An ocean model's response to North Atlantic Oscillation-like wind forcing (original) (raw)
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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...
A Study of the Interaction of the North Atlantic Oscillation with Ocean Circulation
Journal of Climate, 2001
Observed patterns of wind stress curl and air-sea heat flux associated with the North Atlantic oscillation (NAO) are used to discuss the response of ocean gyres and thermohaline circulation to NAO forcing and their possible feedback on the NAO. The observations motivate, and are interpreted in the framework of, a simple mathematical model that couples Ekman layers, ocean gyres, and thermohaline circulation to the atmospheric jet stream. Meridional shifts in the zero wind stress curl line are invoked to drive anomalies in ocean gyres, and north-south dipoles in air-sea flux drive anomalous thermohaline circulation. Both gyres and thermohaline circulation play a role in modulating sea surface temperature anomalies and hence, through air-sea interaction, the overlying jet stream. The model, which can be expressed in the form of a delayed oscillator with ocean gyres and/or thermohaline circulation providing the delay, identifies key nondimensional parameters that control whether the ocean responds passively to NAO forcing or actively couples. It suggests that both thermohaline circulation and ocean gyres can play a role in coupled interactions on decadal timescales. 1 The NAO anomaly fields discussed here were computed by regressing NCEP-NCAR reanalysis fields onto the wintermean (DJF) NAO index of . They correspond to a (Hurrell) NAO index of ϩ1 (See Visbeck et al. 1998).
Decadal Variabilities of the Upper Layers of the Subtropical North Atlantic: An Ocean Model Study
Journal of Physical Oceanography, 1999
Numerical simulations of the Atlantic Ocean during the period 1950 to 1989, using a sigma coordinate, free surface numerical model, show long-term variabilities in the upper ocean subtropical gyre similar to those obtained from observations. The simulations show how westward propagating planetary waves, originated in the eastern North Atlantic, affect interdecadal variabilities of ocean properties such as the Bermuda sea level, the Gulf Stream position and strength, and subsurface temperature anomalies in the western North Atlantic. Special attention is given to the dramatic sea level drop at Bermuda in the early 1970s, which is accompanied by cooling of subsurface layers in the western North Atlantic and a northward shift and weakening of the Gulf Stream. Following these events, between 1970 and 1980, the cold temperature anomalies in the upper layers of the western North Atlantic slowly propagated eastward and downward; the strongest propagating signal in the model is found at 200-m depth, suggesting that advection of anomalies downstream by the Gulf Stream current and changes in winter mixing are involved. Significant correlations were found between the sea level anomalies at Bermuda and sea level anomalies in the eastern North Atlantic up to eight years earlier. Sensitivity experiments with different atmospheric forcing fields are used to study the ocean response to observed sea surface temperature and wind stress anomalies. It is shown that on decadal timescales, the ocean model responds in a linear fashion to the combined effect of SST and wind stress anomalies, a fact that might be exploited in future climate prediction studies.
A Damped Decadal Oscillation in the North Atlantic Climate System
Journal of Climate, 2003
A simple stochastic atmosphere model is coupled to a realistic model of the North Atlantic Ocean. A northsouth SST dipole, with its zero line centered along the subpolar front, influences the atmosphere model, which in turn forces the ocean model by surface fluxes related to the North Atlantic Oscillation. The coupled system exhibits a damped decadal oscillation associated with the adjustment through the ocean model to the changing surface forcing. The oscillation consists of a fast wind-driven, positive feedback of the ocean and a delayed negative feedback orchestrated by overturning circulation anomalies. The positive feedback turns out to be necessary to distinguish the coupled oscillation from that in a model without any influence from the ocean to the atmosphere. Using a novel diagnosing technique, it is possible to rule out the importance of baroclinic wave processes for determining the period of the oscillation, and to show the important role played by anomalous geostrophic advection in sustaining the oscillation.
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.
Journal of Physical Oceanography, 2006
As discussed in Part I of this study, the magnitude of the stochastic component of wind stress forcing is comparable to that of the seasonal cycle and thus will likely have a significant influence on the ocean circulation. By forcing a quasigeostrophic model of the North Atlantic Ocean circulation with stochastic wind stress curl data from the NCAR CCM3, it was found in Part I that much of the stochastically induced variability in the ocean circulation is confined to the western boundary region and some major topographic features even though the stochastic forcing is basinwide. This can be attributed to effects of bathymetry and vorticity gradients in the basic state on the system eigenmodes. Using generalized stability theory (GST), it was found in Part I that transient growth due to the linear interference of nonnormal eigenmodes enhances the stochastically induced variance. In the present study, the GST analysis of Part I is extended and it is found that the patterns of wind stress curl that are most effective for inducing variability in the model have their largest projection on the most nonnormal eigenmodes of the system. These eigenmodes are confined primarily to the western boundary region and are composed of long Rossby wave packets that are Doppler shifted by the Gulf Stream to have eastward group velocity. Linear interference of these eigenmodes yields transient growth of stochastically induced perturbations, and it is this process that maintains the variance of the stochastically induced circulations. Analysis of the large-scale circulation also reveals that the system possesses a large number of degrees of freedom, which has significant implications for ocean prediction. Sensitivity studies show that the results and conclusions of this study are insensitive and robust to variations in model parameters and model configuration.
Dynamics of Atmospheres and Oceans, 2003
A simplified coupled ocean-atmosphere model, consisting of a one-layer bidimensional ocean model and a one-layer unidimensional energy balance atmospheric model [J. Clim. 13 (2000) 232] is used to study the unstable interactions between zonal winds and ocean gyres. In a specific range of parameters, decadal variability is found. Anomalies, quite homogeneous zonally, show small-scale wavelength in latitude: perturbations emerge and grow at the southern limb of the intergyre boundary and propagate southward before decaying. The wind stress anomalies are proportional to the meridional gradient of the atmospheric temperature anomalies: this ratio acts as a positive amplification factor, as confirmed by a parameter sensitivity analysis. Assuming zonally-averaged anomalies harmonic in the meridional direction, a very simple analytical model for the perturbations is derived, based on forced Rossby wave adjustment of the western boundary current and its associated anomalous heat transport: it accounts for the scale selection, the growth and the southward propagation of sea surface temperature anomalies in the subtropical gyre. The latter is not only due to the slow advection by the mean current, but to a prevailing mechanism of self-advecting coupled oceanic and atmospheric waves, out of phase in latitude. Relevance to the observational record is discussed.
Decadal variability of the North Atlantic in an ocean general circulation model
J. Geophys. Res., 1994
Climatic fluctuations on a decadal timescale in the North Atlantic in a global ocean general circulation model were considered. The analysis was carried out for the 3800-year stochastic forcing simulation of Mikolajewicz and Maier-Reimer in which the Hamburg Large-Scale Geostrophic ocean model was driven by monthly climatologies of wind stress, air temperature, and freshwater flux with superimposed white noise freshwater fluxes with an amplitude of about 16 mm/month. We applied a Principal Oscillation Pattern analysis to the vector time series of the upper level salinity fields, so that the examined fluctuations appear as estimated eigenmodes of the system. In addition to an oscillation with a period of 320 years as already described by Mikolajewicz and Maier-Reimer, we found a broadband Principal Oscillation Pattern with a timescale of the order of 10 to 40 years. It describes the generation of salinity anomalies in the Labrador Sea and the following discharge into the North Atlantic. In sensitivity experiments we clarified that the source of the variability lies in the Labrador Sea and showed that the generation of the salinity anomalies is mainly due to an undisturbed local integration of the white noise freshwater fluxes.
Quarterly Journal of The Royal Meteorological Society, 2003
Lead–lag Maximum Covariance Analysis (MCA) between National Centers for Environmental Prediction re-analysis sea surface temperature (SST) and 500 hPa geopotential-height fields shows that autumn tropical Atlantic SST anomalies are significantly linked with the following-winter North Atlantic Oscillation (NAO). The ability of the Météo-France atmospheric general circulation model ARPEGE to reproduce this relationship is tested, by forcing it with autumn tropical SST anomalies derived from lead–lag MCA analysis results. The autumn SST forcing induces a strong wave-like simultaneous response in October and November. The occurrence of the autumn weather regimes is also affected, in agreement with the significant spatial correlation of the midlatitude part of the wave response with the NAO pattern. By coupling the model with a slab ocean in midlatitudes, we show that the thermal coupling between the ocean and the atmosphere allows a better representation of the midlatitude part of the response. A negative autumn tropical SST anomaly triggers an interaction between the midlatitude SST, the low-frequency circulation and the storm-track activity, which reinforces and maintains a positive phase of the NAO until winter. Copyright © 2003 Royal Meteorological Society
Causes of Atlantic Ocean Climate Variability between 1958 and 1998*
Journal of Climate, 2000
Numerical experiments are performed to examine the causes of variability of Atlantic Ocean SST during the period covered by the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis (1958-98). Three ocean models are used. Two are mixed layer models: one with a 75-m-deep mixed layer and the other with a variable depth mixed layer. For both mixed layer models the ocean heat transports are assumed to remain at their diagnosed climatological values. The third model is a full dynamical ocean general circulation model (GCM). All models are coupled to a model of the subcloud atmospheric mixed layer (AML). The AML model computes the air temperature and humidity by balancing surface fluxes, radiative cooling, entrainment at cloud base, advection and eddy heat, and moisture transports. The models are forced with NCEP-NCAR monthly mean winds from 1958 to 1998. The ocean mixed layer models adequately reproduce the dominant pattern of Atlantic Ocean climate variability in both its spatial pattern and time dependence. This pattern is the familiar tripole of alternating zonal bands of SST anomalies stretching between the subpolar gyre and the subtropics. This SST pattern goes along with a wind pattern that corresponds to the North Atlantic Oscillation (NAO). Analysis of the results reveals that changes in wind speed create the subtropical SST anomalies while at higher latitudes changes in advection of temperature and humidity and changes in atmospheric eddy fluxes are important. An observational analysis of the boundary layer energy balance is also performed. Anomalous atmospheric eddy heat fluxes are very closely tied to the SST anomalies. Anomalous horizontal eddy fluxes damp the SST anomalies while anomalous vertical eddy fluxes tend to cool the entire midlatitude North Atlantic during the NAO's high-index phase with the maximum cooling exactly where the SST gradient is strengthened the most. The SSTs simulated by the ocean mixed layer model are compared with those simulated by the dynamic ocean GCM. In the far North Atlantic Ocean anomalous ocean heat transports are equally important as surface fluxes in generating SST anomalies and they act constructively. The anomalous heat transports are associated with anomalous Ekman drifts and are consequently in phase with the changing surface fluxes. Elsewhere changes in surface fluxes dominate over changes in ocean heat transport. These results suggest that almost all of the variability of the North Atlantic SST in the last four decades can be explained as a response to changes in surface fluxes caused by changes in the atmospheric circulation. Changes in the mean atmospheric circulation force the SST while atmospheric eddy fluxes dampen the SST. Both the interannual variability and the longer timescale changes can be explained in this way. While the authors were unable to find evidence for changes in ocean heat transport systematically leading or lagging development of SST anomalies, this leaves open the problem of explaining the causes of the low-frequency variability. Possible causes are discussed with reference to the modeling results. * Lamont-Doherty Earth Observatory Contribution Number 6004.