Decadal Variabilities of the Upper Layers of the Subtropical North Atlantic: An Ocean Model Study (original) (raw)
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An ocean model's response to North Atlantic Oscillation-like wind forcing
Geophysical Research Letters, 1998
The response of the Atlantic Ocean to North Atlantic Oscillation (NAO)-like wind forcing has been investigated using an ocean-only general circulation model coupled to an atmospheric boundary layer model. A series of idealized experiments was performed to investigate the interannual to multi-decadal frequency response of the ocean to a winter wind anomaly pattern. South of 30 N, the sea surface temperature (SST) response of the model was almost exactly in phase with the forcing and largely independent of the forcing frequency suggesting that the subtropical ocean response to the overlying atmosphere is fast and direct. Poleward of 30 N and in particular in the Gulf stream extension region strong SST anomalies lagged the forcing by several years. They were sustained by deep reaching temperature anomalies which were then re-exposed to the atmosphere during the winter season. Overall the strength of the SST response increased slightly with longer forcing periods. In the subpolar gyre, however, the model showed a broad response maximum in the decadal band (12-16 years). The implications for the existence of a decadal coupled mode are discussed.
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.
Journal of Geophysical Research, 1997
Between 1966 and 1995, subsurface temperature data have been collected in the western North Atlantic Ocean using expendable bathythermographs. Data coverage is sparse in both time and space, but evidence for decadal variability in the upper 400 m of the water column is found. The data were averaged by month onto a 2 ø of latitude by 4 ø of longitude grid. Thirty-one quadrangles in the region bounded by 17øN and 43øN and 78øW and 66øW have sufficient data to provide consistent results. Anomaly time series at 0, 100, 200, 300, and 400 m were estimated by subtracting a mean monthly climatology. The individual records were detrended and filtered to highlight the longer-period signals. The analysis resulted in 25-year records (1969-1993) for study. Within the therrnocline of the subtropical gyre and the Gulf Stream at 100 and 200 rn, periods of predominately positive temperature anomaly end in 1971, 1982, and 1990, while periods of negative anomaly end in 1976 and 1985. Only the events ending in 1971, 1976, and 1990 are in the majority of the records at 300 and 400 rn. Most of the events also appear in the sea surface temperature (SST) records but are somewhat masked by significant noise at the surface. Meridional-vertical temperature sections through the subtropical gyre show that transitions from negative to positive anomaly events are characterized by a deepening of the isotherms throughout the section and transitions from positive to negative events by a rising of the isotherms. Significant lateral migration of the axis of the Gulf Stream, although possibly masked by the 2 ø averaging, is not necessary to explain either type of event. The transitions in the SST and 100-m temperature time series occur at essentially the same time as the transitions in an index of the North Atlantic Oscillation (NAO) that has also been detrended (NAO events are also observed at 300 and 400 m as described earlier. Periods of positive subsurface temperature anomaly are coincidental with periods of positive NAO index, and periods of negative subsurface temperature anomaly are coincidental with periods of negative NAO index. Thus earlier results showing connections between the NAO and western Atlantic SST at decadal timescales are now extended to at least 400 m in the water column. Trends were computed from the individual 25-year records. The trends at all depths are predominately negative north of 38øN and positive south of 38øN. Inferences from the horizontal distribution of the trends and results from earlier studies suggest that the 1969-1993 period may be a phase of a 30-to 50-year signal observed in the northern Atlantic since the beginning of the century. [1994], using more extensive data sets than those available to Bjerknes [1964], provided additional evidence to support the earlier findings. Although causal relationships have not yet been firmly established, the SST variability has been shown to be correlated with atmospheric anomalies on the same timescales Deser and Blackman, 1993;. In particular, Bjerknes [1964] and Kushnir [1994] suggested that the SST variability is related This paper is not subject to U.S. copyright. Published in 1997 by the American Geophysical Union.
Journal of Climate, 2000
The Bermuda station ''S'' time series has been used to define the variability of subtropical mode water (STMW) from 1954 to 1995. This record, which shows decadal variability at a nominal period of about 12-14 yr, has been used as a baseline for seeking correlation with large-scale atmospheric forcing and with decadal north-south excursions of the Gulf Stream position defined by the subsurface temperature at 200-m depth. A common time period of 1954-89 inclusive, defined by the data sources, shows a high degree of correlation among the STMW potential vorticity (PV), Gulf Stream position, and large-scale atmospheric forcing (buoyancy flux, SST, and sea level pressure). Two pentads with anomalously small and large STMW PV were further studied and composites were made to define a revised North Atlantic Oscillation (NAO) index associated with the decadal forcing. During years of low PV at Bermuda, the NAO index is low, the Gulf Stream is in a southerly position, and the zero wind stress curl latitude is shifted south as are the composite extratropical winter storm tracks, in comparison to the period of high PV at Bermuda. Because the NAO, Gulf Stream separation latitude, and STMW PV variations are in phase with maximum annually averaged correlation at zero year time lag, the authors hypothesize that all must be either coupled with one another or with some other phenomenon that determines the covariability. A mechanism is proposed that could link all of the above together. It relies on the fact that during periods of high STMW PV, associated with a northerly Gulf Stream and a high NAO, one finds enhanced production of mode water in the subpolar gyre and Labrador Sea. Export of the enhanced Labrador Sea Water (LSW) component into the North Atlantic via the Deep Western Boundary Current can influence the separation point of the Gulf Stream in the upper ocean once the signal propagates from the source region to the crossover point with the Gulf Stream. If the SST signal produced by the 100-km shift of the Gulf Stream along a substantial (1000 km) length of its path as it leaves the coast can influence the NAO, a negative feedback oscillation may develop with a timescale proportional to the time delay between the change of phase of the airsea forcing in the Labrador Basin and the LSW transient at the crossover point. Both a simple mechanistic model as well as a three-layer numerical model are used to examine this feedback, which could produce decadal oscillations given a moderately strong coupling. * Woods Hole Oceanographic Institution Contribution Number 9888.
A Simple Model of the Response of the Atlantic to the North Atlantic Oscillation
Journal of Climate, 2014
The response of an idealized Atlantic Ocean to wind and thermohaline forcing associated with the North Atlantic Oscillation (NAO) is investigated both analytically and numerically in the framework of a reduced-gravity model. The NAO-related wind forcing is found to drive a time-dependent “leaky” gyre circulation that integrates basinwide stochastic wind Ekman pumping and initiates low-frequency variability along the western boundary. This is subsequently communicated, together with the stochastic variability induced by thermohaline forcing at high latitudes, to the remainder of the Atlantic via boundary and Rossby waves. At low frequencies, the basinwide ocean heat content changes owing to NAO wind forcing and thermohaline forcing are found to oppose each other. The model further suggests that the recently reported opposing changes of the meridional overturning circulation in the Atlantic subtropical and subpolar gyres between 1950–70 and 1980–2000 may be a generic feature caused by...
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
Tellus A, 2008
A B S T R A C T Globally forced model simulations with an atmospheric general circulation model of intermediate complexity reveal surface air temperature (SAT) and sea level pressure (SLP) variations at multidecadal time scales. In order to separate the influence of individual ocean basins on the North Atlantic multidecadal variability, we force our model with observed SST data for the period 1856-2000 for Atlantic and Pacific Oceans, separately, while outside the atmosphere is coupled with the ocean via a mixed layer slab model. The experiments indicate the Atlantic Ocean as a principal driver of North Atlantic multidecadal variability, with SAT and SLP highly in phase in the North Atlantic at about 60-70 yr time scale. The Pacific impact is associated to longer period variations in the SLP field over the North Atlantic. We suggest that two distinct physical modes of multidecadal climate variability exist, one of about 70 yr possibly linked with the Atlantic thermohaline circulation, the other linked with the Pacific Ocean and connected to the Atlantic Ocean via Pacific-North America teleconnections. The latter has a time scale of about 80-100 yr.
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.