Low-frequency Atlantic sea level variability (original) (raw)
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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,
Geophysical Research Letters, 2013
Annually averaged sea level (1970-2012) measured by tide gauges along the North American east coast is remarkably coherent over a 1700 km swath from Nova Scotia to North Carolina. Satellite altimetry (1993-2011) shows that this coherent interannual variability extends over the Middle Atlantic Bight, Gulf of Maine, and Scotian Shelf to the shelf break where there is a local minimum in sea level variance. Comparison with National Center for Environmental Prediction reanalysis winds suggests that a significant fraction of the detrended sea level variance is forced by the region's along-shelf wind stress. While interannual changes in sea level appear to be forced locally, altimetry suggests that the changes observed along the coast and over the shelf may influence the Gulf Stream path downstream of Cape Hatteras.
Sea level changes in the North Atlantic by solar forcing and internal variability
Climate Dynamics, 2002
Sea level change due to variations in the thermohaline structure of the North Atlantic has been calculated using a coupled ocean-atmosphere model of intermediate complexity (ECBilt). Two 1000-year simulations are made, one using a constant solar forcing and one using an estimate of historic variations in solar activity. In the solar forced simulation sea level variations are a proxy for climate variations. Anomalies in sea surface temperature (SST) of the northern North Atlantic are generated by the solar radiation changes. These SST anomalies modulate the ocean thermohaline circulation (THC), affecting surface salinities in the northern North Atlantic which are subsequently advected to the deep ocean. The associated deep ocean geopotential thickness anomalies dominate sea level in the North Atlantic and are advected southwards with the overturning circulation. Sea level change in the solar forced simulation is primarily an indirect response to solar radiation changes, which modulate the THC. In the unforced run, changes in the THC affect sea level in a similar way. However, in this simulation THC variability is no longer generated by sea surface temperature variations but by sea surface salinity variations, resulting from internal climate dynamics. The present results will aid in analyses of reconstructed low-frequency sea-level variations based on proxy data.
The role of heating, winds, and topography on sea level changes in the North Atlantic
Journal of Geophysical Research: Oceans, 2016
Seasonal and interannual‐to‐decadal variations of large‐scale altimetric sea surface height (SSH) owing to surface heating and wind forcing in the presence of topography are investigated using simplified models. The dominant forcing mechanisms are time scale dependent. On the seasonal time scale, locally forced thermosteric height explains most of the SSH variance north of 18°N. First‐mode linear long baroclinic Rossby waves forced by changes in the winds and eastern boundary conditions explain most of the variance between 10°N and 15°N and are also important east of Greenland. On interannual‐to‐decadal time scales, local thermosteric height remains important at several locations in the middle and high latitudes. A topographic Sverdrup response explains interannual‐to‐decadal SSH between 53°N and 63°N east of Greenland. Farther south, the linear Rossby wave model explains SSH variations on interannual‐to‐decadal time scales between 30°N and 50°N from mid‐basin to the eastern boundar...
Progress In Oceanography
Simulations with a coarse-resolution global ocean model during 1958 - 2004 are analyzed to understand the inter-annual and decadal variability of the North Atlantic. Analyses of Empirical Orthogonal Functions (EOFs) suggest relationships among basin-scale variations of sea surface height (SSH) and depth-integrated circulation, and the winter North Atlantic Oscillation (NAO) or the East Atlantic Pattern (EAP) indices. The linkages between the atmospheric indices and ocean variables are shown to be related to the different roles played by surface momentum and heat fluxes in driving ocean variability. In the subpolar region, variations of the gyre strength, SSH in the central Labrador Sea and the NAO index are highly correlated. Surface heat flux is important in driving variations of SSH and circulation in the upper ocean and decadal variations of the Atlantic Meridional Overturning Circulation (AMOC). Surface momentum flux drives a significant barotropic component of flow and makes a ...
Deep Sea Research Part II: Topical Studies in Oceanography, 2002
The large-scale sea-level variations in the subtropical north-eastern Atlantic Ocean are characterised by a predominant seasonal fluctuation. Analysis of 6 years (1993-98) of combined TOPEX/POSEIDON and ERS-1/2 altimetric data together with data of concurrent oceanic surface geophysical fields (ECMWF net heat fluxes, ECMWF winds, and Reynolds sea-surface temperature) reveals that the dominant physical forcing of sea-level variations on an annual timescale is the air-sea net heat fluxes. This local forcing generates a seasonal ocean steric response, whose amplitude (of the order of 4-5 cm) varies spatially within the studied area and temporally over the 6-year period. The residual variability is mainly characterised by a non-seasonal signal, which is observed northwards of the Azores Current (B341N), and its time variation resembles that of the North Atlantic Oscillation (NAO). A combination of steric effects (wind-induced heat fluxes) and wind effects (barotropic Sverdrup response) appear to explain this signal. Some evidence of a local Ekman pumping response is also found regionally, both in the seasonal and non-seasonal variations of the observed sea level. At the interannual timescale, an overall sea-level trend with a mean rate of 0.5 cm/ year is detected. However, the magnitude of the trend varies locally, so that the entire sea surface appears to undergo a general tilt, which extends beyond the studied area. Superimposed on this long-term trend, a sharp temporal sea-level rise occurs in 1995, mainly between the Azores and Madeira Islands. All together, these low-frequency sea-level variations seem to co-vary with the large-scale sea-surface temperature (SST) variations. This implies a comparable evolution of the sea level with the upper-ocean thermal content, which suggests a contribution from steric effects. A weaker situation is observed in the northern area, indicating a possible low-frequency wind-forcing-enhanced contribution. r
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.
Scientia Marina, 2000
Data collected at 42ºN, 10ºW in the intergyre region of the Northeast Atlantic show significant year to year variability in the T-S characteristics of the upper 800m of the water column. Taking salinity values on the σ θ = 27.1 kg m -3 isopycnal as representative of the Eastern North Atlantic Central Water mass it was found that the variability correlates well with the wind stress at 43ºN, 11ºW, with cumulative river discharge (which we take as an index of precipitation over the ocean) and with the NAO (which is an index of the strength and position of storm tracks and the state of the evaporationprecipitation balance). The covariation illustrates the close coupling between water mass formation and climate in the North Atlantic, where climate changes affect the deep ventilation by which ENACW is formed and the evaporation-precipitation balance from which the T-S signature results. Hints of a 20 year cycle in the ocean correlate with a 20 year periodicity in the NAO. It remains to be established whether there is a feedback mechanism by which water mass anomalies affect the climate and the intensity and variation of the NAO pattern, and the extent to which upper ocean observations can be used as an indicator of future climate trends.