GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L24602, doi:10.1029/2006GL027350, 2006
Connections between surface fluxes and the deep circulation in the Sea of Japan Maria V. Luneva1,2 and Carol Anne Clayson1,2 Received 24 June 2006; revised 26 October 2006; accepted 13 November 2006; published 16 December 2006.
[1] The extent to which seasonably variable surface fluxes and bottom friction can exert a control on the abyssal circulation above localized bottom topography is analyzed in the Japan Sea region through the use of a threedimensional eddy-resolving ocean model. Our simulations show that the bottom friction exerts a first-order control on the magnitude of the eddy-driven deep circulation over f/H geostrophic contours, which is inversely dependent on the bottom drag, in accordance with theory. Seasonal surface flux variability acts as a modifier to this theory, depending on the timescale of active forcing relative to bottom friction spin-down time. Possible physical mechanisms for a varying bottom drag coefficient are proposed. Winter ventilation of isopycnals during deep convection is followed by enhanced drain energy to small-scale turbulence and strong energy dissipation in intermediate layers leading to a weakening of deep circulation that contradicts the classic ventilation theory. Citation: Luneva, M. V., and C. A. Clayson (2006), Connections between surface fluxes and the deep circulation in the Sea of Japan, Geophys. Res. Lett., 33, L24602, doi:10.1029/2006GL027350.
1. Introduction [2] In the last decade, strong abyssal currents were found in field observations in many regions of the World Ocean [e.g., Watts et al., 1995], including the Sea of Japan (SOJ) or East Sea, where recent field observations revealed the existence of intensive abyssal currents with velocities exceeding 15– 20 cm s1 and a mean velocity up to 7 cm s1 [Takematsu et al., 1999]. These measurements showed that at almost all stations currents at the depths of 1000– 3000 m are quasi-barotropic and have strong seasonal variability with maximums in late winter and spring and minimums in summer. The presence of strong, deep (800 m), eddy-like currents with velocities up to 15 cm s1 in the western part of the Japan Basin was corroborated by two-year observations of PALACE floats trajectories by Danchenkov and Riser [2000]. Their analysis suggested the existence of two large-scale cyclonic gyres between 40°N and 43°N with currents generally following the 3000 – 3500 m isobaths. Recent observations by Senjyu et al. [2005] and Teague et al. [2005] in the Yamato Basin and Ulleung Basin proved the existence of intensive abyssal circulation in these regions also. 1 Department of Meteorology, Florida State University, Tallahassee, Florida, USA. 2 Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, Florida, USA.
Copyright 2006 by the American Geophysical Union. 0094-8276/06/2006GL027350
[3] The Sea of Japan is of interest for studying the basic ideas regarding the formation and variability of abyssal currents due to the region’s strongly meandering currents, intensive frontogenesis, cyclonic and anticyclonic gyre systems, and mesoscale eddy activities that are very similar to energetic zones of the Gulf Stream or Kuroshio currents. The combination of strong abyssal currents and a meandering front, together with other submesoscale processes, may enhance mixing at depth (C. A. Clayson and M. Luneva, Frontal processes and deep convection in the Sea of Japan, submitted to Journal of Physical Oceanography, 2006). The abyssal currents have also been found to significantly affect the generation and modification of frontal jets [e.g., Ikeda, 1981]. This jet – abyssal circulation interaction is capable of intensifying the vertical exchange between the abyssal and upper layers. [4] Three somewhat complementary approaches (or mechanisms) have been proposed concerning the nature of the distribution of mass transport with depth in wind-driven stratified gyres. The Luyten et al. [1983] approach (‘ventilated motion’) asserts that density layers remain motionless if they are not in contact with the surface to gain potential vorticity (PV). Another approach originated from the homogenization theory of Rhines and Young [1982] and was generalized by De Szoeke [1985] and Cessi and Pedlosky [1986]. This approach predicts that the mean oceanic PV field prevents large areas of the deep ocean from establishing vigorous circulations unless Ekman pumping is sufficiently strong to penetrate dynamically to the bottom. Both approaches retain the Sverdrup relation for vertically integrated transport. [5] Dewar [1998] (hereafter D1998) extended the homogenization theory approach for the case of localized bottom topography anomalies by considering the winddriven circulation in a regime where Ekman pumping is not strong and the resulting circulation is too weak to penetrate to the bottom. He proposed that eddy-driven PV fluxes accelerate the abyssal currents following closed PV contours (f/H) similar to the action of topostresses or the Neptune effect described by Holloway [1992]. The flow is in balance with bottom friction and the strength of the deep current is then inversely proportional to the bottom drag coefficient, complementing the Welander [1968] relation for the case of a stratified fluid. At the limit of weak bottom drag coefficient relative to the effective eddy diffusivity of PV, this theory predicts the barotropization of the current and the appearance of closed geostrophic contours in the upper circulation, as seen in the Zapiola Drift and in model results by De Miranda et al. [1999]. [6] The impact of the eddy-driven mechanism and isopycnal outcropping on the formation of abyssal circulation in SOJ in various model simulations was investigated by
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Hogan and Hurlburt [2000, 2005] (hereafter HH2000 and HH2005), who used a four-layered Lagrangian model with resolutions varying from 1/8° to 1/64° to show that the eddy-driven mechanism and the resolution of baroclinic instability (accurately simulated at resolutions of greater than 1/32°) by the model had a first-order effect on the formation of intensive abyssal currents. Isopycnal outcropping and associated vertical mixing provided an alternative mechanism to topographical control. Simulations with varying-strength wind fields in HH2005 demonstrated that the effect of the isopycnal outcropping in the case of strong wind could be of the same order as eddy-driven mechanisms in the Japan Basin with weaker winds. However, the outcropping of the model lower layer isopycnal (at 250 m) does not mean ventilation of deeper waters, so the role of isopycnal outcropping on the formation of abyssal (not intermediate layer) circulation in these results could be overestimated. [7] Recent observations show convection down to roughly 1000 m in normal winters and deeper and more spatially spread mixing in colder winters [Talley et al., 2003; Riser, 2003]. Strong and deep ventilation of isopycnals and the associated transport of PV drives deep convection, while associated production of small-scale turbulence leads to increased dissipation in the intermediate and deep layers. The combined effects of these two opposing processes on the abyssal circulation in the SOJ were not analyzed in HH2000 and HH2005 due to their model setup, but may be important in this region.
2. Model Formulation [8] The numerical model used in this study is the same as in the work of Clayson and Luneva [2004]. The model is modified from the University of Colorado version of the Princeton Ocean Model [Kantha and Clayson, 2000] with sigma-coordinate levels in the upper 100 m (11 levels) and 27 z-levels at the shelf break and deep sea, with a horizontal resolution of 6 km. The mixed layer model uses second moment turbulence closure [Kantha and Clayson, 1994]. The model topography is derived from the 1/12° ETOPO5 database. The model was initialized with data from cruises during summer 1999 and is forced by ECMWF model data with a temporal resolution of 6 hours and a coarse spatial resolution of 1.25° 1.25°. Realistic inflow from the Tsushima Strait and outflow through the Tsugaru and Soya Straits was provided by the data-assimilative variational model of Nechaev et al. [2005], based on observations in the Korea/Tsushima Strait and the Tsugaru Strait (see C. A. Clayson and M. Luneva (Preconditioning studies of deep convection in the Sea of Japan using a numerical ocean model, submitted to Journal of Physical Oceanography, 2006) (hereinafter referred to as Clayson and Luneva, submitted manuscript, 2006) for more details). Model simulations with inflow variability within observed limits demonstrate that variability in the observed August – September increased inflow affects the northward penetration of the deep East Korean Warm Current (EKWC). If the EKWC does not reach 40°N, then the intensive deep eddies shown in Figure 1 near 41– 42°N, 130– 132°E do not form. [9] Model simulations were performed for 10 years with the 2-year cyclic surface forcing of March 1999 to March
Figure 1. (a) Model upper layer mean circulation and Lee and Niiler [2005] data (in red). Inflow and outflow boundaries are also shown. (b) Two-year mean circulation at 1000 m and observations by Takematsu et al. [1999] (red arrows), Senjyu et al. [2005] and Teague et al. [2005] (pink arrows). (c) Square root of double mean kinetic energy (in color) at layer 1000 – 2000 m and currents at 2000 m. 2001, reproducing the effect of a moderate winter as in 1999– 2000 and a very cold winter as occurred in 2000– 2001. Strong winds generally begin in November. The intensity of the Ekman pumping is 30% weaker in the moderate winter but continues until the end of March, whereas the winds in the cold winter are stronger with a very sharp maximum in January to early February and weak winds in March. Figure 1 illustrates the 2-year modeled and observed mean 15 m and 1000 m circulations and energy. The mean deep circulation consists of a system of quasipersistent eddies above the bottom topography features and two large-scale embedded cyclonic gyres following f/H contours in accordance with observations and model results by HH2000 and HH2005, despite the coarser horizontal
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Figure 2. Dependence of amplitudes of mean current at different depth levels on bottom friction. (a, b, c) Results for a = 4, (d, e, f) results for a = 0.25, and (g, h, i) ratio of amplitude of currents shown in Figures 2a – 2c and 2d– 2f. Figures 2a, 2d, and 2g show mass transport, and other plots show amplitude of mean currents in different layers. resolution used here. The level and spatial distribution of kinetic energy reaches 800 (cm s1)2 and 40 (cm s1)2 in upper and deep layers respectively, and is in very good agreement with observations. The amplitude of instantaneous currents at 800– 2000 m in the northern gyres and variable eddy-like currents in the northwestern part of the Japan Basin reaches 15 – 20 cm s1 in wintertime as estimated by Takematsu et al. [1999]. A detailed description of the model itself, spin-up and boundary conditions, and further comparisons with in situ data are given by Clayson and Luneva (submitted manuscript, 2006).
3. The Role of Surface Forcing and Drag Coefficient on the Deep Circulation 3.1. Bottom Drag Coefficient Effects [10] D1998 states that the role of the eddy-driven mechanism leads to dependence of the circulation on bottom friction; if true, increasing the bottom drag coefficient should decrease the amplitude of the bottom flow determining the transport of the abyssal circulation. Alternatively, if closed geostrophic contours are formed by a different mechanism the existence of the lower region of abyssal circulation apart from the eastern boundary should be independent of bottom drag. [11] The bottom drag coefficient Cd is defined in our model from the logarithmic law for a turbulent boundary layer: t b =r ¼ u*2 ¼ Cd U 2 ; Cd ¼ ak2 =ðlnðdz=z0 ÞÞ2 ;
ð1Þ
where t b is the near-bottom shear stress, U is the nearbottom local velocity, u* is dynamical velocity, k is Von Karman constant, z0 is roughness parameter, dz is the distance from bottom, and a is a parameter for varying the effect of the bottom drag, where a = 1 corresponds to the base case. The amplitudes of the currents at different depths for differing a are shown in Figure 2. The abyssal circulation weakens with an increasing Cd, and the f/H circulation in the upper layer is not as pronounced. The barotropisation effect can be seen in the case of weak bottom friction where the bottom Ekman layer mass flux balances the mass input from the surface Ekman layer (Figure 2b). In these simulations, strong abyssal currents correspond to divergence/convergence of PV fluxes above the bottom topography anomalies in accordance with D1998 and strengthen with decreasing bottom drag coefficient (Figure 3). The PV fluxes are defined as w = u0 ðrotðuÞ gf ð@r0 =@zÞ=r0 N 2 Þ, where u is velocity, r0 density, r0 density perturbation, f Coriolis parameter, g gravity, and N is Brunt-Va¨isa¨la¨ frequency. Areas with strong vertical motion are co-located with the area of intensive divergence/convergence of PV fluxes. 3.2. Seasonal and Sub-Seasonal Forcing [12] Another aspect following from the D1998 theory is that, due to the inverse proportionality of the stream function to Cd in a steady state, the bottom friction, which in this theory is linearly proportional to the stream function, is independent of a varying bottom drag coefficient, and bottom stresses reach specific values based on a balance
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Figure 3. Two-year mean PV fluxes and winter (November –March) mean vertical velocities for (a, c) a = 4 and (b, d) a = 0.25. with forcing. Let us assume that a similar conclusion holds for equation (1), then abyssal velocity becomes proportional rather than inversely proportional to the bottom to C1/2 d drag coefficient. In the northern basin (excluding the region of the Liman current along the Siberian coast), the current amplitude increase ranges from 1.8 to 6 for changes over a factor of 16 of a (Figure 3). This is much smaller than the inverse proportionality ratio of 16 and much closer to the 4. law C1/2 d [13] Another reason for the difference from the D1998 4 could be theory and also deflection from the law C1/2 d explained by the strong input of seasonal and sub-seasonal variability in the wind forcing, leading to varying currents. A yearly-mean value of wind stress can be written as a combination of the mean and eddy kinetic energy (KE): 2 t b =r ¼ 2Cd Ke2 ¼ 2Cd ð KEMC þ EKEÞ ¼ Cd U þ 2EKE ; ð2Þ
where the overbar denotes averaging in time, KEMC is the KE of mean currents, and EKE is the eddy KE. The amplitude of the seasonal variation of the monthly and basin-averaged KE is at least twice the KE of two-year mean currents (Figure 4a); this could lead to a discrepancy . Changes in the in the solution from proportionality to C1/2 d mean circulation due to flow instabilities are significantly smaller than annual variations in multi-year model simulations with the same seasonal forcing.
[14] Thus, in order to reach quasi-steady balance in the Sea of Japan the spin-up time for a steady-state bottom Ekman layer must be smaller then the typical timescale of wind forcing, which is here about 4 – 5 months, from November to March. The frictional spin-up time (Tspin) is inversely proportional to the bottom drag D in Rayleigh law for the linear case in D1998, which can be estimated as ~ /(2KeH)1. The mean tb U Tspin D1 t b /(U H) ~ ~ for depths greater than 2000 m is shown in tb U value of ~ Figure 4b. For the case of weak friction, Tspin 1000 days and is much greater then the timescale of forcing, so seasonal forcing is the high-frequency component in comparison to bottom Ekman pumping control. In the case of stronger bottom friction, Tspin 50– 100 days and is of the same order as the timescale of forcing, so local balance could be established within the period of active winds before the wind abruptly decreases in the spring (Figure 4c). 3.3. Winter Ventilation of Deep Waters and Energy Dissipation [15] The area strongly affected by vertical mixing is located in the northern part of the Japan Basin (Figure 4), and coincides with the area of strong isopycnal outcropping in HH2005 and observed locations of deep convection as given by Talley et al. [2003] and Riser [2003] and as modeled by Clayson and Luneva [2004]. Ventilation of the isopycnals is much stronger in winter 2001 (in agreement with Talley et al. [2003]). However, it does not lead to
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Figure 4. Evolution of basin mean characteristics with time for different bottom drag coefficients for (a) kinetic energy at layer 1000– 2000 m and (b) energy loss in bottom boundary layer for depths deeper than 2000 m. Monthly averages are shown by solid lines, and the two-year mean for each a is shown by a dashed line. (c) The intensity of the surface shear stress used in the model as forcing. (d) November and March mean diffusive fluxes of mean kinetic energy and the corresponding dissipation rate of TKE for the underlying layer. Contour interval for dissipation rate is 0.5 at logarithmic scale, with yellow corresponding to 104 cm2 s3, red to 104.5 cm2 s3 and orange to 105 cm2 s3. a strengthening of the mean deep circulation due to the increased level of small scale turbulence and thus dissipation of TKE at depths greater than 400 –800 m (or deeper in cold winters), with winter mean values reaching 104 cm3 s2. This contradicts the results of HH2000 and HH2005 in that in these simulations ventilation of deep waters plays only a positive role in enhancement of abyssal circulation. The dissipation rate of energy at the intermediate level of 400 m– 800 m (or the upper boundary of the abyssal layer) is at least comparable with the effect of dissipation at the ~ for high friction and exceeds tb U bottom Ekman layer ~ this for weak friction.
4. Are Changes in Bottom Drag Coefficient Possible in Regions With Intense Abyssal Circulations? [16] There is a possible mechanism for flow acceleration due to reduction of bottom drag in SOJ. If the local velocity at the bottom turbulent boundary layer exceeds some critical value (typically about 10 –15 cm s1 [e.g., Tengberg et al., 2003]), then sediment particles are uplifted from the bottom and interact with the turbulent flow. For fine particles with a velocity of settling (a) smaller pffiffiffiffiffiffiffiffiffiffithan the dynamic velocity (a < ku*, where u* = t b =r), stratification caused by sediment concentration starts to dynamically affect the turbulence. This leads to laminarisation of the flow and a reduction of bottom drag coefficient [Barenblatt, 1955] [see also Barenblatt et al., 1993; Kagan et al., 1999]. In the critical regime when turbulent flow involves the maximum amount of sediment concentration possible, the asymptotic law for bottom drag predicts the reduction of bottom pffiffiffiffiffiffiffiffiffi drag coefficient with the scaling factor of a = (k b =/a)2 (Barenblatt, 1955). For typical sizes of sediment at the SOJ floor in the range of d 0.1 mcm – 0.1 mm and density 1.8 –2.5 g cm3 (V. N. Lukashin, personal communication, 2006), an estimate of the settling velocity of suspended
sediment based on the Stokes formula for spheres produces 2 gDr 2.0 108 2.0 a wide range of values: a = 29 d rn 102 m s1. In order for the settling velocity to affect the bottom drag coefficient by an order of 16, as shown in the numerical experiments, the abyssal velocity scale would be roughly 10 cm s1. With a 0.1u* in the critical regime and for typical background bottom drag coefficient value (without suspended sediment) of Cd 2.5 103, a < 1.25 103 U 1.25 104 m/s, which is in the estimated range of settling velocities. In reality the situation is more complex because stratification caused by suspended sediment on the slope will create lateral pressure gradients and accelerate/decelerate the flow, and the turbulent regime in the bottom layer is affected by thermal and sediment stratification. However, these simple estimations demonstrate the possibility of physical mechanisms that decrease bottom drag coefficient and could affect the circulation both in the abyssal and upper layers.
5. Summary [17] The abyssal circulation in the northern part of SOJ is caused by the rotational wind component, whereas in the southern part of SOJ the abyssal currents are less dependent on the wind curl. Since the strength of the circulation grows with decreasing bottom drag coefficient, the D1998 theory adequately describes the forcing of the abyssal circulation in this basin. However, there is a strong seasonal variability in the winds. In the case of weak bottom drag, the balance with Ekman pumping cannot be reached, and the strength of the abyssal circulation is dependent on the intensity of Ekman pumping and duration of the high-wind period. In the colder year, the wind duration was about one month shorter. This explains the apparent contradiction in that the stronger abyssal currents occur in the ‘moderate’ year due to the longer duration of winter winds, at least for cases of weak bottom friction. An interannual difference is not apparent in
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the case of strong bottom drag when the solution is near steady-state. The duration of the wind forcing thus has a stronger impact on the intensity of deep currents than the strength of the wind in the case of weak friction. Changes in frictional dissipation at the bottom of the basin are possible over the simulated scales shown here due to the strength of the currents and possible sediment-current feedbacks. Strong deep convection has two competing effects on the deep circulation: the ventilation of isopycnals causes enhanced deep circulation, but the enhanced drain of mechanical energy to small-scale turbulence and dissipation in intermediate and deep layers ultimately weakens the deep circulation. [18] Acknowledgments. We acknowledge with pleasure the support by the Office of Naval Research under grant N00014-03-1-0989. We express our appreciation to Steve Riser for the PALACE buoy data, Lynne Talley for the ship data, and Dmitri Nechaev for the inflow model data. This paper was strengthened by conversations with William Dewar and by comments from two anonymous reviewers.
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C. A. Clayson and M. V. Luneva, Department of Meteorology, 404 Love Building, Florida State University, Tallahassee, FL 3230, USA. (clayson@ met.fsu.edu;
[email protected])
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