Sea Surface Height Signals Research Articles

Sea Surface Elevation and Bottom Pressure Anomalies due to Thermohaline Forcing. Part I: Isolated Perturbations*

Sea surface elevation and bottom pressure anomalies due to thermohaline forcing are examined through analytical and numerical models, including Boussinesq and non-Boussinesq models. It is shown that Boussinesq approximations can introduce noticeable errors, depending on the spatial and temporal scales of the perturbations. According to the theory of geostrophic adjustment, when the initial perturbations have horizontal scales comparable to the barotropic radius of deformation, the initial pressure perturbations will be basically retained through the adjustment. On the other hand, if the initial perturbations have horizontal scales much smaller than the barotropic radius of deformation, the initial pressure perturbations will be largely lost. Precipitation has horizontal scales on the order of 10–100 km, much smaller than the barotropic radius of deformation. Thus, for timescales longer than days, the contribution from individual precipitation events to the local free surface elevation and bottom pressure is small and is difficult to identify from satellite data. On the other hand, thermal forcing has horizontal scales comparable to the barotropic radius of deformation, so its long-term contribution to sea surface height anomaly is noticeable and is easily identified from satellite data. Because Boussinesq models induce faulty sea surface height and bottom pressure signals, the errors introduced by these models are noticeable for anomalies in large-scale [O(1000 km)] thermohaline forcing.

Large-Scale Variability in the Midlatitude Subtropical and Subpolar North Pacific Ocean: Observations and Causes

Altimetric data from the 8-yr TOPEX/Poseidon (T/P) mission (Oct 1992–Jul 2000) are used to investigate large-scale circulation changes in the three current systems of the midlatitude North Pacific Ocean: the North Pacific Current (NPC), the Alaska gyre, and the western subarctic gyre (WSG). To facilitate the understanding of the observed changes, a two-layer ocean model was adopted that includes first-mode baroclinic Rossby wave dynamics and barotropic Sverdrup dynamics. The NPC intensified steadily over the T/P period from 1992 to 1998. Much of this intensification is due to the persistent sea surface height (SSH) drop on the northern side of the NPC. A similar SSH trend is also found in the interior of the Alaska gyre. Both of these SSH changes are shown to be the result of surface wind stress curl forcing accumulated along the baroclinic Rossby wave characteristics initiated from the eastern boundary. In addition to the interior SSH signals, the intensity of the Alaska gyre is shown to depend also on the SSH anomalies along the Canada/Alaska coast, and these anomalies are shown to be jointly determined by the signals propagating from lower latitudes and those forced locally by the alongshore surface winds. The WSG changed interannually from a zonally elongated gyre in 1993–95 to a zonally more contracted gyre in 1997–99. This structural change is due to the interannual SSH anomalies within the WSG as a result of the baroclinic Rossby wave adjustment attenuated by eddy dissipation. Along the western boundary of the subpolar North Pacific, variability of the East Kamchatka Current (EKC) and Oyashio is in balance with that of the interior Sverdrup flow on the annual and year-to-year timescales. On the multiyear timescales, the EKC/Oyashio variability is shown to be determined by the baroclinic SSH signals.

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.

Studying Indian ocean typical dynamical phenomena using TOPEX observations

TOPEX altimeter data of 1993 have been analyzed to study the following three types of oceanographic phenomena in the Indian Ocean: (1) sea level variability of the Indian Ocean (20=S to 25=N. 40=E to 100=E): (2) sea surface height signals of the Somali eddy; and (3) sea surface slope variations of the equatorial Indian Ocean (EIO) spanning 5=S to 5=N and 45=E to 95=E. Root‐mean‐square sea level variability revealed the presence of Rossby waves in the southern Indian Ocean. Fast Fourier technique analysis of a few passes near the Somali region is used to study the formation and dissipation of an anticyclonic eddy.

Long baroclinic Rossby waves detected by TOPEX/POSEIDON

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 direct wind forcing.

Surface manifestation of internal tides in the deep ocean: observations from altimetry and island gauges

The sea-surface height signatures of internal tides in the deep ocean, amounting to a few centimeters or less, are studied using two complementary measurement types: satellite altimetry and island tide gauges. Altimetry can detect internal tides that maintain coherence with the astronomical forcing; island gauges can monitor temporal variability which, in some circumstances, is due to internal tides varying in response to changes in the oceanic medium. This latter mechanism is at work at Hilo and other stations on the northern coasts of the Hawaiian Islands. By detecting spatially coherent low-frequency internal-tide modulations, the tide gauges, along with inverted echo sounders at sea, suggest that the mean internal tide is also spatially coherent; satellite altimetry confirms this. At Hawaii and in many other places, Topex/Poseidon altimetry detects mean surface waves, spatially coherent and propagating great distances (> 1000 km) before decaying below background noise. When temporal variability is small, the altimetry (plus information on ocean density) sets useful constraints on energy fluxes into internal tides. At the Hawaiian Ridge, 15 GW of tidal power is being converted from barotropic to first-mode baroclinic motion. Examples elsewhere warn that a simplistic interpretation of the altimetry, without regard to variability, noise, or in situ information, may be highly misleading. With such uncertainties, extension of the Hawaiian results into a usefully realistic estimate of the global internal-tide energy balance appears premature at this time.

Aliased tidal errors in TOPEX/POSEIDON sea surface height data

Alias periods and wavelengths for the M2, S2, N2, K1, O1, and P1 tidal constituents are calculated for TOPEX/POSEIDON. Alias wavelengths calculated in previous studies are shown to be in error, and a correct method is presented. With the exception of the K1 constituent, all of these tidal aliases for TOPEX/POSEIDON have periods shorter than 90 days and are unlikely to be confounded with long‐period sea surface height signals associated with real ocean processes. In particular, the correspondence between the periods and wavelengths of the M2 alias and annual baroclinic Rossby waves that plagued Geosat sea surface height data is avoided. The potential for aliasing residual tidal errors in smoothed estimates of sea surface height is calculated for the six tidal constituents. The potential for aliasing the lunar tidal constituents M2, N2, and O1 fluctuates with latitude and is different for estimates made at the crossovers of ascending and descending ground tracks than for estimates at points midway between crossovers. The potential for aliasing the solar tidal constituents S2, K1, and P1 varies smoothly with latitude. S2 is strongly aliased for latitudes within 50 degrees of the equator, while K1 and P1 are only weakly aliased in that range. A weighted least squares method for estimating and removing residual tidal errors from TOPEX/POSEIDON sea surface height data is presented. A clear understanding of the nature of aliased tidal error in TOPEX/POSEIDON data aids the unambiguous identification of real propagating sea surface height signals. Unequivocal evidence of annual period, westward propagating waves in the North Atlantic is presented.

Obtaining sea surface height signals from ERS‐1 altimeter data

Abstract Satellite‐borne radar altimeter gave ample opportunity for observing dynamical oceanographic features repeatedly over a larger scale. Before getting sea surface height signals altimeter data have to be corrected for a number of effects. Besides applying corrections given in the geophysical data records (GDR), the altimeter profiles must be processed and further corrections must be applied. This paper describes the various corrections, such as editing for rain cell effects, removal of orbit error and local geoid effects, etc., to be applied to the sea surface height observations. These corrections have to be applied in addition to those given in ERS‐1 altimeter operational products to get dynamic sea surface topographic signals. The signals in the sea surface heights obtained in this analysis are of the expected order.

Tracking mesoscale ocean features in the Caribbean Sea using Geosat altimetry

Geosat Exact Repeat Mission (ERM) altimetry data, collected during 1987–1988 over the Caribbean Sea, are used to detect and track the movement of mesoscale sea surface height anomalies. Along‐track derivatives are used to estimate the mean sea surface height. Contour maps of sea surface height (SSH) anomaly are generated for each 17‐day exact repeat period. Two anticyclonic features are observed during April‐July, one each year, with SSH signatures that reached +30 cm. These features initially are observed near the Beata Ridge and propagate westward with an average speed of 15 cm/s. No mesoscale anomalies associated with the Caribbean Current are apparent during other seasons. The formation of the anticyclones coincides with the seasonal increase to maximum wind stress curl and water transport in the central Caribbean. A quasi‐permanent cyclonic eddy is detected near the southwestern portion of the Colombian Basin, confirming model predictions.

Frontal signals east of Iceland from the GEOSAT altimeter

Frontal Sea Surface Height signals were identified in the Iceland‐Faeroe region for height differences from three sets of GEOSAT tracks (July‐May 1987). These signals were verified with in situ measurements and then combined with them for an improved and synoptic estimate of frontal location. The data assimilation context made the weak signals quantitatively useful. Real time analysis of GEOSAT data contributed to a frontal nowcast and forecast scheme.