Sa lin ity Min i mum in the Up per Layer of the Sea of Ja pan

Transcription

1 ISSN , Russian Meteorology and Hydrology, 2015, Vol. 40, No. 12, pp Allerton Press, Inc., Original Russian Text V.A. Sosnin, N.I. Rudykh, 2015, published in Meteorologiya i Gidrologiya, 2015, No. 12, pp Sa lin ity Min i mum in the Up per Layer of the Sea of Ja pan V. A. Sosnin and N. I. Rudykh Il ichev Pa cific Oceanological In sti tute, Far East Division, Rus sian Acad emy of Sci ences, ul. Baltiiskaya 43, Vladivostok, Rus sia, sosnin@poi.dvo.ru Re ceived April 6, 2015 Ab stract An a lyzed is the intraannual vari abil ity of ver ti cal dis tri bu tion of sa lin ity in the south ern part of the Sea of Ja pan. It is dem on strated that the sign change in fresh wa ter bal ance is the rea son for the for ma tion and dis ap pear ance of min ima and max ima of sa lin ity in the wa ter col umn. The dy nam ics of vari abil ity of sa lin ity min ima pa ram e ters is dem on strated for the first time. The con clu sion is made that the sa lin ity min i mum can not be con sid ered as a water mass be cause it is a tem po rary phe nom e non. DOI: /S Keywords: Vertical distribution of water salinity, salinity minimum, freshwater balance, ocean upper layer, the Sea of Ja pan INTRODUCTION At pres ent the sta tus of sa lin ity min i mum in the ocean up per layer has no un am big u ous in ter pre ta tion. In the North Pa cific it is not con sid ered as a water mass al though it is clearly ob served in the T, S-curves [17, 23]. It is called the min i mum [17] or sea sonal min i mum of sa lin ity [23]. On the con trary, sa lin ity min i mum in the up per layer of the Sea of Ja pan is as sumed to be con sid ered as a water mass [20]. In the au thors opin ion, de ter mi na tion of the sa lin ity min i mum sta tus is so un clear be cause the data of in situ ob ser va tions on which the con cepts of its or i gin are based, are in suf fi cient. So, it could not be stud ied in de tail. The pos si bil ity to im prove the sit u a tion has arisen only re cently due to the ac cu mu la tion of long-term ob ser va tional data. The in ves ti ga tion of the vari abil ity of sa lin ity min i mum pa ram e ters at var i ous time scales should al low the un am big u ous de ter mi na tion of its sta tus and rea sons for its formation. The ob - jec tive of the pres ent pa per is to an a lyze the sea sonal vari abil ity of the ver ti cal dis tri bu tion of sa lin ity in the up per layer and sa lin ity min i mum in one of the ar eas in the south ern part of the Sea of Ja pan. SA LIN ITY MIN I MA IN THE UPPER LAYER OF THE SEA OF JA PAN In the Sea of Ja pan sa lin ity min ima are ob served ev ery where and have dif fer ent pa ram e ters. South wards of the po lar front at the depth of m a sa lin ity min i mum with the value of is ob - served that is called the East Sea In ter me di ate Wa ter (ESIW). It is con sid ered per ma nent [20]. A sa lin ity min i mum with the value of is reg is tered in the east ern part of the sea near the coast of Ja pan. Its depth reaches 100 m and in creases to 350 m as mov ing away from the shore [21]. In the north ern part of the sea at 46 N a sa lin ity min i mum with the value of can be de tected in May against a back ground of slightly higher val ues of sa lin ity (34.1 ) in the sur face layer [18]. In the west ern part of the sea in clud - ing the area north wards of the po lar front, in the Pe ter the Great Gulf, a sa lin ity min i mum is ob served dur - ing the cold sea son ( ) [1, 10]. A sa lin ity min i mum is also found in the south ern part of the sea [13, 15, 21]. In win ter it is also for mally ob served in the Tsushima Strait where it oc cu pies the whole depth and has the val ues of sa lin ity up to 34.6 [19]. A short-term sa lin ity min i mum is also of ten ob served in the sur face layer in the dif fer ent parts of the sea [8]. It is con sid ered that the sa lin ity min i mum in the south ern part of the sea is an ef fect of the trans port of de sa li nated wa ter from the Tumannaya River es tu ary [13, 15] and the Amur River wa ter [16, 22] that can go un der the Tsushima Cur rent wa ter. The opin ion is wide spread that the sa lin ity min i mum in the south ern part of the sea is formed ap prox i mately in the area of the po lar front and is a re sult of subduction of de sa li - 814

2 SA LIN ITY MIN I MUM IN THE UP PER LAYER OF THE SEA OF JA PAN 815 Fig. 1. (a) The re gion un der study at the point with the co or di nates of 38 N, 134 E and typ i cal ver ti cal dis tri bu tion of sa lin ity for the hu mid (b; pro files 1 7) and dry (c; pro files 8 11) sea sons. Dotted lines in fig ure (a) are sur face ther mal fronts ac - cord ing to [12]. nated wa ter to the subsurface depths [20]. Taking into ac count the struc ture of the front [9], ev i dently, it should be ex pected that the sa lin ity min i mum wa ters should prop a gate from their sup posed cen ter in the south ern [20] and east ern [21] di rec tions. There is no clear idea till now where and how sa lin ity min ima are formed and what the con tri bu tion is of the pro cesses of intraannual vari abil ity of sa lin ity. Their life time, spa tial size, and vari abil ity of pa ram e ters still re main un known. This gap is to be filled. DATA AND PRO CESSING METHODS The data of ob ser va tions for re cent years taken from the World Ocean Da ta base-2013 and Ja pan Ocean Data Cen ter were used for the pres ent study. Large datasets ac cu mu lated mainly by Jap a nese re search ers en able study ing the vari abil ity of wa ter char ac ter is tics in the rel a tively small area of the sea for a long time pe riod. The method of T, S-curves was not used and the preliminary division of water column into layers or water masses was not carried out in the present paper. The salinity minimum was not separated as an independent object but was considered as the inseparable part of the salinity vertical profile. The tem po ral vari abil ity of the sa lin ity ver ti cal dis tri bu tion was stud ied in one of the 1 squares in the south ern part of the Sea of Ja pan with the cen ter with the co or di nates of 38 N, 134 E (Fig. 1a). The func - tional re la tion ship be tween the field of sa lin ity and fresh wa ter bal ance was taken into ac count [2]. The vari - abil ity was an a lyzed us ing the ini tial data of long se ries of in situ mea sure ments. Each pro file of sa lin ity and tem per a ture was con sid ered in di vid u ally for its time mo ment with out av er ag ing in time and space.

3 816 SOSNIN, RUDYKH Fig. 2. Vari abil ity of the ver ti cal pro file of sa lin ity at the sign change of fresh wa ter bal ance in RESULTS OF OBSERVATIONS Fresh wa ter bal ance af fect ing the sa lin ity field in the Sea of Ja pan is de fined by the mon soon type of atmospheric cir cu la tion. Ap prox i mately equal amount of pre cip i ta tion is reg is tered through out the sea area with the max i mum dur ing the pe riod of the summer mon soon. The in ten si fi ca tion of evap o ra tion pro cesses and the min i mum amount of pre cip i ta tion are as so ci ated with the win ter mon soon [11]. Let us con sider the intraannual vari abil ity of the ver ti cal dis tri bu tion of sa lin ity in one of the ar eas of the sea south ward of the po lar front (Fig. 1). The ob ser va tional data dem on strate sig nif i cant dif fer ence in sa lin - ity pro files dur ing dry and hu mid sea sons. Sa lin ity min ima are ob served in the wa ter col umn in dry sea sons (Fig. 1c). The variability of salinity for several seasons is presented in more detail in Fig. 2. At the end of the humid season in September 2011 the salinity on the sea surface was equal to (Fig. 2a). At that time the subsurface maximum with the salinity value of was strongly pronounced at the depth of 100 m (September 20, 2011). In the dry season the values of salinity in the surface layer started increasing and reached at the end of November (November 28, 2011). At the depth of 50 m salinity was equal to that turned out to be minimal for this vertical distribution, i.e., this was the salinity minimum. Salinity maximum at the depth of 100 m decreased and was equal to In December the upper homogeneous layer of large thickness was formed under the influence of wind mixing (December 27, 2011). The salinity maximum at subsurface depths disappeared and the weakly pronounced salinity minimum with the value of was kept at the depth of 80 m. The subsequent increase in the values of salinity in surface layers led to the following: at the beginning of the next month the salinity minimum at the depth of 80 m became more pronounced although its value and depth did not change (January 4, 2012). In Feb ru ary, sa lin ity in the up per layer con tin ued in creas ing (Feb ru ary 6, 2012). At the same time, the sa lin ity min i mum deep ened to 110 m and was equal to (Fig. 2a). In the mid dle of March the value of sa lin ity on the sur face reached (Fig. 2b) and the sa lin ity min i mum of was lo cated at the depth of 163 m (March 21, 2012). The sa lin ity min i mum deep en ing con tin ued (March 28, 2012). At the be - gin ning of April it was equal to and was de tected at the depth of 217 m (April 4, 2012). At the end

4 SA LIN ITY MIN I MUM IN THE UP PER LAYER OF THE SEA OF JA PAN 817 of April, the pro cesses of evap o ra tion con tin ued dom i nat ing over pre cip i ta tion that is in di cated by sa lin ity in crease on the sur face up to At that time the sa lin ity min i mum was ob served at the depth of 292 m and was equal to (April 25, 2012). At the be gin ning of May (May 2, 2012) the sa lin ity min i mum (33.98 ) was ob served at the depth of 322 m and was al ready poorly pro nounced against a back ground of sur round ing wa ters as com pared to the pre ced ing pe riod. The sub se quent ob ser va tions dem on strated that the sa lin ity min i mum reached by the mid dle of May and was hardly ob served at the depth of 292 m (May 14, 2012). At the end of June the sa lin ity min i mum was ab sent (June 28, 2012). In July (July 8, 2012) it was not de tected ei ther (Fig. 2b). The dry sea son in the area un der study lasted till the end of April when the sur face val ues of sa lin ity reached the max i mum of (April 25, 2012). In May sa lin ity de crease be gan on the sur face that in di - cates the sign change of fresh wa ter bal ance and the be gin ning of the next hu mid sea son. The for ma tion of a sa lin ity max i mum at the depth of 35 m where the val ues of sa lin ity turned out to be higher than on the sur - face by 0.02, was a con se quence of de sa li na tion (May 14, 2012). At the end of June (Fig. 2b) sa lin ity on the sea sur face be came still lower and was equal to 43.27, and the sa lin ity max i mum of was ob - served at the depth of 49 m (June 28, 2012). In July it was de tected at the depth of 65 m and was equal to (July 8, 2012). The pro cess of vari a tion of the sa lin ity ver ti cal pro file also con tin ued dur ing the next dry sea son (Fig. 2c). Against a back ground of sa lin ity in crease in the sur face layer, the subsurface sa lin ity min i mum of was de tected at the depth of 94 m and the sa lin ity max i mum was ob served at the depth of 150 m (De cem ber 3, 2012). The wind mix ing re sulted in the fact that at the end of De cem ber the sa lin ity min i mum (33.89 ) was ob served at the depth of 143 m. It was not pro nounced against a back ground of slightly higher val ues of surface sa lin ity equal to (De cem ber 31, 2012). The sub se quent mix ing led to in crease in the thickness of the up per ho mo ge neous layer (Jan u ary 7 Feb ru ary 18, 2013) and a sa lin ity min i mum was detected at the depth of about 220 m. All the while the sa lin ity max i mum value grad u ally de creased and its depth in creased (Fig. 2c). At the end of March the max i mum val ues of sa lin ity were ob served on the sea sur face and nei ther min ima nor max ima were ob served in the ver ti cal dis tri bu tion of sa lin ity (March 28, 2013). The sa lin ity pro file (Fig. 2c) in di cates that the den sity con vec tion had al ready oc curred be fore that mo ment of time. The variability of salinity caused by the alternation of dry and humid seasons for the period of is presented in Fig. 3. To plot the graphs the large volume of the data was used that actually means their averaging in time. As a result, the salinity minimum in the water column disappeared and is practically not presented in Fig. 3. Ev ery year the vari abil ity of sa lin ity fol lows the same sce nario and is cy cli cally re peated. In the dry sea son (from late au tumn to early sum mer), the val ues of sa lin ity in the sur face lay ers reach the sea sonal max i - mum. In the hu mid sea son (sum mer and early au tumn), the min i mum val ues of sa lin ity are ob served on the sur face and the sa lin ity max i mum is reg is tered at subsurface lev els. The vari a tions of the fields of sa lin ity and tem per a ture are not syn chro nous that cor rob o rates the in de pend ence of both pa ram e ters (Figs. 3a and 3b). The field of den sity var ies syn chro nously with the field of tem per a ture and mostly de pends on it (Fig. 3c). DISCUSSION OF RESULTS The data of in situ ob ser va tions dem on strate that both min ima and max ima of sa lin ity are formed in the up per layer with equal prob a bil ity. They are formed and dis ap pear in subsurface lay ers against a back ground of sa lin ity vari a tions on the sea sur face and are tem po rary phe nom ena not as so ci ated with the po lar front. The nat u ral and nec es sary con di tion of sa lin ity min i mum for ma tion is the sea sur face de sa li na tion. As a re sult of the sub se quent prev a lence of evap o ra tion over pre cip i ta tion, the con di tions are formed for sa lin ity in crease in the sur face layer of the sea [6]. When the ab so lute val ues of sa lin ity on the sur face be came higher than those at subsurface depths, this means sa lin ity min i mum for ma tion in the wa ter col umn [7, 8]. Model com pu ta tions sim u lat ing the pro cess of sa lin ity min i mum for ma tion dem on strate that the sa lin ity min i mum turns out to be deeper and deeper as the salt flow from the sur face in creases and its value of sa lin - ity be comes higher [8]. The data of in situ ob ser va tions cor rob o rate these re sults in full. At dif fer ent time mo ments (dif fer ent phases of fresh wa ter bal ance) the sa lin ity min i mum is de tected from sur face lev els to the depth of about 300 m. Its ex is tence and the pro cess of vari a tions of its pa ram e ters are in ter rupted by win ter time den sity con vec tion whose real ex is tence in the south ern part of the Sea of Ja pan is cor rob o rated by vast ev i dence [12, 14]. Af ter the con vec tion pro cess the sa lin ity min i mum is not ob served in the wa ter col umn dur ing the

5 818 SOSNIN, RUDYKH Fig. 3. Iso pleths of (a) sa lin ity, (b) tem per a ture, and (c) den sity in the south ern part of the Sea of Ja pan in based on the ac tual data. The val ues of sa lin ity above , tem per a ture above 10 C, and den sity be low 26 con ven tional units are shaded. rest part of the dry sea son and dur ing the sub se quent phase of the hu mid sea son. It is formed again just af ter the be gin ning of the next dry sea son. Prob a bly, sit u a tions are pos si ble when the con vec tion re gime is not formed in some years. In this case, the sa lin ity min i mum will most likely be kept for a lon ger pe riod of time (Fig. 1c). CONCLUSIONS The con stantly in creas ing dataset of in situ ob ser va tions indicates continuous vari a tions in the ocean characteristics, in particular, formation and disappearance of salinity extremes, that is, its maximum and min i mum, in the up per layer of the ocean. This makes it difficult to in ter pret the ver ti cal struc ture of the up per layer wa ters in the frame work of the wa ter mass con cept. The sa lin ity min i mum is pro nounced on the T, S-curves, but it is a tem po rary event and can not be con sid ered as a water mass. The con cepts of con ser va - tive wa ter masses based on the in ter pre ta tion of long-term av er age an nual data [4] no lon ger cor re spond to the state-of-the-art knowl edge about the con tin u ous and dif fer ent-scale vari abil ity of the ocean (and cli mate sys tem) [3, 5]. In or der to over come con tra dic tions and un cer tainty in the in ter pre ta tion of the min ima of sa - lin ity and tem per a ture in the wa ter col umn, the con cept of wa ter masses has to be adapted to the real con di - tions of the con tin u ously chang ing ocean.

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