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Heat balance and regime shifts of the mixed layer in the Kuroshio Extension
Progress in Oceanography 47 (2000) 257–278 www.elsevier.com/locate/pocean
Heat balance and regime shifts of the mixed layer in the Kuroshio Extension Ichiro Yasuda *, Tomoki Tozuka, Masayuki Noto, Shinya Kouketsu Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract We investigated the seasonal variations of the mixed layer depth and temperature in the Kuroshio Extension region (145–180°E, 30–36°N), and studied the causes of the mixed layer depth and temperature ‘regime shifts’ which occurred in the late 1980s, using upper ocean thermal and heat flux datasets incorporated with a bulk mixed layer model. The mixed layer from fall to winter is cooled by the net surface heat flux, the Ekman transport and the entrainment, and warmed by the horizontal heat convergence resulting from the Kuroshio heat advection. The mixed layer depth is controlled by the entrainment and the horizontal transport divergence/convergence which act to slow the mixed layer deepening from fall to early winter and then in winter deepen the mixed layer. The entrainment velocity is significantly influenced by the temperature difference between the mixed layer and the layer below. The mixed layer shift from its deep to shallow phase occurred in 1985. This shift is preceded the SST-shift in 1988 by three years, and was caused by the horizontal transport divergence anomaly. The horizontal heat convergence of the Kuroshio Extension caused the SST-shift in 1988, whereas its anomaly had been already positive since 1983. The delay from 1983 to 1987 can be attributed to the effects of the negative fall–SST anomaly, stronger surface heat flux and Ekman cooling and the shoaling of the mixed layer depth. It is suggested that the Kuroshio current system plays a major role in forcing the SST-shift and thus the subsequent climate regime shift. 2000 Elsevier Science Ltd. All rights reserved.
Contents 1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
* Corresponding author. Tel.: +81-3-5841-4288; fax: 81-3-5841-8791. E-mail address: [email protected] (I. Yasuda). 0079-6611/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 0 0 ) 0 0 0 3 8 - 0
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2.
Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
3.
The mixed layer model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
4. Seasonal variations of the mixed layer depth and temperature . 4.1. Mixed layer temperature . . . . . . . . . . . . . . . . . . . . . . 4.2. The mixed layer depth . . . . . . . . . . . . . . . . . . . . . . . 4.3. Temperature difference ⌬T between the mixed layer and the
. . . . . . . . . . . . . . . . . . . . . . . . layer below
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5.
Regime shift of the mixed layer depth in 1985 . . . . . . . . . . . . . . . . . . . . . 267
6.
Regime shift of the mixed layer temperature in winter of 1987–1988 . . . . . . . 270
7.
Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277
1. Introduction The Kuroshio Extension and its southern recirculation region at 145–180°E and 30–36°N are the areas where a large amount of heat is released from the ocean. Talley (1984) showed that a western boundary current, such as the Kuroshio, loses a significant portion of its heat to the atmosphere. Thus, the Kuroshio Extension region possibly has a substantial impact on the climate variability in the North Pacific. It is well known that the atmosphere–ocean system in the North Pacific undergoes decadal and inter-decadal variability (e.g. Nitta & Yamada, 1989; Trenberth, 1990; Latif & Barnett 1994, 1996; Miller, Cayan, Barnett, Graham & Oberhuber, 1994; Nakamura, Lin & Yamagata, 1997; Minobe, 1997; Yasuda & Hanawa, 1997). The shift in the climate regime, which occurred in the mid 1970s, has been particularly well studied. Changes in the mixed layer depth and temperature in the Kuroshio Extension regions influence biological production. Noto and Yasuda (1999) showed that the winter-SSTs in the Kuroshio Extension regions are significantly related to the mortality of the Japanese sardine. Polovina, Mitchum and Evans (1995) demonstrated the relationship between the mixed layer depth in winter/spring and the biological production using bio-physical models. When the mixed layer deepens, nutrients from subthermocline depths are brought up into the mixed layer. As a result, phytoplankton production increases and the growth of fish is augmented. Studies to clarify the mechanisms underlying the variations in SST and mixed layer depth in the Kuroshio Extension are thus a high priority. The heat balance of the mixed layer of the Kuroshio Extension was studied by Qiu and Kelly (1993) using a 3-dimensional bulk mixed layer model (which was a
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3D extension of the model by Kraus & Turner, 1967) driven by the data of surface heat flux, surface wind-stress and geostrophic velocity fields derived from Geosat Altimetry. They reproduced climatological mixed layer depths and temperatures, and concluded that geostrophic heat transport by the Kuroshio Extension results in a heat divergence; i.e., the Kuroshio cools the mixed layer in the Kuroshio Extension. Qiu and Huang (1995) estimated the rates of ‘subduction’ and ‘obduction’ in the North Pacific, and emphasized the importance of the Kuroshio Extension regions where obduction and subduction take place. Subduction and obduction are processes of interaction between the surface mixed layer and subsurface layer. In subduction waters in the mixed layer escape from the mixed layer and intrude into subsurface; while in obduction, subsurface waters intrude into the surface mixed layers. In this study, we re-examined the heat balance in the Kuroshio Extension using a 3D bulk mixed layer model, similar to the one used by Qiu and Kelly (1993). Qiu and Kelly (1993) calculated the mixed layer depths and temperatures with the 3D mixed layer model, and discussed the effects of the horizontal velocity fields. However, since the velocity field is still not easy to estimate accurately and also the mixed layer model may be sensitive to the processes of obduction and subduction processes, the effects of the horizontal velocity field may be difficult to evaluate from the model. In contrast, we have used observational data for the mixed layer depth and temperature (in addition to the surface heat flux and surface wind stress data) and estimated the effects of horizontal velocity fields, other than Ekman transport, as unknown residuals. Our results showed that the Kuroshio Extension transports large amounts of heat but is cooled by heat losses to the atmosphere, the entrainment of cold deep water and southward Ekman transport. This is different from the conclusion of Qiu and Kelly (1993). We next examined the shifts that occurred in the late 1980s. Noto and Yasuda (1999) showed that in the Kuroshio Extension SST abruptly increased during the winter of 1987–1988, one or two years in advance of the well-known climate shift that occurred in 1989–1990 (e.g. Nakamura et al., 1997; Kachi & Nitta, 1997; Watanabe & Nitta, 1998). We also found there was a ‘regime shift’ in the winter mixed layer depth changing from a thick to a thin phase in 1985. This shift preceded by 3 years the SST-shift that occurred in 1988. We will discuss the causes of these shifts on the basis of heat balance and variations in the mixed layer depth. In the following section, we present the data used in this study. The mixed layer model is then described in Section 3. In Section 4 we discuss how the seasonal variations in the temperature and the depth of the mixed layer are determined. In Sections 5 and 6, the causes of the ‘regime shifts’ of the mixed layer depth and temperature in the late 1980s are sought, and in the final section the results will be summarized. 2. Data The ocean temperature data used in the present study is White’s Ocean Temperature Climatology 1955–1992. This data set has been assembled and maintained by
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the Data Support Section, Scientific Computing Division of the National Center for Atmospheric Research (NCAR). The original source of these data is the Scripps Institute of Oceanography. Temperatures were measured by the XBTs (expendable bathythermograph). The processing of the raw hydrographic data to obtain the monthly temperature anomaly is described by White (1995). The data resolution is 2° in latitude and 5° in longitude on a monthly basis from 1955 to 1992. There are 11 levels in the vertical, six of which are in the upper 120 m (0, 20, 40, 60, 80, 120, 160, 200, 240, 300, 400 m). White (1995) reported that maximum interpolation errors are 0.4°C at surface, 0.5°C at 200 m and 0.2°C at 400 m for biennial signals, and so the data are not adequate for detecting year-to-year changes in upper ocean heat storage; we believe that long-term variations can be discussed as described in Yasuda and Hanawa (1997). For surface heat flux (net heat flux, short-wave radiation, longwave radiation, sensible heat, latent heat), scalar wind speed, zonal and meridional wind stresses, we used monthly data from da Silva, Young and Levitus (1994) for the period from 1955 to 1992. The data resolution is 1° in both latitude and longitude. According to da Silva et al. (1994), r.m.s. errors of monthly wind speed are 0.4–0.8 m/s in the North Pacific. Heat flux uncertainties have been discussed by Weare (1989). Since we examine the seasonal and long-term variations of the heat flux averaged in a wide area, 30–36°N and 145–180°E, the random errors are considerably reduced as Weare (1989) and Yasuda and Hanawa (1997) discussed.
3. The mixed layer model The mixed layer depth can be defined in various ways. Qiu and Kelly (1993) defined it as the depth where the depth-averaged temperature is 1°C higher than the water temperature just below the base of the mixed layer. Polovina et al. (1995) defined the depth of the mixed layer according to the temperature profile. Lamb’s (1984) definition was the depth at which the water temperature is 1°C lower than at the sea surface; and is the definition we use in this study. The mixed layer model used here is similar to Qiu and Kelly’s (1993) 3D bulk mixed layer model. The equation, which governs the mixed layer temperature, is →
∂T ⌬T·we Ue 1 ⫽⫺ ·ⵜT⫹ (Qnet⫺qd)⫺ ⫹(Horizontal). ∂t h r0ch h
(1)
Alternatively in the form of heat content and heat flux, → ∂T r0ch ⫽⫺r0cU e·ⵜT⫹(Qnet⫺qd)⫺r0c⌬T·we⫹(Horizontal), ∂t
(2) →
where T is the mixed layer temperature, h the mixed layer depth, U e=(Ue,Ve) the Ekman transport vector,
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冉冊 冉冊
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1 tx Ve⫽⫺ f r0
(3)
1 ty Ue ⫽ f r0
(4)
r0 is the density of the sea water (=1025 kg/m3), c is the specific heat of the sea water, Qnet is the surface net heat flux, and ⌬T is the temperature difference between the mixed layer and the layer below, and we is the entrainment velocity. tx and ty denote zonal and meridional wind-stresses, respectively. qd=q(⫺h) is the downward radiative flux at the base of the mixed layer and the downward flux at depth of z, q(z) is given by
冋
册
z z q(z)⫽q(0) R exp ⫹(1⫺R)exp g1 g2
(5)
where q(0) is the surface downward radiative flux, R (=0.77) is a separation constant, and g1 (=1.5 m) and g2 (=14 m) are the attenuation length scales (Paulson & Simpson, 1977; Jerlov, 1968). Qiu and Kelly (1993) fix ⌬T at 1°C, where the variation of the temperature in the lower layer cannot induce a change in the entrainment velocity. In the present study, ⌬T is defined as the temperature difference between that at the sea surface (T) and at 20 m below the mixed layer base (Td). In Eq. (1), the first term on the right hand side denotes the horizontal heat convergence resulting from the Ekman transport, the second surface heat flux, and the third entrainment. By using monthly observation data (the area average in the Kuroshio Extension region) on the left hand side and the first three terms on the right hand side, we estimated the horizontal effects, such as the heat advection by the Kuroshio current and eddies other than caused by the Ekman transport. The entrainment velocities we are calculated by the following equation (Davis, de Szoeke & Niiler, 1981; Qiu & Kelly, 1993):
冕
2 q(z)dz 2m0u3∗ 兩Qnet兩−Qnet Qnet+qd we⫽ ⫹ ⫺ ⫺mc , agh⌬T r0ch⌬T r0c⌬T 2r0c⌬T
(6)
where m0 is a coefficient which shows the effect of stirring by wind, mc is the convective efficiency coefficient, and a is the thermal expansion coefficient (=0.00025°C⫺1). From Davis et al. (1981), we use m0=0.5; from laboratory experiments of Deardorff, Willis and Lilly (1969), we take mc to be 0.83. In Eq. (6), the first term on the right hand side denotes the effect of wind stirring (where frictional velocity is defined as u2∗=raCDU 210/r0, CD=0.00125 is a drag coefficient and U10 is the scalar wind speed at 10 m height), the second term denotes radiative penetration flux, and the last two terms are surface buoyancy terms. When the mixed layer is deepening, the entrainment velocity is calculated from the observation data. On the other hand, when the mixed layer is shoaling, the entrainment velocity is set to zero.
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Note that all the parameters used in the present study are the same as in Qiu and Kelly (1993). Finally, the velocity at which the mixed layer deepens is ∂h ⫽we⫹(Horizontal) ∂t
(7)
Horizontal effects as transport convergence are estimated from Eq. (7) using observation data for (∂h)/(∂t) and we.
4. Seasonal variations of the mixed layer depth and temperature 4.1. Mixed layer temperature Fig. 1a shows the monthly averaged climatology of the mixed layer temperature in the Kuroshio Extension region. The temperature increases from April to August, and reaches its maximum in August. The temperature then decreases from September to March reaching its minimum in March. The temperature equations of the mixed layer model (Eq. (1)) were calculated and horizontal effects were estimated for the 38 years from January 1955 to December 1992 at each grid point, and spatial and monthly averages were taken (Fig. 2a). Fig. 2b shows the seasonal contributions by each term to the heat flux equation of Eq. (2). The net surface heat flux is positive from April to August, when the ocean is gaining heat from the atmosphere. The net surface heat flux is negative from September to March, when the ocean is losing heat to the atmosphere. The entrainment mixes cold water from the lower layer and so cools down the mixed layer. So, while the mixed layer is deepening from September to March, the entrainment always acts to lower the temperature of the mixed layer. In general, the Westerly Wind blows over the Kuroshio Extension region. This results in a southward Ekman transport. Since the negative northward temperature gradient is present, the Ekman transport acts to cool the mixed layer in this region. The horizontal effects include the advection of the Kuroshio current and the eddy diffusive fluxes, and are found to contribute significantly to the heat balance, especially from fall to winter. Except in summer, this term results in warming of the mixed layer, since the Kuroshio is transporting large amounts of warm water into the area. Note that this result is different from the conclusion reached by Qiu and Kelly (1993) that the horizontal effects act to cool the mixed layer. This difference might arise from the geostrophic velocity fields used in Qiu and Kelly (1993), interannual variability (1986–1989 in Qiu and Kelly (1993) and climatology for 38 years in the present study) or the mixed layer model. We believe that the result that the Kuroshio warms the mixed layer obtained in the present study is more intuitive than the Qiu and Kelly (1993) conclusion. Finally, if integrated annually, both the surface heat flux (3.60×10⫺7°C/s) and the
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Fig. 1. Monthly climatologies of (a) the mixed layer temperature T (in °C), (b) the mixed layer depth MLD (in m) and (c) the temperature difference between the mixed layer and the layer below ⌬T (=T⫺Td) (in °C), averaged in the Kuroshio Extension region of 145–180°E and 30–36°N. Vertical bars denote standard deviations.
horizontal effects (27.13×10⫺7°C/s) act to warm the mixed layer in the Kuroshio Extension region, whereas an entrainment (⫺22.73×10⫺7°C/s) and the Ekman transport (⫺7.43×10⫺7°C/s) both cool the mixed layer. The horizontal effects have the largest absolute value, while the net surface heat flux has the smallest absolute value.
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Fig. 2. Monthly climatologies of the heat balance and contributing terms, to the mixed layer temperature in Eq. (1) (a), and to the mixed layer heat content (b), averaged in the Kuroshio Extension region. Values are in 10⫺7°C/s (a) and in W/m2 (b). In (a), ‘dT/dt’ denotes the temperature time change rate, ‘heat f’ surface net heat flux, ‘entrain’ entrainment, ‘Ekman’ Ekman heat convergence and ‘Hor’ the horizontal effects in Eq. (1). In (b), notations are the same as in (a) but for ‘dH/dt’ of the heat content time change rate and Eq. (2).
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Our estimate of surface heat flux is much smaller than the one obtained by Qiu and Kelly (1993) (38.67×10⫺7°C/s). One possible reason is the difference in the areas assessed: 30–36°N and 145–180°E in the present study and 30–40°N and 140–180°E in Qiu and Kelly (1993). Another possible cause is the mixed layer depth in summer from which subtle differences would generate large differences in the values because the contribution of the surface heat flux is a function of Qnet/h. 4.2. The mixed layer depth Seasonal variations of the mixed layer depth are shown in Fig. 1b. The mixed layer depth is the shallowest during the summer, at about 20 m. It then deepens slowly until March, when the mixed layer depth is at its deepest at around 200 m, and then shoals sharply in April. The climatology of entrainment velocities and term-balances in Eq. (6) shown in Fig. 3 are monthly averages for 38 years from January 1955 to December 1992. The entrainment velocity is largest in January. Generally, comparing the three terms, the wind stirring and the surface buoyancy are dominant, whereas the radiation penetration flux term is insignificant. Wind stirring has the largest effect except in February and March. Next, we discuss how the vertical and horizontal effects determine the mixed layer
Fig. 3. Monthly climatologies of the entrainment velocity we and contributing terms in Eq. (6). Values are averaged over 38 years. ‘We’ denotes the entrainment velocity, ‘wind’ denotes the wind stirring term, ‘p.f.’ denotes the radiative penetration flux term, and ‘s.f.’ denotes the surface buoyancy flux term. Values are in 10⫺6 m/s.
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depth (Fig. 4). This is done by calculating Eq. (7) for 38 years and taking monthly averages to evaluate the horizontal effects. The mixed layer in the Kuroshio Extension region deepens from August to March (Fig. 1b), most sharply in February. However, the vertical effect (entrainment velocity) is the largest in January. The horizontal effects make the mixed layer shallower from October to December, whereas the horizontal effects play a role in the deepening of the mixed layer in January and especially in February, making the rate of mixed layer deepening the largest in February. This dominant contribution of the horizontal effects in February implies that obduction is important then. 4.3. Temperature difference ⌬T between the mixed layer and the layer below Finally, we discuss the temperature difference between the mixed layer and the lower layer ⌬T (Fig. 1c). ⌬T shows significant seasonal variation ranging from 1 to 4°C. Thus the fixed ⌬T employed by Qiu and Kelly (1993) was probably inappropriate except in winter. ⌬T reaches its maximum in August and then decreases to a minimum in January. It then increases slightly but again reaches a minimum in April. If the temperature difference is large (small), the entrainment velocity becomes small (large). Interannual and decadal variations in the mixed layer depth might also be influenced by these temperature difference as will be shown below. The systematic difference in ⌬T between that in the present study (⌬T=1⫺3.5°C)
Fig. 4. Monthly balances between the vertical (entrainment) and the horizontal effects in the time change rate of mixed layer deepening (see Eq. (7)). Values are averaged for 38 years. ‘dh/dt’ denotes the rate of mixed layer deepening, ‘We’ denotes the entrainment velocity, and ‘Hor’ denotes the horizontal effects. Values are in 10⫺6 m/s.
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and in Qiu and Kelly (1993) (⌬T=1°C) has no influence on the estimated heat balance, because, as shown in Eqs. (1) and (6), the mixed layer temperature is not a function of ⌬T. Hence the conclusion that the fall–winter mixed layer is cooled by the net surface flux, entrainment and Ekman cooling but is warmed by the Kuroshio heat transport remains unchanged. However, the systematic difference in ⌬T between the two studies may generate changes in the mixed layer depth balance (Eq. (7)) because we estimate the entrainment velocities we as being smaller than those of Qiu and Kelly (1993), especially from September to November when ⌬T=2–3.5°C. If we were to assume ⌬T=1°C as in Qiu and Kelly (1993), the horizontal effects would shift to the negative direction in Fig. 4 because of the larger we; the tendency of the transport divergence from September to December would be enhanced, and the transport convergence suggesting obduction would occur only in February. Further studies are needed on how best to determine ⌬T. 5. Regime shift of the mixed layer depth in 1985 We next discuss the long-term variations in the mixed layer depth and temperature. The wintertime mixed layer depth in the Kuroshio Extension region underwent a transition from a deep to a shallow phase in 1985. Fig. 5 shows the time series of the mixed layer depth in this region for each month from October to March. The shift is seen most clearly in the January time series, in which the average mixed layer depth was 苲140 m from the late 1970s to the early 1980s but from 1985 to 1992 was 苲110 m. The horizontal distribution of this mixed layer depth difference between that in the periods of 1977–1984 and 1985–1992 in January and February are shown in Fig. 6. The largest change occurred in the central part of the Kuroshio Extension regions (145–180°E, 30–36°N) where we now focus on. In this section, we will seek the causes for this abrupt change. To see what caused this ‘shift’, we first estimated the difference in the termbalance of the vertical and the horizontal effects in Eq. (7) between in the shallow period of 1986–1992 and in the deep period of 1978–1984 (Fig. 7). In the fall from September and October, the entrainment velocity was slightly larger in 1986–1992, one reason for this being the smaller values of temperature ⌬T as shown in Fig. 8, resulting in larger entrainment velocities. From Eq. (6) the 0.5°C decrease of ⌬T during this period will have increased entrainment velocities by about 10–20% (0.5–1×10⫺6 m/s). This estimate explains most of the observed increase (1×10⫺6 m/s) (Fig. 7). The agreement between the spatial distributions of the anomalies of ⌬T and the mixed layer depth supports the above explanation. For example, in October of 1986, the areas where there were positive mixed layer depth anomalies coincided also exactly with the areas of negative temperature difference (Fig. 9). However, from November to January, the rate of the mixed layer deepening became slower in the period of 1986–1992 compared with the earlier 1978–1984 period (Fig. 7). As a result, in January and February the mixed layer depth was much shallower in 1986–1992. Since the entrainment velocity became larger after
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Fig. 5. Time series of the mixed layer depth (in m) in the Kuroshio Extension region from October to March (solid lines). 5-Year running mean (dotted lines) and the average values are also shown.
the transition (Fig. 7), the vertical effects cannot be the cause of the shift in the mixed layer depth that occurred in 1985. On the other hand, the horizontal effects are acting to make the mixed layer shallower, especially in December and January. Thus, it can be concluded that the 1985 transition was caused by horizontal and not by the vertical effects (entrainment). The shifts in the mixed layer depth are also seen in the time series of anomalous
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Fig. 6. Horizontal distributions of the mixed layer depth change (1977–1984) minus (1985–1992) in January (a) and February (b). The areas where the differences are over 20 m (a) and 30 m (b) are shaded. The areas north of 42°N where salinity largely contributes to the surface density are not displayed.
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Fig. 7. Changes in the rates of the deepening of the mixed layer for the periods (1986–1992) minus (1978–1984) and the contributions from the vertical (entrainment) and the horizontal effects. As in Fig. 4, ‘dh/dt’ denotes the rate of mixed layer deepening, ‘We’ denotes entrainment velocity, and ‘Hor’ denotes horizontal effects. Values are in 10⫺6 m/s.
time change rate of the mixed layer depth, anomalous entrainment velocity and anomalous horizontal transport convergence (Fig. 10). In Fig. 10, these anomalies were averaged for November to January in each year, in order to elucidate the February mixed layer depth anomalies. Once again since 1985 the shoaling of the wintermixed layer depth was evidently caused by the successive horizontal transport divergence anomalies. Why and how these horizontal effects induced the shift in the mixed layer depth may be related to the subduction and/or obduction processes; but clearer explanations will need further study.
6. Regime shift of the mixed layer temperature in winter of 1987–1988 As shown in Fig. 11, the mixed layer temperature T in the Kuroshio Extension region increased by about 1°C in the winter of 1987–88 (see also Noto & Yasuda, 1999). In January the temperature increased from 17.8°C in 1987 to 18.8°C in 1988. From the late 1970s to the early 1980s, the temperature was in a cold phase, changing to a warm phase after the shift in 1988. In this section, we will analyze the causes for this transition in the temperature. Let us examine the temperature change in Eq. (1) by separating each variable into monthly climatological component and anomaly such as A=A¯(x,m)+A⬘, where A¯(x,m) is a monthly climatology at the position of x and the month of m and A⬘ is
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Fig. 8. Time series of the temperature difference ⌬T (=T⫺Td) between the mixed layer and the layer below from July to December. 5-Year running mean (dotted lines) and the average values are also shown.
an anomaly. When we put these into Eq. (1) and linearize it assuming 兩A⬘/A¯兩¿1, we then get
冋
册 冋
册
∂T⬘ 1 h⬘ ¯ ⌬T ⬘ h⬘w¯e V⬘e ∂T¯ we⫺ ¯ ⫺ ¯ ⫽ ¯ Q⬘net⫺ ¯ Q ⫹(Horizontal) net ⫺ ¯ ∂t r0ch h h h h ∂y
(8)
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Fig. 9. Horizontal distributions of mixed layer depth anomaly (in m) (a) and ⌬T anomaly (b) in the North Pacific in October 1986. Areas with negative anomalies are shaded in (a); while positive anomalies are shaded in (b). Contour intervals are 20 m in (a) and 1°C in (b).
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Fig. 10. Time series of anomalies in the time change rate of the mixed layer depth (a), entrainment velocity (b) and horizontal effects (c), averaged from November to January to evaluate anomalies in February. Values are in 10⫺5 m/s. ‘dh/dt’ denotes the anomalous rate of mixed layer deepening, ‘We’ entrainment velocity anomaly, and ‘Hor’ the anomaly of the horizontal effects.
This equation represents the formation of interannual temperature anomalies by the anomalies of net surface heat flux (the first term on right hand side), entrainment (second) and Ekman heat convergence (third). The fourth term represents the horizontal heat convergence anomaly generated by horizontal current heat advection other than the Ekman convergence. The terms multiplied by h⬘ denote the effect of ¯ net⬍0), shoaling always the change in the mixed layer depth: in cooling period (Q creates colder anomalies. Positive temperature anomalies occur in the cases where Q⬘net⬎0, h⬘⬎0, w⬘e⬍0 and V⬘e⬎0. Fig. 12 shows the time series of the anomalies contributing to the time change rate of the mixed layer temperature ((∂T⬘)/(∂t)) averaged from November to January, to evaluate roles of each term in Eq. (8) in forming the February SST anomalies. Fig. 12 was generated, not from the approximated Eq. (8), but from the anomalies for each term in the original Eq. (1) from which have been subtracted the monthly climatological values obtained in Section 4.1. (∂T⬘)/(∂t) (Fig. 12a) has a similar pattern to the February-SST: in the 1950s and late 1980s it was positive, but was negative in the 1970s and early 1980s. In spite of the February-SST anomalies being negative from 1985 to 1987 and having switched to being positive since 1988, (∂T⬘)/(∂t) (Fig. 12a) had already become positive in 1985. This is because the October-SST anomalies were negative in 1984–1986 (Fig. 11). It is clear that the warm February-SST anomalies observed since 1988 were caused by the horizontal heat convergence anomaly generated by the Kuroshio Extension because all the other three effects (net surface flux, entrainment and Ekman heat convergence) tended to make the anomalies negative. The horizontal heat convergence anomaly had been already positive since 1983; stronger cooling because of net surface flux, entrainment and Ekman transport had maintained the February-SST anomaly negative from 1983 to 1987. Also the shoaling of the mixed layer depth
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Fig. 11. Time series of the mixed layer temperature (in °C) in the Kuroshio Extension region from October to March (solid lines). 5-Year running mean (dotted lines) and the average values are also shown.
since 1985 also contributed to the maintenance of the negative SST anomalies from 1985 to 1987.
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Fig. 12. Time series of anomalies contributing to the time change rate of the mixed layer temperature anomaly (in 10⫺7°C/s), averaged from November to January to evaluate roles of each term in Eq. (8) to form February SST anomalies. (a) Time change rate anomaly (‘dT/dt’), (b) net surface flux anomaly (‘S.f.’), (c) entrainment anomaly (‘En.’), (d) Ekman heat convergence anomaly (‘Ek.’) and (e) horizontal heat convergence anomaly due to the Kuroshio heat advection (‘Hor.’).
7. Summary and discussion We have studied the seasonal variations of the mixed layer depth and temperature in the Kuroshio Extension region (145–180°E, 30–36°N), and studied the causes of the mixed layer depth and temperature ‘regime shifts’ which occurred in the late 1980s, using upper ocean thermal and heat flux datasets incorporated with a bulk mixed layer model. Main results are summarized as follows: 1. The mixed layer from fall to winter is cooled by the net surface heat flux, the Ekman transport and the entrainment and warmed by the horizontal heat convergence of the Kuroshio heat advection. 2. The mixed layer depth is controlled by the entrainment and the horizontal effects which, from fall to early winter, act to slow the deepening of the mixed layer, and then in winter act to deepen the mixed layer, suggesting obduction. 3. The entrainment velocity is also significantly influenced by the temperature differ-
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ence ⌬T between the mixed layer and the layer below. ⌬T fluctuated both annually and interannually. The decrease in ⌬T has effectively deepened the mixed layer in fall since 1985. 4. The winter mixed layer shift from a deep to a shallow phase in 1985. This shift occurred 3 years prior to the SST-shift in 1988. The horizontal transport divergence anomaly caused the shift of the mixed layer depth in 1985. 5. The horizontal heat convergence anomaly resulting from the Kuroshio heat advection caused the winter SST-shift since 1988, although the anomaly had been already positive since 1983. The delay in the expression of the anomaly in SST from 1983 to 1987 can be attributed to the negative October-SST, stronger surface forcing and Ekman cooling and the shoaling of the mixed layer depth. The result (1) is consistent with the intuitive assessment that Kuroshio transports a large amount of heat and is cooled by surface heat flux and cold Ekman transport. However, it is contrary to the conclusion by Qiu and Kelly (1993) that the Kuroshio current cools the mixed layer. This difference in interpretation may arise from the velocity field used in Qiu and Kelly (1993) in which the southward recirculating flow may have been too strong or their mixed layer model omitted subduction and obduction processes. Both subduction and obduction occur in the Kuroshio Extension region as shown by Qiu and Huang (1995) and as also suggested in this study. The difference may also come from the interannual variability. The result (2) indicates that horizontal advection processes such as transport convergence/divergence and including subduction and obduction are important if the mixed layer depth is to be represented properly and thus SST in the Kuroshio Extension region understood. From the result (3), ⌬T was also found to be an important factor in properly representing mixed layer depths and SST in the Kuroshio Extension. The identification that the change in the mixed layer depth (4) preceded the 1988 SST change is believed to indicate that it was important not only as being a precursor of the SST-shift but also for the prediction of the Japanese sardine stocks (Noto & Yasuda, personal communication). The inter-decadal variation pattern of the January and February mixed layer depth is quite similar to winter-SST, both of which reversed their signs in the mid-1960s and in the mid-late 1980s. The relationship between the mixed layer depth and SST, and the mechanism by which the mixed layer depth shift is mediated requires further studies. The result (5) has quite important implications. The SST-shift in 1988 was caused by the increase in the transport of heat by the Kuroshio Extension. Furthermore, the heat transport had already increased since 1983, whereas the atmospheric climate shift was not reported until the winter of 1989–1990. This indicates that variations in the Kuroshio current system are a major driving force in the control of the longterm climate variability in the North Pacific. This scenario is consistent with that of Latif and Barnett (1994, 1996). This increase in the heat transport may be related to spin-up of the subtropical gyre spin-up as shown in Yasuda and Hanawa (1997). They pointed out that there was an increase in the annual mean Sverdrup transport following the mid-1970s climate shift. The baroclinic response may have generated
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the positive heat convergence anomalies observed after 1983, several years after the barotropic transport increased. In the time series of horizontal term contributions for the mixed layer depth timechange rate and temperature change rate (Figs. 10 and 12), year-to-year variation are large, especially in 1974. The positive large horizontal effect observed in 1974, may have been the enhancement of either the Kuroshio Current system or eddy heat flux convergence because the temperatures upstream of the study region were as usual or colder. However, such discussions of the year-to-year changes might not be satisfactory because the available data are of inadequate quality as described by White (1995) and Yasuda and Hanawa (1997).
Acknowledgements The authors thank S. Minobe, W. Wooster and S. Hare who gave an opportunity to present a paper in PICES science board symposium and also thank B. Qiu for fruitful discussion. Thanks are extended to Tamaki Yasuda and Shinya Kouketsu who improved the manuscript. This work was partially supported by grants from the Ministry of Education, Science and Culture of the Japanese Government.
References da Silva, A., Young, A.C., & Levitus, S. (1994). Algorithms and procedures. In Atlas of surface marine data NOAA ATLAS NESDIS, 6, vol. 1, Washington, DC: US Department of Commerce. Davis, R. E., de Szoeke, R., & Niiler, P. (1981). Variability in the upper ocean during MILE. Part II: modeling the mixed layer response. Deep-Sea Research, 28, 1453–1475. Deardorff, J. W., Willis, G. E., & Lilly, D. K. (1969). Laboratory investigation of non-steady penetrative convection. Journal of Fluid Mechanics, 35, 7–35. Kachi, M., & Nitta, T. (1997). Decadal variations of the global atmosphere–ocean system. Journal of Meteorological Society of Japan, 75, 657–675. Kraus, E. B., & Turner, J. S. (1967). A one-dimensional model of the seasonal thermocline: II. The general theory and its consequences. Tellus, 19, 98–106. Jerlov, N. G. (1968). Optical oceanography. Amsterdam: Elsevier. 194 pp. Lamb, P. J. (1984). On the mixed-layer climatology of the north and tropical Atlantic. Tellus, 36A, 292–305. Latif, M., & Barnett, T. P. (1994). Causes of decadal climate variability over the North Pacific and North America. Science, 266, 634–637. Latif, M., & Barnett, T. P. (1996). Decadal climate variability over the North Pacific and North America: dynamics and predictability. Journal of Climate, 9, 2407–2423. Miller, A. J., Cayan, D. R., Barnett, T. P., Graham, N. F., & Oberhuber, J. M. (1994). Interdecadal variability of the Pacific Ocean: model response to observed heat flux and wind stress anomalies. Climate Dynamics, 9, 287–302. Minobe, S. (1997). A 50–70 year climatic oscillation over the North Pacific and North America. Geophysical Research Letters, 24, 683–686. Nakamura, H., Lin, G., & Yamagata, T. (1997). Decadal climate variability in the North Pacific during the recent decades. Bulletin of American Meteorological Society, 78, 2215–2225. Noto, M., & Yasuda, I. (1999). Population decline of the Japanese sardine in relation to sea surface
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temperature in the Kuroshio Extension. Canadian Journal of Fisheries and Aquatic Sciences, 56, 973–983. Paulson, C. A., & Simpson, J. J. (1977). Irradiative measurements in the upper ocean. Journal of Physical Oceanography, 7, 952–956. Polovina, J. J., Mitchum, G. T., & Evans, G. T. (1995). Decadal and basin-scale variation in mixed layer depth and the impact on biological production in the Central and Northern Pacific. Deep-Sea Research, 42, 1701–1716. Talley, L. D. (1984). Meridional heat transport in the Pacific Ocean. Journal of Physical Oceanography, 14, 231–241. Trenberth, K. E. (1990). Recent observed interdecadal climate changes in the Northern Hemisphere. Bulletin of American Meteorological Society, 71, 988–993. Qiu, B., & Huang, R. X. (1995). Ventilation of the North Atlantic and North Pacific: subduction versus obduction. Journal of Physical Oceanography, 25, 2374–2390. Qiu, B., & Kelly, K. A. (1993). Upper-ocean heat balance in the Kuroshio Extension region. Journal of Physical Oceanography, 23, 2027–2041. Watanabe, M., & Nitta, T. (1998). Relative impacts of snow and sea surface temperature anomalies on an extreme phase in the winter atmospheric circulation. Journal of Climate, 11, 2837–2857. Weare, B. C. (1989). Uncertainties in estimates of surface heat fluxes derived from marine reports over the tropical and subtropical oceans. Tellus, 41A, 357–370. White, W. B. (1995). Design of a global observing system for gyre-scale upper ocean temperature variability. Progress in Oceanography, 36, 169–217. Yasuda, T., & Hanawa, K. (1997). Decadal changes in the mode waters in the midlatitude North Pacific. Journal of Physical Oceanography, 27, 858–870.
Heat balance and regime shifts of the mixed layer in the Kuroshio Extension Ichiro Yasuda *, Tomoki Tozuka, Masayuki Noto, Shinya Kouketsu Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract We investigated the seasonal variations of the mixed layer depth and temperature in the Kuroshio Extension region (145–180°E, 30–36°N), and studied the causes of the mixed layer depth and temperature ‘regime shifts’ which occurred in the late 1980s, using upper ocean thermal and heat flux datasets incorporated with a bulk mixed layer model. The mixed layer from fall to winter is cooled by the net surface heat flux, the Ekman transport and the entrainment, and warmed by the horizontal heat convergence resulting from the Kuroshio heat advection. The mixed layer depth is controlled by the entrainment and the horizontal transport divergence/convergence which act to slow the mixed layer deepening from fall to early winter and then in winter deepen the mixed layer. The entrainment velocity is significantly influenced by the temperature difference between the mixed layer and the layer below. The mixed layer shift from its deep to shallow phase occurred in 1985. This shift is preceded the SST-shift in 1988 by three years, and was caused by the horizontal transport divergence anomaly. The horizontal heat convergence of the Kuroshio Extension caused the SST-shift in 1988, whereas its anomaly had been already positive since 1983. The delay from 1983 to 1987 can be attributed to the effects of the negative fall–SST anomaly, stronger surface heat flux and Ekman cooling and the shoaling of the mixed layer depth. It is suggested that the Kuroshio current system plays a major role in forcing the SST-shift and thus the subsequent climate regime shift. 2000 Elsevier Science Ltd. All rights reserved.
Contents 1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
* Corresponding author. Tel.: +81-3-5841-4288; fax: 81-3-5841-8791. E-mail address: [email protected] (I. Yasuda). 0079-6611/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 0 0 ) 0 0 0 3 8 - 0
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2.
Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
3.
The mixed layer model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
4. Seasonal variations of the mixed layer depth and temperature . 4.1. Mixed layer temperature . . . . . . . . . . . . . . . . . . . . . . 4.2. The mixed layer depth . . . . . . . . . . . . . . . . . . . . . . . 4.3. Temperature difference ⌬T between the mixed layer and the
. . . . . . . . . . . . . . . . . . . . . . . . layer below
. . . .
. . . .
. . . .
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5.
Regime shift of the mixed layer depth in 1985 . . . . . . . . . . . . . . . . . . . . . 267
6.
Regime shift of the mixed layer temperature in winter of 1987–1988 . . . . . . . 270
7.
Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277
1. Introduction The Kuroshio Extension and its southern recirculation region at 145–180°E and 30–36°N are the areas where a large amount of heat is released from the ocean. Talley (1984) showed that a western boundary current, such as the Kuroshio, loses a significant portion of its heat to the atmosphere. Thus, the Kuroshio Extension region possibly has a substantial impact on the climate variability in the North Pacific. It is well known that the atmosphere–ocean system in the North Pacific undergoes decadal and inter-decadal variability (e.g. Nitta & Yamada, 1989; Trenberth, 1990; Latif & Barnett 1994, 1996; Miller, Cayan, Barnett, Graham & Oberhuber, 1994; Nakamura, Lin & Yamagata, 1997; Minobe, 1997; Yasuda & Hanawa, 1997). The shift in the climate regime, which occurred in the mid 1970s, has been particularly well studied. Changes in the mixed layer depth and temperature in the Kuroshio Extension regions influence biological production. Noto and Yasuda (1999) showed that the winter-SSTs in the Kuroshio Extension regions are significantly related to the mortality of the Japanese sardine. Polovina, Mitchum and Evans (1995) demonstrated the relationship between the mixed layer depth in winter/spring and the biological production using bio-physical models. When the mixed layer deepens, nutrients from subthermocline depths are brought up into the mixed layer. As a result, phytoplankton production increases and the growth of fish is augmented. Studies to clarify the mechanisms underlying the variations in SST and mixed layer depth in the Kuroshio Extension are thus a high priority. The heat balance of the mixed layer of the Kuroshio Extension was studied by Qiu and Kelly (1993) using a 3-dimensional bulk mixed layer model (which was a
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3D extension of the model by Kraus & Turner, 1967) driven by the data of surface heat flux, surface wind-stress and geostrophic velocity fields derived from Geosat Altimetry. They reproduced climatological mixed layer depths and temperatures, and concluded that geostrophic heat transport by the Kuroshio Extension results in a heat divergence; i.e., the Kuroshio cools the mixed layer in the Kuroshio Extension. Qiu and Huang (1995) estimated the rates of ‘subduction’ and ‘obduction’ in the North Pacific, and emphasized the importance of the Kuroshio Extension regions where obduction and subduction take place. Subduction and obduction are processes of interaction between the surface mixed layer and subsurface layer. In subduction waters in the mixed layer escape from the mixed layer and intrude into subsurface; while in obduction, subsurface waters intrude into the surface mixed layers. In this study, we re-examined the heat balance in the Kuroshio Extension using a 3D bulk mixed layer model, similar to the one used by Qiu and Kelly (1993). Qiu and Kelly (1993) calculated the mixed layer depths and temperatures with the 3D mixed layer model, and discussed the effects of the horizontal velocity fields. However, since the velocity field is still not easy to estimate accurately and also the mixed layer model may be sensitive to the processes of obduction and subduction processes, the effects of the horizontal velocity field may be difficult to evaluate from the model. In contrast, we have used observational data for the mixed layer depth and temperature (in addition to the surface heat flux and surface wind stress data) and estimated the effects of horizontal velocity fields, other than Ekman transport, as unknown residuals. Our results showed that the Kuroshio Extension transports large amounts of heat but is cooled by heat losses to the atmosphere, the entrainment of cold deep water and southward Ekman transport. This is different from the conclusion of Qiu and Kelly (1993). We next examined the shifts that occurred in the late 1980s. Noto and Yasuda (1999) showed that in the Kuroshio Extension SST abruptly increased during the winter of 1987–1988, one or two years in advance of the well-known climate shift that occurred in 1989–1990 (e.g. Nakamura et al., 1997; Kachi & Nitta, 1997; Watanabe & Nitta, 1998). We also found there was a ‘regime shift’ in the winter mixed layer depth changing from a thick to a thin phase in 1985. This shift preceded by 3 years the SST-shift that occurred in 1988. We will discuss the causes of these shifts on the basis of heat balance and variations in the mixed layer depth. In the following section, we present the data used in this study. The mixed layer model is then described in Section 3. In Section 4 we discuss how the seasonal variations in the temperature and the depth of the mixed layer are determined. In Sections 5 and 6, the causes of the ‘regime shifts’ of the mixed layer depth and temperature in the late 1980s are sought, and in the final section the results will be summarized. 2. Data The ocean temperature data used in the present study is White’s Ocean Temperature Climatology 1955–1992. This data set has been assembled and maintained by
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the Data Support Section, Scientific Computing Division of the National Center for Atmospheric Research (NCAR). The original source of these data is the Scripps Institute of Oceanography. Temperatures were measured by the XBTs (expendable bathythermograph). The processing of the raw hydrographic data to obtain the monthly temperature anomaly is described by White (1995). The data resolution is 2° in latitude and 5° in longitude on a monthly basis from 1955 to 1992. There are 11 levels in the vertical, six of which are in the upper 120 m (0, 20, 40, 60, 80, 120, 160, 200, 240, 300, 400 m). White (1995) reported that maximum interpolation errors are 0.4°C at surface, 0.5°C at 200 m and 0.2°C at 400 m for biennial signals, and so the data are not adequate for detecting year-to-year changes in upper ocean heat storage; we believe that long-term variations can be discussed as described in Yasuda and Hanawa (1997). For surface heat flux (net heat flux, short-wave radiation, longwave radiation, sensible heat, latent heat), scalar wind speed, zonal and meridional wind stresses, we used monthly data from da Silva, Young and Levitus (1994) for the period from 1955 to 1992. The data resolution is 1° in both latitude and longitude. According to da Silva et al. (1994), r.m.s. errors of monthly wind speed are 0.4–0.8 m/s in the North Pacific. Heat flux uncertainties have been discussed by Weare (1989). Since we examine the seasonal and long-term variations of the heat flux averaged in a wide area, 30–36°N and 145–180°E, the random errors are considerably reduced as Weare (1989) and Yasuda and Hanawa (1997) discussed.
3. The mixed layer model The mixed layer depth can be defined in various ways. Qiu and Kelly (1993) defined it as the depth where the depth-averaged temperature is 1°C higher than the water temperature just below the base of the mixed layer. Polovina et al. (1995) defined the depth of the mixed layer according to the temperature profile. Lamb’s (1984) definition was the depth at which the water temperature is 1°C lower than at the sea surface; and is the definition we use in this study. The mixed layer model used here is similar to Qiu and Kelly’s (1993) 3D bulk mixed layer model. The equation, which governs the mixed layer temperature, is →
∂T ⌬T·we Ue 1 ⫽⫺ ·ⵜT⫹ (Qnet⫺qd)⫺ ⫹(Horizontal). ∂t h r0ch h
(1)
Alternatively in the form of heat content and heat flux, → ∂T r0ch ⫽⫺r0cU e·ⵜT⫹(Qnet⫺qd)⫺r0c⌬T·we⫹(Horizontal), ∂t
(2) →
where T is the mixed layer temperature, h the mixed layer depth, U e=(Ue,Ve) the Ekman transport vector,
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冉冊 冉冊
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1 tx Ve⫽⫺ f r0
(3)
1 ty Ue ⫽ f r0
(4)
r0 is the density of the sea water (=1025 kg/m3), c is the specific heat of the sea water, Qnet is the surface net heat flux, and ⌬T is the temperature difference between the mixed layer and the layer below, and we is the entrainment velocity. tx and ty denote zonal and meridional wind-stresses, respectively. qd=q(⫺h) is the downward radiative flux at the base of the mixed layer and the downward flux at depth of z, q(z) is given by
冋
册
z z q(z)⫽q(0) R exp ⫹(1⫺R)exp g1 g2
(5)
where q(0) is the surface downward radiative flux, R (=0.77) is a separation constant, and g1 (=1.5 m) and g2 (=14 m) are the attenuation length scales (Paulson & Simpson, 1977; Jerlov, 1968). Qiu and Kelly (1993) fix ⌬T at 1°C, where the variation of the temperature in the lower layer cannot induce a change in the entrainment velocity. In the present study, ⌬T is defined as the temperature difference between that at the sea surface (T) and at 20 m below the mixed layer base (Td). In Eq. (1), the first term on the right hand side denotes the horizontal heat convergence resulting from the Ekman transport, the second surface heat flux, and the third entrainment. By using monthly observation data (the area average in the Kuroshio Extension region) on the left hand side and the first three terms on the right hand side, we estimated the horizontal effects, such as the heat advection by the Kuroshio current and eddies other than caused by the Ekman transport. The entrainment velocities we are calculated by the following equation (Davis, de Szoeke & Niiler, 1981; Qiu & Kelly, 1993):
冕
2 q(z)dz 2m0u3∗ 兩Qnet兩−Qnet Qnet+qd we⫽ ⫹ ⫺ ⫺mc , agh⌬T r0ch⌬T r0c⌬T 2r0c⌬T
(6)
where m0 is a coefficient which shows the effect of stirring by wind, mc is the convective efficiency coefficient, and a is the thermal expansion coefficient (=0.00025°C⫺1). From Davis et al. (1981), we use m0=0.5; from laboratory experiments of Deardorff, Willis and Lilly (1969), we take mc to be 0.83. In Eq. (6), the first term on the right hand side denotes the effect of wind stirring (where frictional velocity is defined as u2∗=raCDU 210/r0, CD=0.00125 is a drag coefficient and U10 is the scalar wind speed at 10 m height), the second term denotes radiative penetration flux, and the last two terms are surface buoyancy terms. When the mixed layer is deepening, the entrainment velocity is calculated from the observation data. On the other hand, when the mixed layer is shoaling, the entrainment velocity is set to zero.
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Note that all the parameters used in the present study are the same as in Qiu and Kelly (1993). Finally, the velocity at which the mixed layer deepens is ∂h ⫽we⫹(Horizontal) ∂t
(7)
Horizontal effects as transport convergence are estimated from Eq. (7) using observation data for (∂h)/(∂t) and we.
4. Seasonal variations of the mixed layer depth and temperature 4.1. Mixed layer temperature Fig. 1a shows the monthly averaged climatology of the mixed layer temperature in the Kuroshio Extension region. The temperature increases from April to August, and reaches its maximum in August. The temperature then decreases from September to March reaching its minimum in March. The temperature equations of the mixed layer model (Eq. (1)) were calculated and horizontal effects were estimated for the 38 years from January 1955 to December 1992 at each grid point, and spatial and monthly averages were taken (Fig. 2a). Fig. 2b shows the seasonal contributions by each term to the heat flux equation of Eq. (2). The net surface heat flux is positive from April to August, when the ocean is gaining heat from the atmosphere. The net surface heat flux is negative from September to March, when the ocean is losing heat to the atmosphere. The entrainment mixes cold water from the lower layer and so cools down the mixed layer. So, while the mixed layer is deepening from September to March, the entrainment always acts to lower the temperature of the mixed layer. In general, the Westerly Wind blows over the Kuroshio Extension region. This results in a southward Ekman transport. Since the negative northward temperature gradient is present, the Ekman transport acts to cool the mixed layer in this region. The horizontal effects include the advection of the Kuroshio current and the eddy diffusive fluxes, and are found to contribute significantly to the heat balance, especially from fall to winter. Except in summer, this term results in warming of the mixed layer, since the Kuroshio is transporting large amounts of warm water into the area. Note that this result is different from the conclusion reached by Qiu and Kelly (1993) that the horizontal effects act to cool the mixed layer. This difference might arise from the geostrophic velocity fields used in Qiu and Kelly (1993), interannual variability (1986–1989 in Qiu and Kelly (1993) and climatology for 38 years in the present study) or the mixed layer model. We believe that the result that the Kuroshio warms the mixed layer obtained in the present study is more intuitive than the Qiu and Kelly (1993) conclusion. Finally, if integrated annually, both the surface heat flux (3.60×10⫺7°C/s) and the
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Fig. 1. Monthly climatologies of (a) the mixed layer temperature T (in °C), (b) the mixed layer depth MLD (in m) and (c) the temperature difference between the mixed layer and the layer below ⌬T (=T⫺Td) (in °C), averaged in the Kuroshio Extension region of 145–180°E and 30–36°N. Vertical bars denote standard deviations.
horizontal effects (27.13×10⫺7°C/s) act to warm the mixed layer in the Kuroshio Extension region, whereas an entrainment (⫺22.73×10⫺7°C/s) and the Ekman transport (⫺7.43×10⫺7°C/s) both cool the mixed layer. The horizontal effects have the largest absolute value, while the net surface heat flux has the smallest absolute value.
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Fig. 2. Monthly climatologies of the heat balance and contributing terms, to the mixed layer temperature in Eq. (1) (a), and to the mixed layer heat content (b), averaged in the Kuroshio Extension region. Values are in 10⫺7°C/s (a) and in W/m2 (b). In (a), ‘dT/dt’ denotes the temperature time change rate, ‘heat f’ surface net heat flux, ‘entrain’ entrainment, ‘Ekman’ Ekman heat convergence and ‘Hor’ the horizontal effects in Eq. (1). In (b), notations are the same as in (a) but for ‘dH/dt’ of the heat content time change rate and Eq. (2).
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Our estimate of surface heat flux is much smaller than the one obtained by Qiu and Kelly (1993) (38.67×10⫺7°C/s). One possible reason is the difference in the areas assessed: 30–36°N and 145–180°E in the present study and 30–40°N and 140–180°E in Qiu and Kelly (1993). Another possible cause is the mixed layer depth in summer from which subtle differences would generate large differences in the values because the contribution of the surface heat flux is a function of Qnet/h. 4.2. The mixed layer depth Seasonal variations of the mixed layer depth are shown in Fig. 1b. The mixed layer depth is the shallowest during the summer, at about 20 m. It then deepens slowly until March, when the mixed layer depth is at its deepest at around 200 m, and then shoals sharply in April. The climatology of entrainment velocities and term-balances in Eq. (6) shown in Fig. 3 are monthly averages for 38 years from January 1955 to December 1992. The entrainment velocity is largest in January. Generally, comparing the three terms, the wind stirring and the surface buoyancy are dominant, whereas the radiation penetration flux term is insignificant. Wind stirring has the largest effect except in February and March. Next, we discuss how the vertical and horizontal effects determine the mixed layer
Fig. 3. Monthly climatologies of the entrainment velocity we and contributing terms in Eq. (6). Values are averaged over 38 years. ‘We’ denotes the entrainment velocity, ‘wind’ denotes the wind stirring term, ‘p.f.’ denotes the radiative penetration flux term, and ‘s.f.’ denotes the surface buoyancy flux term. Values are in 10⫺6 m/s.
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depth (Fig. 4). This is done by calculating Eq. (7) for 38 years and taking monthly averages to evaluate the horizontal effects. The mixed layer in the Kuroshio Extension region deepens from August to March (Fig. 1b), most sharply in February. However, the vertical effect (entrainment velocity) is the largest in January. The horizontal effects make the mixed layer shallower from October to December, whereas the horizontal effects play a role in the deepening of the mixed layer in January and especially in February, making the rate of mixed layer deepening the largest in February. This dominant contribution of the horizontal effects in February implies that obduction is important then. 4.3. Temperature difference ⌬T between the mixed layer and the layer below Finally, we discuss the temperature difference between the mixed layer and the lower layer ⌬T (Fig. 1c). ⌬T shows significant seasonal variation ranging from 1 to 4°C. Thus the fixed ⌬T employed by Qiu and Kelly (1993) was probably inappropriate except in winter. ⌬T reaches its maximum in August and then decreases to a minimum in January. It then increases slightly but again reaches a minimum in April. If the temperature difference is large (small), the entrainment velocity becomes small (large). Interannual and decadal variations in the mixed layer depth might also be influenced by these temperature difference as will be shown below. The systematic difference in ⌬T between that in the present study (⌬T=1⫺3.5°C)
Fig. 4. Monthly balances between the vertical (entrainment) and the horizontal effects in the time change rate of mixed layer deepening (see Eq. (7)). Values are averaged for 38 years. ‘dh/dt’ denotes the rate of mixed layer deepening, ‘We’ denotes the entrainment velocity, and ‘Hor’ denotes the horizontal effects. Values are in 10⫺6 m/s.
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and in Qiu and Kelly (1993) (⌬T=1°C) has no influence on the estimated heat balance, because, as shown in Eqs. (1) and (6), the mixed layer temperature is not a function of ⌬T. Hence the conclusion that the fall–winter mixed layer is cooled by the net surface flux, entrainment and Ekman cooling but is warmed by the Kuroshio heat transport remains unchanged. However, the systematic difference in ⌬T between the two studies may generate changes in the mixed layer depth balance (Eq. (7)) because we estimate the entrainment velocities we as being smaller than those of Qiu and Kelly (1993), especially from September to November when ⌬T=2–3.5°C. If we were to assume ⌬T=1°C as in Qiu and Kelly (1993), the horizontal effects would shift to the negative direction in Fig. 4 because of the larger we; the tendency of the transport divergence from September to December would be enhanced, and the transport convergence suggesting obduction would occur only in February. Further studies are needed on how best to determine ⌬T. 5. Regime shift of the mixed layer depth in 1985 We next discuss the long-term variations in the mixed layer depth and temperature. The wintertime mixed layer depth in the Kuroshio Extension region underwent a transition from a deep to a shallow phase in 1985. Fig. 5 shows the time series of the mixed layer depth in this region for each month from October to March. The shift is seen most clearly in the January time series, in which the average mixed layer depth was 苲140 m from the late 1970s to the early 1980s but from 1985 to 1992 was 苲110 m. The horizontal distribution of this mixed layer depth difference between that in the periods of 1977–1984 and 1985–1992 in January and February are shown in Fig. 6. The largest change occurred in the central part of the Kuroshio Extension regions (145–180°E, 30–36°N) where we now focus on. In this section, we will seek the causes for this abrupt change. To see what caused this ‘shift’, we first estimated the difference in the termbalance of the vertical and the horizontal effects in Eq. (7) between in the shallow period of 1986–1992 and in the deep period of 1978–1984 (Fig. 7). In the fall from September and October, the entrainment velocity was slightly larger in 1986–1992, one reason for this being the smaller values of temperature ⌬T as shown in Fig. 8, resulting in larger entrainment velocities. From Eq. (6) the 0.5°C decrease of ⌬T during this period will have increased entrainment velocities by about 10–20% (0.5–1×10⫺6 m/s). This estimate explains most of the observed increase (1×10⫺6 m/s) (Fig. 7). The agreement between the spatial distributions of the anomalies of ⌬T and the mixed layer depth supports the above explanation. For example, in October of 1986, the areas where there were positive mixed layer depth anomalies coincided also exactly with the areas of negative temperature difference (Fig. 9). However, from November to January, the rate of the mixed layer deepening became slower in the period of 1986–1992 compared with the earlier 1978–1984 period (Fig. 7). As a result, in January and February the mixed layer depth was much shallower in 1986–1992. Since the entrainment velocity became larger after
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Fig. 5. Time series of the mixed layer depth (in m) in the Kuroshio Extension region from October to March (solid lines). 5-Year running mean (dotted lines) and the average values are also shown.
the transition (Fig. 7), the vertical effects cannot be the cause of the shift in the mixed layer depth that occurred in 1985. On the other hand, the horizontal effects are acting to make the mixed layer shallower, especially in December and January. Thus, it can be concluded that the 1985 transition was caused by horizontal and not by the vertical effects (entrainment). The shifts in the mixed layer depth are also seen in the time series of anomalous
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Fig. 6. Horizontal distributions of the mixed layer depth change (1977–1984) minus (1985–1992) in January (a) and February (b). The areas where the differences are over 20 m (a) and 30 m (b) are shaded. The areas north of 42°N where salinity largely contributes to the surface density are not displayed.
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Fig. 7. Changes in the rates of the deepening of the mixed layer for the periods (1986–1992) minus (1978–1984) and the contributions from the vertical (entrainment) and the horizontal effects. As in Fig. 4, ‘dh/dt’ denotes the rate of mixed layer deepening, ‘We’ denotes entrainment velocity, and ‘Hor’ denotes horizontal effects. Values are in 10⫺6 m/s.
time change rate of the mixed layer depth, anomalous entrainment velocity and anomalous horizontal transport convergence (Fig. 10). In Fig. 10, these anomalies were averaged for November to January in each year, in order to elucidate the February mixed layer depth anomalies. Once again since 1985 the shoaling of the wintermixed layer depth was evidently caused by the successive horizontal transport divergence anomalies. Why and how these horizontal effects induced the shift in the mixed layer depth may be related to the subduction and/or obduction processes; but clearer explanations will need further study.
6. Regime shift of the mixed layer temperature in winter of 1987–1988 As shown in Fig. 11, the mixed layer temperature T in the Kuroshio Extension region increased by about 1°C in the winter of 1987–88 (see also Noto & Yasuda, 1999). In January the temperature increased from 17.8°C in 1987 to 18.8°C in 1988. From the late 1970s to the early 1980s, the temperature was in a cold phase, changing to a warm phase after the shift in 1988. In this section, we will analyze the causes for this transition in the temperature. Let us examine the temperature change in Eq. (1) by separating each variable into monthly climatological component and anomaly such as A=A¯(x,m)+A⬘, where A¯(x,m) is a monthly climatology at the position of x and the month of m and A⬘ is
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Fig. 8. Time series of the temperature difference ⌬T (=T⫺Td) between the mixed layer and the layer below from July to December. 5-Year running mean (dotted lines) and the average values are also shown.
an anomaly. When we put these into Eq. (1) and linearize it assuming 兩A⬘/A¯兩¿1, we then get
冋
册 冋
册
∂T⬘ 1 h⬘ ¯ ⌬T ⬘ h⬘w¯e V⬘e ∂T¯ we⫺ ¯ ⫺ ¯ ⫽ ¯ Q⬘net⫺ ¯ Q ⫹(Horizontal) net ⫺ ¯ ∂t r0ch h h h h ∂y
(8)
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Fig. 9. Horizontal distributions of mixed layer depth anomaly (in m) (a) and ⌬T anomaly (b) in the North Pacific in October 1986. Areas with negative anomalies are shaded in (a); while positive anomalies are shaded in (b). Contour intervals are 20 m in (a) and 1°C in (b).
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Fig. 10. Time series of anomalies in the time change rate of the mixed layer depth (a), entrainment velocity (b) and horizontal effects (c), averaged from November to January to evaluate anomalies in February. Values are in 10⫺5 m/s. ‘dh/dt’ denotes the anomalous rate of mixed layer deepening, ‘We’ entrainment velocity anomaly, and ‘Hor’ the anomaly of the horizontal effects.
This equation represents the formation of interannual temperature anomalies by the anomalies of net surface heat flux (the first term on right hand side), entrainment (second) and Ekman heat convergence (third). The fourth term represents the horizontal heat convergence anomaly generated by horizontal current heat advection other than the Ekman convergence. The terms multiplied by h⬘ denote the effect of ¯ net⬍0), shoaling always the change in the mixed layer depth: in cooling period (Q creates colder anomalies. Positive temperature anomalies occur in the cases where Q⬘net⬎0, h⬘⬎0, w⬘e⬍0 and V⬘e⬎0. Fig. 12 shows the time series of the anomalies contributing to the time change rate of the mixed layer temperature ((∂T⬘)/(∂t)) averaged from November to January, to evaluate roles of each term in Eq. (8) in forming the February SST anomalies. Fig. 12 was generated, not from the approximated Eq. (8), but from the anomalies for each term in the original Eq. (1) from which have been subtracted the monthly climatological values obtained in Section 4.1. (∂T⬘)/(∂t) (Fig. 12a) has a similar pattern to the February-SST: in the 1950s and late 1980s it was positive, but was negative in the 1970s and early 1980s. In spite of the February-SST anomalies being negative from 1985 to 1987 and having switched to being positive since 1988, (∂T⬘)/(∂t) (Fig. 12a) had already become positive in 1985. This is because the October-SST anomalies were negative in 1984–1986 (Fig. 11). It is clear that the warm February-SST anomalies observed since 1988 were caused by the horizontal heat convergence anomaly generated by the Kuroshio Extension because all the other three effects (net surface flux, entrainment and Ekman heat convergence) tended to make the anomalies negative. The horizontal heat convergence anomaly had been already positive since 1983; stronger cooling because of net surface flux, entrainment and Ekman transport had maintained the February-SST anomaly negative from 1983 to 1987. Also the shoaling of the mixed layer depth
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Fig. 11. Time series of the mixed layer temperature (in °C) in the Kuroshio Extension region from October to March (solid lines). 5-Year running mean (dotted lines) and the average values are also shown.
since 1985 also contributed to the maintenance of the negative SST anomalies from 1985 to 1987.
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Fig. 12. Time series of anomalies contributing to the time change rate of the mixed layer temperature anomaly (in 10⫺7°C/s), averaged from November to January to evaluate roles of each term in Eq. (8) to form February SST anomalies. (a) Time change rate anomaly (‘dT/dt’), (b) net surface flux anomaly (‘S.f.’), (c) entrainment anomaly (‘En.’), (d) Ekman heat convergence anomaly (‘Ek.’) and (e) horizontal heat convergence anomaly due to the Kuroshio heat advection (‘Hor.’).
7. Summary and discussion We have studied the seasonal variations of the mixed layer depth and temperature in the Kuroshio Extension region (145–180°E, 30–36°N), and studied the causes of the mixed layer depth and temperature ‘regime shifts’ which occurred in the late 1980s, using upper ocean thermal and heat flux datasets incorporated with a bulk mixed layer model. Main results are summarized as follows: 1. The mixed layer from fall to winter is cooled by the net surface heat flux, the Ekman transport and the entrainment and warmed by the horizontal heat convergence of the Kuroshio heat advection. 2. The mixed layer depth is controlled by the entrainment and the horizontal effects which, from fall to early winter, act to slow the deepening of the mixed layer, and then in winter act to deepen the mixed layer, suggesting obduction. 3. The entrainment velocity is also significantly influenced by the temperature differ-
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ence ⌬T between the mixed layer and the layer below. ⌬T fluctuated both annually and interannually. The decrease in ⌬T has effectively deepened the mixed layer in fall since 1985. 4. The winter mixed layer shift from a deep to a shallow phase in 1985. This shift occurred 3 years prior to the SST-shift in 1988. The horizontal transport divergence anomaly caused the shift of the mixed layer depth in 1985. 5. The horizontal heat convergence anomaly resulting from the Kuroshio heat advection caused the winter SST-shift since 1988, although the anomaly had been already positive since 1983. The delay in the expression of the anomaly in SST from 1983 to 1987 can be attributed to the negative October-SST, stronger surface forcing and Ekman cooling and the shoaling of the mixed layer depth. The result (1) is consistent with the intuitive assessment that Kuroshio transports a large amount of heat and is cooled by surface heat flux and cold Ekman transport. However, it is contrary to the conclusion by Qiu and Kelly (1993) that the Kuroshio current cools the mixed layer. This difference in interpretation may arise from the velocity field used in Qiu and Kelly (1993) in which the southward recirculating flow may have been too strong or their mixed layer model omitted subduction and obduction processes. Both subduction and obduction occur in the Kuroshio Extension region as shown by Qiu and Huang (1995) and as also suggested in this study. The difference may also come from the interannual variability. The result (2) indicates that horizontal advection processes such as transport convergence/divergence and including subduction and obduction are important if the mixed layer depth is to be represented properly and thus SST in the Kuroshio Extension region understood. From the result (3), ⌬T was also found to be an important factor in properly representing mixed layer depths and SST in the Kuroshio Extension. The identification that the change in the mixed layer depth (4) preceded the 1988 SST change is believed to indicate that it was important not only as being a precursor of the SST-shift but also for the prediction of the Japanese sardine stocks (Noto & Yasuda, personal communication). The inter-decadal variation pattern of the January and February mixed layer depth is quite similar to winter-SST, both of which reversed their signs in the mid-1960s and in the mid-late 1980s. The relationship between the mixed layer depth and SST, and the mechanism by which the mixed layer depth shift is mediated requires further studies. The result (5) has quite important implications. The SST-shift in 1988 was caused by the increase in the transport of heat by the Kuroshio Extension. Furthermore, the heat transport had already increased since 1983, whereas the atmospheric climate shift was not reported until the winter of 1989–1990. This indicates that variations in the Kuroshio current system are a major driving force in the control of the longterm climate variability in the North Pacific. This scenario is consistent with that of Latif and Barnett (1994, 1996). This increase in the heat transport may be related to spin-up of the subtropical gyre spin-up as shown in Yasuda and Hanawa (1997). They pointed out that there was an increase in the annual mean Sverdrup transport following the mid-1970s climate shift. The baroclinic response may have generated
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the positive heat convergence anomalies observed after 1983, several years after the barotropic transport increased. In the time series of horizontal term contributions for the mixed layer depth timechange rate and temperature change rate (Figs. 10 and 12), year-to-year variation are large, especially in 1974. The positive large horizontal effect observed in 1974, may have been the enhancement of either the Kuroshio Current system or eddy heat flux convergence because the temperatures upstream of the study region were as usual or colder. However, such discussions of the year-to-year changes might not be satisfactory because the available data are of inadequate quality as described by White (1995) and Yasuda and Hanawa (1997).
Acknowledgements The authors thank S. Minobe, W. Wooster and S. Hare who gave an opportunity to present a paper in PICES science board symposium and also thank B. Qiu for fruitful discussion. Thanks are extended to Tamaki Yasuda and Shinya Kouketsu who improved the manuscript. This work was partially supported by grants from the Ministry of Education, Science and Culture of the Japanese Government.
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