As in the case of the temperature transport by the Florida Current, the time series of Kuroshio temperature transport in the ETC is found to be dominated by the volume transport variations such that the temperature transport can be estimated from the volume transport with an almost constant flow mean temperature.

Volume and temperature transport of the Kuroshio have large fluctuations on 100-day time scales, caused by perturbations associated with energetic eddies propagating westward from the interior ocean (Zhang et al., 2000).

The seasonal cycle of trans-Pacific heat flux across 24°N has a minimum of southward heat flux, -0.07 PW, in January and February, and becomes stronger in the second half of the year with a maximum of 1.01 PW in July and a secondary maximum of 0.9 PW in November. The annual mean is 0.55 PW and the associated error bar is about 0.21 PW.

Final estimate of trans-Pacific heat flux at 24°N with error bar. (a) through (c) are the Kuroshio, interior geostrophic baroclinic, and Ekman components, with annual means: 1.58±0.07 PW, -1.81±0.15 PW, and 0.78±0.07 PW. (d) is the total heat flux, whose annual mean is 0.55±0.21 PW. The error bar included the upper bound of possible barotropic correction term (0.11 PW) related to the East Ryukyu Current.

The total meridional heat flux at 24°N in the POP numerical simulation compares favorably with the observations in terms of both its seasonal variability and annual mean (below, panel d).

(a) QK from observation (thick) and POP simulation (thin). K+T: includes Kuroshio in the ETC and currents in the Taiwan Strait. K: Kuroshio in the ETC only. (b) QI from Levitus94, the high resolution climatology, and POP model results. (c) Q,sub>EK from observation and POP. Both use ECMWF wind. (d) Total Qnet using Levitus94 and high resolution climatology for the interior ocean and from POP simulation, as the sum of the three components in (a), (b), and (c). The heat transport directly integrated from the modeled absolute v and temperature fields across the whole section is plotted as the thin solid line.

On the annual mean, the vertical and horizontal cells are found to contribute equally to the heat flux, and the deep ocean (>800 m) contribution is negligible, in agreement with Bryden et al. (1991). However, on seasonal time scales, the vertical cell of the heat flux extends much deeper than 800 m. The seasonal heat flux variation is dominated by the vertical cell, while the contribution of the horizontal cell to the heat flux remains nearly constant.

Interestingly, the model results for the annual mean trans-Pacific heat flux and meridional ocean circulation patterns, in terms of the volume transport distribution in temperature classes as well as in vertical and horizontal cells, are very close to those for April-June, which is close to the time of the April-May P03 trans-Pacific section used by Bryden et al. (1991).

The heat flux in the Pacific derived from this study, together with recently updated estimates in the Atlantic from Fillenbaum et al. (1997), yield the first direct estimate of the seasonal cycle of oceanic heat flux across the latitude circle of 24°N. The combined annual mean heat flux is 1.99±0.39 PW. The annual range of the total oceanic heat flux at this latitude is 1.7 PW, with a maximum in August (2.72 PW) and a minimum in February (1.03 PW).