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A. J. Meijers, N. L. Bindoff, and J. L. Roberts

1. Introduction Mesoscale eddies play an important role in the oceanic transport of heat and freshwater, in addition to the main thermohaline and upper-ocean circulations. Sea surface height measurements indicate that eddies are ubiquitous over the global ocean, although they are more active in regions of strong, narrow flow and current confluences, such as western boundary currents and the Antarctic Circumpolar Current (ACC) ( Rhines 2001 ). The difficulties inherent in gathering and observing

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Reiner Schlitzer

notation follows that in Schlitzer (2000) . The model is of global extent and uses a nonuniform grid with horizontal resolution ranging between 1° × 1° and 4° × 5° latitude × longitude. High resolution is implemented in regions with narrow currents (Drake Passage, Atlantic Ocean part of the Antarctic Circumpolar Current, the Indonesian and Caribbean archipelagos), along coastal boundaries with strong currents (Florida Current, Gulf Stream, Brazil Current, Agulhas Current, Kuroshio), over steep

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Martin Losch and Patrick Heimbach

overturning circulation (the Gulf Stream/North Atlantic Current) and the lower branch (Deep Western Boundary Current) in part determine the overturning strength (see also Fig. 5 in section 4c ). Both branches are boundary currents (barely resolved in the present configuration) that are strongly influenced by the topography. Because of the chosen color scale, many second-order features appear in the North Atlantic that are difficult to interpret. In section 4 , we will see that these patterns depend

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Dimitris Menemenlis, Ichiro Fukumori, and Tong Lee

cross-shelf wind component in producing coastal sea level changes at low frequency. Consider a semi-infinite sea of uniform depth H bounded on the right by a straight boundary at x = 0. A wind stress parallel to the coast and of magnitude τ y is applied starting at time t = 0. The wind forcing produces an Ekman current to the right of the wind in the Northern Hemisphere, which causes the surface to rise at a constant rate within a distance on the order of the barotropic Rossby radius of

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Peter Huybers, Geoffrey Gebbie, and Olivier Marchal

of F tot = −15 ± 0.1 Sv is prescribed at each latitude. A further transport of F deep = 5 ± 1 Sv of northward transport is prescribed east of the boundary current between 2 and 4 km. The individual transports through each box face are loosely constrained at 0 ± 20, 0 ± 15, and 0 ± 10 Sv in the upper, middle, and deepest layers, respectively (for vertical velocities this corresponds to the upper face). These large transport uncertainties permit the model to freely adjust toward a circulation

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A. Köhl, D. Stammer, and B. Cornuelle

although in the optimized run the rms differences are also significantly smaller below 200 m (see Fig. 3 ). The largest temperature bias can be found in the subpolar North Atlantic and near boundary currents. It appears that one of the main reasons for the remaining model–data inconsistencies with many datasets is caused by incorrect water mass formation processes in the model’s subpolar North Atlantic. This points to the necessity of including overflow parameterizations and mixed layer parameters as

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Lee-Lueng Fu

. (1999) discussed the relation between the resonance and the semiannual Wyrtki jet. The existence of the semiannual basin mode was also noted by Gent et al. (1983) and Clarke and Liu (1993) . The variability at higher intraseasonal frequencies was studied by Han et al. (2001) and Han (2005) through comparisons of model simulations with observations. Enhanced variance in both sea level and zonal current near a period of 90 days was noted in the studies. The resonance mechanism of Cane and

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Victor Zlotnicki, John Wahr, Ichiro Fukumori, and Yuhe T. Song

is converted to geostrophic circulation, is predominantly zonal ( Wunsch 1998 ; Wang and Huang 2004 ) and balanced by pressure gradients across the bottom topography because the current has no meridional boundaries ( Munk and Palmén 1951 ). Modern estimates of the time-averaged volume transport of the ACC range between 120 and 157 Sv [1 Sv ≡ 10 6 m 3 s −1 ; Cunningham et al. (2003) ; Whitworth and Peterson (1985) ; Peterson (1988) has a long list of estimates dating back to 1933]. Many of

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Manfred Wenzel and Jens Schröter

, giving a global rms value of 11.3 cm. The largest deviations (up to ±30 cm) are found in the regions with strong currents, that is, the western boundary currents as well as the ACC. In particular, the signature in the ACC region implies that these currents are represented too broadly by the model. The temporal rms differences between the modeled SSH and the data are shown in Fig. 2 . The global rms value, which is the measure of success in the assimilation, is 2.8 cm; although locally we find higher

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Serguei Sokolov and Stephen R. Rintoul

the southern ACC front (sACCf). A fourth feature, the southern boundary of the ACC (Bdy), marks the southern limit of the current. The fact that the fronts coincide with particular water mass features means that simple phenomenological criteria such as the location of a particular isotherm at a particular depth can be used to locate the fronts (e.g., the PF is often associated with the northern limit of the temperature minimum layer near 200 m). Perhaps surprisingly, the same simple criteria can

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