JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C5, 3039, /2001JC000926, 2002

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C5, 3039, /2001JC000926, 2002 A Lagrangian investigation of vertical turbulent heat fluxes in the upper ocean during Tropical Ocean Global Atmosphere/Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) Yves du Penhoat and Gilles Reverdin Laboratoire d Etudes en Géophysique et Océanographie Spatiales, Toulouse, France Guy Caniaux Centre National Recherche Meteorologique, Toulouse, France Received 13 April 2001; revised 22 October 2001; accepted 26 October 2001; published 14 May [1] We investigated the diurnal cycle of the near-surface ocean temperatures in the western Pacific Warm Pool, using temperature data from drifters with 20 m thermistor chains, during the Coupled Ocean-Atmosphere Response Experiment (COARE) Intensive Observation Period (December 1992 to February 1993). We estimated turbulent heat fluxes at 20 m as a residual in the hourly heat budget along each drifter trajectory using hourly surface fluxes. Fortnightly averaged turbulent heat fluxes at 20 m were upward during periods of sustained winds, indicating a warming of the daily thermocline by the deeper layers. During calm periods the inferred turbulent heat fluxes were downward. We also calculated the day-to-night differences of the turbulent heat fluxes at 20 m as a residual in the budget of the daily cycle of heat content in the upper 20 m. This method largely avoids the uncertainties of surface turbulent heat fluxes. The residual averaged over the 2 month period was negative for the drifters that experienced strong westerly wind conditions (with heat fluxes positive downward). These surprising results for the warm pool region are at odds with what was found earlier from moored measurements in the central Pacific. Interestingly, however, periods characterized by a westerly wind burst during December 1992 to March 1993 contrasted with the periods with weaker winds: during westerly wind bursts the difference was negative, whereas it was positive during calm or warming periods. This is consistent with our analysis of the daily budget and also with other studies in the COARE domain. It confirms that the nighttime mixed layer was usually much deeper than 20 m during this period. However, our estimates are different for drifters deployed north of 1 N than for drifters near or south of the equator, indicating marked spatial variability in the heat budget of the warm pool. INDEX TERMS: 4572 Oceanography: Physical: Upper ocean processes; 4504 Oceanography: Physical: Air/sea interactions (0312); 4568 Oceanography: Physical: Turbulence, diffusion, and mixing process; KEYWORDS: surface drifter, Pacific Warm Pool, TOGA-COARE, turbulent heat fluxes, upper ocean processes, Lagrangian measurements 1. Introduction Copyright 2002 by the American Geophysical Union /02/2001JC [2] Sea surface temperature (SST) is directly coupled to the atmosphere, and processes contributing to its variability are key to climate variability. SST varies over a wide range of timescales from hours to years. High-frequency (hourly to weekly) variability of SST is often ignored in investigations of climate variability. The climate system being very nonlinear, it is possible that this range of frequency could impact low-frequency fluctuations of the climate system. This is expected to apply more where SST is high (warmer than 28.5 C), which corresponds to areas where climate is very sensitive to SST variability [e.g., Webster and Lukas, 1992]. There, even small daytime variations of SST may result in strong interactions with the overlying atmosphere because of the nonlinear dependence between SST and latent heat flux. The largest region of surface warm water with SST >28.5 C lies in the western tropical Pacific. This area is recognized as important for the development of El Niño events. What role high-frequency variability plays in the warm pool needs to be investigated. Improved understanding of how SST is controlled in this area was one objective of the Tropical Ocean Global Atmosphere/Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) [Webster and Lukas, 1992]. The equatorial warm pool usually has weak horizontal SST gradients, so that it is common to assume that the horizontal temperature advection is small [Niiler and Stevenson, 1982]. It is therefore expected that to a first order the heat budget of the near-surface layers is controlled by vertical processes in the mixed layer. This paper will provide further discussion on this issue with data from drifters collected during TOGA-COARE. [3] During the Tropic Heat Experiment (November 1984) microstructure measurements were carried out in the central equatorial Pacific (140 W), indicating diurnal variations in turbulent dissipation rates reached to depths greater than the base of the mixed layer [Peters et al., 1994]. Imawaki et al. [1988] estimated the turbulent heat flux at the bottom of the daily mixed layer at 140 W using a 9 day record of the daily cycle of the near-surface heat content in November 1984 and showed that it was characterized by a diurnal cycle. Bond and McPhaden [1995] investigated the mean evolution of hourly time series of turbulent heat fluxes estimated from data of 7-1

2 7-2 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE the TOGA-TAO mooring at 140 W. They deduced vertical turbulent heat fluxes at the base of the mixed layer, which had a magnitude and phase similar to those inferred from microstructure data. Thus there is a reason to expect that computing vertical heat flux as a residual in a heat budget analysis for the warm pool region may lead to reasonable results as well. [4] During the TOGA-COARE experiment, air-sea fluxes were measured using diverse platforms, which led to the development of new bulk formulae adapted to the warm pool region [Fairall et al., 1996a, 1996b]. Microstructure measurements were also carried out during TOGA-COARE, near S, 156 E (the COARE Intensive Flux Area (COARE IFA)). Wijesekera and Gregg [1996] found that during calm days the mixed layer heat content was driven by the balance between the residual net heat flux at the surface minus the penetrative radiative flux out of the layer. Ando et al. [1995] calculated the variations of the 10 day heat storage rate at 0, 156 E in February 1993 from repeated conductivitytemperature-depth (CTD) casts. Their study suggests that the mixed layer temperature (0 60 m) was mainly controlled by the local surface heat fluxes during these 10 days. Nevertheless, the heat advection term was very large during a westerly burst in November Furthermore, Cronin and McPhaden [1997] found the zonal advective heat flux divergence to be as large as 30 W m 2 at 156 E on the equator during the onset of the December 1992 westerly wind burst (WWB). Feng et al. [1998, 2000] also found important advective heat flux during the same WWB in the center of the COARE IFA array. Feng et al. found that heat advection at short time period was significant in the surface layer heat balance. Smyth et al. [1996a, 1996b] also suggested that inertial motions influenced the upper heat and salt budgets in the COARE IFA. [5] In the present study we will revisit, in a Lagrangian frame, some of these conclusions based on simultaneous data from surface drifters and heat fluxes across the air-sea interface computed in or near the COARE IFA. We will investigate the vertical turbulent heat fluxes at the base of the diurnal layer and that layer s variability in the western equatorial Pacific using drifter data. Lagrangian drifters offer a unique data source to study the upper ocean current and nearsurface temperature variabilities in response to atmospheric forcing. They efficiently provide large spatial coverage. Moreover, budgets, done along trajectories, minimize the contribution of horizontal advection, which is usually difficult to assess. [6] In section 2 we present temperature data collected during the Intensive Observation Period (IOP) of TOGA-COARE (November 1992 to February 1993) by Bouées de TOGA (BOD- EGA) surface drifters. In section 3 the oceanic and atmospheric conditions are described as well as the temporal variability of near-surface heat content. In section 4, the thermal evolution of the upper layer is described, and the net heat budget of the 20 m layer is estimated for periods characterized by differences in the winds. This is done on each drifter using solar irradiance estimated from GMS 4 satellite data [Gautier and Landsfeld, 1997]. In section 5 we then present estimates of the night-day differences in turbulent fluxes following Imawaki et al. s [1988] method. We discuss the errors in the fluxes, the applicability of the drifter-following one-dimensional (1-D) heat budget in this region, and what this implies for the vertical turbulent fluxes at a depth of 20 m. In section 6 the results of the present study are summarized and discussed. 2. Data [7] TOGA surface drifters (379) were deployed in the western Pacific between 1986 and March 1993 during various programs (Western Equatorial Pacific Ocean Circulation Study (WEPOCS), United States/China cruises (US/PRC), and Surveillance Trans- Océanique du Pacifique (SURTROPAC)). All drifters included a surface buoy tethered to a drogue at a depth of 15 m and were designed to follow the mixed layer currents at the drogue depth to within a few cm s 1 [Niiler et al., 1987]. The BODEGA drifters were qualified TOGA drifters fitted with a Tristar float [Niiler et al., 1995]. They were also equipped with a minithermistor chain containing five thermistors at the nominal depths of 2, 5, 8, 12, and 20 m. The precision of the reported data is 0.01 C; the measurement accuracy is probably closer to 0.2 C. The main sources of error are microleaks in the wiring harness and warming of the pods by absorbed visible solar energy. The former can induce bias errors in the measured temperature; the latter can result in overestimated midday temperatures. In addition to the thermistor chain sensors, there was a hull temperature sensor within the surface float, sampled at a lower resolution of 0.04 C, which is used as a control of the other sensors. This sensor is located close to the bottom of the surface float, and we expect some slight contamination from the hull temperature. During daytime in low wind conditions this measurement is often rather different than at 2 m depth, which is consistent with the surface daytime stratification. All sensors were calibrated in a temperature-controlled bath and tested prior to deployment. Hourly temperatures from a 15 min average were transmitted. Buoy positions were determined by satellite positioning via Service Argos. [8] The first deployment of BODEGA drifters took place in the central Pacific in October Subsequently, they were deployed in the western tropical Pacific through February During TOGA-COARE IOP, 12 BODEGA drifters were deployed along 156 E together with 21 other Lagrangian mixed layer drifters. These BODEGA drifters were programmed to record continuous hourly temperature data for 2 months, before switching to a 1 day on 2 days off transmission mode [du Penhoat et al., 1995]. Therefore we are able to follow the heat content evolution on a day-to-day basis for the first 2 month period. In the present study we use these data to evaluate turbulent fluxes at the base of the diurnal layer. [9] In order to derive terms in the local heat budget following each float we need to estimate the different components of the surface fluxes at each drifter s position. This is done by combining the following atmospheric data sets: weather prediction model analyses for the wind stress, satellite data for the solar shortwave radiation, and in situ data for the other components. The wind stress is from the European Centre Medium-Range Weather Forecast (ECMWF) special TOGA-COARE data set provided every 3 hours during the IOP on a grid. The wind stress fields were interpolated with a bicubic spline to the drifter positions. We do not use the surface heat fluxes from the ECMWF model as they exhibit significant differences from observations. In a comparison with direct estimates from the central mooring in the IFA (deployed by Woods Hole Oceanographic Institution, hereafter WHOI mooring), Weller and Anderson [1996] noticed that during short-lived deep convective events the average net ECMWF heat flux over several days differed by more than 70 W m 2 from the estimated mooring fluxes and had opposite sign. [10] For the shortwave component we use GMS 4 satellite solar irradiance data provided hourly on a 5 km 5 km grid (the method is described by Gautier and Landsfeld [1997]). Shortwave radiative fluxes were also interpolated to the drifter positions. A comparison was made with the shortwave radiation flux from the WHOI mooring. We found a bias in the GMS 4 product with generally too small values for cloudy conditions (about 30%) and too high values for clear-sky conditions (about 15%). We corrected the satellite values based on a linear regression with the WHOI mooring data, assuming that the correction was valid for the 2 month period and wider area investigated here.

3 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE 7-3 [11] Other components of the surface fluxes (longwave radiation and sensible and latent heat fluxes) were computed with the TOGA-COARE bulk formula [Fairall et al., 1996a, 1996b] at the drifter position. They combined drifter SST, ECMWF wind stress, and atmospheric TOGA/TAO mooring data (air temperature and humidity) [McPhaden et al., 1998] interpolated linearly to the drifter positions from the closest moorings. 3. Upper Ocean Conditions and Meteorological Forcing During COARE IOP [12] From earlier BODEGA drifter deployments in the western Pacific ( ), simple statistics computed on all diurnal cycles show that the daily minimum to maximum temperature differences range between 0.02 and roughly 2 C at 2 m and between 0.02 and 0.5 C at 20 m. For 80% of all recorded diurnal cyles the diurnal temperature range does not exceed 0.57 C at 2 m and 0.3 C at any deeper level [Teisseyre and Vigneaux, 1993]. These variations in amplitude of the daily cycle are related to the surface conditions, with wind stress and solar radiation being the most important controlling factors. For wind speed >6 m s 1 the amplitude is usually reduced and the diurnal thermocline is weak, or the surface layers are completely mixed to below 20 m. Reduced solar radiation also leads to a weaker diurnal thermocline. The daily heat wave in the western Pacific seldom penetrates deeper than 30 m (based on TAO mooring temperature records). [13] A sense of the spatial structure of temperature variability is given by grouping observations from 12 TOGA-COARE drifters in three latitude bands: 15 5 S, 5 S 5 N, and 5 15 N (between 130 E and 180 ). For the period, the equatorial strip had the warmest near-surface waters, with an average >29.6 C between 0 and 20 m. The equator was also characterized by a minimum in rms variability (0.2 C). Waters south of 5 S were warmer than north of 5 N and had a greater variance. The 2 m level diurnal amplitude was also much larger for the southern band compared to the northern strip. At the 20 m level the diurnal variation was approximately the same for the two off-equator bands, indicating a stronger diurnal stratification south of the equator. The smaller amplitude north of 5 N could be a consequence of the mean position of the Intertropical Convergence Zone. This atmospheric pattern resulted in stronger trades to the north that induced more stirring and reduced the diurnal surface warming. [14] The COARE IOP experiment occurred at a time when the ocean structure was under the influence of ENSO conditions, corresponding to the unusually long warm event. A major WWB occurred near the equator at 156 E on 10 December, with maximum wind strength reached at the end of December/early January. This WWB episode, which was the most energetic equatorial event during the IOP, was centered south of the equator. It led to the formation of a near-equatorial eastward jet, which resulted in significant advective cooling [Ralph et al., 1997]. The greatest SST change (temperature drop exceeding 1 C) occurred near 2 S at the end of the WWB. After calm days in January a strong surge of easterly trade winds was experienced first north of the equator and then to the south. This was followed by another strong WWB in connection with the formation of tropical cyclones in the Coral Sea but centered 4 5 south of equator. This latter WWB did not lead to the formation of an oceanic eastward jet near the equator. Figure 1 summarizes these conditions using interpolated TAO wind vectors along 156 E and near-surface temperature from hydrographic cruises [Eldin et al., 1994]. [15] The BODEGA drifters were deployed along 156 E by R/V Le Noroit. Table 1 gives the dates and positions of the deployments and Figure 2 gives their trajectories up until 1 March The trajectories in December are different depending on whether the deployment was north or south of the equator. North of 2 N, the drifters moved northward in response to Ekman drift for almost the whole period, whereas south of 1 N, the drifters converged in the jet centered near 1.5 S. This jet had a zonal extension of roughly 30 in longitude, based on the trajectories of all the surface drifters present in the western Pacific. The drifters remained within the jet until the end of the westerly burst (4 January), then flowed to the south-southwest in response to the predominant northeasterlies. At the end of January and in early February, drifters south of the equator near 4 5 S flowed eastward under the influence of westerlies (Figure 2). Drifter 17626, deployed at the end of January, went to the west and remained close to the equator. Consequently, it did not report the same near-surface variability as other drifters deployed north of the equator (located near 6 8 N at that time), as it encountered different atmospheric conditions. Drifter 17628, a few days after deployment, presented abnormal behavior of the 2 m sensor (with excessive warming) and was consequently not used for heat budget calculations. [16] The influence of wind stress on the surface layer s thermal stratification can be seen in the temperature daily amplitudes from the sensors at 2 and 20 m (Figure 3). At 20 m the diurnal temperature variability is weak. Therefore the daytime temperature difference between 2 and 20 m is generally a good indicator of the diurnal SST range at 2 m. Drifters deployed north of the equator exhibited a weaker diurnal amplitude than drifters flowing south of the equator as they experienced stronger winds (Table 1). Drifter has a comparable amplitude, but it was deployed later and stay close to the equator and flew west (Figure 2). The diurnal SST range at 2 m is reduced during the December westerly burst when the atmospheric convection was active, the mixed layer turbulence was strong, and the 0 20 m layer was homogeneous. During periods of weak winds and reduced convection the diurnal temperature range was significant at 2 m but remained weak at 20 m, indicating a pronounced daily stratification. The depth of the mixed layer along 156 E, calculated on a density gradient criterion [Lukas and Lindstrom, 1991], is shown in Figure 4 based on CTD measurements aboard R/V Le Noroit. The mixed layer deepened during periods of sustained winds and shoaled during times of weak winds. At night the mixed layer always reached depths deeper than the 20 m of the drifter thermistor chain. [17] To illustrate the character of the heat content variability, we present the different fluxes and the 0 20 m heat content for the period mid-december to mid-february for drifter (Figures 5 and 6). This drifter was deployed in the nearequatorial jet (19 December). Its heat content evolution shows a cooling during the December WWB that lasted until 5 January. Then the drifter moved away from the equator as the near-equatorial jet vanished. This was a period of light winds, and the heat content increased until January when it fell and reached a second minimum (most pronounced for drifter 17618). At that time the drifter (and drifter as well) veered to the southeast because the influence of an easterly wind surge, which was more energetic north of the equator and east of the region. After 28 January, drifter was near 4 S and experienced strong westerly wind associated with the formation of a tropical cyclone pair in the Coral Sea. The heat content decreased during that period. [18] The heat content curve also shows variability in the amplitude of the diurnal cycle. The diurnal heat content cycle results from the daily cycle of the heat fluxes, in particular of the solar shortwave radiation (Figure 5) and is damped by the turbulence flux across 20 m, which is expected to intensify when wind stirring is large. Periods of strong winds coincided both with rapid cooling and the disappearance of diurnal variation. All

4 7-4 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE Figure 1. (a) Objective analysis of TAO wind stresses between 5 N and 5 S along 156 E during the TOGA/ COARE IOP (December 1992 to February 1993). (b) Ocean temperature at 5 m as seen by hydrographic surveys along 156 E, between 5 N and 5 S, during the same period. drifters deployed south of the equator had a similar response for the heat content (Figure 6a). There were differences of 1 2 days in the time of minimum heat content between drifter 17618, which was farther east, and drifters and near 157 and 160 E (i.e., 2 3 west of the drifter), which experienced different atmospheric forcing. [19] Heat content variations of drifters deployed north of 1 N such as drifter differ from those deployed south of the equator (Figure 6b). These drifters flowed northward and encountered different wind and surface flux regimes. Drifter was deployed north of the equator during the WWB at the end of December. However, the WWB centered south of the

5 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE 7-5 Table 1. Deployment Dates and Positions of the Surface Drifters a Period P1 20 Dec to 3 Jan WWB, Cloudy Period P Jan Period P Jan Period P Feb Argos Deployment Positions Post WWB, Weak Winds, Clear Sky Weak Winds, Weakly Cloudy WWB, Increasing to the Southeast, Cloudy Number Date Latitude Longitude t Q sw t Q sw t Q sw t Q sw Dec N E Dec N E Dec S E Jan S E Jan S E Jan S E Jan S E NE Winds, Cloudy NE Winds, Clear Sky Sustained NE Winds, Cloudy Sustained NE Winds, Weakly Cloudy t Q sw t Q sw t Q sw t Q sw Dec N E Dec N E Dec N E Jan N E a Mean winds stress (t, Nm 2 ) and solar radiations (Q sw,wm 2 ) are listed for each drifter for the four periods corresponding to different atmospheric conditions. Northward and southward deployments are separated. Drifter was deployed north of the equator but encountered different atmospheric conditions than other drifters deployed earlier. equator had little effect on the float drift that presented an average motion to the north. The heat content anomalies indicated a significant cooling during the period January corresponding to a surge of easterly trade winds (Figure 1), whose effects were mainly felt north of the equator. The cooling period also corresponds to weaker daily cycles as depicted by drifters near the equator. [20] In the following sections we synthesize the drifter data set by grouping in periods according to the wind regimes and separating the equatorial and south equatorial zone from the Figure 2. Drifter trajectories between December 1992 and February Solid dots indicate the deployment positions, and open dots indicate positions of the drifters at the end of each of the four periods defined in Table 1. The solid lines indicate the trajectories for the first and fourth period; the dotted lines indicate the trajectories for the second period, and the dashed lines indicate the trajectories for the third period.

6 7-6 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE Figure 3. Daily range of the near-surface temperature differences between 2 and 20 m, measured by the drifters (a) deployed at or south of the equator and (c) north of the equator. Daily wind stress collocated at drifter positions shown (b) for the southern deployments and (d) for the northern deployments. Unit for wind stress is 10 2 Nm 2, and unit for temperature is C.

7 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE 7-7 Figure 4. Mixed layer depth (in meters) measured between December 1992 and February 1993 by hydrographic surveys along 156 E, between 5 N and 5 S. north equatorial zone (Table 1). The first period, 19 December to 4 January, is characterized by WWB conditions and strong atmospheric convection. Following that is a calm period with a warming of the oceanic near-surface layer. Then there are a few days of cooling when strong easterly winds occurred north of the equator, followed by another quiet period. In February 1993 there are again increasing westerly winds especially south of the equator. North of the equator, moderate Figure 5. Time series fluxes for drifter with shortwave radiation (Sw), sensible heat flux (Hsen), latent heat flux (Hlat), and longwave radiation (Lw). Units are in W m 2. Wind stress is also plotted with scale on the right vertical axis (in N m 2 ).

8 7-8 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE Figure 6. Cumulated heat content (in MJ m 2 ) between 0 and 20 m for (a) drifters deployed at or south of the equator and (b), drifters deployed north of the equator.

9 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE 7-9 Table 2. Mean Rate of Heating (H t ), Net Heat Flux at the Surface (Q surf + Q sw [1 T r ( h)]), Shortwave Radiation Penetrating at 20 m (Q surf + Q sw* [1 T r ( h)]), and Turbulent Heat Fluxes at 20 m (Q h )for Drifters for the Southward Deployment (South of 1 N) and the Northward Deployment (North of 1 N) a H t Q surf + Q sw* [1 T r ( h)] T r ( h)*q sw Q h South Deployment P P P P P P P P P P P P P P P P P P P P North Deployment P P P P P P P P P P P P P a Units are in W m 2. P1 is from 20 December 1992 to 3 January 1993; P2 is from 4 to 17 January 1993; P3 is from 18 to 31 January 1993; and P4 is from 1 to 15 February to strong northeasterly trade winds prevailed for the four periods (Figure 1). As drifters went north (Figure 2), they encountered stronger winds at higher latitudes. 4. Heat Budget [21] In this section we explore a heat budget approach to estimate turbulent heat fluxes at 20 m. They can be estimated directly as a residual to the heat budget but with the caveat that surface fluxes are poorly known, in particular, the latent heat flux. The errors on these fluxes can be quite large (see discussion below). We computed hourly values of the turbulent heat fluxes as a residual to the 1-D heat budget integrated over the upper 20 m layer. [22] We start with the equation describing the heat content evolution of the near-surface layer of thickness hti r c p þ UrhTi ¼ Q 0 Q h : Here h is set by the deepest sensor depth, 20 m, and is usually shallower than (or equal to) the late night thermocline depth in the western tropical Pacific. [23] Q 0 = Q surf + Q sw [1 T r ( h)] is the net surface heat flux where Q surf is the net nonsolar heat flux at the surface, Q sw the solar radiation and T r the solar transmission coefficient at depth h =20m. Siegel et al. [1995] estimated during IOP a transmission factor of 15% for the penetrating solar radiation at 20 m; we used this value in our budget. Q h is the net turbulent heat flux at depth h, r is the density, and c p is the specific heat. We set Z 0 hti ¼ h Tdz as the vertically integrated temperature and U as the horizontal velocity averaged over depth h = 20 m. By convention, fluxes are positive downward. [24] The effect of vertical advection across the 20 m surface has been neglected, which is usually expected in this level characterized by weak vertical gradients. The horizontal heat flux convergence hr(rc p U 0 T 0 )i is also neglected, U 0 and T 0 being the deviations of velocity and temperature from their respective vertical averages. r c þ Ur dt h i hti r c p ¼ H t dt is the Lagrangian rate of heating following the vertically averaged velocity. Therefore H t approximates the time rate of change in heat content for the depth interval occupied by the diurnal thermocline. [25] We also derived an alternative estimate of the turbulent fluxes by fitting a 1-D turbulence model to observed temperature at 2 m via an iterative process [Gaspar et al., 1990]. This model provides both corrected estimates of the surface heat fluxes and of

10 7-10 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE Figure 7. Fortnightly averaged turbulent heat fluxes at 20 m for different drifters. Fluxes (in W m 2 ) are counted positive downward. the vertical turbulent heat fluxes at 20 m. However, because of extreme sensitivity to the model vertical resolution, results were not robust and will not be presented. They were, however, critical for detecting periods when advective effects were probably important and the 1-D model was not applicable. [26] The heat budget is summarized by averaging over the periods defined in section 3 (Table 1). Notice that the periods start in the middle of local night, when the heat content is best defined. As previously noted, these periods correspond to different wind regimes. Table 2 and Figure 7 give the results of the heat budget calculation averaged over these periods (fortnightly averaging). Period 1, corresponding to the WWB in late December, was sampled by three drifters of the southern deployment. The period is characterized by a cooling of the 20 m layer at between 20 and 40 W m 2 ; the net surface heat flux was negative ( 29 W m 2 ) for drifter 17618, slightly negative for drifter 17629, and slightly positive for drifter The inferred turbulent heat flux at the base of the 20 m layer was upward (negative) for drifter 17618, indicating a warming of the upper layer from below. The turbulent heat flux was near zero and slightly positive for drifters and 17625, but they experienced only the last part of the WWB with more incoming shortwave radiation and less or no deep convection [Feng et al., 1998]. For drifter 17625, wind stress was also smaller, and the drifter was not entrained into the eastward jet. This result also still holds for the period of strong winds south of the equator in February, but only drifter experienced strong winds. During other periods of weak wind all drifters experienced a positive net surface heat flux and a downward (positive) residual turbulent heat flux at 20 m (the upper layer warmed the deeper layer). [27] The drifters north of the equator showed a decrease in heat content for all periods, and the turbulent heat fluxes at the base of the layer were strongly upward (negative) only during period 3. This period corresponded to the strong incursion of northeasterly trade wind down to the equator (Figure 1). [28] In conclusion, during strong winds, H t was negative and turbulent heat fluxes at the base of the layer were negative (with stronger values for the drifters that moved north). This means that during these periods the upper layer was warmed by turbulent exchange with the deeper layer, in accordance with the results of Feng et al. [2000] in the IFA. This corresponds to times when the mixed layer was deeper than 20 m (as seen by the lack of temperature gradient between 12 and 20 m) and when a cooling of the mixed layer was observed. It also can be associated with temperature inversions when SST is smaller than ocean subsurface temperature. [29] The largest errors in those budgets averaged over 2 weeks originate from the surface heat flux estimates (as mentioned in section 2). Another source of error was related to relative advection of the drifters with respect to the currents averaged over the mixed layer. This was particularly important when the mixed layer was very deep and in the presence of horizontal temperature gradients. According to Figure 4 (and generally for drifters north of 5 N as confirmed by SST data, not shown), this was particularly the case during period 3. We therefore considered suspect the results for drifters 17619, 17623, and (flowing north of 4 N) during this period, a belief supported by the 1-D turbulence model. However, as the mixed layer was very deep (deeper than 90 m) and as there was strong cooling at the surface, we also expect strong upward turbulent flux at 20 m during this period. 5. Night-Day Difference in Turbulent Heat Flux [30] In this section we follow the approach developed by Imawaki et al. [1988] in order to interpret further the turbulent heat fluxes of section 4. This method estimates the difference of turbulent heat fluxes between day and night at the base of the diurnal layer and is less sensitive to errors in nonsolar surface heat fluxes. We start with (1) that we average from 0600 to 1800 LT and from 1800 to 0600 LT. We get the following heat budget equations, with superscripts D and N standing for day and night average, respectively: H D t ¼ Q D 0 Q D h H N t ¼ Q N 0 Q N h :

11 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE 7-11 Figure 8. Fortnightly night-to-day differences of turbulent heat fluxes for different drifters. Fluxes (in W m 2 )are counted positive downward. Sampling errors on the averages are indicated by vertical bars. The terms Q 0 and Q h are defined as in section 4. Subtracting the nightly average from the daily average, we end up with H D t H N t ¼ Q D 0 Q N 0 Q D h þ Q N h : ð2þ [31] As we follow drifter trajectories, we expect the divergence of the advective heat flux to have no daily cycle, arguing that it varies with a timescale much longer than the daily cycle. At times a velocity shear in the upper layer can be significant, causing the velocity at 15 m to differ from the average depth 0 20 m. We also neglect the effect of horizontal diffusion. Imawaki et al. [1988] showed that day-to-night differences in nonsolar radiation fluxes are negligible in the central Pacific. This result also holds in our study (Figure 5). The day-to-night difference =(H t D H t N )of the heat content change is calculated by subtracting the evening-tomorning change in temperature from the sea surface to 20 m according to (2). Note that possible spurious daytime warming of the sensors is not going to affect greatly these quantities, which depend on heat content at sunset and sunrise. [32] For all drifters this budget presented strong day-to-day variations as both negative and positive values occurred, mostly because of high-frequency variations of the heat content. The mean difference, averaged over the whole period of study (nearly 58 days), varied from 200 to 300 W m 2 for the different drifters. [33] The difference was generally positive, as the layer usually warmed during daytime under the influence of solar radiation. However, there were exceptions when the layer cooled during the day or warmed during the night. A cooling can happen under the influence of the wind, usually with reduced solar flux. There were examples (17 January and 8 February; Figure 6) when drifter experienced a negative day-night difference, even though the heat content continued to decrease at night (albeit at a more moderate rate). There were also cases when at 2 m a cooling during the day was followed by a temperature increase at night, the temperatures at a deeper level remaining stable and warmer. This happened usually on a day with weak wind conditions when, most likely, the daytime temperature inversion between the surface and 20 m was maintained by an input of cooler fresh water by rain. In such cases there was possibly velocity shear between the shallow freshwater lenses and the 15 m level at which the drifter was anchored, so that the Lagrangian assumption did not hold as well. [34] The random error dh in the estimate of the day-to-night difference of the heat change is given by [Imawaki et al., 1988] dh ¼ ð6þ1=2 rc p he tðnmþ ; 1=2 where N (5) is the number of sensors deployed throughout the layer of thickness h (20 m), e (in C) is the accuracy of sensed temperature, and M is the number of daily cycles averaged. Our sensors have an estimated accuracy of 0.2 C, which results in an estimated accuracy of 40 W m 2 for daily estimates, 11 W m 2 for 2 week averages, and 5 W m 2 for 2 month averages (57 daily cycle). The assumption is made here that the sensors have no systematic biases. [35] From (2) the daily estimates show strong variations of (Q N h Q D h ) between positive and negative values. To obtain more reliable estimates, 2 week averages are computed for the drifters south of the equator (Table 1) with the same periods defined in the previous sections. One intriguing result is that on average the night-to-day difference in turbulent flux at the base of the 20 m layer is negative (Figure 8), especially for the drifters experiencing strong winds. This is the case for the three drifters deployed south of the equator in December that reported negative differences of between 40 and 80 W m 2. The difference between morning and evening heat content was small during the periods of strong winds compared to periods of weak winds. Periods 2 and 3 of weak winds presented more positive turbulent heat fluxes at night with positive values between 50 and 100 W m 2. Our result for the period of strong winds differs from the central Pacific where the differences are generally positive (or on occasion weakly negative) [Imawaki et al., 1988]. We note that these two regions have different stratification and wind conditions, with a usually shallower thermocline and a shallower undercurrent in the central Pacific. [36] For consistency we carried out a similar analysis based on data from the WHOI mooring [Anderson et al., 1996; R. Weller and S. Anderson, personal communication, 1993]. We found periods at the mooring when the turbulent flux differences were negative or close to 0, and they never reached positive values for the period 20 January to 28 February. For the mooring data the H t

12 7-12 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE Figure 9. Fortnightly averaged daytime (open diamonds) and nighttime (shaded squares) turbulent heat fluxes versus wind stress. The dashed line is the trend for the daytime flux, and the solid line is the trend for the nighttime flux. (a) Drifters deployed south of 1 N. (b) Drifters deployed north of 1 N. term is a local change, and we should also take into account the advective term, although we do not expect this to affect significantly the day-to-night differences. However, Feng et al. [1998, 2000] suggested that advection in the upper ocean was not negligible at times in the IFA zone during the IOP. This is beyond the scope of our study, but it is worth noting that both the surface drifters and the mooring data suggest no increase in turbulent heat flux across 20 m at night. 6. Discussion [37] The mini-thermistor chain on BODEGA drifters allowed us to follow the hourly variations of the daily thermocline. Compared to earlier Eularian studies, our calculations were from drifters that are expected to be good followers of the currents at 15 m. In our Lagrangian framework the advective contribution to the heat budget is small (limited to effects of shear and slippage). Several terms contribute to the surface heat budget studied in our investigation of heat content variations. We estimated the fluxes from a combination of in situ data from the TAO moorings, ECMWF wind fields, and satellite radiative fluxes interpolated at the position of the drifters. We did not include the sensible heat fluxes associated with rain. This term, estimated, for example, by Cronin and McPhaden [1997] and Feng et al. [2000], was expected to be small compared to uncertainties in the fluxes, in particular, the uncertainty in shortwave radiation. This latter flux, critical for evaluating night-to-day difference in turbulent fluxes at the base of the layer, was derived from satellite estimation interpolated to the drifter positions. Satellite-derived fluxes were compared to measurements in the TOGA-COARE IFA and were adjusted to shortwave radiation measurements made at the WHOI mooring position [Weller and Anderson, 1996]. Empirically, we hypothesized that the correction was the same for the whole region. All these corrections could lead to significant error in the night-to-day differences in turbulent heat flux. The shortwave radiation is the main term contributing to the differences, as other components of the surface budget presented no daily cycle. A negative night-today difference was found during periods of strong winds. This

13 DU PENHOAT ET AL.: VERTICAL TURBULENT HEAT FLUXES DURING TOGA-COARE 7-13 result was robust, as similar results were found for the WHOI mooring data. We found that the coarse vertical resolution provided by the thermistor chain had little effect on the results. We also expect that anomalous heating of the sensors by direct solar heating was limited because the estimate is based on only the sunset and sunrise values of heat content. There is a remaining uncertainty on the heat budget caused by thermal inertia of the thermistor chain. However, our results were not sensitive to 1 hour shifts of the temperature data, which, according to tests, is the magnitude expected for this effect, but this remains poorly known. [38] A difficulty of the approach with drifters equipped with short thermistor chains is that we do not usually know how deep the mixed layer is at night. Thus our turbulent heat flux estimates are not representative of the fluxes at the base of the mixed layer. Mixed layer depth could be estimated if longer thermistor chains were used, but this is detrimental to the Lagrangian character of the device. The advantage of a Lagrangian approach is that in theory the budget does not require estimates of advection. However, it is quite possible that the drift velocity at 15 m depth might differ from the velocity averaged over the nighttime mixed layer, resulting in unknown advective errors in the budget. This situation probably happened for some drifters north of the equator in February 1993 when they entered regions of large horizontal temperature gradients. [39] The drifters provided an interesting sample of spatial variability in the heat budget of the warm pool. Our heat budgets differed for drifters in different areas, in particular, north and south of the equator. This is not surprising because the meteorological conditions encountered, wind bursts, and cloudiness varied significantly across the area. During a large part of the period, drifters north of the equator experienced heat loss at the air-sea interface as they drifted northward in areas of deep mixed layers (Figure 4). The situation south of the equator was characterized by recurring WWB with cooling and deepening of the mixed layer interpersed by calm situations with warming and shoaling mixed layers. [40] The average heat budget calculated from hourly estimates was consistent with the day-to-night difference in turbulent fluxes and involved heat gain from below the 20 m thick upper layer during strong wind events and heat loss at other times. These results confirm the model calculation by Shinoda and Hendon [1998], who found that near 165 E, entrainment cooling at the base of the mixed layer tends to be minimum and even acted to warm the mixed layer when the surface net heat flux was most negative. (It results because a statically stable temperature inversion can develop at the base of the fresh mixed layer.) Our results were also consistent with the heat budget of the diurnal mixed layer at 2 S, 156 E for the first WWB (period P1, in late December 1992 to early January 1993 presented by Smyth et al. [1996b]). They inferred a small downward turbulent flux at 34 m that would be consistent for this period with an upward turbulent flux of 40 W m 2 at 20 m. This is greater than the average heat budget for the drifters in this area and near the equator. However, it is consistent with the night-to-day differences in turbulent fluxes calculated from our drifter data, assuming that daytime turbulent fluxes were small at 20 m. Furthermore, we expect the turbulent heat fluxes to be larger at 20 m than at the nighttime mixed layer base (that was usually deeper than 20 m [Shinoda and Hendon, 1998; Smyth et al., 1996b]. [41] Because of the uncertainties on the daily fluxes, we group the data over rather long but quite homogeneous periods to obtain significant results. (There is some arbitrariness in defining the periods relative to the dates of major meteorological changes that vary by 1 or 2 days across the domain.) What emerges from this averaging is a dependency of the turbulent heat fluxes (and of the day-night differences) with respect to the wind intensity (Figure 9). South of the Equator, the average nighttime turbulent heat flux at 20 m is negative, and the daytime turbulent heat flux is positive, indicating a net diurnal reversal of the flux at this depth. Positive nighttime turbulent fluxes occurred on occasion but only under weak wind conditions (Figure 9) for drifters located south of 2 N. Under strong wind conditions the nighttime turbulent flux is stronger (in absolute value) than the daytime turbulent flux. This means that there is a global heat transfer from the top layers to the deeper layers by weak wind and a global transfer in the opposite direction by stronger winds. These results are in agreement with results using Imawaki et al. [1988] method in section 5. We should, however, notice that by no means do we think that mixing processes in this area are only a function of wind stress. Clearly, insolation (and the net heat budget at the sea surface) also plays a major role. The fact that near the equator, windy conditions are often associated with rather cloudy situations is certainly important. This should be contrasted with the situation north of the equator where the strong winds, in early February 1993, are northeasterlies trade winds that are not associated with increased cloudiness. The small set of drifters we used certainly did not experienced all possible situations for the area, and therefore our study should be thought as only representative of a subset of conditions. [42] Acknowledgments. Support by Maurice du Chaffaut, Annie Kartavtseff, and MarieJo Langlade contributed to the success of our participation in the surface drifter experiment in TOGA-COARE. Advice and support by Peter Niiler were essential. Steve Anderson and Bob Weller are gratefully acknowledged for an early release of their previous data of the WHOI mooring. Thanks also are due to Catherine Gautier and Martin Landsfeld for the GMS data and to Serge Planton and Gérand Eldin for discussion on the data. We are also grateful to Marie-Hélène Radenac for carefully reading the manuscript and to two anonymous reviewers as well as the editor, John Toole, for their constructive comments. We wish to thank the TAO project office for their effort in providing mooring data. The ECMWF TOGA-COARE special data set was provided by Ilana Stern at UCAR, and its preparation by Anthony Hollingsworth and Ernst Klinker is gratefully acknowledged. The project was primarily funded by IFREMER. References Anderson, S. P., R. A. Weller, and R. B. Lukas, Surface buoyancy forcing and mixed layer of the western Pacific Warm Pool: Observations and 1D model results, J. Clim., 9, , Ando, K., Y. Kuroda, K. Yoneyama, K. Muneyama, and K. Takeuchi, Variations of hydrographic properties and heat budget at 0, 156 E during the TOGA/COARE R/V Natsushima cruise, J. Meteorol. Soc. Jpn., 73, , Bond, N. A., and M. J. McPhaden, An indirect estimate of the diurnal cycle in upper ocean turbulent heat fluxes at the equator, 140W, J. Geophys. Res., 100, 18,369 18,378, Cronin, M. F., and M. J. McPhaden, The upper ocean heat balance in the western equatorial Pacific warm pool during September December 1992, J. Geophys. Res., 102, , du Penhoat, Y., G. Reverdin, A. Kartavtseff, and M. J. Langlade, BODEGA surface drifter measurements in the western equatorial Pacific during TOGA COARE experiment, Notes Tech. Sci. Mer ORSTOM-Nouméa, 11, 241 pp., Eldin, G., T. Delcroix, C. Hénin, K. Richards, Y. du Penhoat, J. Picaut, and P. Rual, Large-scale current and thermohaline structures along 156 E during the COARE intensive Observation period, Geophys. Res. Lett., 21, , Fairall, C. W., E. F. Bradley, D. P. Rogers, J. B. Edson, and G. S. Young, Bulk parameterization of air-sea fluxes for Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment, J. Geophys. Res., 101, , 1996a. Fairall, C. W., E. F. Bradley, J. S. Godfrey, G. A. Wick, J. B. Edson, and G. S. Young, Cool-skin and warm-layer effects on sea surface temperature, J. Geophys. Res., 101, , 1996b. Feng, M., P. Hacker, and R. Lukas, Upper ocean heat and salt balances in response to a westerly wind burst in the western equatorial Pacific during TOGA COARE, J. Geophys. Res., 103, 10,289 10,311, Feng, M., R. Lukas, P. Hacker, R. A. Weller, and S. P. Anderson, Upperocean heat and salt balances in the western equatorial Pacific in response to intraseasonal oscillation during TOGA COARE, J. Clim., 13, , Gaspar, P., Y. Grégoris, and J. M. Lefèvre, A simple eddy kinetic energy