Dynamical Response of the Oceanic Eddy Field to the North Atlantic Oscillation: A Model Data Comparison (original) (raw)
Related papers
2004
Observational studies based on altimeter data have shown that in many regions of the World Ocean the oceanic eddy kinetic energy (EKE) significantly varies on interannual timescales. Comparing altimeter-based EKE maps for 1993 and 1996 discuss a significant meridional redistribution of EKE in the North Atlantic and speculated about the possible influence of the NAO cycle. This hypothesis is examined using 7 years of T/P altimeter data and three 1/6 o -resolution Atlantic Ocean model simulations from the French CLIPPER numerical experiment performed over the period 1979-2000. The subpolarsubtropical meridional contrast of EKE actually appears to vary on interannual timescales in the real ocean, and the model reproduces it realistically. The NAO cycle forces the meridional contrast of energy input by the wind. Our analysis suggests that after 1993, the large amplitude of the NAO cycle induces an adjustment of the large-scale circulation (Gulf Stream/North Atlantic Current), of its baroclinically unstable character, and, in turn, of the distribution of EKE. Model results suggest that before 1994, most NAO-like fluctuations were not followed by EKE redistributions, probably because NAO oscillations were weaker. Strong NAO index modulations are followed by gyre-scale EKE fluctuations with a 9-10 months lag, suggesting that complex, nonlinear adjustment processes are involved in this oceanic adjustment.
Observational studies based on altimeter data have shown that in many regions of the World Ocean the oceanic eddy kinetic energy (EKE) significantly varies on interannual timescales. Comparing altimeter-based EKE maps for 1993 and 1996 discuss a significant meridional redistribution of EKE in the North Atlantic and speculated about the possible influence of the NAO cycle. This hypothesis is examined using 7 years of T/P altimeter data and three 1/6 o -resolution Atlantic Ocean model simulations from the French CLIPPER numerical experiment performed over the period 1979-2000. The subpolarsubtropical meridional contrast of EKE actually appears to vary on interannual timescales in the real ocean, and the model reproduces it realistically. The NAO cycle forces the meridional contrast of energy input by the wind. Our analysis suggests that after 1993, the large amplitude of the NAO cycle induces an adjustment of the large-scale circulation (Gulf Stream/North Atlantic Current), of its baroclinically unstable character, and, in turn, of the distribution of EKE. Model results suggest that before 1994, most NAO-like fluctuations were not followed by EKE redistributions, probably because NAO oscillations were weaker. Strong NAO index modulations are followed by gyre-scale EKE fluctuations with a 9-10 months lag, suggesting that complex, nonlinear adjustment processes are involved in this oceanic adjustment.
Journal of Physical Oceanography, 2005
The distribution of surface eddy kinetic energy (EKE) depicts main oceanic surface circulation features. The interannual variability of EKE and associated geostrophic velocity anomalies in the North Atlantic Ocean were analyzed to describe the variations in oceanic currents between 1993 and 2002. The sea level anomaly maps of the combined TOPEX/Poseidon ϩ ERS-1/2 and TOPEX/Poseidon-alone satellite altimetry data were used to derive EKE. The study focused on the areas of the Gulf Stream extension (GS), North Atlantic Current (NAC), Azores Current (AC), and the northeastern (Rockall Channel and Iceland Basin) and northwestern (Irminger Basin and Labrador Sea) North Atlantic. The interannual variability of the altimetry-derived EKE field in the GS extension area reflected the meridional displacements of the GS core described in earlier studies. The interannual change of EKE in the AC was characterized by high values in 1993-95 followed by lower EKE in subsequent years. The interannual variability of EKE in the NAC area west of the Mid-Atlantic Ridge exhibited an out-of-phase change between the band centered at ϳ47°N and two bands on either side centered at ϳ43°and ϳ50°N. In the Rockall Channel the geostrophic velocity anomalies indicated an intensified northeastward flow in 1993-95 followed by a relaxation in 1996-2000. The EKE band associated with the NAC branch in the Iceland Basin was found to be extended farther west after 1996, possibly following the North Atlantic Oscillation (NAO)-induced shift of the subpolar front. A rise of EKE was observed in the Irminger Basin from 1995 to 1999. This rise may have been associated with large anticyclonic geostrophic velocity anomalies, which indicated significant weakening of the cyclonic circulation in the Irminger Basin after 1996, and/or with possibly intensified eddy generation mechanisms due to the NAO-induced approach of the subpolar front. The interannual change of EKE in the Labrador Sea did not appear to always follow the atmospheric forcing expressed by NAO. Therefore other eddy generation mechanisms in the Labrador Sea can be important.
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).
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...
On eddy scales in the eastern and northern North Atlantic Ocean as a function of latitude
Journal of Geophysical Research, 1990
Infrared satellite images and two ensembles of satellite-tracked buoys have been used to analyze eddy scales in the central and eastern North Atlantic between 35øN and 60øN. Both data sets yield eddy scales which decrease northward from the subtropical Atlantic and can be closely related to the Rossby radius of the first baroclinic mode.
Spatio-Temporal Variability of the Eddy Kinetic Energy in the South Atlantic Ocean
IEEE Geoscience and Remote Sensing Letters, 2014
The spatio-temporal variability of the eddy kinetic energy (EKE) in the South Atlantic Ocean (SAO) is investigated using 19 years of satellite altimetry observations. The EKE in this region presents different significant frequency modes. The interannual to intrannual cycles dominate the EKE variability spectrum. Spatial patterns of the EKE variance were also determined and associated with the propagation of the Agulhas Current eddies across the SAO. At the annual frequency, EKE anomalies are generated in the Agulhas leakage (AL) region and propagate westward. The interannual signal was associated with the Antarctic oscillation and displays a stationary spatial oscillation pattern in the Agulhas eddy corridor (AEC). This is the first time that a full spectral analysis of the EKE variability in the SAO is produced. Results show that the AEC is an important feature in the SAO, with low (high) frequencies associated to the west (east) part of the basin. The AL is a significant source of mesoscale variability to the South Atlantic subtropical gyre. Index Terms-Agulhas eddy corridor (AEC), Agulhas leakage (AL), Eddy kinetic energy (EKE), South Atlantic Ocean (SAO).
Eddies in the northeastern North Atlantic: Statistics from observations from a moving ship
Journal of Geophysical Research, 1997
Statistics of currents and hydrographic data collected from a moving ship in the northeastern North Atlantic have been used to investigate the role of eddies in the circulation of the ocean. The mean currents show, besides a North Atlantic Current of about 12 cm s -1, a significant westward flow, perhaps 4 cm s -1 just north of the Azores. Eddy kinetic energies in the Western Basin reach about 1000 cm 2 s -2, while there is indication of a weak secondary maximum of about 300 cm 2 s -2 in the Eastern Basin. Both the eddy kinetic energy and the thermohaline variability show a strong asymmetry with greater variability on the south side of the main branch of the North Atlantic Current at about 51øN. The momentum budget of the North Atlantic current shows a dominant geostrophic balance with terms of order l0 cm s -1. The gradients of the Reynolds stresses contribute about 0.1 cm s -1 and the nonlinear terms less than 0.01 cm s -1. The residual meridional velocity implied by the meridional heat transport is of order 3 cm s -1. The eddy scale was estimated by means of the average zero-crossing length scale of the transverse velocity correlation function to be 46 km. There is significant interannual variability of both mean conditions and mesoscale variability in the region of the North Atlantic Current. The northward eddy heat transport appears to be zero in the mixed layer but may be a significant proportion of the total oceanic heat transport at the intergyre boundary in the thermocline. Estimates of the magnitudes of eddy diffusivities for both momentum and heat give values of about 104 m 2 s -•. Richardson [19831, Krauss and Kdse [1984], and Br•gge [1994, 1995] have used satellite-tracked drifters; their results show in the NE North Atlantic a maximum east of the Grand Banks, in Richardson's case of over 1000 cm 2 s -2 centered at about 48øN, 40øW, in Krauss and K/ise's case over 600 cm 2 s -2 centered at 50øN, 42øW, and in Briigge's case 1200 cm 2 s -2 at 47øN, 40øW. Their figures also show weak maxima of about 200 cm 2 s -•' in the Eastern Basin, though Krauss and K•ise principally draw attention to the minimum in their data west of Iberia. Wyrtki et al.'s [1976] earlier paper based on ship drift also shows some hint of increased activity off Biscay. Analysis of satellite altimeter data is able to show the variance of sea surface height (SSH), and Cheney et al. [1983] present the analysis of Seasat data from the whole world ocean. There is no clear indication of any secondary maximum in the east of the
Spatial and Temporal Variability of North Atlantic Eddy Field at Scale less than 100km
2019
Ocean circulation is dominated by turbulent geostrophic eddy fields with typical scales ranging from 10 km to 300 km. At mesoscales (¿ 50 km), the size of eddy structures varies regionally following the Rossby radius of deformation. The variability of the scale of smaller eddies is not well known due to the limitations in existing numerical simulations and satellite capability. But it is well established that oceanic flows (¡ 50km) generally exhibit strong seasonality. In this study, we present a basinscale analysis of coherent structures down to 10 km in the North Atlantic Ocean using two submesoscale-permitting ocean models, a NEMO-based North Atlantic simulation with a horizontal resolution of 1/60 (NATL60) and an HYCOM-based Atlantic simulation with a horizontal resolution of 1/50 (HYCOM50). We investigate the spatial and temporal variability of the scale of eddy structures with a particular focus on eddies with scales of 10 to 100 km, and examine the impact of the seasonality of submesoscale energy on the seasonality and distribution of coherent structures in the North Atlantic. Our results show an overall good agreement between the two models in terms of surface wavenumber spectra and seasonal variability. The key findings of the paper are that (i) the mean size of ocean eddies show strong seasonality; (ii) this seasonality is associated with an increased population of submesoscale eddies (10-50 km) in winter; and (iii) the net release of available potential energy associated with mixed layer instability is responsible for the emergence of the increased population of submesoscale eddies in wintertime.
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.