DISCUSSION

Comparison with Model Simulation

Heat Transport Mechanisms: Horizontal and Vertical Cells

Comparison with existing direct meridional heat transport estimates at 24° in the Pacific

World ocean meridional heat flux across 24°N

Comparison with Model Simulation

A surprising result from this analysis is the large amplitude of the inferred seasonal cycle of oceanic heat flux across 24° in the Pacific, including the nonintuitive finding of a negative (southward) heat flux in winter. The annual heat flux cycle is dominated by the Ekman heat flux variation, with a secondary influence from the Kuroshio annual cycle. The same basic results are also found in the POP numerical model simulation.

(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) QEK 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.

The QK in the ETC from both the model simulations and the observations resemble their respective volume transport cycles (Lee et al., 2000). They both have distinct summer maximum, but the observed QK seasonal amplitude is about twice that simulated by the model. The smaller amplitude of the Kuroshio seasonal heat flux cycle in the model is due to the smaller amplitude of the modeled transport in the ETC (1 Sv) compared to the observed seasonal amplitude (~2 Sv). Although the mean transport of Kuroshio in the ETC simulated by the model is slightly larger (23 Sv) than the one (21.8 Sv) derived from the 7-year SLD calibrated by the PCM-1 measurements, the simulated Kuroshio heat flux is smaller than the observation because of a cooler flow-averaged Kuroshio temperature (18.35°C in POP vs. 19.72°C by PCM-1) and a warmer cross-section averaged potential temperature of the interior in the model (4.45°C in POP vs. 3.78°C in Levitus94 and high resolution climatologies).

The bimonthly mean cumulative QI from the eastern boundary to the Ryukyu Islands. The much larger seasonal cycle of QI in POP is confined mostly to the western boundary layer off the Ryukyus with little interior seasonal variation, suggested by the more divergent of the accumulated QI curves near the western boundary, and less in the interior ocean. Consistent with Gill and Niiler (1973) and Anderson et al. (1979) this small baroclinic adjustment of the subtropical gyre on seasonal time scales is because of the slow propagation of baroclinic Rossby waves at this latitude. Interestingly, while the modeled seasonal signal of the Kuroshio heat flux (QK) in the ETC is smaller than in the observations, the seasonal signal of QI to the east of the Ryukyus is larger than in the observations. The deficiency of the model in simulating the seasonal signal of the heat transport in the ETC and to the east of the Ryukyus may thus be related to the local dynamics in the western boundary region.

Bimonthly cumulative geostrophic baroclinic heat flux in the interior ocean from POP, Levitus94, and high resolution climatology. For intercomparison, all the March-April mean in three data sets are plotted as dashed curves and the January-February mean in Pop as dotted, which forms the largest seasonal variation with March-April mean.

Heat Transport Mechanisms: Horizontal and Vertical Cells

Based on the P03 trans-Pacific section data at 24°N, Bryden et al. (1991) demonstrated that the northward meridional heat flux is due half to the zonally averaged, vertical meridional circulation cell and half to a horizontal circulation cell. Since the POP model simulation is shown to give promising results in comparison to the observations in the western boundary (here includes the east of the Ryukyus) and the trans-Pacific heat flux at 24°N, we can use the model results here to study the heat flux mechanisms in relation to the seasonal variation.

The figure below shows the seasonal variation of the heat flux from H_v and HH. A striking feature is the dominance of Hv in the total heat flux variation while HH contributes an almost constant value of about 0.22 PW throughout the year. The resemblance of the Hv and QEK seasonal variation indicates that they are closely related.

POP simulations at 24°N. Total meridional heat transport (solid) and heat transports by vertical (dashed) and horizontal (dotted) cells.

To understand the mechanism of meridional heat flux across 24°N, it is better to look at the volume transports in temperature classes (figure below). A clear impression from the figure below is the similarity between the annual and April-May mean, which explains why the heat flux in April and May is so close to the annual average. The modeled transport distribution in the temperature classes compares remarkably well with the P03 transect data from Bryden et al. (1991) (their Fig. 5 and 6).

Modeled net northward transport (Sv) across 24°N in the Pacific as a function of temperature.

The southward flow between 1.5-2.5°C in the figure above can be related to the North Pacific Deep Water at 1.05-1.9°C in the P03 section (Bryden et al., 1991), which was suggested as a return flow formed by the mixing of the Antarctic Intermediate and Bottom Waters flowing northward across 24°N. But its large seasonal variation (9 Sv) suggests that not all the southward flow at about 2°C in the model can be viewed as the thermohaline circulation but it should be related to higher frequency forcing. The almost identical transport variations shown by the solid curves in the figure below (panel b) for different temperature classes, and the similar transport variation of the deep ocean currents at temperature colder than 2°C, indicate that the ocean adjusts to the large-scale forcing barotropically on the seasonal time scale, consistent with theoretical and model studies (Gill and Niiler, 1973; Anderson et al., 1979).

POP simulation at 24°N.
(a) volume transport of waters of 1.5 to 2.5°C.
(b) Ekman transport and transports of various temperature classes in the interior ocean east of the Ryukyus. Dashed: transport colder than 2°C. Bold curves (transports colder than 6°C and 16°C) mark the range which most close to the reverse of Ekman transport.
(c) Seasonal transport anomalies of the Kuroshio (solid, ETC + Taiwan Strait) and the balance of Ekman transport and transport colder than 6°C.

Comparison with existing direct meridional heat transport estimates at 24° in the Pacific

World ocean meridional heat flux across 24°N

Since the Indian Ocean is confined to latitudes south of 24°N, the sum of the heat flux in the Pacific and the Atlantic gives the meridional heat flux in the world ocean across the 24°N latitude circle. A new estimate of the global ocean heat flux across 24°N and its seasonal variation can be obtained by combining the results of this study with a similar analysis in the Atlantic (Fillenbaum et al., 1997).

Table 3 compares the annual mean heat flux obtained by these combined studies with previous direct and indirect estimates at this latitude. The mean value obtained from this study is 1.99±0.39 PW, which falls about midway in the range of the reported values of 1.6 to 2.5 PW.

Perhaps the most important contribution from this study is that it provides a first direct estimate of the global seasonal cycle of oceanic heat transport across 24°N. Though both the mean and seasonal range of the heat flux from Hsiung at al. (1989) in the Pacific are larger than our estimates, their seasonal variation of the heat flux across the world ocean is quite similar to our seasonal cycle of the Pacific and Atlantic together. Note that the error bar of Hsiung et al. (1989)'s estimate is as large as 0.7 PW in the Pacific and 0.5 PW in the Atlantic due to the limitation of their indirect method.

(a) 3-month smoothed seasonal variation of meridional heat flux across 24-27°N in the Pacific (this study), Atlantic (from Fillenbaum et al., (1997)), and the world ocean.
(b) Meridional Heat flux across 24-27°N in the Pacific and Pacific + Atlantic, in comparison to Hsiung et al., (1989) using sea surface heat flux and upper ocean heat storage change.
ABSTRACT INTRODUCTION
METHOD AND DATA HEAT TRANSPORT COMPONENTS
DISCUSSION SUMMARY
MAIN PAGE