Wintertime shoaling of oceanic surface mixed layer

GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 22, 2152, doi:10.1029/2003gl018511, 2003 Wintertime shoaling of oceanic surface mixed layer Emiri Takeuchi and Ichiro Yasuda Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Japan Received 28 August 2003; accepted 8 October 2003; published 21 November 2003. [1] Wintertime oceanic surface mixed layer has been believed to deepen due to cooling and wind stirring. However, it is shown that in the latitudes of 20 30 in the world ocean, there are regions where wintertime oceanic surface mixed layer shoals. The mixed layer shoaling is mostly accompanied by sea surface temperature (SST) cooling from January to February (from July to August), and SST warming from February to March (from August to September) in the northern (southern) hemisphere ocean. Further studies on the evolution of the mixed layer in these areas are suggested as these shoaling phenomena cannot be explained by existing theories of oceanic mixed layer based on monthly surface flux data. INDEX TERMS: 4572 Oceanography: Physical: Upper ocean processes; 4504 Oceanography: Physical: Air/sea interactions (0312); 4283 Oceanography: General: Water masses; 4227 Oceanography: General: Diurnal, seasonal, and annual cycles. Citation: Takeuchi, E., and I. Yasuda, Wintertime shoaling of oceanic surface mixed layer, Geophys. Res. Lett., 30(22), 2152, doi:10.1029/2003gl018511, 2003. 1. Introduction [2] Oceanic surface mixed layer (ML), which is a vertically well-mixed and homogeneous layer in density, temperature and salinity, is very important for various reasons. First, sea surface temperature (SST) that affects the atmospheric circulation critically depends on mixed layer depth (MLD). In addition, ML has great impacts on biological production, global carbon cycles, and water mass formation [e.g., Polovina et al., 1995; Suga and Hanawa, 1990]. [3] Wintertime ML has been generally believed to deepen due to wind stirring and sea surface cooling, reaching its maximum depth in late February and/or March [e.g., Bathen, 1972; Stommel, 1979; Suga and Hanawa, 1990; Marshall et al., 1993; Kara et al., 2002]. In this study, horizontal distribution of MLD and its wintertime variation are re-examined by analyzing the climatological data [WOA98: Antonov et al., 1998a, 1998b, and 1998c; Boyer et al., 1998a, 1998b, and 1998c] and the temperature time series data [White, 1995]. 2. Mixed Layer Shoaling Based on the Climatological Data [4] Figure 1 shows the horizontal distribution of MLD in January, February and March (July, August and September) in the northern (southern) hemisphere ocean from WOA98. MLD is defined as the depth whose density is 0.125 kg/m 3 denser than the sea surface density. The results here are Copyright 2003 by the American Geophysical Union. 0094-8276/03/2003GL018511 OCE 4-1 qualitatively robust even if the density difference in the MLD definition is varied from 0.05 kg/m 3 to 0.5 kg/m 3. Wintertime deepening of ML is seen in mid to high latitude on the polar side of 30, being consistent with the previous studies that wintertime ML deepens under the strong wind and cooling. In the North Pacific, for example, the deep ML regions are seen in the latitudes of 38 N 40 N and 30 N 35 N, corresponding to the formation area of Central Mode Water (CMW) [e.g., Nakamura, 1996; Suga et al., 1997] and Subtropical Mode Water (STMW) [e.g., Masuzawa, 1969] respectively. [5] In contrast, there are regions where ML becomes shallower in winter in the latitudes of 18 30, where MLD changes from about 100m in early winter to less than 50m in late winter (Figure 1). The shoaling regions are located roughly in the latitudes between 15 N and 30 N in the northern hemisphere and between 10 S and 35 S inthe southern hemisphere, especially between 20 N(S) and 30 N(S) in the North (western South) Pacific and South Indian Ocean (Figure 2). In each ocean, the shoaling regions are wider and the shoaling speeds are larger in February March (August September) than in January February (July August) in the northern (southern) hemisphere. Note that the regions where ML becomes shallower are located in the similar latitude bands in each ocean. Also, the mixed layer front becomes prominent from early to late winter around the latitude of 30 between the deepening region and shoaling region in all the oceans (Figure 1). [6] In the ML shoaling regions, Ekman downwelling prevails, and wind speed is weaker compared with the higher latitude. The shoaling regions correspond to the formation area of Tropical Water [e.g., Cannon, 1966 for the North Pacific] in the North Pacific and to the equatorial side of the formation area of STMW and Eastern Subtropical Mode Water [e.g., Hautala and Roemmich, 1998 for the North Pacific; Provost et al., 1999 for the South Atlantic; Tsuchiya and Talley, 1996 for the South Pacific]. The southern rim of the shoaling regions around the latitude of 20 N corresponds to the northern rim of the westward North Equatorial Current. Southwestern part of ML shoaling region in the North Pacific corresponds to the Subtropical Counter Current [Uda and Hasunuma, 1969]. [7] The column a of Table 1 lists the areal averages of monthly MLD change (minus indicates ML shoaling) in the latitudes of 20 N(S) 30 N(S) from January to March (from July to September) in the five oceans and their 95% confidence interval, which is defined here as 1:96s p ffiffiffiffi N f, where s and N f denote standard deviation and degree of freedom respectively. N f is estimated as N f = N total N dcs where N total is total grid number and N dcs is the grid number inside of the decorrelation scale (= 222 km [Antonov et al., 1998a]). Statistically significant ML shoaling occurs in the North Pacific and the North Atlantic from February to March and

OCE 4-2 TAKEUCHI AND YASUDA: WINTERTIME SHOALING OF MIXED LAYER Figure 1. Horizontal distribution of oceanic surface mixed layer depth (MLD, in m). Color changes in 25 m interval and contour interval is 100 m. Upper panel: January (July), middle panel: February (August), lower panel: March (September) in the northern (southern) hemisphere. in the South Pacific and the South Indian Ocean from August to September. [8] Sea surface density increases in most of the shoaling regions from January to February (from July to August) and in half of the shoaling regions from February to March (from August to September) in the climatological data (not shown). [9] Monthly climatological net surface heat flux (henceforth, nshf) which is the sum of latent heat flux, sensible heat flux, longwave radiation, and shortwave radiation, in January and February (July and August) in the northern (southern) hemisphere is negative poleward of 15, indicating ocean cooling (nshf in February (August) is shown in the column f, g and h of Table 1). In March (September), there are some heating region, but the areal average of 20 N(S) 30 N(S) denotes cooling in every data set (the column f, g and h of Table 1). Above results are confirmed by da Silva [da Silva et al., 1994], NCEP/NCAR [Kalnay et al., 1996], and corrected Southampton Oceanography Centre (SOC) Air-Sea Flux [Grist and Josey, 2003]. 3. Mixed Layer Shoaling Based on the Time Series Data [10] Wintertime shoaling of ML is also seen in the time series data [White, 1995]. We use 2 in latitude and 5 in longitude gridded monthly temperature time series data for 39 years from January 1955 to December 1993. We estimate MLD whose temperature is 0.5 C colder than SST, corresponding to the density difference 0.125 kg/m 3 around the shoaling region. We took the monthly changes of MLD and SST at each grid from the time series data. The column b of Table 1 shows the areal averages of monthly MLD change in the latitudes of 20 N(S) 30 N(S) in the five oceans and their 95% confidence interval. From January to February (from July to August) in the northern (southern)

TAKEUCHI AND YASUDA: WINTERTIME SHOALING OF MIXED LAYER OCE 4-3 Figure 2. The monthly change of mixed layer depth (in m). Upper panel: in the period from January to February (July to August), lower panel: in the period from February to March (August to September) in the northern (southern) hemisphere, The regions where MLD shoals (deepens) are shown in the blue-purple (yellow-red) color. hemisphere, there are both deepening and shoaling in the areal average of MLD change, but these deepening and shoaling are both not statistically significant, except for the deepening in the North and South Pacific (the column b of Table 1). It is noted that ML shoaling indeed occurs at 30 60% with the shoaling of 10 to 20 m (the column c of Table 1), although ML shoaling is not significant in the mean MLD change. From February to March (from August to September) in the northern (southern) hemisphere, shoaling of 2 to 24 m occurs significantly, except in the South Atlantic (the column b of Table 1). ML shoaling occurs at 50 80% with the shoaling magnitude of 10 to 40 m. The regions where the ML shoaling phenomena from January to March (from July to September) in the northern (southern) hemisphere occur, well agree with the shoaling regions seen in the climatological data. [11] We next examine whether the ML shoaling phenomena are accompanied by warming or cooling. If heating occurs and the SST becomes warmer, it is reasonable to expect ML to shoal as ML would be capped with warm water and shallower ML would develop near the sea surface. From January to February (from July to August) in the northern (southern) hemisphere, SST cooling dominantly occurs at nearly 70% of the ML shoaling phenomena, except that SST cooling occurs less or equal to SST warming in the South Atlantic and South Indian Ocean (the column d and e of Table 1). From February to March (from August to September) in the northern (southern) hemisphere, SST warming dominantly occurs at nearly 60% of the ML shoaling phenomena, while SST cooling and warming evenly occurs in the South Indian Ocean. ML shoaling of 10 to 50 m in the case of SST warming in each ocean tends to be greater than ML shoaling of 10 to 30 m in the case of SST cooling. 4. Summary and Discussion [12] Using the monthly climatology data and the time series data, we have investigated the mixed layer variation and found an unexpected feature; mixed layer becomes shallower in winter in the latitudinal band of 20 30 in the world ocean, in contrast to the previous thought that mixed layer deepens under the wintertime cooling and strong wind condition. Based on the analysis of the time series data, wintertime shoaling of mixed layer occurs at 30 60% from January to February (from July to August) and at 50 80% from February to March (from August to September) in the northern (southern) hemisphere. Since the shoaling phenomena are mainly accompanied by SST cooling from January to February (from July to August) in the northern (southern) hemisphere, it cannot be explained by existing theories of mixed layer. Therefore, the change in the wintertime mixed layer depth should be examined in more detail to understand the mechanism of wintertime mixed layer shoaling. [13] Although the mixed layer shoaling phenomena are accompanied by SST warming from February to March (from August to September) in the northern (southern) hemisphere, the analysis of monthly mean surface heat flux indicates that the ocean is actually losing heat to the atmosphere. One possible explanation for this apparent inconsistency is a surface heat flux variability with a time

OCE 4-4 TAKEUCHI AND YASUDA: WINTERTIME SHOALING OF MIXED LAYER Table 1. Statistics of wintertime mixed layer depth change (MLD) and net surface heat flux (nshf) in the latitudes between 20 and 30 a (a) MLD(m) (b) MLD(m) (c) ML Shoaling (d) SST Warming (e) SST Cooling (f ) nshf (g) nshf (h) nshf (WOA98) (White) Ratio (MLD) Ratio (MLD) Ratio (MLD) Month (da Silva) (NCEP) (SOC) Area Month North Pacific Jan. to Feb. 4.1 ± 5.5 5.1 ± 4.7 40.6% ( 17.8m) 22.1% ( 25.1m) 73.2% ( 15.7m) Feb. 132.7 102.7 128.1 (119.5 E 170.5 W) Feb. to Mar. 24.9 ± 5.9 23.8 ± 5.5 79.1% ( 33.9m) 60.9% ( 38.5m) 32.6% ( 26.2m) Mar. 44.1 28.2 21.1 North Atlantic Jan. to Feb. 4.2 ± 5.5 2.5 ± 6.4 39.5% ( 22.7m) 22.1% ( 28.2m) 70.4% ( 22.0m) Feb. 80.8 69.7 78.4 (80.5 W 40.5 W) Feb. to Mar. 17.6 ± 6.9 16.4 ± 7.9 65.1% ( 33.9m) 59.2% ( 37.0m) 31.8% ( 30.2m) Mar. 20.7 17.4 29.5 South Pacific Jul. to Aug. 2.3 ± 6.7 10.1 ± 7.2 30.9% ( 21.5m) 22.9% ( 29.3m) 71.7% ( 19.2m) Aug. 91.4 72.5 101.4 (149.5 E 170.5 W) Aug. to Sep. 25.1 ± 8.0 21.8 ± 9.6 68.4% ( 40.6m) 62.6% ( 46.7m) 28.6% ( 29.1m) Sep. 34.8 20.6 30.3 South Atlantic Jul. to Aug. 21.1 ± 9.5 4.0 ± 5.5 62.4% ( 9.5m) 58.9% ( 8.2m) 31.8% ( 11.5m) Aug. 36.8 40.4 50.3 (30.5 W 14.5 W) Aug. to Sep. 16.5 ± 22.9 2.4 ± 7.9 50.4% ( 12.2m) 72.5% ( 12.2m) 19.9% ( 13.2m) Sep. 5.6 0.2 9.8 South Indian Ocean Jul. to Aug. 5.2 ± 7.0 2.9 ± 5.2 52.6% ( 19.6m) 41.3% ( 19.7m) 51.4% ( 19.3m) Aug. 106.3 102.6 111.4 (74.5 E 114.5 E) Aug. to Sep. 10.6 ± 8.5 7.8 ± 5.2 61.5% ( 19.8m) 49.8% ( 20.2m) 42.4% ( 19.1m) Sep. 51.5 43.6 52.4 Average 0.2 3.4 45.2% ( 19.2m) 33.5% ( 21.7m) 59.7% ( 18.2m) 101.1 84.9 103.7 19.9 17.5 64.9% ( 31.8m) 61.0% ( 35.8m) 31.1% ( 25.5m) 36.1 25.0 29.7 a (a) Areal average of change of MLD (MLD) (in m) from WOA98, (b) areal average of monthly MLD change (MLD) (in m), (c) ML shoaling ratio (in %) and mean MLD change (in m), (d) SST warming ratio (in %) in the ML shoaling and mean MLD change (in m), (e) SST cooling ratio (in %) in the ML shoaling and mean MLD change (in m) ((b e) from White s time series data), (f h) areal average of nshf (in W/m 2 ) from (f ) da Silva, (g) NCEP/NCAR and (h) corrected SOC climatological data. 95% confidence interval in (a) and (b) is defined as 1:96s p ffiffiffiffi, where s is a standard deviation and Nf a degree of freedom = Ntotal. N total denotes Nf Ndcs the total grid number and Ndcs the grid number inside of the decorrelation scale (222 km for WOA98 and 10 in latitude and 20 in longitude square for the White s time series data http://jedac.ucsd.edu/) respectively). scale shorter than one month; mixed layer would be shallower and SST would be warmer if observation were carried out under a heating condition. Further studies using daily heat flux data to clarify the mixed layer shoaling phenomenon are underway. The result here also suggests that we need to be careful in estimating subduction rate based on the maximum mixed layer depth [e.g., Marshall et al., 1993] as a mixed layer does not reach its maximum depth in March at least in the mixed layer shoaling regions. [14] Acknowledgments. 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TAKEUCHI AND YASUDA: WINTERTIME SHOALING OF MIXED LAYER OCE 4-5 Stommel, H., Determination of watermass properties of water pumped down from the Ekman layer to the geostrophic flow below, Proc. Nat. Acad. Sci. U.S.A, 76, 3051 3055, 1979. Suga, T., and K. Hanawa, The mixed-layer climatology in the northwestern part of the North Pacific subtropical gyre and the formation area of Subtropical Mode Water, J. Mar. Res., 48, 543 566, 1990. Suga, T., Y. Takei, and K. Hanawa, Thermostad distribution in the North Pacific subtropical gyre: The central mode water and the subtropical mode water, J. Phys. Oceanogr., 27, 140 152, 1997. Tsuchiya, M., and L. D. Talley, Water-property distributions along an eastern Pacific hydrographic section at 135W, J. Mar. Res., 54, 541 564, 1996. Uda, M., and K. Hasunuma, The Eastward Subtropical Countercurrent in the Western North Pacific Ocean, J. Oceanogr. Soc. Japan, 25, 201 210, 1969. White, W. B., Design of a global observing system for gyre-scale upper ocean temperature variability, Prog. Oceanogr., 36, 169 217, 1995. E. Takeuchi and I. Yasuda, Department of Earth and Planetary Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. (emiri@aos.eps.s.u-tokyo.ac.jp; ichiro@eps.s.u-tokyo.ac.jp)