Soy. J. Phys. Oceanogr., Vol. 1, No. 5, pp. 399-403 (1990) @ VSP 1990.

Sub-arctic frontal zone in the central northern Pacific during autumn*

T. P. K I L ' M A T O V

Abstract--We examine the cross structure of the sub-arctic frontal zone in the central Pacific using data obtained during the fifth cruise of the RVAkademik M. A. Lavrent'ev (autumn 1986). It is noted that the zone's position is closely related to the Ekman circulation (northern boundary of the area) and the North Pacific current (southern boundary). An estimate of the cross circulation is given.

INTRODUCTION

The impor tance o f s tudying oceanic frontal zones is dictated by their pract ical use in fisheries, and weather and climatic predict ion [I, 3]. The sub-arctic f ronta l zone, shaped as a na r row band, crosses the nor th Pacific f rom west to east near 40 ~ N and separates the sub-arctic water mass f rom the subtropical one [1 ,2] . The available publ icat ions [1,2, 4 -6 ] concentra te mainly on the s tudy o f the western par t o f the ocean, i.e. the f ront in the Kuroshio current area. Da ta on the sub-arct ic f ronta l zone in the central Pacific are considerably less abundant .

Da ta on the meridional hydrologic sections a long 180 ~ 170 ~ 160 ~ and 150 ~ W and between 36 ~ and 46 ~ N (fifth cruise o f the R V A k a d e m i k M. A . Lavren t ' e v ) are used to analyse the posit ion and cross s t ructure o f the sub-arctic f rontal zone. STD stations were taken along the sections at intervals o f 30 miles between 16 September and 6 Oc tober 1986, using a M A R C - I I I C probe. It should be recalled that in reference [6] hydrologic data on the sub-arctic f rontal zone, compi led at approximate ly the same area in S e p t e m b e r - O c t o b e r 1975, have been published. A l though the area studied [6] was smaller in size, it would be o f interest to c o m p a r e these data with the new ones.

SALINITY FIELD

The sub-arct ic f rontal zone has conspicuously large horizontal t empera tu re and salinity gradients [1,4] . The cruise data show that the frontal zone is bet ter p ronounced in the salinity field than at temperature sections. This is conf i rmed in reference [6]. The max imum amoun t by which the gradients between stat ions exceed the b a c k g r o u n d meridional one is as fol lows--~n tempera ture 2 to 4 times; in salinity 3 to 6 times. The background gradients have such an o r d e r - - i n t empera tu re 1 . 7 - 1 0 -2 C/mile, in salinity 2 . 8 x 10-3%0/mile. In Fig. 1 the salinity field at sect ion

* Translated by Vladimir A. Puchkin.. UDK 551.465.

400 T. P. Ki l 'matov

180 ~ 170 ~ 160 ~ 150 ~ ; t �9

O

Figure 1. Position of the sub-arctic frontal zone, cross structure of the salinity field (autumn 1986); pattern of the cross circulation in the sub-arctic frontal zone and the computed salinity gradient.

180~ is presented; a similar pattern is, in general, reproduced at the other sections. The frontal zone 's northern boundary delineates the sub-arctic waters f rom the south, the southern one f rom the north [1,2]. Climatically, the northern Pacific frontal zone is traceable at 39-43~ N [2]. According to references [5, 6], the term ' a sub-arctic f ront ' implies the existence of a group of fronts located in the vicinity of the zone 's northern boundary north of 40 ~ N. It is readily visualized in Fig. 1 that these frontal boundaries are geometrically connected via isohalines with the sub- arctic halocline located at a depth of 150 m. The positions of the zone 's northern boundary , as given by the data of 1975 [6] and 1986, coincide roughly (40-42 ~ N). There seems to be a difference between our observations and the data in reference [6]. In the early au tumn of 1975 in the vicinity o f the sub-arctic zone 's nor thern boundary , a well-pronounced frontal surface was documented approximate ly at 4 4 ~ which resulted f rom the outcropping of the sub-arctic seasonal halocline f rom a depth of 40 m [6]. According to the data of 1986, no marked seasonal halocline was observed.

I~is seen that the frontal zone 's southern boundary , separating the subtropical region by one or several fronts, is located below 4 0 ~ (Fig. 1). At the ocean surface, the m ax i m um temperature and salinity gradients along the sub-arctic f rontal zone 's boundaries concur in general, and also coincide at the nor thern boundary for the 33-4%0 isohaline, and the southern one for the 34.3%o isohaline. The highest horizontal surface temperature and salinity gradients are observed at the zone 's nor thern boundary fronts, which probably explains the identification of this boundary with the sub-arctic front in references [5, 6]. Thus, the sub-arctic f rontal zone seems to have a two-frontal structure associated with its nor thern and southern boundaries . A similar structure has been documented and described in reference [1] for the sub-arctic frontal area in the north-western Pacific ocean in the Kuroshio current area.

Sub-arctic frontal zone 401

CIRCULATION PATTERN

Computations of the geostrophic surface current velocity component based on the measurement data show that at all sections a general geostrophic transport eastward takes place, the mean velocity being 15 cms -1. At the 180 ~ 170 ~ and 160~ sections a stream current of velocity 45 cm s-1 is observed, whose route coincides roughly with the sub-arctic frontal zone's southern boundary position. At 150 ~ W, this current is diffused and is not emphatically pronounced. The geostrophic current velocity components (7 cms -1) at a depth of 200 m are generally directed eastward.

Let us examine the cross water transport with respedct to the front in a surface layer that occurs at the southern boundary owing to the above-mentioned geostrophic stream current. In Fig. 1 the origin of coordinates is situated on the current 's axis. The current 's regime across the front is ageostrophic [3] ; the lateral diffusion is counterbalanced by the Coriolis force

- f v = Auyy . (1)

Here the inertial lateral force is essentially less than the Coriolis one, [ VUy[ .,g I f v I; the scales of the values involved are given below. Hence we obtain a scale of the current velocity component along the y-axis in the upper layer v = A u o / f L 2 Here L is the current 's cross dimension and Uo is the geostrophic velocity at the current 's axis. We have the following estimates: u0 - 1 0 -I ms - l , f - 10 -4 s -1, A - 104 mEs -1 [3], L - 105 m. From these and (1) we obtain the estimates for the southern boundary of the frontal zone: v - 1 0 - 3 m s -1, vertical velocity w - [ H / L ] v ~ 1 0 - 6 m s -I at H - 100m.

The northern boundary location is related to the Ekman circulation [5, 6]. The Ekman transport computed for mean October values using 1 ~ squares in the northern Pacific is presented in reference [6]. For the sub-arctic frontal zone, the mean water mass transport velocity from north southward in a 0-100 m layer is 1 0 - 3 - 1 0 - 4 m s - t . Ekman's vertical velocity component at a pycnocline depth (100 m), according to reference [6], is downward and attains a maximum of 2 -10-6 m s -1 at the sub-arctic front (the zone's northern boundary). Comparison of the magnitudes of cross water circulations in the upper layer, induced by a stream current at the southern boundary, and of the Ekman water circulation at the northern one yields coinciding scales.

ON THE CAUSES OF FRONT FORMATION AT THE SUB-ARCTIC FRONTAL ZONE

BOUNDARIES

The possible formation of a frontal surface associated with the maximum water sinking due to Ekman convergence has been demonstrated using a simple model in reference [7]. This mechanism [6] helps to explain the front formation at the sub- arctic zone's northern boundary. It has also been shown, through the analysis of field-documented data, that the northern boundary is located in the area o f maximal Ekman convergence, i.e. the border 's position depends on the wind field configuration in the near-surface layer.

The sub-arctic zone's southern boundary is represented by a density f ront and is associated with the geostrophic chrrent~ This type of front has been studied in more

402 7". P. Kil'matov

detail than the others [3]. The numerical experimental data and analytical estimates allow us to reproduce the general features of the cross structure and secondary circulation of such a front. The circulation pattern according to Fedorov [3] (an upwelling is on the right-hand side, a downwelling on the left-hand side of the current ' s axis) is shown in Fig. 1. The salinity gradient at the surface due to the cross circulation is easily evaluated in the following manner. The salinity field at the surface S across a stationary, zonally-homogeneous front (a i r -sea interaction in deeper layers has not been considered) satisfies the equation

vsy = AsSy~ (2)

where A s is a constant turbulent exchange coefficient for salinity along the y-axis. Substituting equat ion (1) into (2) and integrating, we obtain a fo rmula for calculating the salinity gradient:

S y = c, e x p ( - A ~ u y ) . (3)

Here c~ is an integration constant which can be identified with the salinity gradient at the current ' s axis where uy = 0. The estimation of scales yields A A s 2 - - 10 2. It is visualized f rom formula (3) that the salinity gradient is directly linked with the jet current ' s shape through the velocity gradient. Sy increases exponentially with growth of - uy. As the current 's geometry indicates, the southern boundary of the sub-arctic frontal area must be slightly shifted to the left nor thward with respect to the current ' s core. Deflection of the density front towards the cold water has been noted, e.g. in reference [3]. F rom the previously obtained data it follows that the southern boundary and the current 's axis coincide roughly. To find the indicated displacement, a more detailed survey is needed, since this effect must encompass less than half the width of the current.

CONCLUSION

The sub-arctic frontal area in the central northern Pacific in au tumn has a two- f ront structure, similar to the one suggested by Bulgakov et al. [1] for the north- western part o f the ocean. The observations show that horizontal tempera ture and salinity gradients at the northern boundary are larger than those at the southern one; therefore, the northern boundary is more sharply delineated than the southern one. The location of the northern boundary is related in reference [6] to the area of extreme E k m a n convergence, and the position of the southern border to the nor th Pacific current. Thus the structure, location, and width of the sub-arctic area depend largely on the combinat ion of these dynamic factors.

REFERENCES

1. Bulgakov, N. P., Glushchuk, B. A., Kozlov, V. F., Novozhilov, V. N. and Starodubtsev, E. G. The Sub-arctic Front in the North-Western Pacific Ocean. Vladivostok: FERC, USSR Acad. of Sci., POI (1972), 133 p.

2. Moroz, K. N. On the position and structure of the sub-arctic front in the mid-Pacific ocean, lzv. T[NRO (1972) 81, 3-15.

3. Fedorov, K. N. Physical Nature and Structure o f Oceanic Fronts. Leningrad: Gidrometeoizdat (1983), 272 p.

Sub-arctic frontal zone 403

4. Gruzinov, V. M. Hydrology of the World Ocean's Frontal Zones. Leningrad: Gidrometeoizdat (1986), 272 p.

5. Roden, G. J. On the north Pacific temperature, salinity, sound velocity and density fronts and their relation to the wind and energy flux fields. J. Phys. Oceanogr. (1975) 5, 557-571.

6. Roden, G. J. Oceanic sub-arctic fronts of the central Pacific: structure and response to atmospheric forcing. Z Phys. Oceanogr. (1977) 7, 761-778.

7. Welander, P. Mixed layers and fronts in simple ocean circulation models. J. Phys. Oceanogr. (1981) 11, 148-152.