters are pr...