Seasonal Variations of the Upper Ocean in the Western North Pacific Observed by an Argo Float

Transcription

1 Journal of Oceanography, Vol. 62, pp. 481 to 492, 2006 Seasonal Variations of the Upper Ocean in the Western North Pacific Observed by an Argo Float NAOTO IWASAKA 1,2 *, FUMIAKI KOBASHI 2, YOSUKE KINOSHITA 2 and YUKO OHNO 2 1 Institute of Observational Research for Global Change, JAMSTEC, Natsushima-cho, Yokosuka, Kanagawa , Japan 2 Department of Marine Technology, Tokyo University of Marine Science and Technology, Etchujima, Koto-ku, Tokyo , Japan (Received 28 June 2005; in revised form 10 February 2006; accepted 16 February 2006) A seasonal evolution of surface mixed layer in the western North Pacific around 24 N between 143 E and 150 E was observed by using an Argo float for more than 9 months, from December 2001 through August The result showed that the mixed layer deepened gradually in the first two months. It reached its maximum depth of about 130 m at the end of January, after which the mixed layer varied largely and sometimes the pycnocline below the mixed layer was much weakened until the summer mixed layer formed in late April. The thin surface mixed layer was maintained during the rest of the observation period. Heat budget analysis suggests that the vertical and horizontal temperature advections are the two most dominant terms in the heat balance in the upper layer on time scales from a few days to a month. The vertical motions that are possibly responsible for the vertical temperature advection are discussed. Keywords: Argo float observation, oceanic mixed layer, heat balance, internal tide, barotropic response. 1. Introduction The oceanic mixed layer plays an important role in both oceanic and atmospheric structures and circulations through air-sea heat exchange, formation of water masses and ventilation. Because of the lack of spatially and temporally dense observations, the features of the mixed layer have been described mostly using data observed by ships in local areas for short periods of time (e.g., Brainerd and Gregg, 1995; Otobe et al., 2003) or based on climatologies produced by averaging hydrographic data distributed unevenly in both space and time domains (e.g., Levitus, 1982; Suga and Hanawa, 1990; Kara et al., 2000). Although there are several mixed layer observations that have longer than seasonal duration with high time resolution (1 hr to a few days) (e.g., Freeland et al., 1997; Weller et al., 2002; Cronin and Kessler, 2002), such observations are limited to particular areas, while the mixed layer in most of the world ocean still left unobserved. Thus, our present knowledge of the synoptic distribution and seasonal evolution of the mixed layer is still quite limited. * Corresponding author. iwasaka@e.kaiyodai.ac.jp Copyright The Oceanographic Society of Japan/TERRAPUB/Springer The International Argo Project started in 2000, with the goal of building a system for monitoring the upper and middle layer temperature and salinity fields of the world oceans with about 3000 profiling floats (The Argo Science Team, 2000). As of February 2006, more than 2100 floats are active, of which more than 300 are Japanese contributions, and the number of the floats is increasing year by year. The Argo observations have been contributing many opportunities to study the upper and middle layer of the world oceans, including the oceanic mixed layer (e.g., Oka and Suga, 2003; Ohno et al., 2004). Although the observation period of the Argo float is normally 10 days, a few floats exist, the observation of which period was accidentally shortened to about 3 days. One of the floats had observed for about 9 months in the western Pacific, where there are no strong current systems or SST fronts. Thus, we are going to investigate the mixed layer variations on seasonal and much shorter time scales in the center region of the subtropical gyre from the float observations, where surface and subsurface layers have rarely been observed with high time resolution for long periods of time. The present paper is composed as follows: data and data processing procedure are described in Section 2. We reveal a seasonal evolution of upper layer conditions such as potential temperature, salinity and potential density and 481

2 the mixed layer depth based on the profiles obtained by an Argo float in order to understand the seasonal changes of the mixed layer. We also analyze the heat budget of the upper layer based on the float observations to infer the processes contributing to the heat budget in the upper ocean in the subtropical western North Pacific. 2. Data and Data Processing Procedure The profile data used in the present study are those obtained by the Argo float (WMO ID ) that was deployed at N, E on December 2nd, The float had been designed to measure temperature and salinity profiles from Pa to about Pa every 10 days as standard Argo floats (The Argo Science Team, 2000). However, it made an emergency surfacing only 14 hours after deployment and it repeated emergency surfacing from its parking depth of Pa every 3 days thereafter. The cause of the emergency surfacing is thought to be inappropriate design of the mechanism and software to keep the float at the parking depth of about Pa. The float repeated the emergency surfacing until August 23rd, Eighty nine sets of temperature and salinity profiles from Pa to about Pa were obtained during the observation period. The trajectory of the float is shown in Fig. 1. The float generally moved to the west, although climatological sea surface height data suggest an eastward current in the region. Average zonal speed of the float was about 0.03 ms 1. No other Argo float data were available in the present study because there were no floats within 500 km around float concurrently. The Akima method (Akima, 1970), an interpolation scheme, was applied to the Argo data. Potential temperature and density were first calculated at each observation level. They were then interpolated with a 1 m depth interval. Salinity was also interpolated on the same grid. Sea surface height (SSH) and sea surface temperature (SST) data are used to understand background sea state around the float. The SSH data used in the present study is the AVISO weekly mean maps of sea level anomalies from TOPEX/Poseidon, Jason and ERS/ENVISAT altimetry (Ducet et al., 2000), which are provided through the AVISO website ( The spatial resolution of the maps provided is 1/3 degree. The SSH anomalies are linearly interpolated into every 0.25 degree by 0.25 degree grid, which is the same grid size as the SST data described later. Absolute SSH was calculated by adding a climatological sea level to the SSH anomaly. The climatology evaluated by Niiler et al. (2003) is employed in the present study. The SST data used here are the Tropical Rainfall Measuring Mission, TMI SST, which is processed and provided by Remote Sensing Systems (RSS: Wentz and Meissner, 2000) through its website ( Fig. 1. Trajectory of the Argo float (WMO ID ). Background contour lines in the upper panel show the climatological mean sea level estimated by Niiler et al. (2003). Close-up of the trajectory is shown in the lower panel. In the lower, the star symbol indicates the deployment point and the open circle shows the last surfacing point. In order to evaluate the Ekman current we also use the gridded QuikScat wind data provided by RSS. Weekly mean, 0.25 degree by 0.25 degree averaged SST and wind data are used. The National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data (Kistler et al., 2001) of surface heat and radiation fluxes are used to estimate surface heat flux. The flux data contain net short- and longwave radiation fluxes and sensible and latent heat fluxes. Daily mean values of the reanalysis data are used. The reanalysis data sets were downloaded from the NOAA Climate Diagnostics Center web site ( N. Iwasaka et al.

3 The Horizontal Distribution Maps (later referred to as HDMap) for the Pacific Ocean (Hosoda and Minato, 2003) and the World Ocean Atlas 2001 (WOA2001: Conkright et al., 2002) are used to infer subsurface temperature and salinity fields around the float. The HDMap is a product of an objective analysis for the temperature and salinity fields in the North Pacific based on the Argo float and the TRITON buoy data and is available through the Japan Argo web site ( ARGO/J_ARGOe.html). Both of the data sets give monthly mean values. 3. Results 3.1 Time evolution of the upper layer structure a. Time-depth section Time-depth sections of the potential temperature, salinity, and the potential density in the upper 500 m depth are shown in Fig. 2. The time series of the mixed layer depth (MLD) is shown in Fig. 3(a), as well as in each time-depth section in Fig. 2. A simple definition of the MLD is employed in the present study: the MLD is defined as the depth at which the potential density is larger than the sea surface by kgm 3, following Ohno et al. (2004). Since the surface observation was not available due to the sensor mechanism, the potential density at 10 m depth was used instead of the surface value to define the MLD. The time change of MLD during the observation period shows a clear seasonal evolution, which can be divided into three stages: in the first stage the mixed layer was gradually deepened from December through the end of January. The maximum MLD was about 130 m in late January. In the second stage the MLD fluctuated largely with a short period, from more than 100 m to 30 m, until the middle of April. During the third stage, the mixed layer maintained a depth of about 20 to 30 m to the end of the observation period. In the first and the third stages thermoclines and pycnoclines, which are recognized by large vertical gradient of the potential density, shown by shading in Fig. 2, were enhanced beneath the bottom of the mixed layer, while in the second stage they were not clearly distinctive and sometimes much weakened. Short term variations of the MLD are also seen in the first and the third stages, although the amplitudes are much smaller than that in the second stage. The period of the high frequency variation is estimated as about 10 days. The net surface heat flux along the float trajectory during the observation period is calculated by linearly interpolating the NCEP/NCAR reanalysis data. The result is shown in Fig. 3(b). The net surface heat flux was almost negative during the first stage, i.e., surface cooling occurred, while the third stage corresponded to the season of sea surface warming. The sea surface generally cooled during the second stage but warming events sometimes occurred. This seasonal variation of the surface heat flux coincided well with those of the MLD, as expected. The net heat flux also showed short term changes. A spectrum analysis suggests that the periods of the high frequency variations in the surface heat flux are about 7 10 days (figures not shown). Large amplitudes of the short term variations are found in the first and the second stages. Comparing the observed MLDs and the 3-day average surface heat flux, the correlation coefficient between them for the entire observation period is 0.67, which is well beyond the rejection region (<0.22) at 5% significance level. The relatively high correlation coefficient must be a reflection of the similarity in seasonal variations of the two time series. On the other hand, the correlation coefficient for the period of the first and the second stages is 0.38, which is barely significant at 5% significance level (the rejection limit is 0.31). Although the correlation coefficient suggests that the MLD tended to deepen when the surface cooling was strong, the relationship is rather obscure. Isopleths in the time-depth sections (Fig. 2) under the mixed layer also fluctuate. The period of the large fluctuations seems to be one to two months, while small amplitude variations have a period of about ten days. These fluctuations do not seem to have seasonality. An ordinary EOF analysis of the temperature profiles demonstrates vertically coherent changes in the temperature below 150 m depth, which is represented by the leading EOF mode that accounts for 63.7% of total variance (Fig. 4). The vertically coherent temperature fluctuations below 150 m depths are well recognized in Fig. 2. The second and the third EOFs account for 17.8 and 5.2% of the total variance, respectively. The vertical structure of the second EOF has large amplitudes in m layer, indicating that variations represented by the second mode are confined to the subsurface layer. The third EOF has two nodes at depths of 125 m and 310 m and the amplitude is generally about one third of that of the first EOF. b. Background fields Figures 5(a) and (b) show the SSH and SST anomalies observed by satellites along the trajectory of the float. The SSH relative to Pa, which is estimated from the float observation, and the temperature anomaly observed at the depth of 10 m by the float (treated as the SST observed by the float), are also shown in Figs. 5(a) and (b), respectively. The satellite and float SST anomalies here are the temperature deviations from the satellite SST averaged zonally in each latitude belt, the width of which is 0.5 degree in latitude, for each period. The SSH and SST anomalies derived from the float observations coincide well with those of the satellite observations except for high-frequency variations. Thus, it can be said that the float observations are reasonably consistent with Upper Layer Variations in the Western North Pacific 483

4 (a) (b) Fig. 2. Time-depth sections of potential temperature (a), salinity (b) and potential density (c). Contour intervals are 1.0 degree (a), 0.05 (b) and 0.25 kgm 3 (c), respectively. Light (dark) shadings in (c) indicate the vertical potential density gradient greater than 0.01 (0.03) kgm 3 /10 4 Pa. In each figure, the thick line indicates the mixed layer depth (MLD). 484 N. Iwasaka et al.

5 (c) Fig. 2. (continued). Fig. 3. Time series of the MLD (a), net surface heat flux (b), time change of the heat content (TCHC) (c), vertical (d) and horizontal (e) temperature advections in the upper 500 m layer during the float observation. Upper Layer Variations in the Western North Pacific 485

6 (a) (b) Fig. 4. First three leading EOFs of the time-depth temperature section of the sea water temperature, shown in Fig. 2(a). Vertical temperature structures (a) and time coefficients (b) are shown for each mode. Time coefficient for each mode is normalized by its standard deviation. the background SSH and SST fields. Figure 6 shows the time-longitude section of the SSH and SST anomalies for the latitude belt between 22 N and 25 N, from 140 E to 155 E, where the float drifted during its lifetime (Fig. 1). The float positions are also indicated. The figure shows that the float encountered two positive and two negative SSH anomalies during its lifetime. The SSH anomalies can be considered as westward propagating mesoscale eddies. The time scale of the eddies was more than two months and the horizontal scale was about km. The phase speed of the propagating anomalies was about 0.07 ms 1. The time and horizontal scales and the phase speed are similar to those observed in previous studies (e.g., Kobashi and Kawamura, 2001). 3.2 Thermal variations in the upper layer a. Estimation of heat budget of a water column In order to understand the heat budget in the upper layer, the time change of the heat content (TCHC) in the upper 500 m layer is examined. The TCHC is generally determined by three major terms, i.e., net surface heat 486 N. Iwasaka et al.

7 (a) (b) Fig. 5. Time series of the anomalies of sea surface height (a) and sea surface temperature (b) along the float trajectory. In each panel, a thick line denotes the anomaly calculated from the float observation and a thin line indicates the satellite-derived anomaly. See the text for details. flux, vertical and horizontal temperature advections, as follows: t H r r cpρtdz = c U T + w T dz F pρ H h h, z () 1 where H is the depth of the water column (500 m), c p is the specific heat of the sea water ( Jkg 1 K 1 ), ρ is the sea water density ( kgm 3 ), T is the water temperature, r U h and w are the horizontal and vertical velocities and F is the net surface heat flux, respectively. In the present study, three terms on the right-hand side of Eq. (1) are evaluated at the mean position of two successive Argo observations. (1) Vertical temperature advection The vertical temperature advection term in (1) is evaluated in the following way. We assumed that the vertical temperature advection occurred between the two successive observations of the Argo float, with no change in the mixed layer temperature and the shape of the temperature profile under the mixed layer, but with change in the MLD, that is, Tz ( + zt, )= Tzt, + t Tzt (, )= Tzt (, + t) ( ) under the mixed layer in the mixed layer, where t is time interval of the successive observations and z is vertical displacement of the temperature profiles below the mixed layer. We employed these assumptions because vertical displacement of the isotherms is almost vertically uniform below the mixed layer, as can be seen in the results of the EOF analysis shown in Subsection 3.1a (Fig. 4(a)). We chose the 15 C isotherm as an indicator of vertical displacement z because the mean depth of the 15 C isotherm during the observation period is 340 m, which is almost middle depth of the layer that exhibits the vertically uniform displacement in Fig. 4(a). We did not use the MLD difference as z because the MLD varied much more widely than the depth of isopleths below the mixed layer from January to April. The vertical temperature advection term is therefore estimated as follows, w T z Tz zt Tzt z 1 { t z ( +, ) (, )}. (2) Horizontal temperature advection The horizontal temperature advection cannot be estimated directly for the entire water column because there are no concurrent observations that can be used to infer horizontal temperature gradient or horizontal velocities below the sea surface around the float. Thus, we first estimated the horizontal temperature advection for the surface mixed layer, assuming that the horizontal temperature gradient and velocities in the mixed layer were vertically uniform and were equal to those on the sea sur- Upper Layer Variations in the Western North Pacific 487

8 Fig. 6. Time-longitude section of the SSH (contour lines) and the SST (colors) anomalies for the latitudes from 22 N to 25 N. Float positions are indicated by circles. Table 1. Root mean square of the variations of the TCHC and vertical and horizontal temperature advections in the upper 500 m layer and the surface heat flux ( 10 3 Wm 2). TCHC Vertical temperature advection Horizontal temperature advection Net surface heat flux face. The surface temperature gradients and velocities are estimated from the RSS SST data and the geostrophic current derived from the absolute SSH fields, respectively. We then adjusted it to the advection for the entire water column by multiplication by a correction factor, computed as described below. The correction factor was estimated based on WOA2001 and HDMap. We calculated the horizontal advections from each dataset around the point of 24 N, E, the mean position of the float, using geostrophic velocity relative to Pa level and horizontal temperature gradient. The horizontal advections were computed in the surface MLD and the entire water col488 N. Iwasaka et al. No time average 30-day average umn (0 500 m). For HDMap the calculation was done in the period from January 2003 through July 2005, during which the number density of the Argo floats in the analysis area had became large enough to reconstruct monthly mean, synoptic fields. The results from the two datasets both reveal that the horizontal advections in the MLD and the entire water column have similar variations but the amplitude for the entire column was a few times larger than that for the surface MLD (figures not shown). Root mean square (RMS) variability of the horizontal temperature advection for the MLD and the entire water column calculated from HDMap is 17.9 Wm 2 and 88.9 Wm 2, and that computed

9 (a) (b) Fig day averaged time series of the TCHC (asterisks), vertical temperature advection (open circles), corrected horizontal temperature advection (open squares) and surface heat flux (crosses) (a). Time series of the sum of each component (solid squares) is shown with the TCHC (asterisks) in the lower panel (b). from WOA2001 is 9.0 Wm 2 and 49.9 Wm 2, respectively. Since WOA2001 is the long term mean, spatially smoothed climatology, the RMSs from WOA2001 are much smaller than those from HDMap, which contains smaller horizontal scale structure and relatively short term variations. The variability of the entire water column is about five times greater than that of the MLD, indicating that the horizontal temperature advection in the MLD corresponds to 20% of the magnitude of the advection for the entire water column. Thus, we chose the factor of 5 as the correction factor. We do not explicitly include the float movement in Eq. (1) although the float generally moved to the west. We believe that the float movement can be negligible as long as we look at Eq. (1) over short time scale because the typical distance between two successive profiles is 9.5 km, which is much smaller than an internal Rossby radius of deformation in the analysis area (e.g., Emery et al., 1994) and also because there are no strong temperature fronts around the float (e.g., Fig. 1). The effect of the float motion on the horizontal temperature advection will be discussed later. We do not include the advection induced by the Ekman current either. Possible errors in evaluation of the horizontal advections due to the lack of knowledge will be addressed later. b. Heat budget in the water column observed by the float TCHC, the horizontal and vertical temperature advections, and the net surface heat flux along the float trajectory are shown in Fig. 3. RMS of each term is shown in Table 1. The TCHC varied between to Wm 2 (Fig. 3(c)) and its RMS is Wm 2. The TCHC variations do not show a clear seasonal cycle. The amplitude of the vertical temperature advection (Fig. 3(d)) is comparable to that of the TCHC and the variations of the two time series appear very similar. The horizontal temperature advection (Fig. 3(e)) has a significantly large magnitude, comparable to that of the TCHC, in December, middle of January and late February through early March, but the time scale of the variation is longer than that of the TCHC and the other components. The large horizontal temperature advection may be related to the mesoscale eddies passing the float, as shown in Fig. 6. On the other hand, the surface heat flux (Fig. 3(b)) is one order of magnitude smaller than the other two components (Table 1). Time series of the 30-day averaged TCHC and each heat flux component are shown in Fig. 7 and their variability is also summarized in Table 1. On a monthly time scale, the vertical and horizontal advections are still major components of the heat budget. The net surface heat flux makes a smaller but significant contribution to the TCHC. The sum of the each component shows a variation quite similar to the TCHC (Fig. 7(b)). Upper Layer Variations in the Western North Pacific 489

10 4. Discussion 4.1 Expected errors in the heat budget a. Float motion The average float movement is estimated as 0.06 ms 1 based on the distance between successive two profiles observed by the float. This is about 31% of the scalar mean, surface geostrophic current speed ( ms 1 ) around the float during the observation period, which is calculated from the SSH data. Thus, the float movement would result in an error of 30% of the horizontal advection at a maximum. However, since the horizontal temperature advection is fairly small, except for certain periods (Fig. 3(e)), the error due to the float movement may not significantly change the relationships among the TCHC, the horizontal and vertical temperature advections in the water column observed by the float. b. Ekman current The Ekman current speed induced by surface wind is evaluated based on the satellite observations of the wind and SST. The average Ekman current is about ms 1, which is much weaker than the surface geostrophic current speed ( ms 1 ) estimated from the altimeter data. The RMS of horizontal temperature advection by the Ekman current is 29.9 Wm 2, which is only about 13% of the horizontal advection in the mixed layer estimated above (Table 1). Therefore, we conclude that the effect of the horizontal heat advection by the Ekman current on the time variations of the heat budget is negligible compared to the other terms. c. Vertical temperature advection The method to evaluate the vertical temperature advection shown in the previous section may overestimate of the advection because a part of the vertical displacement of the isotherm might be induced by horizontal temperature advection. In order to assess the possible overestimation, we subtracted the sum of the corrected horizontal temperature advection and the surface heat flux from the TCHC and the residual was compared to the vertical temperature advection. The comparison shows that the RMS of the residual is larger by about 30% than the vertical advection and larger by 50% in the case of 30-day means, although the residual and the vertical advection have quite similar variations (figures not shown). Thus, we do not believe that the vertical temperature advection estimated in the present study is overestimated so much as to contradict the relationship among the TCHC and heat budget components shown in the previous section. 4.2 Seasonal evolution of surface mixed layer The MLD changes discontinuously from winter regime (from December to late January) to summer regime (from middle of April to August). During the transition period from a deep winter mixed layer in the end of January to a shallow summer one in the middle of April, the mixed layer is sometimes obscured or disappears. Similar features of the transition period have been observed by moorings in the Arabian Sea (Weller et al., 2002), and in the region south of the Kuroshio Extension by Argo floats (Ohno et al., 2004), for example. This kind of characteristic in the surface mixed layer during the transition season may not be widely recognized because high time resolution, long-term mixed layer observations have rarely been performed in open oceans. The relationship between the MLD and the surface heat flux (Figs. 3(a) and (b)) on a seasonal time scale is very good, as expected. On the short time scale (order of 10 days), a deep MLD tends to follow strong surface cooling but the relationships are sometimes unclear. Uncertainty in the heat flux dataset could cause a part of the obscurity in the relationships. The obscurity also suggests that not only the surface forcing but also subsurface processes may contribute to the mechanism of mixed layer formation. During the transition period the MLD undergoes intermittent but significant shoaling, which precedes the formation of the shallow mixed layer in late spring (Fig. 2). The shoaling events start in the beginning of February, which is much earlier than late winter when the surface mixed layer is generally believed to achieve its deepest depth. This early shoaling of the mixed layer may have important implication for a recent finding by Takeuchi and Yasuda (2003), who investigated the climatological monthly change of the MLD and pointed out that the mixed layer in the latitudinal band of N tends to be shallower in winter. The shoaling events observed in the present study may be consistent with the phenomenon found by Takeuchi and Yasuda (2003). In the present study, the MLD is defined using a simple method (see Subsection 3.1). The method is widely used and apparently works well for most of the observation period (Fig. 2). It should be noticed, however, that as seen from the vertical gradient of the potential density in Fig. 2, the pycnocline beneath the mixed layer is weakened and sometimes doubled during the transition period. In such a case, there is a possibility that a simple method like that used in this study may be unable to detect the MLD properly. Caution will be needed in applying a simple method to observations, especially profiles in the mixed layer transition period. 4.3 Vertical displacement of the thermal structure The heat budget in the upper layer is strongly controlled by the short time scale, periodic vertical displacement of the thermal structure. One might argue that the short term, vertical displacement would be induced by mesoscale eddy activities. In fact, the mesoscale eddies 490 N. Iwasaka et al.

11 can contribute to the vertical temperature advection on a monthly time scale or longer, as well as significant horizontal temperature advections, as mentioned before. However, Fig. 6 clearly shows that the mesoscale eddies passed the float several times but the time scale of the eddy passage is much longer than the period of the short term variations. Thus, it is not likely that mesoscale eddies cause the short term variations. Effects of internal tide may also appear in the observations. Brainerd and Gregg (1995), for example, observed that the seasonal thermocline moved through a range of m with a semidiurnal frequency. Niwa and Hibiya (2001), based on their model study, suggested that the M 2 internal tide had some amplitude in the area of the present study. If the M 2 tide is observed with a sampling interval of 72 hours, the aliasing period is about 14.6 days, which is rather close to the period of the vertical displacement of isotherms observed in Fig. 2. Since the float moved slowly to the west, the sampling interval is not exactly the same as that at a particular point, but this estimation suggests that the vertical motion observed by the float could be the aliasing of the internal tide. On the other hand, some numerical model studies have suggested that there are high frequency (order of 0.1 cycle per day), significant amplitude barotropic sea level changes in the world oceans and the signal is aliased in the TOPEX/POSEIDON altimeter data in the low frequency spectral band (e.g., Chao and Fu, 1995; Fukumori et al., 1998; Stammer et al., 2000). The barotropic motions are considered as responses to the variations of surface wind stress fields (Fu and Davidson, 1995). The period of the predicted barotropic motions is quite similar to that observed in the vertical changes of the isotherms. Thus, one of the major contributors to the short time scale vertical motions could be the barotropic motions. Since the float generally moved to the west of its deployed position, it might capture some spatially periodic structure in the upper ocean. We do not exclude such a possibility of explaining the observed vertical displacement, but there is no clear observational/theoretical evidence, so far, to prove the hypothesis. 5. Conclusion The present study shows that the float observation continuously captured a seasonal evolution of the surface mixed layer in the western North Pacific for more than 9 months, from December 2001 through August The result showed that the mixed layer gradually deepened until it reached its maximum depth at the end of January. From February through the middle of April, the mixed layer varied largely and sometimes disappeared, although MLD gradually decreased until the summer mixed layer formed in late winter through spring. In summer, the shallow surface mixed layer was maintained by the accompanying sharp thermocline and pycnolcine beneath it. Heat content analysis suggested that the vertical temperature advection, or vertical displacement of the thermal structure and the horizontal temperature advection have comparable magnitude and are dominant terms in the heat balance in the upper layer. The present study demonstrates that the profiling float is a useful tool to observe the surface mixed layer of the ocean. Acknowledgements The authors are grateful to Dr. Hosoda of JAMSTEC for kindly providing a digital form of HDMap data and two anonymous reviewers for useful comments and suggestions. The NCEP/NCAR Reanalysis data was provided by the NOAA-CIRES ESRL/PSD Climate Diagnostics branch, Boulder, Colorado, USA, from their web site at The altimeter products were produced by Ssalto/Duacs as part of the Environment and Climate EU Enact project (EVK2-CT ) and distributed by Aviso, with support from Cnes. References Akima, H. (1970): A new method of interpolation and smooth curve fitting based on local procedures. J. Associ. 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