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Tong Lee and Peter Cornillon

Abstract

Positions of the Gulf Stream path from 74° to 45°W were obtained from satellite infrared images for the period of April 1982–December 1989. The propagation of meanders between 74° and 70°W was studied through spectral analysis in wavenumber-frequency space, empirical orthogonal function analysis in time and frequency domains, and direct measurements of individual meander properties. Progressive meanders are found to have a broad range of periods from days to years, and wavelengths from about 200 to 1100 km. Good agreement is found between the satellite and Inverted Echo Sounder data for short-period (<80 days) progressive propagation. Retrogressive meanders with wavelengths longer than 1100 km are found to coexist with progressive ones at periods longer than 4 months. The empirical dispersion relation is in qualitative agreement with the linear prediction of a recent equivalent-barotropic,β-plane thin-jet model, the comparison also suggests that topographic β may need to be considered in order to account for the magnitudes of observed retrogressive phase speeds. Amplitude dependence of propagation is observed. with the phase speed decreasing as the amplitude increases. Standing meanders are observed at periods when both progressive and retrogressive propagation are present; their wavelengths fall between those of oppositely traveling meanders. These standing meanders are responsible for the standing wave pattern of the path envelope between Cape Hatteras and 69°W It is argued that they are formed by near-stationary meanders of a similar wavelength but different amplitudes propagating in opposite directions as a result of the combined amplitude-dependent and β effect.

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Tong Lee and Ichiro Fukumori

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Interannual-to-decadal variations of tropical–subtropical mass exchange in the Pacific Ocean are investigated using a near-global ocean general circulation model along with satellite observations of sea level and wind and a data assimilation product. The analysis focuses on the variability of pycnocline transports through the western boundary and interior near 10°N and 10°S. In contrast to time-mean exchange, where boundary and interior pycnocline transports are both equatorward, the variations of boundary and interior pycnocline transports are found to be generally anticorrelated to each other. Moreover, the variation of the boundary pycnocline transport is smaller than that of the interior, again different from time-mean exchange, where the boundary transport at 10°N is substantially larger than that through the interior. Interannual variations of the boundary and interior transports are consistent with near-surface geostrophic flow inferred from sea level data. Interior pycnocline flow into the Tropics is weaker in the 1990s than that in the 1980s, in agreement with recent observations. However, approximately half of it is compensated by an opposite change in boundary flow at 10°N. The results indicate that the interior pathway is more important to interannual and decadal variability of tropical–subtropical exchange than the boundary pathway, despite a much larger time-mean transport of the western boundary current at 10°N. To a large extent, the counteracting tendency of the boundary and interior flow and the larger variation of the latter can be explained by the combined effect of variability in off-equatorial wind stress curl in the western Pacific and near-equatorial zonal wind stress. The former changes the strength of horizontal circulation and results in a variation of boundary pycnocline flow that is opposite in direction but comparable in magnitude to that of the interior pycnocline flow. The latter primarily affects the strength of the shallow meridional overturning circulation with net pycnocline flow (mostly in the interior) opposing the surface Ekman flow. The covariability of these two forcings leads to an enhancement of interior transport. The relative variability of boundary and interior pycnocline flow is insensitive to whether the Indonesian Throughflow is present or not.

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Tong Lee and Peter Cornillon

Abstract

Analysis of the Gulf Stream path between 75° and 60°W indicates that the spectral signature of propagating and standing meanders is qualitatively similar to that observed for the upstream region 74°–70°W. Progressive, retrogressive,and standing meanders coexist at periods of several months and longer.

The amplitude-dependent dispersion relation obtained for the region 75°–45°W demonstrates the decrease of phase speed as the amplitude increases; the dependence of phase speed on amplitude is found to be stronger than that on wavelength. The average phase speed decreases with downstream distance primarily due to the downstream increase of meander amplitude. Consequently, a relation between phase speed and wavelength for the region west of 70°W, averaged over all amplitudes, is not uniformly valid for a larger domain. Furthermore, downstream propagating meander troughs are steeper and travel more slowly than meander crests. The average stationary wavelength, 700–800 km for 75°–60°W, is much shorter than that predicted based on an equivalent barotropic,,β-plane thin-jet model.

The most energetic meanders have a period of 46 days and a wavelength of 427 km. The period of the fastest-growing meanders is approximately 40 days, close to the period of the most energetic meanders. The wavelength of the fastest-growing meanders, about 350 km, is shorter than the wavelength of the most energetic meanders.

The New England Seamounts do not have a significant effect on the most energetic meanders. However, meanders having periods either shorter or longer than the period of the most energetic meanders are affected by the seamounts. For long-period meanders, their lateral excursions seem to be constrained by the seamounts.

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Tong Lee and Jochem Marotzke

Abstract

A general circulation model of the Indian Ocean is fitted to monthly averaged climatological temperatures, salinities, and surface fluxes using the adjoint method. Interannual variability is minimized by penalizing the temporal drift from one seasonal cycle to another during a two-year integration. The resultant meridional overturning and heat transport display large seasonal variations, with maximum amplitudes of 18 and 22 (× 106 m3 s−1) for the overturning and 1.8 and 1.4 (× 1015 W) for heat transport near 10°S and 10°N, respectively. A dynamical decomposition of the overturning and heat transport shows that the time-varying Ekman flow plus its barotropic compensation can explain a large part of the seasonal variations in overturning and heat transport. The maximum variations at 10°N and 10°S are associated with monsoon reversal over the northern Indian Ocean and changes of the easterlies over the southern Indian Ocean. An external mode with variable topography has a moderate contribution where the Somali Current and the corresponding gyre reverse direction seasonally. Contribution from vertical shear (thermal wind and ageostrophic shear) is dominant near the southern boundary and large near the Somali Current latitudes. The dominant balance in the zonally integrated heat budget is between heat storage change and heat transport convergence except south of 15°S.

Optimization with seasonal forcings improves estimates of sea surface temperatures, but the annual average overturning and heat transport are very similar to previous results with annual mean forcings. The annual average heat transport consists of roughly equal contributions from time-mean and time-varying fields of meridional velocities and temperatures in the northern Indian Ocean, indicating a significant rectification to the heat transport due to the time-varying fields. The time-mean and time-varying contributions are primarily due to the overturning and horizontal gyre, respectively.

Inclusion of TOPEX data enhances the seasonal cycles of the estimated overturning and heat transport in the central Indian Ocean significantly and improves the estimated equatorial zonal flows but leads to unrealistic estimates of the velocity structure near the Indonesian Throughflow region, most likely owing to the deficiencies in the lateral boundary conditions.

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Cécile Cabanes, Tong Lee, and Lee-Lueng Fu

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The authors investigate the nature of the interannual variability of the meridional overturning circulation (MOC) of the North Atlantic Ocean using an Estimating the Circulation and Climate of the Ocean (ECCO) assimilation product for the period of 1993–2003. The time series of the first empirical orthogonal function of the MOC is found to be correlated with the North Atlantic Oscillation (NAO) index, while the associated circulation anomalies correspond to cells extending over the full ocean depth. Model sensitivity experiments suggest that the wind is responsible for most of this interannual variability, at least south of 40°N. A dynamical decomposition of the meridional streamfunction allows a further look into the mechanisms. In particular, the contributions associated with 1) the Ekman flow and its depth-independent compensation, 2) the vertical shear flow, and 3) the barotropic gyre flowing over zonally varying topography are examined. Ekman processes are found to dominate the shorter time scales (1.5–3 yr), while for longer time scales (3–10 yr) the MOC variations associated with vertical shear flow are of greater importance. The latter is primarily caused by heaving of the pycnocline in the western subtropics associated with the stronger wind forcing. Finally, how these changes in the MOC affect the meridional heat transport (MHT) is examined. It is found that overall, Ekman processes explain a larger part of interannual variability (3–10 yr) for MHT (57%) than for the MOC (33%).

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Dimitris Menemenlis, Ichiro Fukumori, and Tong Lee

Abstract

Green's functions provide a simple yet effective method to test and to calibrate general circulation model (GCM) parameterizations, to study and to quantify model and data errors, to correct model biases and trends, and to blend estimates from different solutions and data products. The method is applied to an ocean GCM, resulting in substantial improvements of the solution relative to observations when compared to prior estimates: overall model bias and drift are reduced and there is a 10%–30% increase in explained variance. Within the context of this optimization, the following new estimates for commonly used ocean GCM parameters are obtained. Background vertical diffusivity is (15.1 ± 0.1) × 10−6 m2 s−2. Background vertical viscosity is (18 ± 3) × 10−6 m2 s−2. The critical bulk Richardson number, which sets boundary layer depth, is Ric = 0.354 ± 0.004. The threshold gradient Richardson number for shear instability vertical mixing is Ri0 = 0.699 ± 0.008. The estimated isopycnal diffusivity coefficient ranges from 550 to 1350 m2 s−2, with the largest values occurring at depth in regions of increased mesoscale eddy activity. Surprisingly, the estimated isopycnal diffusivity exhibits a 5%–35% decrease near the surface. Improved estimates of initial and boundary conditions are also obtained. The above estimates are the backbone of a quasi-operational, global-ocean circulation analysis system.

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Peter Cornillon, Tong Lee, and Gregory Fall

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Eight years of Gulf Stream data were examined to determine the percentage of meander crests that give rise to warm core rings. Because of cloud cover in many of the satellite images it was not possible to associate a specific meander crest with each warm core ring formed, nor was it possible to follow each meander crest from the point at which its amplitude was first detectable to the point at which it left the study area, was absorbed by another meander, or split to form two meanders. Despite these problems, a lower bound of 0.24 could be placed on the probability that a meander crest detaches to form a warm core ring. This was obtained by considering all disturbances in the path of the Gulf Stream that could be tracked over a several day period. If consideration is restricted to disturbances with a length scale the size of warm core rings or larger, the probability of formation increases to 0.40. The authors argue that this is a more interesting number in that the wavelength of the meanders observed rarely (less than 20% of the time) increased by more than 50% of their initial value; hence only ring-scale meander crests can result in warm core rings, of these more than 40% do. Finally, it could be argued that the numbers for troughs forming cold core rings are similar.

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Dimitris Menemenlis, Ichiro Fukumori, and Tong Lee

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Observations and numerical simulations show that winds near Gibraltar Strait cause an Atlantic Ocean to Mediterranean Sea sea level difference of 20 cm peak to peak with a 3-cm standard deviation for periods of days to years. Theoretical arguments and numerical experiments establish that this wind-driven sea level difference is caused in part by storm surges due to alongshore winds near the North African coastline on the Atlantic side of Gibraltar. The fraction of the Moroccan coastal current offshore of the 284-m isobath is deflected across Gibraltar Strait, west of Camarinal Sill, resulting in a geostrophic surface pressure gradient that contributes to a sea level difference at the stationary limit. The sea level difference is also caused in part by the along-strait wind setup, with a contribution proportional to the along-strait wind stress and to the length of Gibraltar Strait and adjoining regions and inversely proportional to its depth. In the 20–360-day band, average transfer coefficients between the Atlantic–Alboran sea level difference and surface wind stress at 36°N, 6.5°W, estimated from barometrically corrected Ocean Topography Experiment (TOPEX)/Poseidon data and NCEP–NCAR reanalysis data, are 0.10 ± 0.04 m Pa−1 with 1 ± 5-day lag and 0.19 ± 0.08 m Pa−1 with 5 ± 4-day lag for the zonal and meridional wind stresses, respectively. This transfer function is consistent with equivalent estimates derived from a 1992–2003 high-resolution barotropic simulation forced by the NCEP–NCAR wind stress. The barotropic simulation explains 29% of the observed Atlantic–Alboran sea level difference in the 20–360-day band. In turn, the Alboran and Mediterranean mean sea level time series are highly correlated, ρ = 0.7 in the observations and ρ = 0.8 in the barotropic simulation, hence providing a pathway for winds near Gibraltar Strait to affect the mean sea level of the entire Mediterranean.

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Seung-Bum Kim, Ichiro Fukumori, and Tong Lee

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Entrainment is an important element of the mixed layer mass, heat, and temperature budgets. Conventional procedures to estimate entrainment heat advection often do not permit the closure of heat and temperature budgets because of inaccuracies in its formulation. In this study a rigorous approach to evaluate the effect of entrainment using the output of a general circulation model (GCM) that does not have an explicit prognostic mixed layer model is described. The integral elements of the evaluation are 1) the rigorous estimates of the temperature difference between mixed layer water and entrained water at each horizontal grid point, 2) the formulation of the temperature difference such that the budget closes over a volume greater than one horizontal grid point, and 3) the apparent warming of the mixed layer during the mixed layer shoaling to account for the weak vertical temperature gradient within the mixed layer.

This evaluation of entrainment heat advection is compared with the estimates by other commonly used ad hoc formulations by applying them in three regions: the north-central Pacific, the Kuroshio Extension, and the Niño-3 areas in the tropical Pacific. In all three areas the imbalance in the mixed layer temperature budget by the ad hoc estimates is significant, reaching a maximum of about 4 K yr−1.

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Ichiro Fukumori, Dimitris Menemenlis, and Tong Lee

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A new basin-wide oscillation of the Mediterranean Sea is identified and analyzed using sea level observations from the Ocean Topography Experiment (TOPEX)/Poseidon satellite altimeter and a numerical ocean circulation model. More than 50% of the large-scale, nontidal, and non-pressure-driven variance of sea level can be attributed to this oscillation, which is nearly uniform in phase and amplitude across the entire basin. The oscillation has periods ranging from 10 days to several years and has a magnitude as large as 10 cm. The model suggests that the fluctuations are driven by winds at the Strait of Gibraltar and its neighboring region, including the Alboran Sea and a part of the Atlantic Ocean immediately to the west of the strait. Winds in this region force a net mass flux through the Strait of Gibraltar to which the Mediterranean Sea adjusts almost uniformly across its entire basin with depth-independent pressure perturbations. The wind-driven response can be explained in part by wind setup; a near-stationary balance is established between the along-strait wind in this forcing region and the sea level difference between the Mediterranean Sea and the Atlantic Ocean. The amplitude of this basin-wide wind-driven sea level fluctuation is inversely proportional to the setup region’s depth but is insensitive to its width including that of Gibraltar Strait. The wind-driven fluctuation is coherent with atmospheric pressure over the basin and contributes to the apparent deviation of the Mediterranean Sea from an inverse barometer response.

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