Steric height variability in the Northern Atlantic on seasonal and interannual scales (original) (raw)
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Comparing the steric height in the Northern Atlantic with satellite altimetry
Ocean Science, 2007
Anomalies of dynamic height derived from an analysis of Argo profiling buoys data are analysed to assess the relative roles of contributions from temperature and salinity over the North Atlantic for the period of 1999-2004. They are compared with dynamic topography anomalies based on TOPEX/Poseidon and Jason altimetry. It is shown that the halosteric contribution to the anomalies of dynamic height is comparable in magnitude to the thermosteric one over the period analyzed. Taking both salinity and temperature into account improves the agreement between zonally averaged trends in the satellite dynamic topography and dynamic height increasing the correlation between them to 0.73 from 0.63 when only temperature variability is taken into account. The implication of this result is that the salinity contribution cannot be neglected in the North Atlantic and that one cannot rely on estimating the thermosteric part by anomalies in the sea surface dynamic topography derived from the satellite altimetry.
Russian Journal of Earth Sciences, 2020
Five data sets were used to estimate steric level fluctuations in the North Atlantic for 2003-2015. We compare estimates made by a combination of altimetry and GRACE gravity data (ALT-GRV) with assessments obtained from vertical density profiles derived from SODA reanalysis, ARMOR, and EN4 objective analyses. We analyze the datasets without linear trends, and the seasonal signals are also removed. The resulting signals demonstrate the steric sea-level anomalies not related to the linear trends and the seasonal cycles and can be connected with short-period intra-annual variability as well as vortex dynamics of the region since mesoscale eddies can transfer heat and salt and influence thereby the thermohaline water structure from the sea surface to the depth. The deep convection, as well as meandering of the currents also influences the variability of residual time series. The steric sea-level fluctuations, obtained from the ARMOR dataset, which incorporates results of satellite observations, shows the best fit for those, derived from ALT-GRV data. The correlation coefficient between ARMOR and ALT-GRV varies between 0.6 and 0.8 over the study region (0.7 on average). Steric sea-level variations derived from SODA or EN4 show good matches with ALT-GRV only for the steric sea-level fluctuations spatially averaged over central regions of the North Atlantic. The discrepancies between the data sets increase northwards and towards the coast. Of the considered data sets, ARMOR is the most suitable for climate studies and research of the sea-level change effects; however, it should be used with caution in the study of the spatial distribution of the steric level.
Seasonal sea surface height variability in the North Atlantic Ocean
Journal of geophysical research, 2000
We investigate the seasonal sea surface height (SSH) variability on large spatial scales in the North Atlantic by using both a numerical simulation and in situ data. First, an ocean general circulation model is run with daily forcing from the European Centre for Medium-Range Weather Forecasts reanalysis. We evaluate the different contributions to the seasonal SSH variability resulting from the surface heat fluxes, advection, salt content variability, deep ocean steric changes, and bottom pressure variability. These terms are compared with estimates from in situ data. North of 20øN, there is an approximate balance between hQ, the air-sea heat flux induced changes in steric height, and SSH variability. The next important component is the advection (its contribution to the annual amplitude is of the order of 1 cm except near the western boundary); other contributions are found to be smaller. Between 10øN and 10øS the advection variability induced by the seasonal wind stress cycle is the primary source of SSH variability. We then compare the sea surface height annual harmonic from TOPEX/Poseidon altimetry with the steric effect from the heat flux and with model and/or in situ estimates of the other terms. In many areas north of 20øN the balance between h and the altimetric SSH seasonal cycle is closed within the Q uncertainty limit of each of the terms of the SSH budget. However, h o and the SSH do not balance each other in the eastern North Atlantic, and the results are sensitive tb the choice of the heat flux product, suggesting that significant errors, typically 20-40 W m-2 for the seasonal cycle amplitude, are present in the meteorological model heat fluxes. 1. Introduction Gill and Niiler's [1973] analysis of seasonal variations of upper ocean temperature identified air-sea heat exchange as the prime agent of the seasonal oceanic heat content change. This is particularly expected to hold for large spatial scales away from strong currents and outside of the tropics and has been verified at a few locations with good sampling, for example, in the northeastern Pacific at station Papa [ Tabata et al., 1986] and at other weather ship sites [Alexander and Deser, 1995]. Direct investigations of the large-scale ocean heat budget also seem to confirm this hypothesis both in the Pacific [Moisan and Niiler, 1998] and in the Atlantic [Cayan, 1992]. Changes in upper ocean temperature and salinity result in a steric contribution to the sea level variability which was observed first by Patullo et al. [1955]. Wang and Koblinsky's [1996] analysis of altimetric sea level data show that this contribution dominates the large-scale seasonal variability in the northeast Atlantic. This is also supported by the work of Chambers et al.
Interannual sea surface height variations in the Atlantic Ocean are examined from 10 years of highprecision altimeter data in light of simple mechanisms that describe the ocean response to atmospheric forcing: 1) local steric changes due to surface buoyancy forcing and a local response to wind stress via Ekman pumping and 2) baroclinic and barotropic oceanic adjustment via propagating Rossby waves and quasi-steady Sverdrup balance, respectively. The relevance of these simple mechanisms in explaining interannual sea level variability in the whole Atlantic Ocean is investigated. It is shown that, in various regions, a large part of the interannual sea level variability is related to local response to heat flux changes (more than 50% in the eastern North Atlantic). Except in a few places, a local response to wind stress forcing is less successful in explaining sea surface height observations. In this case, it is necessary to consider large-scale oceanic adjustments: the first baroclinic mode forced by wind stress explains about 70% of interannual sea level variations in the latitude band 18°-20°N. A quasi-steady barotropic Sverdrup response is observed between 40°and 50°N.
Journal of Geophysical Research, 1998
TOPEX/Poseidon altimetric height is compared with 20 transpacific eddy-resolving realizations of steric height. The latter are calculated from temperature (expendable bathythermograph (XBT)) and salinity (expendable conductivity and temperature profiler (XCTD)) profiles along a precisely repeating ship track over a period of 5 years. The overall difference between steric height and altimetric height is 5.2 cm RMS. On long wavelengths (•, <500 km), the 3.5 cm RMS difference is due mainly to altimetric measurement errors but also has a component from steric variability deeper than the 800 m limit of the XBT. The data sets are very coherent in the long-wavelength band, with coherence amplitude of 0.89. This band contains 65% of the total variance in steric height. On short wavelengths (•, >500 km), containing 17% of the steric height variance, the 3.0 cm RMS difference and lowered coherence are due to the sparse distribution of altimeter ground tracks along the XBT section. The 2.4 cm RMS difference in the basin-wide spatial mean appears to be due to fluctuations in bottom pressure. Differences between steric height and altimetric height increase near the western boundary, but data variance increases even more, and so the signal-to-noise ratio is highest in the western quarter of the transect. Basinwide integrals of surface geostrophic transport from steric height and altimetric height are in reasonable agreement. The 1.9 x 104 m 2 s 'i RMS difference is mainly because the interpolated altimetric height lacks spatial resolution across the narrow western boundary current. A linear regression is used to demonstrate the estimation of subsurface temperature from altimetric data. Errors diminish from 0.8øC at 200 m to 0.3øC at 400 m. Geostrophic volume transport, 0-800 m, shows agreement that is similar to surface transport, with 4.8 Sverdrup (Sv) (106m 3 s -1) RMS difference. The combination of altimetric height with subsurface temperature and salinity proffiing is a powerful tool for observing variability in circulation and wansport of the upper ocean. The continuing need for appropriate subsurface data for verification and for statistical estimation is emphasized. This includes salinity measurements, which significantly reduce errors in specific volrune and steric height. 1. Introduction Changes in the height of the sea surface may be caused either by changes in the mass of water at a given location or by changes in the density at constant mass. The latter, known as steric height change, is thought to dominate sea surface variability at low frequency [e.g. Patullo et al., 1955]. A column of warm water stands higher than a column of cold water of equal mass. In the present work, sea surface height measured by the TOPEX/Poseidon (T/P) satellite altimeter is compared with steric height from a data set consisting of repeating temperature (expendable bathythermograph (XBT)) and temperature/salinity (expendable conductivity and temperature profiler (XCTD)) profiles. The XBT/XCFD data set consists of 20 quarterly eddy-resolving XBT transects, together with sparse XCTDs, along a 12,650 km track during Copyright 1998 by the American Geophysical Union. Paper number 98JC01680. 0148-0227/98/98 JC-01680509.00 the T/P era. The comparison, carried out over spatial scales ranging from tens of kilometers to the basin width, will serve to determine (1) the degree to which sea level height i s, indeed, controlled by upper ocean steric change, (2) an upper bound on errors in the T/P measurements as a function of spatial scale, (3)the limits imposed by across-track spatial resolution in the T/P data (i.e., interpolation errors), and (4) the errors in steric height when salinity information is missing or withheld. In places where the difference between steric height and altimetric height exceeds measurement and interpolation errors, deep-ocean steric change or redistribution of mass must have signlficant effects on sea surface height. The high correlations linking altimetric height to steric height and steric height to subsurface temperature variability permit an inversion of altimetric height in order to estimate temperature as a function of position, depth, and time (T( ). A substantial fraction of subsurface temperature variability can be recovered in this way. The relationship between temperature (or density) variability and altimetric height can be exploited to reduce the sampling deficiencies of 27,947
Annual and interannual variability of sea level in the northern North Atlantic Ocean
Journal of Geophysical Research, 2003
300 cycles) were processed. The annual and interannual signals were examined, and their contribution to the total variance is estimated. Both signals appeared to be responsible for a greatest portion of variability outside the North Atlantic Current (NAC) and its branches. The annual signal manifested consistency with seasonal changes in the heat storage, possibly enhanced by seasonal variations of advection. A considerable interannual change was monitored in the vast areas of the northern North Atlantic Ocean as well as along the NAC. The interannual change in SLA was found to be consistent with the North Atlantic Oscillation (NAO) index when it changed from its positive to its negative phase in 1996. The significant negative correlation coefficient between the annually averaged SLA and NAO index time series was estimated in all dynamically calm areas outside the NAC. Hydrographic data obtained during several cruises for the time interval studied were also investigated and coupled with the variations of SLA and NAO index. The analysis showed a good agreement between the altimeter-derived SLA and dynamic height anomalies suggesting that the sea level changes in the northern North Atlantic have a baroclinic nature. Citation: Volkov, D. L., and H. M. van Aken, Annual and interannual variability of sea level in the northern North Atlantic Ocean,
Low Frequency Change of Sea Level in the North Atlantic Ocean as Observed with Satellite Altimetry
International Association of Geodesy Symposia, 2003
The Topex/Poseidon and ERS-1/2 satellite altimetry missions have revealed significant low-frequency changes of sea level in the North Atlantic Ocean. This work describes the changes that occurred from 1993 to 2001. Decomposition of total variance in three main modes of variabilityannual, inter-annual and high frequency (basically eddies) -has been performed. As the analysis has shown, the annual and inter-annual signals are responsible for a greatest portion of variability in the northern North Atlantic outside the North Atlantic Current and its branches. In the areas where the inter-annual change is found to be significant, the sea level change followed the North Atlantic Oscillation index when it changed from its positive phase to negative in winter
Spatial and long-term variability of steric height in the Nordic Seas
A major task of the GOCINA project is the determination of a Mean Dynamic Topography (MDT) for the region between Greenland and the UK, north and south of the Greenland-Scotland Ridge. The MDT is the temporal average of the ocean's surface as measured by satellite altimetry relative to the geopotential height (or Geoid). The MDT gradient determines the geostrophic part of the surface circulation.
Journal of Physical Oceanography, 2006
Interannual sea surface height variations in the Atlantic Ocean are examined from 10 years of highprecision altimeter data in light of simple mechanisms that describe the ocean response to atmospheric forcing: 1) local steric changes due to surface buoyancy forcing and a local response to wind stress via Ekman pumping and 2) baroclinic and barotropic oceanic adjustment via propagating Rossby waves and quasi-steady Sverdrup balance, respectively. The relevance of these simple mechanisms in explaining interannual sea level variability in the whole Atlantic Ocean is investigated. It is shown that, in various regions, a large part of the interannual sea level variability is related to local response to heat flux changes (more than 50% in the eastern North Atlantic). Except in a few places, a local response to wind stress forcing is less successful in explaining sea surface height observations. In this case, it is necessary to consider large-scale oceanic adjustments: the first baroclinic mode forced by wind stress explains about 70% of interannual sea level variations in the latitude band 18°-20°N. A quasi-steady barotropic Sverdrup response is observed between 40°and 50°N.