## 1. Introduction

[2] Heat transport by ocean currents plays an important role in determining the rate of global climate change and regional climate patterns [*U.S. World Ocean Circulation Experiment*, 1991]. Observations and model experiments show that 24°N is close to the latitude of maximum poleward ocean heat transport [*Vonder Haar and Oort*, 1973; *Carissimo et al.*, 1985; *Semtner and Chervin*, 1992]. At this latitude the oceanic northward heat flux takes place in two basins, the Atlantic and the Pacific. With well-established Florida Current transport estimates, the transatlantic heat flux across this latitude has been extensively investigated, and its annual mean value and seasonal cycle have been quantified [*Hall and Bryden*, 1982; *Roemmich and Wunsch*, 1985; *Gordon*, 1986; *Molinari et al.*, 1990; *Fillenbaum et al.*, 1997]. A thorough study of the heat transport in the Pacific sector is thus necessary to supplement these values and to understand the world ocean heat transport across this latitude circle.

[3] Both indirect and direct methods have been used to estimate the oceanic heat flux through zonal sections using observational data. There are two indirect methods, the “air-sea exchange” method and the “radiation budget” method, the details of which are reviewed by *Bryden and Imawaki* [2001]. The sparsity of observations in the oceanic and atmospheric boundary layer and errors in the bulk formulae used to calculate the sea surface heat fluxes can result in large uncertainties in the estimates of meridional oceanic heat transport using the “air-sea exchange” method [e.g., *Talley*, 1984]. Until recently, results from the radiation method were substantially different from those derived from direct ocean observations [*Bryden*, 1993]. Trenberth and his colleagues [e.g., *Trenberth and Solomon*, 1994] presented more reasonable results using compatible top-of-atmosphere radiation from the Earth Radiation Budget Experiment combined with European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis, the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, and Comprehensive Ocean Atmosphere Data Set (COADS). They showed that earlier residual calculations suffered from biases in the satellite measurements and poor estimates of the atmospheric heat transport over the oceans, where there were few observations.

[4] Direct methods for determining the meridional ocean heat transport involve calculation of the meridional heat flux integral from transoceanic zonal hydrographic data coupled with current measurements near the boundaries. *Hall and Bryden* [1982] used annual mean volume and temperature transport of the well-defined Florida Current in addition to hydrographic data along 24°N to estimate the transatlantic heat flux and to conduct an error analysis. Their work established the direct method as the most reliable one for estimating the oceanic heat transport. *Bryden et al.* [1991] estimated the transpacific heat flux across 24°N to be 0.76 ± 0.3 PW northward from the one-time transpacific hydrographic survey (P03) along this latitude. A similar estimate of 0.75 PW was obtained by *Roemmich and McCallister* [1989] using an inverse model of the North Pacific constrained by sections at 24°N, 35°N, and 47°N, and additional meridional hydrographic sections made in different years (and different seasons), under an assumption that the ocean was in steady state. However, more recent global inverse models by *Macdonald and Wunsch* [1996] and *Ganachaud and Wunsch* [2000], using the same 24°N section data and other available transbasin hydrographic sections, suggest a lower northward heat flux of 0.5 ± 0.3 PW across 24°N in the Pacific. The available mean transpacific heat flux estimates using either direct or indirect methods at 24°N are summarized in Table 1, which shows a rather large range of values.

Reference | Heat Flux, PW | Method |
---|---|---|

- a
Error bars that are available in these studies are also listed.
| ||

Talley [1984] | ∼0 ± 0.3 | surface heat flux |

Moisan and Niiler [1998] | 0.3 ± 0.15 | and heat storage |

Esbensen and Kushnir [1981] | 0.74 | |

Oberhuber [1988] | 0.79 ± 0.3 | |

Hsiung et al. [1989] | 0.83 ± 0.6 | |

Hastenrath [1980] | 1.10 | radiation, atmospheric heat transport |

Keith [1995] | 1.2 ± 0.5 | |

Trenberth and Solomon [1994] | 0.96 ± 0.18 | |

Trenberth [1997] | 0.7 ± 0.3 | |

Trenberth et al. [2001] | ||

NCEP | 0.7 | |

ECMWF | 0.5 | |

COADS | 0.9 | |

Wilkin et al. [1995] | 0.37 | Semtner and Chervin [1992] |

Roemmich and McCallister [1989] | 0.75 | hydrographic |

Bryden et al. [1991] | 0.76 ± 0.3 | |

Macdonald and Wunsch [1996] | 0.5 ± 0.3 |

[5] The error analysis performed by *Hall and Bryden* [1982] suggests that errors in estimating the transport of western boundary currents on continental slopes and over shallow regions can introduce large errors into meridional oceanic heat flux estimates. The oceanic state measured by a single hydrographic section can be aliased by eddies and seasonal and interannual variability, especially near the western boundary where the variability is most energetic. The uncertainty of the Kuroshio volume and temperature transport may thus lead to significant errors in presently available transpacific heat flux estimates.

[6] Here we make a new estimate of the annual mean meridional ocean heat transport across 24°N in the Pacific and its seasonal variation using the WOCE PCM-1 array data [*Johns et al.*, 2001], various wind data sets, and an improved interior hydrographic climatology. The main purpose of this paper is to use the new PCM-1 Kuroshio transport measurements to constrain the 24°N Pacific meridional heat transport estimate. By incorporating the PCM-1 transport time series measurements, errors introduced into snapshot surveys by eddies and seasonal variations can be largely reduced or removed. A second purpose of this paper is to compare the observed heat flux estimate with model simulations by the Parallel Ocean Program (POP) in the Los Alamos National Laboratory. These model-data comparisons not only provide the necessary validation for model simulations to be used for climate studies but also test the methods used to calculate meridional heat flux from limited observational data and to help understand the associated heat flux mechanisms.

[7] After this introduction the paper is organized as follows: Section 2 briefly introduces the heat flux calculation method and procedures used to prepare the hydrographic data fields in the interior ocean, as well as the POP model configuration. In section 3 we present the time series of heat transport in terms of three principal (mass-balanced) components: the Kuroshio heat transport, the interior ocean baroclinic (geostrophic) heat transport, and the Ekman heat transport. In section 4 we compare the mean heat transport and its seasonal cycle with that of the POP simulation, followed by an investigation of the heat transport mechanisms. The heat transport estimates from this study are then combined with the climatological estimates in the Atlantic near 24°N [*Fillenbaum et al.*, 1997] to form an estimate of the seasonal cycle of the heat transport across the world ocean at 24°N. Section 5 presents a summary and conclusions.