## 1. Motivation

[2] The ocean's role in climate is primarily determined by its ability to transport heat from the tropics towards the poles. The time-mean oceanic heat transport is of the same order of magnitude as the atmospheric heat transport [e.g., *Trenberth and Caron*, 2001]. The oceanic temperature flux across any given latitude *y* at time *t* is well approximated by:

where the integration is carried out over depth and zonally, V = V(x, y, z, t) is the meridional component of the absolute velocity across the latitude, θ = θ(x, y, z, t) is potential temperature, *ρ* is the density of seawater, and C_{p} is the specific heat capacity of seawater at constant pressure [*Hall and Bryden*, 1982] (hereinafter referred to as HB1982). In general, the value of Q depends on the definition of zero temperature (e.g., degree K or C). In the absence of the net mass flux across zonal sections Q represents a meaningful heat transport [*Montgomery*, 1974]. The time-mean of the oceanic heat transport has been reasonably well addressed by combining hydrographic observations using inverse models. For the World Ocean the estimates of the northward heat transport across 24°N range from 1.5 ± 0.3 PW [*Macdonald and Wunsch*, 1996] to 1.8 ± 0.3 PW [*Ganachaud and Wunsch*, 2000, 2003] (1 PW = 10^{15} W). The remaining uncertainties are mainly attributed to the temporal variability unresolved by the one-time hydrographic measurements. The temporal variability of the oceanic heat transport is not well known. Usually, neither the seasonal cycle nor the eddy-induced variability is resolved by full-depth hydrographic surveys. Most of the ocean circulation models, used to compute the heat flux do not adequately reproduce the mesoscale eddy field. HB1982 showed that the eddy heat flux at 24°N could be as much as 25% of the total and it was the largest error in their estimate.

[3] The oceanic meridional eddy heat transport can be estimated by calculating the deviations from the time-mean heat transport:

where the angle brackets indicate averaging over a certain time interval. It has been perceived that globally eddies play only a minor role in the time-dependent heat transport, but in a number of locations they contribute to the time-mean heat transport [*Jayne and Marotzke*, 2001, 2002] (hereinafter referred to as JM2001 and JM2002). It has been shown that in some locations the meridional eddy heat transport may be compensated by the mean flow, which transports heat in the opposite direction [*Bryan*, 1996]. JM2002 estimated the time-mean eddy heat transport using the output of the Parallel Ocean Climate Model (POCM). They found that the zonally integrated eddy heat transport makes a significant contribution to the total time-mean heat transport in the tropics and in the Antarctic Circumpolar Current. In the northern mid-latitudes they estimated a small eddy heat transport with a peak amplitude of 0.2 PW. JM2002 acknowledged, however, that POCM with an average horizontal resolution of 1/4° and 20 vertical levels does not adequately resolve the eddy field. The eddy kinetic energy, simulated by the model, was too weak by a factor of 4 compared to the altimetry observations. Therefore, there is a need to revisit the subject of the eddy heat transport using a higher-resolution and more realistic model. Moreover, the calculations of the eddy heat transport by JM2002 included variability on all time scales, i.e. everything that is time varying, because they used the mean values of θ and V in the equation (2) for the entire 9-year period of their study, which includes temporal variability that is not due to the mesoscale eddies (e.g., semiannual, seasonal and interannual). In addition to the issue of defining the eddy heat transport in terms of time scales, we want to discuss the effects of eddies not only on the time-mean, as has been done earlier [JM2002; *Bryan*, 1996], but also on the temporal variability of the heat transport.

[4] In this paper we revisit the findings of JM2002 and present results obtained using high-resolution global-ocean data syntheses from the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project (www.ecco2.org).