东印度洋水体输运的季节变化及相关盐度变化

L.A.A.N.Wickramasinghe 王卫强 刘雨 M.K.Abeyratne

W.C.Hemamali1 K.B.S.S.J.Ekanayake1

0 Introduction

The Eastern Indian Ocean (EIO) is a well-known tropical ocean confined by the landlocked northern Bay of Bengal.Under the influence of seasonal reversing monsoon winds and semi-enclosed nature,the EIO is unique compared with other world oceans.The winds blow over the EIO from southwest during summer monsoon (June-September) and northeast during winter monsoon (December-February).This dramatic wind field has a profound effect on upper hydrospheric structure and seasonal variation of surface current system[1-10].

The surface circulation of winter monsoon and summer monsoon reverses its direction westward and eastward respectively.The area south of Sri Lanka is an important bottleneck region where transport of water masses flows into and out the EIO,accordingly currents transport masses in the region as follows.The monsoon reversing currents are the Northeast Monsoon Current (hereafter NMC) and the Southwest Monsoon Current (SMC).The transport of the NMC in winter monsoon generally was about 11 Sv,whereas the SMC transport was 8 Sv during summer monsoon[11-12].

March-April and October-November are periods of monsoon transition.However,in the equatorial region,the eastward strong Wyrtki Jets (WJs) were generated by sustained westerly wind during both monsoon transition periods.The jets transport both heat and salt eastward thereby influence water mass property of the EIO.The Equatorial Under Current (EUC) exists just beneath the WJs only in spring (March-April) due to the presence of eastward subsurface pressure gradient.The EUC is absent during autumn especially in the Indian Ocean.

Thus,the above-mentioned currents significantly contribute to the water mass transport in the EIO,which is important to identify each current in more detail[12-21].

The westward NMC appears in the BoB during boreal winter.It starts to develop in November and peaks in February.The current can be observed weaken further and subside in April.The NMC contains low saline water from the BoB and flows westward upon the AS along the coast of India[14,22].And the Southwest Monsoon Current (SMC),a surface ocean

current contributes to significant zonal mass transport from western basin(AS) to eastern basin(BoB) in boreal summer (JJAS).In general,the SMC flows in the region between 3°45′N and 6°N and makes a northeast-ward turn into the BoB.There are cyclonic and anticyclonic circulations along with the current such as Sri Lanka Dome (SLD) and other eddies[4,11,15,17,19-21,23-24].

The Wyrtki Jets are surface narrow jet-like eastward currents in equatorial region during both monsoon transition periods of April to May and October to November[25-26].The WJs are wind driven surface jets which converge at the equator.The jets become stronger between 60°E and 90°E,often exceeds 50 m·s-1.It is believed that direct wind force is the dominant mechanism of the WJs,though the effects of reflected Rossby waves and mixing layer shear are also important.The WJs contribute to water mass transport from western basin to eastern basin and drastically affect the water mass property of the eastern basin(EIO)[2,22,27].

During March and April,the EUC exists beneath the WJs.The EUC appears in equatorial current profile between 53°E and 93°E.In response to the equatorial wind forcing,the optimum condition for the EUC (the pressure gradient) depends on the traveling time and reflection of equatorial Kelvin and Rossby waves.Generally,the EUC appears at the top of the thermocline and can be either eastward or westward[12,16].The above mentioned surface currents with the EUC contribute to the basing zonal transportation,while the equatorial Indian Ocean water act as a barrier to transequatorial movement[28].

Hence,the current study is proposed to examine critical water mass transport in the EIO with respect to the selected three boundaries considering salinity distribution.

1 Data and method

The whole study is carried out using ECCO2 global ocean model simulation data for 20 years (1992-2012),which consists of monthly fields of different data.ECCO2 state estimate is obtained by using Green’s function approach,and the mean horizontal resolution of 18 kilometers are used.The particular simulation is labeled as “cube 92” and the data are available at http:∥apdrc.soest.hawaii.edu/las/v6/dataset?catitem=4 931.

The study domain is covered by three distinctive sections of the Eastern Indian Ocean as depicted in Fig.1.The coordinates of the three sections include,from 80°E to 100°E along the equator,the equator to 6°N at 80°E,and from 82°E to 100°E at 6°N.Initially,the mass transport was calculated using zonal (80°E) and meridional (equator and 6°N) current velocity of climatological data,from surface to bottom for 12 months.And the upper transport (from surface to 300 m) was calculated separately.The following equation was used for the calculation of total mass transport (M).

Wherehis depth,Usis Stokes velocity,zis width of the current[29].Mass transport variations of each section are analysed with respect to the salinity variation of ocean currents such as SMC,NMC,WJ,and EUC which appear either once during the year[30].To study the water mass property,seasonalT-S(potential temperature vs.salinity) curves are ploted for upper 300 m;and to understand the water mass distribution,water currents against salinity at depth of 50 cm and 100 cm are ploted ultimately.

Our study focuses on the investigation of mass transport variation in the EIO.It is expected that wider range of data and modal simulations with longer duration and better resolution can reveal more accurate results of the EIO mass transport and water mass property variation.

2 Results

In winter (DJF),the total upper transport is generally westward at 80°E and northward at the equator.In December,the upper transport across 80°E is 6 Sv westward,for which a narrow westward current within 2°N to 6°N and an eastward current near the equator to 2°N above 100 m are responsible (Fig.3d).Previous studies suggest that the northeast monsoon current (NMC) in winter is responsible for this prominent westward mass transport south off Sri Lanka[16,31].As to eastward current near the equator,it might be the residuals of the WJs,since the WJs flow eastward in the equatorial region during monsoon transitions[26,32].At equator,the total upper transport is 3 Sv northward.As a result,the total upper transport is less than 1 Sv northward.Though the net mass transport across 6°N is small,there are strong southward boundary current east off Sri Lanka coast,which is compensated by interior weaker northward current at the rest of the section.Strong boundary currents across 6°N flow southward into the study domain and then turn westward to the western Indian Ocean as a part of the NMC.Correspondingly,the eastward inflow across 80°E as the residual of WJs is mainly balanced by northward interior flow at 6°N.The currents in January and February are similar to that in December,except that at 80°E section (Fig.3).In contrast to that in December,the NMC is dominant at 80°E and the total upper transport increases to 10 Sv in January.Simultaneously,the residual of the WJs disappears.For the subsurface layer,it experiences the transition from westward current to eastward current near the equator from December to February at 80°E.The subsurface westward current in December might be resulted from the zonal gradient set up by EUC in autumn,whereas the eastward current in February corresponds to the emergence of the EUC.

In spring (MAM),significant changes occur at 80°E (Figs.4d,e,and f).First,NMC becomes weak and almost disappears in May.Nevertheless,the strong boundary current at 6°N does not change much,suggesting that the balanced southward current at the equator plays more important role.Second,the eastward strong subsurface EUC gradually disappears and the westward gradient current emerges from March to May.In addition,the surface WJs emerge and become the strongest.Accompanying with the summer monsoon onset,the SMC starts to prevail.They both result in broad eastward current from the equator to 5°N at 80°E.It is worthwhile to note that,there is strong narrow northward current close to the boundary current at 6°N.It is because the prevailing summer monsoon drives part of SMC turning to the BoB.As a result,a cyclonic circulation starts to appear east off Sri Lanka,which is called Sri Lanka Dome (SLD).Correspondingly,similar variations of the total upper mass transport are found both at 80°E and at 6°N.At the equator,the total upper mass transport slightly decreases from 2 Sv to -6 Sv.

In summer (JJAS),two distinctive transports with opposite signs can be identified at 80°E during the whole summer season,in which there are westward transport near the equator and strong eastward transport within 2°N to 6°N (Figs.5e,f,g,h).The strong eastward transport is the prevailing SMC consistent with results in previous studies[4,19,23].Since both WJs and EUC are gone in summer,most of the upper transport near the equator is westward at 80°E until closing to transition season of September.Similar to that in May,more transports of SMC bend to the BoB and form a strong northward core at 6°N.More northward transport corresponds to stronger interior return flow at the same time.

In contrast,the boundary current at 6°N becomes weak and disappears in July.It suggests that the SLD gradually moves away from 6°N since then.Accompanying with prevailing summer monsoon,there is gradually increased upper transport from June to August due to the intensified SMC.The upper transport across the equator keeps unchanged with ～ 8 Sv southward.Similar to that in winter,mass transport of the subsurface layer is dominated by EUC and its transition.The westward gradient current is significant in June and gradually turns to eastward EUC in September.

In autumn (October and November),the eastward WJs dominate the upper 100 m near the equator and SMC secedes in transition season covering 2°N to 5°N at 80°E (Figs.6c,d).There is only a small gap for westward transport south off Sri Lanka.As the signs of monsoon transition,the SMC weakens and the coastal current south off Sri Lanka changes its direction from eastward to westward.The latter corresponds to the transition of boundary current east off Sri Lanka coast.Simultaneously,losing the drag force from monsoon and local cyclonic wind curl,there is no obvious SMC transports into the BoB at 6°N.The upper transport across 80°E decreases from 10 Sv to 5 Sv,most of which can be explained by the upper transport decreasing from 6 Sv to 1 Sv southward at the equator.For the subsurface layer,the residue of the EUC weakens in October and is replaced by changes of westward gradient current in November.

The sections of 80°E and 6°N are key sections to examine the freshwater and saline water exchange between the AS and the BoB.Big salinity differences exist between the AS and the BoB because of the excessive evaporation over the Arabian Sea in contrast to the large freshwater input into the Bay of Bengal.The zonal currents in the northern Indian Ocean play important roles,in which the SMC,WJs,and EUC carry cool and high saline water from the AS into the BoB in summer,while the NMC transports warm and low saline water to the western Indian Ocean basin in winter[4,23,33].As a result,it shows that the fresh water from the BoB is in the upper 50 m and the saline water from the AS is in the subsurface layer.This vertical structure is stable,so it is not surprising that the salinity pattern in these three sections does not change much in the study domain all the year round.Nevertheless,the monsoon currents have visible signs,in which the prevailing SMC results in visible subsurface high saline water intrusion east off Sri Lanka in summer at 6°N and the prevailing NMC results in visible surface low saline water south off Sri Lanka in winter at 80°E.The WJs and EUC transport high saline water near equator in the transition seasons,corresponding to a larger wedge of saline water intrusion than that in summer and winter at the equator.It is evidenced by the seasonal variations ofT-Splot at these three sections shown in Fig.7.The two ends of theT-Scurve at 80°E connects one end of 6°N section and one end of the equator section in all seasons,respectively.It corresponds to the close relationships of saline water both at 6°N and 80°E transported by the SMC and the NMC and that both at the equator and 80°E transported by the WJs and the EUC.The depth of the selected 35.1 psu isohaline can further explain how water mass transports into the BoB (Fig.8).The 35.1 psu isohaline is rough at depth of 80 m at 80°E.The depth of 35.1 psu isohaline gradually deepens eastward to the eastern basin and southward to the BoB.In summer and autumn,it is clear that there is saline water intrusion east off Sri Lanka from western basin at ～80 m by SMC.While in winter and spring,there is visible westward saline water intrusion south of Sri Lanka by the NMC at ～100 m.

To better trace the salinity changes brought by the equatorial currents,we further examine these changes in a larger region,especially in summer,since there are more complex circulation patterns than other seasons in the eastern Indian Ocean (Fig.9,10).The WJs and EUC are relaxed in summer.As a result,the zonal gradient set up by the WJs and EUC in previous months forces significant westward equatorial zonal current in June.The outflow of the BoB derived from the SMC bend also contributes to the westward flow because of Ekman effect.It shows that visible westward currents originate from Sumatra coast both at 50 m and 100 m depth,which are used to represent the depths of WJs and EUC for simplicity.The westward currents are weakened because of the prevailing SMC and the gradually weakened zonal gradient since July,particularly at depth of 100 m.In the study domain,the salinity exchange is significant due to the eastward SMC and the adjacent westward gradient current (Fig.10).Since June,the SMC carries saline water eastward north of 5°N at 80°E and transports northeastward into the BoB,whereas the gradient current parallels to the SMC but carries fresh water and returns back.The salinity exchange process is more violent at depth of 50 m since there are more strong contrasts of concurrent opposite currents and salinity.While at depth of 100 m,the salinity contrast is small though the concurrent opposite currents still exist.

3 Summary and discussions

The upper 300 m water mass transport is examined in the study domain enclosed by three sections of 80°E,6°N,and the equator.The major water mass transports into and out of the study domain are from that across 80°E and the equator,whereas that across 6°N is small because of the water mass conservation in the BoB.The results show that the seasonal variations of the upper mass transport across 80°E are mainly compensated by that across the equator.The maximum transports across 80°E and across the equator are roughly ±12 Sv throughout the year.

Since the EIO is strongly influenced by the monsoon circulation,the monsoon currents of the SMC and the NMC are key to understand the mass transport variations.The monsoon currents display the significant changes at 80°E,therefore the mass transport across 80°E plays a predominant role in variations of the study domain.Besides,the WJs and the EUC are important as well in surface and subsurface layer during transition seasons.At 80°E,the WJs and the EUC are near the equator,whereas the monsoon currents are north of 2°N and south off Sri Lanka.

In winter,the westward NMC dominates the mass transport at 80°E.It is enhanced by strong boundary current east off Sri Lanka at 6°N.But in summer,the eastward SMC only dominates north of 2°N,while the return flow resulted from the zonal gradient and the SMC bend are responsible for the near equator transports.Most of the SMC enters the BoB after leaving the 80°E section.As a result,the interior flow at 6°N is southward for compensation,which favors the westward return flow as well.During transition seasons,when the monsoon currents weakens,the WJs and EUC dominate the near equator transports.Simultaneously,the inflow/outflow transport to the BoB weakens as well due to the weakened monsoon,which is responsible for the weakened boundary current east off Sri Lanka.

Since there is a big salinity difference between the AS and the BoB,it is worthy to note the roles of the tropical currents in salinity exchange between western and eastern basin.In general,eastward currents across 80°E carry cool and high saline water to eastern basin,such as the SMC,the WJs,and the EUC.In contrast,the westward currents like the NMC and the return gradient flow carry warm and low saline water to western basin(Fig.11).As a result,it shows a stable vertical salinity structure both at 80°E and at 6°N all the year round,in which the fresh water from the BoB is in the upper 50 m and the saline water from the AS is in the subsurface layer.From 80°E to 6°N,the depths of high saline water from western basin tend to descend under less saline water.Horizontally,the contrast of the saline water from the AS and the fresh water from the BoB is significant in the study domain,which is sometimes an interlocking pattern and sometimes a non-interference pattern.The monsoon variation is the drive force for variation of this contrast through currents under prevailing monsoon and the WJs/EUC during monsoon relaxation.