阿拉伯海小暖池混合层盐度的年度变化

A.U.MARASINGHE 张玉红 王东晓 Tilak P.D.GAMAGE 黎大宁 姚景龙

摘要采用ECMWF的ORAS4盐度资料（1999—2009年）来研究阿拉伯海暖池区（ASMWP）混合层盐度的年际变化.基于ORAS4盐度的季节循环特征与观测资料高度一致，发现了ASMWP混合层盐度变化的异常高（低）盐度时期，即在2003、2005年的7—10月混合层盐度异常偏大，而2002年2—4月则有明显减弱.进一步研究发现，混合层盐度的强度和持续时间与ENSO和IOD有一定的关系，ASMWP混合层盐度异常在IOD负位向和ENSO负位相的时候偏高，反之亦然.最后，通过盐度收支分析发现，水平输运是导致ASMWP混合层盐度异常年际变化的主要因子.

关键词阿拉伯海暖池区；年际变化；混合层；盐度收支；气候模态指数

中图分类号P715.6；P731.12

文献标志码A

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低纬度海区在表层淡水的作用下障碍层普遍存在.深厚的障碍层会抑制海洋上层的垂向混合和热盐交换，进而影响海洋大气相互作用的方式.阿拉伯海暖池在夏季西南季风爆发前（3—4月）发生，它是阿拉伯海东南海区温度最高的海区.阿拉伯海暖池的形成与该区域的低盐水密切相关.阿拉伯海暖池对印度季风槽的形成和印度季风的爆发有着重要的影响.因此研究阿拉伯海暖池的盐度变化有着重要的意义.

本文应用1999—2009年间欧洲中期天气预报中心（ECMWF）提供的ORAS4盐度、经向及纬向流速、温度和位势密度数据，研究了阿拉伯海暖池区混合层盐度的年际变化及影响因素.在正常年份，阿拉伯海暖池区域混合层盐跃层从7月逐渐形成，主要分布在上层60 m水体中，于10月达到最大.夏季风将阿拉伯海内部的高盐水输送到暖池区是造成盐度跃层加强的主要原因.现场观测的盐度垂向分布显示阿拉伯海暖池区盐度有着强烈的季节周期和明显的年际变化特征.在研究时段期间，阿拉伯海暖池区混合层盐度存在显著的年际变化.在2003、2005年的7—10月混合层盐度异常偏高，同时阿拉伯暖池异常偏弱；而2002年的2—4月混合层盐度异常偏低，阿拉伯暖池异常偏强.盐度收支分析方法研究结果表明，影响阿拉伯海暖池区的因素包括水平盐度平流、混合层深度、蒸发和降水以及艾克曼抽吸.在年際变化中，水平平流异常对阿拉伯海暖池区盐度异常发展的贡献最为显著.在2003、2005年，夏季风引起的高盐水平流和蒸发的增强是造成后期盐度显著升高的主要因素.暖池区障碍层的存在减弱艾克曼抽吸，抑制了低盐度的深层水和高盐度的表层水的混合.

本文结合由美国海洋和大气管理局（NOAA）提供的Nino指数以及偶极子指数（DMI）进行分析，发现混合层盐度的强度和持续时间与ENSO和IOD有一定的关系.单纯的IOD负位相对阿拉伯海暖池区混

合层盐度的影响较小，但当IOD负位相伴随着ENSO负位相发生时，阿拉伯海暖池区混合层盐度异常偏高，反之亦然.这是由于ENSO和IOD影响北印度洋季风，季风异常引起海洋平流输送，造成混合层盐度的异常.

本文研究发现混合层盐度的增加会影响障碍层的厚度，并可能通过障碍层深度变化影响海表温度.在年际时间尺度上，混合层盐度变化与障碍层和逆温层的关系值得更为深入的研究.

Abstract Salinity data from the Ocean Reanalysis System （ORAS4） that implemented by the European Centre for Medium-Range Weather Forecasts （ECMWF），have been used to study the interannual variability of mixed layer salinity in the Arabian Sea mini warm pool （ASMWP） during the period of 1999-2009.Apart from the broad agreement with previous studies on the seasonal salinity observations，we investigated several abnormal interannual high （low） salinity episodes in the mixed layer of the ASMWP.This study concludes the anomalous salinity growth in the mixed layer during July-October in the years of 2003 and 2005 and declining in February-April of 2002 for the study period （1999-2009）.Furthermore，the analysis showed that the intensity of the mixed layer salinity and the duration of the ASMWP high salinity period increased when negative Indian Ocean Dipole（nIOD） coincides with the La Nina.Apart from that，the mixed layer salinity budget analysis revealed the dominant role of horizontal advection for the interannual mixed layer salinity changes in the ASMWP.

Key words Arabian Sea mini warm pool；interannual variability；mixed layer；salinity budget；climate mode indices

1 Introduction

The Arabian Sea mini warm pool （ASMWP） is an oceanic region （65-76°E，3-13°N） with exceeding 30 ℃ sea surface temperature （Vinayachandran and Shetye[1]）.The Arabian Sea mini warm pool becomes the warmest area of the Indian Ocean in April-May，prior to the onset of the summer monsoon over the Indian subcontinent [2].Vinayachandran & Shetye[1] and Preenu et al.[3] have explained that ASMWP exerts considerable influence on the monsoon onset vortex and other ocean-atmospheric interactions.Since the ASMWP represents a noted mechanism for regulating regional climate system，it is important to study the factors affecting the dynamics of ASMWP.

Salinity is one of the most studied physical parameters particularly at the lower latitudes where the barrier layer exist.Accordingly，the southeastern Arabian Sea （ASMWP area） presents a peculiar thermodynamic structure which prevails one of the highest sea surface salinity variation observed in the Indian Ocean.The key point is that the salinity limits the thickness of the mixed layer by creating a barrier layer which directly influences the ocean-atmospheric interactions [4-6].Furthermore，salinity controls the thermohaline circulation and vertical mixings[7-8] as well.Preenu et al.[3]，Da-Allada et al.[9]，Nyadjro et al.[10]，Rao[11]，and Rao & Ramakrishna[12] have suggested that salinity structure of the Arabian Sea mini warm pool （ASMWP） combined with physical processes affect the regional rainfall or atmospheric temperature of the near Indian subcontinent.Josepth[13] pointed out that the large Sea Surface Salinity （SSS） variability in the ASMWP area，the buildup of highest SST，and monsoon onset are intimately linked.

Previous studies have examined ASMWP seasonal salinity variabilities of sea surface and mixed layer with use of various available data sets.Nyadjro et al.[10] used Argo and HYCOM data to investigate salinity budgets of the ASMWP.They found clear seasonal salinity variability along with significant interannual mixed layer salinity variability.However，due to the data scarcity in the period，they only investigated some of the processes，and did not focus on processes such as vertical advection，entrainment at the mixed layer base，and diffusion terms.Later on，Da-Allada et al.[9] carried out an extensive investigation to close the mixed layer salinity tendency equation with some physical processes that were neglected by Nyadjro et al.[10]and with the use of salinity data from In-Situ Data Analysis System（ISAS）.Furthermore，several researchers have studied the ASMWP，surface （mixed layer） temperature and heat budget and discussed the important role of the mixed layer salinity.Among the ASMWP studies，Kurian and Vinayachandran[14] have investigated the mechanism and formation of the ASMWP using general ocean circulation model and highlighted the importance of identifing accurately the arrival time and the quantity of low salinity water （<35 psu） into the ASMWP.Previous studies of Arabian Sea have clearly observed large mixed layer salinity variability in the ASMWP，but most of the studies mainly focused only on the seasonal scale salinity evolution.However，due to the limitations of the studies and observations，the interannual variation of the mixed layer salinity in the ASMWP is still under investigation.

The rest of this paper is organized as follows.Data and methodology are described in section 2.Observed interannual variability of mixed layer salinity and its regulating mechanisms are explained in section 3.Finally，the discussion and conclusions are presented in section 4.

2 Data and method

We used European Centre for Medium-Range Weather Forecasts （ECMWF） ORAS4 from 1999 to 2009 for salinity，zonal and meridional wind speed，temperature，and potential density data，available at 0.75°×0.75° resolution from 1958 to 2009.The ECMWF ORAS4 data were assimilated in many ways and bias corrected before the final product available in public databases.The ECMWF temperature and salinity profiles were obtained from the EN3 v2a XBT，CTD，Argo，Mooring，and from real-time GTS （Balmaseda et al.[15]）.These data （1958-2009） were assimilated to reduce bias and can be found at https：//www.ecmwf.int/.Meanwhile，global ocean gridded L4 sea surface heights and derived variables （1993-present） were used and reprocessed to get ocean current velocity and sea surface height data.

Large-scale ocean-atmosphere coupled climate signals as El Nio/Southern Oscillation （ENSO） have a large influence on interannual scale salinity evolution in the tropical Indian Ocean [21-23].

Oceanic Nio Index （ONI） is one of the most commonly used index to define El Nio and La Nia events in the Nio3.4 region of the central Pacific （58°N-58°S，120°W-170°W）[24].The ONI uses a 3-month running mean，and can be classified as a full-fledged El Nio or La Nia when its anomalies exceed +0.5 ℃ or -0.5 ℃ for at least five consecutive months[24].Here we used ONI index which is available at （http：∥origin.cpc.ncep.noaa.gov/）.During the period of 1999-2009，ONI represents 2002-2003，2005-2006 and 2009-2007 as El Nio years （marked in red color） and 1999-2000，2007-2008 as La Nia years （marked in blue color）.

The intensity of the IOD is commonly measured by an index called Dipole mode index （DMI）.This index is calculated by considering the anomalous SST gradient between the western equatorial Indian Ocean （50-70°E and 10°S-10°N） and the southeastern equatorial Indian Ocean （90-110°E and 10°S-0°）[25].Here we use the DMI index based on Reynolds OIv2 SST[26]

on a threshold of ±0.48 ℃，（b）Oceanic Nio Index （ONI） 3 month running mean of ERSST.v5 SST anomalies，

with warm （red） and cold （blue） periods based on a threshold of ±0.5 ℃

which is available at（http：//ioc3.unesco.org/oopc/state_of_the_ocean/sur /ind/dmi.php）.The positive IOD （negative IOD） events are identified as the years in which DMI is above （below） one standard deviation （±0.48 ℃） during September-November of that year[25].

3 Results and discussion

To examine the high salinity （low salinity） anomalies at interannual scale，firstly we need to understand the ordinary evolution of the mixed layer salinity in the ASMWP.The first part of this section describes the ordinary seasonal evolution and relative importance of different mechanisms which are involved in the evolution of the ASMWP mixed layer salinity.The latter parts discuss the abnormal MLS observations during 1999-2009 and the involved possible mechanisms.

Both the Arabian Sea and the Bay of Bengal have huge salinity differences throughout the year.The Bay of Bengal always exhibits salinity in fresh bias，compared with other areas of the Indian Ocean since the colossal amount of freshwater flux from the big rivers of the Indian subcontinent （eg：Irrawaddy，Ganga-Brahmaputra，Mahanadi，Godavari-Krishna，etc.）.Average mixed layer salinity of the Arabian Sea shows a clear gradient from the west Arabian Sea to the east Arabian Sea （Fig.2a）.The salinity gradient of the western Arabian Sea is basically created by the uneven distribution of precipitation，evaporation and horizontal advection from the east part of the Arabian Sea[27].The west Arabian Sea receives relatively less precipitation throughout the year（Fig.3b）.Meanwhile，the west Arabian Sea shows high evaporation rates when compared with other parts of the Arabian Sea （Fig.3b）.Therefore，positive E-P observations in the Arabian Sea are created by less precipitation and higher evaporation （Fig.3c）.When we examine the salinity standard deviation of the Arabian Sea （Fig.2b），a considerable fluctuation of salinity level is observed in Lakshadweep area.Seasonal reversing of the transporting of high （low） saline water from the west Arabian Sea （Bay of Bengal） creates this dynamic salinity observation in the Arabian Sea mini warm pool which will be discussed in the following sections.

3.1 General evolution of the ASMWP

We first state the seasonal evolution of MLS in the ASMWP considering the period of 1999-2009.Depth-time section （Fig.6a） of the ASMWP mixed layer shows a clear seasonal pattern with a repeating of low and high salinity episodes for the whole period （1999-2009）.The general evolution of Arabian Sea Salt Dome （65-73°E，3-13°N） is varied within minimum salinity （about 34.5 psu during December-February） and maximum salinity （about 36 psu during June-August） all round the year[28].

Salt and heat transport limit the barrier layer thickness and thin （or thick） barrier layer can regulate the regional ocean physical properties[29].The observed high saline water of the Arabian Sea and low saline water of BoB connected with the ocean currents play critical roles in regulating salinity structure of the Indian Ocean[12，30-31].Therefore，studying currents pattern of the Indian Ocean is important to understand how it influences the ASMWP mixed layer salinity.

According to Figure 4b，the East Indian Coastal Current （EICC） flows equatorward while bringing low salinity water from the Bay of Bengal during the winter monsoon[32-33].Low salinity water is transported by the EICC and merged with Northeast Monsoon Current （NMC） which flows towards the ASMWP.Cumulative transport of the EICC and the NMC feeds the ASMWP with low salinity water which significantly reduce the mixed layer salinity during the winter monsoon.Simultaneously in this period （December-January），West Indian Coastal Current （WICC） flows northward along the west Indian coast，further enhancing the transport of low salinity water towards the north-west Arabian Sea where high salinity water preoccupied when compared to other regions of the Indian Ocean.The Bay of Bengal has fresher water （about 32 psu） by high freshwater The current flow observed in the winter monsoon starts to reverse during the southwest monsoon （June-August） period.The East Indian Coastal Current （EICC） starts to flow northward along the east coast of India and cuts off the freshwater feed to southern tips of Sri Lankan ocean （Fig.4a）.Meanwhile，the southwest monsoonal currents

start to flow westward with notable kinetic energy.Although they have less energy，the current vectors （Fig.4a） show that the West Indian Coastal Currents （WICC） flow towards ASMWP from the east Arabian Sea where high salinities are observed in the Arabian[33].During winter monsoon，the largest kinetic energy are observed along east Indian coast （Fig.4b） because of the East Indian Coastal Current （EICC） flow.According to Figure 4b，below the southern tip of Sri Lanka during winter monsoon，notable kinetic energy is observed in geostrophic current （NMC） which flows northwest.High energy band and ocean currents vectors （Fig.4b） show westward flow of NMC which transport the water moved by the EICC.

The Indian Ocean has a dynamic wind pattern with the monsoon year around.Wind-induced mechanisms and mixed layer depth changes are responsible for the ASMWP area salinity distribution.The calculating of the mixed layer salinity would be helpful to understand the relative importance of each mechanism （explained in section 3.2.1）.

3.2 Interannual variability

3.2.1 Interannual salinity change connected with the tropical climate modes

Understanding the interannual dominant climate modes in the tropic assists to understand the interannual salinity anomalies[25].Here we study into the DMI and ONI indices to understand the abnormal high salinity episodes in 2003 and 2005（Fig.1）.As we observed （Fig.1），2005 shows negative IOD event along with La Nia observation.But in 2003 we observed in mixed layer salinity anomaly a positive IOD event without ENSO signal.Meanwhile，year 2002 shows positive IOD event along with La Nia observation and lowest salinity （marked in blue arrow in Fig.7） within this study period.

The ONI index in 1999 shows high amplitude LA Nia observations and in 2007 it shows no significant negative or positive signal from the DMI index in particular period，and also no anomalous salinity record of mixed layer salinity is observed in that period.

According to the observations，the cumulative effect （overlapping incidence） of nIOD and La Nia conditions promotes to increase the salinity structure in the ASMWP.While the combination of pIOD and La Nia phenomenon tends to decrease the ASMWP mixed layer salinity.But individual anomaly of the tropical climate modes shows little influence on disturbing the general salinity structure of the ASMWP.

Interannual variability of the mixed layer salinity can be revealed in amplitude，vertical extent，and temporal duration.Taking a close look at the depth-time section of the ASMWP salinity and salinity anomaly （Fig.6） for the period 1999-2009，we indentified two interesting abnormal salinity events in 2003 and 2009.

The ASMWP shows intra-annual cycles of near-surface salinity variability within the first 60 m in a general year.The ASMWP shows two major salinity and temperature seasons，a low salinity phase during February-June and a high saline phase during September-November in the ASMWP within upper layers.During the low salinity phase，the salinity generally reaches its lowest value about 35 psu in April.High salinity phase of the ASMWP peaks are observed in October for the 11 years averaged results of near-surface salinity in the ASMWP.

In this study MLS extreme cases for eleven years （1999-2009） are investigated and two interesting anomalies are identified in 2003 and 2005.The observed evolution of near-surface salinity anomalies （marked in black arrows in Fig.6） are expanded to represent in Figure 6 with average intra-annual salinity evolution.

In contrasting，two years with generally mixed layer salinity evolution of the ASMWP，notable salinity anomalies are observed in aspects of amplitude （>36 psu），extended temporal duration and mild change in the vertical extent （about 70 m）.

The observed anomalies （Figs.7b，c） in the salinity peaks （>36 psu） indicate the temporal duration extension from August to November during both abnormal years compared with average mixed layer salinity.

Mixed layer salt budget is analyzed to understand these two abnormal events （marked in Fig.6 by black arrows） of the ASMWP.For better understanding of salinity behaviors of the ASMWP，the eleven years mean budget term closings are given in the first part of the following section.The second part of the following section explains the individual abnormal salinity years with most possible mechanisms identified in the first part of the section.

3.2.2 Mixed layer salt budget

The salt budget of the ASMWP is studied for the period of 1999-2009 （Fig.8） to examine the relative contribution of the freshwater flux，Ekman and Geostrophic advection，and entrainment.The observed salinity tendency （LHS.Eq.1） and calculated salinity tendency （RHS.Eq.1） show good correlation of 0.942 5 at 95% significant level （Fig.8a）.The sum of the budget terms （LHS.Eq.1） overestimates the salinity tendency except for January and April.Physical processes like turbulent mixing，diffusion，eddy process are found to be responsible for the residual of the budget closing （Hasson et al.[34] and Schiller & Oke[35]）.

Horizontal advection is the most significant contributor to the salinity tendency in the ASMWP，while freshwater flux and entrainment give second and third contributions respectively （Fig.8b）.The composite analysis of the horizontal advection shows that the zonal and meridional Ekman salt advection is more significant than geostrophic advections.Zonal component of the horizontal advection gives little contribution to the advection term，but Ekman zonal advection is significant （Figs.8c，d） compared with geostrophic zonal advections.The observed contribution of the Ekman advection describes the importance of wind force for the modification of mixed layer salinity in the ASMWP.

During the summer monsoon，the southwestward trade winds are strengthened and lead to a strong southward Ekman transport.During this period （May-October）the meridional Ekman salt transport becomes dominant and leads to transport of high saline water from the north-west Arabian Sea.During the winter monsoon，the northeasterly trade winds are reinforced，driving strong northward Ekman transport over the ASMWP [36].

During the period when meridional Ekman transport significantly decreases and a zonal component of Ekman is activated，low salinity water advects into the ASMWP which creates low salinity season of the ASMWP.

3.2.3 Governing mechanisms

As observed in previous section，the horizontal advection and freshwater flux are the most influencing factors for the ASMWP mixed layer salinity.This section tested both horizontal advection and freshwater flux to understand their relative contributions to 2003 and 2005 abnormal mixed layer salinity in the ASMWP.

The salt budget analysis in the ASMWP shows strong increasing trend （0.5 psu/month） in salinity for 2003 and 2005 during June-September for the extended time period （Fig.9）.For both 2003 and 2005，strong positive tendency anomalies corresponding with mixed layer salinity tendency are observed during June-September period.This clearly indicates that the observed anomalous increasing of salinity during the initial phase is primarily modulated by horizontal advection.

Local freshwater flux shows a mild negative influence by a decreasing trend （-0.1 psu/month） during December-May for both years.The relative contributions to salinity budget terms conclude that horizontal advection plays a major role in creating abnormal MLS in the ASMWP during 2003 and 2005.

Horizontal advection controls general salinity evolution of the ASMWP，abnormal high salinity observation also controls during 2003 and 2005，and both years show positive anomalies in horizontal advection term （Figs.9c，9d）.As we observed，this positive anomaly keeps November high salinity during this period.Parallel to this observation，mixed layer depth also decreased in June compared to normal years.As （Shankar and Shetye[33]） suggested，the decrease of MLD of the ASMWP is due to the increased evaporation （Fig.10b）.According to the figure during June-August，ASMWP receives high rainfalls causing the decrease of MLD，which moderates the salinity anomaly.Identically，the E-P anomaly （Fig.10d） shows less evaporation than the rainfalls.Although both years observed high salinity of the mixed layer （Fig.10f），it is interesting to observe a positive Ekman pumping anomaly in June of 2003 but a negative Ekman pumping velocity in the same period of 2005.

4 Summary and conclusion

Eleven years （1999-2009） of salinity observations of the ASMWP represent the salient interannual variability of mixed layer salinity in 2003 and 2005.Two notable episodes of vertically and temporally extended anomalies are observed during August-December in 2003 and August-November in 2005 respectively.Mixed layer salt budget analysis illustrates the relative roles of various possible mechanisms occurring with anomalous high salinity events.The mixed layer salinity budget analysis shows that horizontal advection is responsible for temporal anomaly and the mixed layer salinity intensification.Simultaneously，horizontal advection is also responsible to shoal the mixed layer depth and tends to intensify the evaporation.This intensified evaporation induces notable high salinity episode in the ASMWP.

The scope of current study is bounded to investigate the influence of salinity on temperature in interannual scale，except studying the barrier layer changes.We strongly suggest that the influences of salinity variations on interannual modulation of the barrier layer，temperature inversion，and sea surface temperature are worth investigating further.

Acknowledgments：The authors gratefully acknowledge the joint program of China Sri Lanka Research and Education Center （CSL-CER），and this work is funded by International Partnership Program of Chinese Academy of Sciences with grant no.131551KYSB20160002.We also thank the European Centre for Medium-Range Weather Forecasts （ECMWF） for providing open access databases.

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