Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA ... revised 9 October 2001; accepted 25 October 2001; published 26 July 2002. ... The resulting seasonal cycle of meridional heat transport has a ... The vertical and horizontal cells are found to contribute about 3/5 and 2/5 of the.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C7, 3083, 10.1029/2001JC001011, 2002
The seasonal cycle of meridional heat transport at 24 N in the North Pacific and in the global ocean D. Zhang Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, USA
W. E. Johns and T. N. Lee Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA Received 20 December 2000; revised 9 October 2001; accepted 25 October 2001; published 26 July 2002.
[1] An improved estimate of the Kuroshio transport and its seasonal variation derived from the World Ocean Circulation Experiment (WOCE) moored current meter array east of Taiwan (referred to as PCM-1) is combined with various wind products and available hydrographic data in the ocean interior to determine the meridional heat transport at 24N in the North Pacific. The resulting seasonal cycle of meridional heat transport has a minimum heat flux of 0 PW in January and February and a broad maximum in the second half of the year, with a peak of 1.1 PW in July and a secondary maximum of 1.0 PW in November. The annual mean heat transport is 0.62 PW, with an uncertainty of 0.2 PW. Of the total heat transport, 0.37 PW is associated with the ‘‘overturning’’ circulation, and 0.25 PW is contributed by the horizontal ‘‘gyre’’ circulation. The Parallel Ocean Program (POP) model simulation of the meridional heat flux at 24N compares favorably with the observations with regard to both the seasonal variation and the annual mean value. The vertical and horizontal cells are found to contribute about 3/5 and 2/5 of the total heat transport, respectively, and they are confined in the upper ocean on the annual mean and longer timescales. However, for the seasonal variation the vertical cell dominates the variation and involves circulation changes through the entire water column, while the horizontal cell heat flux remains nearly constant year-round. The new estimate of meridional ocean heat flux across 24N in the Pacific is combined with an updated estimate in the Atlantic at this latitude to yield a total oceanic heat flux across the latitude circle of 24N of 2.1 ± 0.4 PW, with an annual cycle that ranges from 1.1 PW in February to 2.8 PW in August. This is the first such estimate of the seasonal cycle of the world INDEX TERMS: 4576 ocean heat flux across 24N from direct oceanographic observations. Oceanography: Physical: Western boundary currents; 4532 Oceanography: Physical: General circulation; 4227 Oceanography: General: Diurnal, seasonal, and annual cycles; 4283 Oceanography: General: Water masses; 4255 Oceanography: General: Numerical modeling; KEYWORDS: oceanic heat transport, Pacific 24N, kuroshio, seasonal cycle, World Ocean Circulation Experiment, model-data comparison
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 24N 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, Copyright 2002 by the American Geophysical Union. 0148-0227/02/2001JC001011
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 ‘‘airsea 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
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ZHANG ET AL.: OCEANIC HEAT TRANSPORT AT 24N Table 1. Previous Estimates of Oceanic Heat Flux Across 24 – 25N in the Pacific Oceana Reference
Heat Flux, PW
Method
Talley [1984] Moisan and Niiler [1998] Esbensen and Kushnir [1981] Oberhuber [1988] Hsiung et al. [1989] Hastenrath [1980] Keith [1995] Trenberth and Solomon [1994] Trenberth [1997] Trenberth et al. [2001] NCEP ECMWF COADS Wilkin et al. [1995] Roemmich and McCallister [1989] Bryden et al. [1991] Macdonald and Wunsch [1996]
0 ± 0.3 0.3 ± 0.15 0.74 0.79 ± 0.3 0.83 ± 0.6 1.10 1.2 ± 0.5 0.96 ± 0.18 0.7 ± 0.3
surface heat flux and heat storage
0.7 0.5 0.9 0.37 0.75 0.76 ± 0.3 0.5 ± 0.3
radiation, atmospheric heat transport
Semtner and Chervin [1992] hydrographic
a
Error bars that are available in these studies are also listed.
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 24N 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 24N 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 24N, 35N, and 47N, 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 24N section data and other available transbasin hydrographic sections, suggest a lower northward heat flux of 0.5 ± 0.3 PW across 24N in the Pacific. The available mean transpacific heat flux estimates using either direct or indirect methods at 24N are summarized in Table 1, which shows a rather large range of values. [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 24N 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 24N 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 (massbalanced) 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 24N [Fillenbaum et al., 1997] to form an estimate of the seasonal cycle of the heat
ZHANG ET AL.: OCEANIC HEAT TRANSPORT AT 24N
transport across the world ocean at 24N. Section 5 presents a summary and conclusions.
2. Methodology and Data 2.1. Method [8] Jung [1952], Bryan [1962], and Warren [1999] have demonstrated that meridional oceanic heat flux Q net, through a zonal cross section, can be adequately approximated by Z
HðxÞ
Z
L
Qnet ¼ rCp
v q dxdz; 0
0
where Cp is the specific heat capacity at constant pressure, r is the density of the sea water, v is the north-south component of absolute velocity, q is the potential temperature, L is the width of the basin at the particular latitude being considered, and H(x) is the depth of the ocean. A meaningful estimate of the meridional ocean heat flux requires the above integration over a full oceanic section with a zero net mass transport; otherwise, the calculated heat transport depends on an arbitrary temperature reference [Montgomery, 1974]. In the following analysis we will refer to the integration over a zero net mass transport as the heat transport or heat flux and to the integration over a nonzero transport section (say the PCM-1 section across the Kuroshio) as the temperature transport (referenced to 0C). The mass transport across the 24N Pacific section is indeed nearly zero because there is only a small transport occurring through the Bering Strait with negligible effects on the poleward heat flux calculation at midlatitudes [Hall and Bryden, 1982; Bryden et al., 1991]. [9] Following the general methodology of Hall and Bryden [1982], we break down the total meridional heat flux into components which represent the western boundary current (Kuroshio) contribution QK, the interior baroclinic contribution due to geostrophic flow QI, and the Ekman contribution QEK, Qnet ¼ QK þ QI þ QEK :
These contributions are given by Z Z QK ¼ rCp
v q dxdz rCp VK hqI i K
¼ rCp VK ðh~qK i hqI iÞ Z Z v0 q0 dxdz QI ¼ rCp
ð1Þ
I
QEK ¼ rCp VEK ðh~qEK i hqI iÞ:
QK can also be broken down into barotropic and baroclinic contribution terms as !
Z Z QK ¼ rCp
v q dxdz VK hqI i K |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} QKbt
Z Z
þ rCp
v0 q0 dxdz ; |fflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflffl} K
QKbc
R R where VK = K vdxdz is volume transport; VEK is Ekman transport, qðxÞ; vðxÞ is vertically averaged q, v; q0(x, z), v0(x, z)
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is deviation from vertical average; hqi is vertically and zonally averaged q; and h~qi is flow-weighted average q. Subscripts K, EK, and I represent the region of the Kuroshio, the cross-basin Ekman layer, and the interior ocean, respectively, over which the integrals or averages are calculated. For the detailed derivation of these breakdowns the reader is referred to Bryden and Hall [1980], Hall and Bryden [1982], and Molinari et al. [1990]. The above quantities represent long-term mean or climatological (monthly mean) values and do not include eddy transport terms that can arise due to temporal correlations of the velocity and temperature fluctuations, which are later shown to be small at this latitude. For each of the terms above we calculate climatological annual cycles and the annual mean values, which then are combined to yield the total 24N transpacific heat flux and its seasonal variation. 2.2. Kuroshio Measurements by the PCM-1 Moored Array [10] The estimate of heat transport contribution by the Kuroshio is primarily based on the current and temperature observations collected by a current meter array moored in the East Taiwan Channel (ETC). This array (Figure 1) is referred to as the PCM-1 array, deployed between September 1994 and May 1996 as part of the WOCE. Figure 1 displays the mooring locations of this array that survived the harsh sea conditions and heavy fishing activities in this region. Formed by the Ilan Ridge between the east coast of Taiwan and the southern Ryukyu island of Iriomote, the ETC serves as a choke point for the Kuroshio flowing across the 24N section into the East China Sea (ECS) and provides a natural location for monitoring the Kuroshio. A detailed description of mooring deployments and data recovery is given by Johns et al. [2001]. [11] Prior to the deployment of the PCM-1 array, estimates of the mean transport of the Kuroshio near 24N ranged from 20 to 33 Sv, mainly determined from hydrographic sections across the East China Sea [Guan, 1981; Nitani, 1972; Ichikawa and Beardsley, 1993; Roemmich and McCallister, 1989; Bingham and Talley, 1991]. From the PCM-1 observations [Johns et al., 2001; Zhang et al., 2001; Lee et al., 2001] the Kuroshio transport is found to have large fluctuations of O (±10 Sv), which can occur on timescales of days when the crests of 100-day period Kuroshio meanders collide with the southern Ryukyu Islands and cause large amounts of Kuroshio water to bypass the entrance to the East China Sea. This short-term transport variation reaches 50% of the mean Kuroshio transport and makes transport estimates from limited number of hydrographic sections less suitable for representing the annual mean. Nevertheless, among the historical transport estimates the mean transports of Guan [1981] and Ichikawa and Beardsley [1993], 21.3 and 23.7 Sv, respectively, are very close to the mean transport, 21.5 ± 2.5 Sv, derived from the 600-day continuous PCM-1 moored current records [Johns et al., 2001]. The above two historical estimates both used a large number of hydrographic sections to estimate the mean transport in the ECS. Recently, Liu et al. [1998] estimated the mean Kuroshio transport to be 22.6 Sv using 11 shipboard acoustic Doppler current profile (ADCP) sections across the ETC to average out the tidal signals.
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ZHANG ET AL.: OCEANIC HEAT TRANSPORT AT 24N
Figure 1. (top) Schematic of the western boundary currents with estimated mean transports in the vicinity of the East China Sea [Nitani, 1972]. The striped area east of Taiwan marks the East Taiwan Channel, where the WOCE PCM-1 moored current meter array was deployed. (bottom) Moorings and tide gauge locations on the topography of the Ilan Ridge. Shaded area indicates depth
