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  • View in gallery

    Location map showing XBT and sea surface salinity stations (dots) collected during May 2002–March 2007. The densely covered Kochi–Kavaratti transect is shown by a shaded strip. Argo float locations during different winters are also shown.

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    Monthly mean climatology of (a) sea surface salinity (Conkright et al. 2002) overlaid with mean surface currents (Ekman and geostrophic) derived from climatology of QuikSCAT winds and AVISO T/P merged sea level anomalies for November–February. Schematic of the East India Coastal Current and Winter Monsoon Current are shown by white arrows. (b) Monthly mean climatology of the TMI SST for November–February.

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    XBT station locations for W23, W34, W45, W56, and W67 in the Lakshadweep Sea. Dark (light) dots represent XBT stations with (without) temperature inversions.

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    Formation of the temperature inversion through horizontal advection of (a) low salinity waters and (b) cool waters. The TS profiles from Argo float ID 2900091 are used.

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    Hovmöller plot of TMI SST along the Kochi–Kavaratti XBT transect from September to December during 2002–06.

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    Vertical thermal sections for the upper 125 m along the Kochi–Kavaratti XBT transect from September to February during 2002–07.

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    (a) Typical thermal structure from GODAS in the SEAS region showing the thermal inversions. (b) Thermal structure in the upper 100 m averaged in the SEAS region.

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    a. Evolution of QuikSCAT monthly mean climatology of wind speed during (left) September–February and the wind speed anomaly during (right) September 2005–February 2006. (b) Evolution of monthly mean climatology of NCEP net surface heat flux during (left) September–February (note that the color scales are not at regular intervals) and net heat flux anomaly during (right) September 2005–February 2006.

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    Upper-ocean heat budget terms derived from GODAS in the upper 0–35 m for (a) 2003–04 and (b) 2005–06.

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    Hovmöller plot of sea surface salinity (psu) along the Koch–Kavaratti XBT transect. Black dots indicate station locations.

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    Comparison of Argo float (ID 2900530 and 2900091) vertical salinity profiles for W56 (black solid lines) in the Lakshadweep Sea with corresponding climatology (dashed line) and available mean monthly salinity profiles (blue lines for 2004; red lines for 2005; pink lines for 2006; green line for 2007).

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    Comparison of (a) Argo float (ID 2900530 and 2900091) vertical temperature profiles for W56 in the Lakshadweep Sea (floats locations shown Fig. 1) and corresponding climatology (dashed lines) and (b) individual XBT profiles (light-shaded lines in the left panel) and climatology (dark lines in the left panel) and station locations (right panel) during October–December 2005 in the Lakshadweep Sea.

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    Comparison of river discharge (May–November) for the rivers Mahanadi and Godavary.

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    (a) Areas (Box A and Box B) considered for comparison of total rainfall (mm) during June 2005–February 2006. (b) Histograms showing the total rainfall (June–February) over (i) Box A and (ii) Box B for the years 2002–06.

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    Monthly mean near-surface currents (Ekman and geostrophic): (arrows represent current vectors and the background color represents current speed) derived from QuikSCAT winds and AVISO T/P merged anomalies for September–February during 2002–06.

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    (a) Salt budget terms (salinity tendency, horizontal and vertical advection) derived from GODAS in the upper 35 m in the SEAS region and (b) freshwater flux.

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    Time series of SST differences between the Boxes B and A during the winter seasons of 2002–07.

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Reduced Near-Surface Thermal Inversions in 2005–06 in the Southeastern Arabian Sea (Lakshadweep Sea)

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  • + National Institute of Oceanography, Dona Paula, Goa, India
  • | # Indian Institute of Tropical Meteorology, Pashan, Pune, India
  • | @ Naval Physical and Oceanographic Laboratory, Kochi, India
  • | 4 Indian National Center for Ocean Information Services, Hyderabad, India
  • | * *Space Application Centre, Ahmedabad, India
  • | ++ Cochin University of Science and Technology, Kochi, India
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Abstract

Repeat XBT transects made at near-fortnightly intervals in the Lakshadweep Sea (southeastern Arabian Sea) and ocean data assimilation products are examined to describe the year-to-year variability in the observed near-surface thermal inversions during the winter seasons of 2002–06. Despite the existence of a large low-salinity water intrusion into the Lakshadweep Sea, there was an unusually lower number of near-surface thermal inversions during the winter 2005/06 compared to the other winters. The possible causative mechanisms are examined. During the summer monsoon of 2005 and the following winter season, unusually heavy rainfall occurred over the southwestern Bay of Bengal and the Lakshadweep Sea compared to other years in the study. Furthermore, during the winter of 2005, both the East India Coastal Current and the Winter Monsoon Current were stronger compared to the other years, transporting larger quantities of low salinity waters from the Bay of Bengal into the Lakshadweep Sea where a relatively cooler near-surface thermal regime persisted owing to prolonged upwelling until November 2005. In addition, the observed local surface wind field was relatively stronger, and the net surface heat gain to the ocean was weaker over the Lakshadweep Sea during the postmonsoon season of 2005. Thus, in winter 2005/06, the combination of prolonged upwelling and stronger surface wind field resulting in anomalous net surface heat loss caused weaker secondary warming of the near-surface waters in the Lakshadweep Sea. This led to a weaker horizontal sea surface temperature (SST) gradient between the Lakshadweep Sea and the intruding Bay of Bengal waters and, hence, a reduced number of thermal inversions compared to other winters despite the presence of stronger vertical haline stratification.

Corresponding author address: Ms. K. Nisha, Physical Oceanography Division, National Institute of Oceanography, Dona Paula, Goa-403004, India. Email: knisha@nio.org

Abstract

Repeat XBT transects made at near-fortnightly intervals in the Lakshadweep Sea (southeastern Arabian Sea) and ocean data assimilation products are examined to describe the year-to-year variability in the observed near-surface thermal inversions during the winter seasons of 2002–06. Despite the existence of a large low-salinity water intrusion into the Lakshadweep Sea, there was an unusually lower number of near-surface thermal inversions during the winter 2005/06 compared to the other winters. The possible causative mechanisms are examined. During the summer monsoon of 2005 and the following winter season, unusually heavy rainfall occurred over the southwestern Bay of Bengal and the Lakshadweep Sea compared to other years in the study. Furthermore, during the winter of 2005, both the East India Coastal Current and the Winter Monsoon Current were stronger compared to the other years, transporting larger quantities of low salinity waters from the Bay of Bengal into the Lakshadweep Sea where a relatively cooler near-surface thermal regime persisted owing to prolonged upwelling until November 2005. In addition, the observed local surface wind field was relatively stronger, and the net surface heat gain to the ocean was weaker over the Lakshadweep Sea during the postmonsoon season of 2005. Thus, in winter 2005/06, the combination of prolonged upwelling and stronger surface wind field resulting in anomalous net surface heat loss caused weaker secondary warming of the near-surface waters in the Lakshadweep Sea. This led to a weaker horizontal sea surface temperature (SST) gradient between the Lakshadweep Sea and the intruding Bay of Bengal waters and, hence, a reduced number of thermal inversions compared to other winters despite the presence of stronger vertical haline stratification.

Corresponding author address: Ms. K. Nisha, Physical Oceanography Division, National Institute of Oceanography, Dona Paula, Goa-403004, India. Email: knisha@nio.org

1. Introduction

The Lakshadweep Sea situated between the southwest coast of India and the Lakshadweep island chain in the southeastern Arabian Sea (Fig. 1) exhibits strong variability in the near-surface hydrography and circulation under the influence of seasonally reversing monsoons (Cutler and Swallow 1984; Shetye et al. 1991). During winter (November–February) the occurrence of near-surface thermal inversions in the Lakshadweep Sea is a well-documented phenomenon in the literature (Thadathil and Gosh 1992; Shankar et al. 2004; Gopalakrishna et al. 2005). These thermal inversions show amplitudes typically in the range of 0.25°–1.0°C and occur in the depth range from 10 to 80 m (Gopalakrishna et al. 2005). These inversions begin to appear during October–November and their population reaches its peak during January–February.

The formation of thermal inversions suggests the confluence of different water masses in the Lakshadweep Sea and is primarily attributed to the vertical salinity stratification. In winter, the intrusion of low salinity and cooler waters from the Bay of Bengal, through the East India Coastal Current and Winter Monsoon Current, is responsible for strong salinity stratification in this region (Cutler and Swallow 1984; Johannessen et al. 1987; Shetye et al. 1991, 1996; Rao and Sivakumar 1999, 2003; Shenoi et al. 1999; Prasanna Kumar et al. 2004; Gopalakrishna et al. 2005). Intrusion of these low salinity waters results in the formation of a barrier layer (a layer embedded between the top of the thermocline and bottom of the surface mixed layer) in the Lakshadweep Sea (Sprintall and Tomczak 1992; Rao and Sivakumar 1999; Durand et al. 2004; Masson et al. 2005). Using an ocean general circulation model, Durand et al. (2004) have shown that the heat trapped within these thermal inversions makes a significant contribution to increasing the SST at least by 1.1°C during November–March, contributing to the seasonal buildup of the warm pool in the Lakshadweep Sea. Masson et al. (2005), using a coupled general circulation model, demonstrated that the lack of heating associated with the barrier layer in the Lakshadweep Sea results in late onset of the summer monsoon. Another study by Masson et al. (2002) has shown that a barrier layer enhances the spring SST warming and leads to a statistically significant increase of precipitation in May, linked to an early monsoon onset.

The remote forcing is also well known to play an important role in the dynamics of the Lakshadweep Sea through propagation of coastal Kelvin waves that trigger westward propagating Rossby waves (McCreary et al. 1993; Shankar et al. 2002). During January–February the near-surface isothermal layer is also deeper due to downwelling caused by the anticyclonic eddy circulation popularly known as the “Lakshadweep High” seen in the satellite altimetry (Bruce et al. 1994; Shankar and Shetye 1997; Bruce et al. 1998).

Associated with the East India Coastal Current and Winter Monsoon Current (shown schematically as white arrows in Fig. 2a), a progressive drop in sea surface salinity in the Lakshadweep Sea is clearly noticed from November to January/February. In addition, the surface waters in the southwestern Bay of Bengal are cooler by ∼2°C compared to the temperature in the Lakshadweep Sea region (Fig. 2b) and these currents transport relatively cooler surface waters into this region. During their passage the intruded low salinity waters encounter intense surface cooling south of the Indian tip due to strong winds that blow through the orographic gap between the Indian tip and Sri Lanka, enhancing the turbulent heat losses, resulting in lowering the SST by about 1°C in the region south of Gulf of Mannar (Luis and Kawamura 2000). Recently Kurian and Vinayachandran (2006) have examined the possible mechanisms of thermal inversions in a numerical model simulation of the Lakshadweep Sea and concluded that the haline stratification is an important prerequisite for the formation of thermal inversions.

Gopalakrishna et al. (2005) have reported large differences in the life cycle and the depth of occurrence of these thermal inversions between the winters of 2002/03 and 2003/04. During the latter winter more thermal inversions occurred at shallower depths than in the former winter. These differences are primarily attributed to differences in the intrusion of low salinity waters into the Lakshadweep Sea. Based on near-fortnightly XBT surveys in the Lakshadweep Sea, we report here an unusual occurrence of the reduced number of thermal inversions in the winter of 2005/06 when compared to any other observed winter. The plausible causative mechanisms are examined to explain this unusual observed feature.

The paper is organized as follows: In section 2, the datasets, satellite-derived winds, SST, surface net heat flux, currents, and in situ vertical temperature and salinity profiles collected from ships and by Argo floats and a three-dimensional ocean GCM assimilation product—Global Ocean Data Assimilation (GODAS)—used in this study are described. A brief description of the evolution of near-surface thermal inversions is presented in section 3. The probable governing mechanisms responsible for the observed reduced number of thermal inversions in the Lakshadweep Sea during the winter of 2005/06 are addressed in section 4. The salient results are summarized in section 5.

2. Data

The repeat XBT measurement program in the Lakshadweep Sea is a major long-term observational initiative supported by the Ministry of Earth Sciences in India. Under this initiative, near-fortnightly XBT surveys are being systematically organized for the first time since May 2002 using passenger ships that ply regularly between Kochi and the Lakshadweep island chain. During each XBT survey, a minimum of 10–13 (at 50-km spatial intervals) vertical temperature profiles (T7 Sippican XBT probes and MK21 data acquisition system) and 20–25 sea surface water samples (bucket samples) are collected (black dots in Fig. 1 depicts the XBT and sea surface salinity stations). These water samples are analyzed for sea surface salinity using Guild Line 8400 Autosal. In all, 1324 vertical temperature profiles and 1755 surface salinity samples were collected for the study region for all months and analyzed. The period during November 2002–February 2003 is considered to represent the winter season 2002/03 (W23). The winter seasons of 2003/04, 2004/05, 2005/06, and 2006/07 are referred as W34, W45, W56, and W67, respectively. To examine the evolution of the near-surface temperature/salinity structure, the available Argo float profile data are extracted for the winters from the Web sites ftp://ftp.ifremer.fr/ifremer/argo/dac and http://www.coriolis.eu.org, for the study region. No Argo float data are available for W23. However, temperature–salinity (TS) data are extracted for W34 one float [identification number (ID) 2900263] with four TS profiles (Fig. 1a), W45 one float (ID 2900193) with five TS profiles (Fig. 1b), W56 two floats (ID 2900530 and 2900091) with 24 TS profiles (six TS profiles during each month of this winter season) (Fig. 1c), and W67 one float (ID 2900347) with ten TS profiles (Fig. 1d). The Kochi–Kavaratti transect (shaded strip in Fig. 1) is the most densely covered XBT transect used to construct Hovmöller plots of sea surface salinity (1048 salinity samples) and the snapshot vertical thermal sections (320 XBT profiles). The monthly means of National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) net surface heat flux; Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) SST; Quick Scatterometer (QuikSCAT) winds; currents derived from QuikSCAT winds (Ekman); and Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) blended Ocean Topography Experiment (TOPEX)/Poseidon (T/P) sea surface height (SSH) anomalies (geostrophic) are utilized to show the observed large-scale variability.

The GODAS products from NCEP are utilized to estimate heat and salt budgets of the upper 30-m layer of the Lakshadweep Sea region (Behringer and Xue 2004). The GODAS is based on a quasi-global configuration of the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model, version 3 (MOM.v3). The model domain extends from 75°S to 65°N and has a resolution of 1° by 1° enhanced to 1/3° in the north–south direction within 10° of the equator. The model has 40 levels with a 10-m resolution in the upper 200 m. Other new features include an explicit free surface, the Gent–McWilliams isoneutral mixing scheme, and the K-profile parameterization (KPP) vertical mixing scheme. The GODAS is forced by the momentum flux, heat flux, and freshwater flux from the NCEP atmospheric reanalysis 2. In this reanalysis GODAS assimilates temperature profiles from XBTs; from Tropical Atmosphere Ocean (TAO), Triangle Trans-Ocean Buoy Network (TRITON), and Pilot Research Moored Array in the Tropical Atlantic (PIRATA) moorings; and from Argo profiling floats. The GODAS data are created and available online at http://cfs.ncep.noaa.gov/cfs/godas. Heat and salt budgets of the upper 30-m water column is carried out following the methodology adopted in Vinayachandran et al. (2002) and Rao and Sivakumar (2003) for the region between 8° and 12°N, 72° and 76°E:
i1520-0485-39-5-1184-e1
i1520-0485-39-5-1184-e2
where D are diffusion terms, Qs is the near-surface heat flux, S the vertically averaged mixed layer salinity, t the time unit (month), E evaporation and P rainfall (m month−1), h mixed layer depth [MLDts (m)], u the zonal component of flow and wh the vertical advection below mixed layer (m month−1); H is the Heaviside step function [= 0 if (wh + dh/dt < 0, = 1 if (wh + dh/dt > 0] and Sh is the salinity just below the mixed layer base.

3. Analysis

The vertical temperature profile data are examined to describe and understand the evolution of the near-surface thermal inversions. The spatial distribution of all XBT stations with (dark dots) and without (light dots) thermal inversions respectively for each month of the individual winter season are shown in Fig. 3. The near-surface thermal inversions are only considered when their amplitude exceeds the SST value by at least 0.25°C and the inversion layer thickness exceeds 5 m. The distribution of these thermal inversions shows a distinct life cycle during each winter. They first appear few in number during November and their population increases with progress of the season. They peak during January and disappear by March when the SST begins to increase. They also show large year-to-year variability in their characteristics and population density (Gopalakrishna et al. 2005). Interestingly, a relatively reduced number of thermal inversions occurred throughout W56. During the winters of 2002–06 the percentage of occurrence of thermal inversions observed from the XBT data as well as from the model output (numbers in brackets) have shown a large spread from a minimum of 16% (6%) in W56 to a maximum of 49% (43%) in W23 (Table 1). The W56 can be cited as the winter season with the least occurrence of thermal inversions among winters 2002 through 2006. In addition, it is also interesting to note that these inversions have occurred at shallower depths (∼10 m) and occupied a thicker water column (∼35 m) in W56 compared to the other winters in the study (Table 1). The means, standard deviations, and ranges (numbers in brackets) for the depth of occurrence, layer thickness, and amplitude of the inversions for all five winters are also shown in Table 1. The plausible causative mechanisms for this unusual reduced number of thermal inversions are examined in the following sections.

4. Governing mechanisms

The mechanisms of modification of the near-surface thermohaline structure in the Lakshadweep Sea due to the horizontal advection of low-salinity and low-temperature waters are examined utilizing the observed Argo TS profiles. The deep isohaline layer (30 m) observed on 19 December 2005 (broken line in Fig. 4a; Argo float ID 2900091) was modified into a shallow isohaline layer (20 m) by 29 December 2005 (solid line in Fig. 4a; float ID 2900091) owing to the intrusion of low salinity waters into this region. Similarly, as a result of the intrusion of low temperature waters, the corresponding temperature profile, which does not show a thermal inversion (dashed line in Fig. 4b), was modified, indicating an occurrence of inversion (solid line in Fig. 4b). The amplitude of the near-surface thermal inversions depends upon the horizontal SST gradient between the Lakshadweep Sea and the intruding waters. The depth of occurrence of the near-surface inversion layer depends upon the vertical thickness of the near-surface isohaline layer.

All previous studies have suggested that the thermal inversions in the Lakshadweep Sea occur due to (i) intrusion of cooler and low salinity waters from the Bay of Bengal (Thadathil and Gosh 1992; Gopalakrishna et al. 2005, 2008) and (ii) penetrative radiation into the barrier layer (Anderson et al. 1996; Kurian and Vinayachandran 2006). The first mechanism will be active if the SST gradient between the intruding Bay of Bengal waters and the Lakshadweep Sea is positive and the mixed layer is deep enough. The second mechanism will be active if the mixed layer is shallow, when the shortwave radiation penetrates into the barrier layer below the mixed layer base. In the following sections the possible mechanisms are examined to identify the actual mechanism responsible for the observed reduced number of inversions in the Lakshadweep Sea during W56.

a. Anomalous background state of the Lakshadweep Sea

To understand the role of the background state of the Lakshadweep Sea, several parameters are examined. The observed evolution of TMI SST along the Kochi–Kavaratti XBT transect (shaded strip in Fig. 1) during September–December is examined for all of the five years (Fig. 5). Usually a secondary warming occurs in the Lakshadweep Sea after the summer monsoon cooling (Colborn 1975; Hastenrath and Lamb 1979). However, perceptible differences in the secondary warming are seen among all of these five years. The most noteworthy feature is the occurrence of an unusually weaker secondary warming during October–November 2005 unlike any other year. This anomalous feature is also noticed in the near-surface thermal structure along Kochi–Kavaratti transect during October–November 2005 (Fig. 6). The cooling regime continued beyond the summer monsoon season of 2005 and persisted almost until the end of the year.

A typical thermal structure associated with thermal inversions from model assimilated temperature is shown in Fig. 7a. Just as in observations, thermal inversions in model assimilations are also seen clearly. The percentage of occurrence of temperature inversions from the model output (numbers in brackets) observed in a region between 8° and 12°N, 72° and 76°E are listed in Table 1. As in the XBT observations the percent occurrence of thermal inversions in the region of interest is lower in W56. The model assimilated temperature fields (Fig. 7b) also shows weak secondary surface warming in 2005 as observed in the TMI SST. Therefore, the heat and salt budget terms derived from model assimilated fields can be used to interpret the various governing mechanisms.

The governing mechanisms responsible for this observed anomalous weaker secondary warming in 2005 are examined. The surface winds and the surface turbulent heat losses play an important role in the cooling process of the surface mixed layer of the ocean through vertical mixing. The observed mean monthly wind speed derived from QuikSCAT for winter climatology and its anomaly for W56 is shown in Fig. 8a. The positive anomaly of the surface wind field is distinctly stronger during September 2005–January 2006 compared to the corresponding climatological wind field, particularly in the Lakshadweep Sea region. The net surface heat flux anomalies during September–December 2005 are also distinctly more negative (less heat gain by the ocean) in the Lakshadweep Sea compared to any other year considered in this study (Fig. 8b). Thus, both the observed stronger winds and decreased heat gain by the ocean during the postmonsoon season might have also contributed to the weaker secondary warming observed during October–November 2005 in association with the prolonged upwelling until November 2005, noticed in the snapshot vertical thermal sections along the Kochi–Kavaratti XBT transect (Fig. 6). This has resulted in a weaker horizontal SST gradient between the Lakshadweep Sea and the intruding low salinity waters from the Bay of Bengal. Such a situation potentially contributes to formation of a reduced number of thermal inversions. The same is further confirmed with the GODAS product through the heat budget analysis of the upper 30-m layer in the Lakshadweep Sea.

Heat budget terms computed from the GODAS reanalysis are shown in Fig. 9a for 2003/04 and (Fig. 9b) for 2005/06. In mid-May 2003, a strong cooling tendency (in the upper 35 m) in response to strong vertical advection of cool subsurface waters associated with upwelling (Fig. 9a) is observed. This cooling tendency lasted until end of August. In 2005 the cooling tendency associated with upwelling started by early June and lasted until end of October, with brief fluctuations of a warm tendency in between. However, the vertical advection of cool subsurface waters associated with upwelling continued until November. The cooling tendency during the summer monsoon, in general, is supported initially by the net surface heat fluxes. After August the net surface heat fluxes support a warm tendency of temperature. Horizontal advection of heat played a neutral role for the temperature tendency in summer monsoon months during both the years. Due to continued vertical advection of cooler waters, even after September 2005, the temperatures in the southeastern Arabian Sea (SEAS) region (Fig. 7b) remained below 29°C after the summer monsoon months. Reduced net surface heat fluxes might have played an important role in keeping the surface temperature lower.

b. Intrusion of low saline waters from the Bay of Bengal

The low salinities observed in the Lakshadweep Sea during winter are associated with both the intrusion of low salinity waters from the Bay of Bengal and unusually high precipitation. The observed variability of sea surface salinity along the Kochi–Kavaratti XBT transect (Fig. 10) is examined to understand its evolution during May 2002–March 2007. The Hovmöller field of sea surface salinity along the Kochi–Kavaratti XBT transect clearly shows the evolution of the seasonal cycle, in conformity with the earlier published climatologies (Conkright et al. 2002). The seasonal cycle is characterized by the appearance of low (high) salinity waters during winter extending into premonsoon (summer monsoon and extending into postmonsoon) with decreasing (increasing) values toward the southwest coast of India (Kavaratti/Amini Islands). This high salinity noticed in the vicinity of Kavaratti/Amini Islands is attributed to horizontal advection from the northern Arabian Sea. The observed drop from November to February in the Conkright et al. (2002) sea surface salinity climatology along the Kochi–Kavaratti transect is 1.6 psu. Interestingly, the present observations show a much greater drop of 4.03 psu during W56. As hypothesized earlier by Gopalakrishna et al. (2005), this large freshening is primarily attributed to the heavy freshwater input into the Bay of Bengal and Lakshadweep Sea through river discharges and rainfall during the preceding summer monsoon and winter monsoon seasons. On the year-to-year time scale, the most dominant signals noticed are the occurrence of low salinity waters during W56 and W34, among which the freshening is greater during W56.

The vertical TS profiles collected by Argo floats in the study region (shown in Fig. 1) are examined to understand the observed evolution of near-surface TS structure during W56. During November 2005, the salinity is uniformly high and varied between 34.5 and 35.9 psu (black lines in Fig. 11) except the left extreme profile (black line, of 29 November 2005), which showed a dramatic decrease (32.2 psu) indicating the arrival of low salinity waters. With progress of the season, the surface layer salinity dropped further and the strengthened halocline persisted until January 2006. In general, the observed salinities during W56 are much lower when compared with the corresponding climatological salinity profile (dashed line). A comparison of available mean salinity profiles for each winter season (colored solid lines in Fig. 11) further confirms that the vertical salinity values are much lower during all months of W56. The Argo salinity profiles during W56 also show shallow near-surface isohaline layers. This implies the occurrence of inversion layers at shallower depths during W56. To show this, the corresponding Argo temperature profiles are presented in Fig. 12a. For example, during December 2005 the inversions are seen at depths ∼25 m coinciding with the observed shallower isohaline layer. In broad agreement with the TMI SST (Fig. 5), the observed composite of Argo and XBT (Fig. 12b) temperature profiles also confirm that the secondary warming in the near-surface layers is weaker than the climatological normal (dark profiles) during October–December 2005.

The low salinity waters that intrude from the Bay of Bengal through the East India Coastal Current and the Winter Monsoon Current determine the near-surface freshening in the Lakshadweep Sea region. To examine the year-to-year variability of river discharge in to the Bay of Bengal, we have used the data on monthly total river discharges for two major rivers, Mahanadi and Godavary, which are situated along the east coast of India (shown in Fig. 1). Among the years, the total river discharge during May to November is greater during 2003 and 2005 (Fig. 13) lending support to the observed excessive freshening in the Lakshadweep Sea during W34 and W56. Further, the available Global Precipitation Climatology Project (GPCP) satellite precipitation estimates over the southwestern Bay of Bengal and the Lakshadweep Sea (for two boxes shown in Fig. 14a) also clearly show the occurrence of more rainfall during June 2005 to February 2006 compared to the same period during the other years (Fig. 14b). The larger rainfall signal noticed during 2005/06 and 2003/04 coincided with greater surface freshening during the following winter. While precipitation and the river discharges primarily control the freshening in the Bay of Bengal and in the Lakshadweep Sea, the intrusion of these low salinity waters into the Lakshadweep Sea depends upon the strength of the East India Coastal Current and the Winter Monsoon Current. This is also confirmed by the observed relatively stronger currents during W56 (Fig. 15). Thus it is clearly seen that, in addition to the influence of the river discharges, the cumulative rainfall and strong local surface currents have also contributed to the observed intense freshening noticed during W56 and W34. The salt budget analysis in the Lakshadweep Sea region, using GODAS, clearly confirms the above arguments. The observed variability in the salinity of the model 30-m layer in the Lakshadweep Sea region in January/February is primarily determined by the horizontal advection of low salinity waters in W56 and more freshwater flux in November/December (Figs. 16a and 16b).

c. Intrusion of cooler waters (SST gradient between Lakshadweep Sea and the intruding Bay of Bengal waters)

It is well known that the low salinity waters that intrude from the Bay of Bengal into the Lakshadweep Sea are cooler than the local ambient waters (Luis and Kawamura 2002). The horizontal SST differences between these two regions, the vertical haline stratification, the strength of East India Coastal Current, and Winter Monsoon Current determine the amplitude of the observed thermal inversions. These thermal inversions can only occur when the Bay of Bengal surface waters are both low saline and cooler than the near-surface ambient waters in the Lakshadweep Sea. To test this hypothesis, the difference between the SSTs for two selected boxes representing the Lakshadweep Sea and the intruding Bay of Bengal waters shown in Fig. 17 is examined for all five winters. The SST of Box B is, by and large, lower than that of Box A (positive gradients) during all five winters, thus clearly indicating that the intruding waters from the bay that enter the Lakshadweep Sea are cooler than the local ambient waters. However, during W56 the difference was very weak from December 2005 through January/February 2006, suggesting that the SST of the intruding waters from the bay is of comparable magnitude of the SST of Box A. This weak horizontal gradient in SST resulted in the occurrence of a reduced number of thermal inversions during January/February 2006 as shown in Fig. 3.

5. Summary and discussion

The thermal inversions in the Lakshadweep Sea are very common during winter and they begin to appear from November to December and peak in January–February. The strong haline stratification in the Lakshadweep Sea is conducive for the formation of these inversions. In spite of relatively stronger haline stratification observed in the Lakshadweep Sea region, a reduced number of thermal inversions occurred in W56. Utilizing both in situ and satellite measurements and ocean reanalysis products, several processes that are responsible for the reduced number of thermal inversions during W56 in the study region are examined.

The characteristics of the observed near-surface thermal inversions in the Lakshadweep Sea for five winters (2002–06) are examined utilizing repeat near-fortnightly XBT measurements. During W56 a reduced number of thermal inversions (10%) occurred at relatively shallower depths (∼10 m) compared to any other winter in the study. Surprisingly this has occurred in spite of a large drop of 4.03 psu in the observed sea surface salinity during W56, which is more than twice that observed during W23 (1.9 psu). This dramatic drop is attributed to heavy rainfall over the southwestern Bay of Bengal and Lakshadweep Sea during June 2005 to February 2006 compared to the other years. The observed vertical salinity profiles in the southern region of the Lakshadweep Sea also suggest the intrusion of relatively low salinity waters favoring the occurrence of intense haline stratification during W56, where the reduced number of thermal inversions occurred at very shallow depths (∼10 m).

The salt budget analysis for the upper 35-m layer of the Lakshadweep Sea region clearly suggests that the horizontal advection of low salinity waters from the Bay of Bengal into the Lakshadweep Sea region is primarily responsible for the drop in salinity in the study region. Even though these low salinity waters are advected in large (small) amounts during W56 (W34), relatively fewer (more) thermal inversions occurred in W56 (W34).

During normal years, off the southwest coast of India, strong upwelling is observed (Gopalakrishna et al. 2008) from May to September. Following the summer monsoon months, a secondary surface warming occurs in October–November associated with deepening of the thermocline in November due to downwelling favorable wind stress curl and propagating Kelvin waves (figure not shown). During winter a thin layer of low salinity and cooler water from the Bay of Bengal is advected and results in the formation of thermal inversions in the Lakshadweep Sea. However, in 2005 the upwelling persisted unusually long until November, and the secondary warming in the Lakshadweep Sea is very weak. Owing to this, the temperature gradient between the Lakshadweep Sea and the intruding waters from the Bay of Bengal is very weak. This study highlights the importance of the secondary surface warming in Lakshadweep Sea region for the formation of thermal inversions. It also illustrates that even the large amount of low salinity water advection from Bay of Bengal into the Lakshadweep Sea may not be a sufficient condition for the formation of thermal inversions in the Lakshadweep Sea region. The advection of low salinity waters from the Bay of Bengal may play an important role in the formation of thermal inversions only if the advected waters are relatively cooler than the ambient waters in the Lakshadweep Sea. Recently, Kurian and Vinayachandran (2006), using model simulations, have suggested that factors other than low salinity advection are responsible for formation of temperature inversions in the Lakshadweep Sea region. Our study, based on observations, suggests that the background state of the Lakshadweep Sea prior to the arrival of low salinity waters is an important condition for the formation of thermal inversions. In a previous study, Gopalakrishna et al. (2005) have suggested that anomalous low salinity water influx into the Lakshadweep Sea region is partly responsible for the observed greater number of thermal inversions in W34. However, in this study, we have shown the importance of a strong secondary warming in the Lakshadweep Sea and the horizontal temperature gradient between the Lakshadweep Sea and the Bay of Bengal in the formation of thermal inversions. The observed SST during W56 in the Lakshadweep Sea is relatively cooler in November owing to prolonged upwelling, stronger wind field, and weaker net surface heat gain during the postmonsoon season compared to the other years. Thus the occurrence of a reduced number of thermal inversions during W56 is primarily attributed to the weak horizontal SST gradient between the Lakshadweep Sea and the intruding Bay of Bengal waters in spite of strong haline stratification. A few moored buoys with subsurface thermohaline sensors and current meters are needed to be deployed in the Lakshadweep Sea during October–February to capture the evolution of the full life cycle and to test the governing mechanisms proposed in the study.

Acknowledgments

We thank the Lakshadweep Administrator for permitting XBT measurements on board their passenger ships. This work was supported by the Ministry of Earth Sciences through INCOIS, Hyderabad. The editor and both the anonymous reviewers have provided excellent suggestions that helped us to improve the quality of the paper, and the authors profusely thank them. Mr. Shamkant Akerkar prepared the figures for publication.

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Fig. 1.
Fig. 1.

Location map showing XBT and sea surface salinity stations (dots) collected during May 2002–March 2007. The densely covered Kochi–Kavaratti transect is shown by a shaded strip. Argo float locations during different winters are also shown.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 2.
Fig. 2.

Monthly mean climatology of (a) sea surface salinity (Conkright et al. 2002) overlaid with mean surface currents (Ekman and geostrophic) derived from climatology of QuikSCAT winds and AVISO T/P merged sea level anomalies for November–February. Schematic of the East India Coastal Current and Winter Monsoon Current are shown by white arrows. (b) Monthly mean climatology of the TMI SST for November–February.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 3.
Fig. 3.

XBT station locations for W23, W34, W45, W56, and W67 in the Lakshadweep Sea. Dark (light) dots represent XBT stations with (without) temperature inversions.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 4.
Fig. 4.

Formation of the temperature inversion through horizontal advection of (a) low salinity waters and (b) cool waters. The TS profiles from Argo float ID 2900091 are used.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 5.
Fig. 5.

Hovmöller plot of TMI SST along the Kochi–Kavaratti XBT transect from September to December during 2002–06.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 6.
Fig. 6.

Vertical thermal sections for the upper 125 m along the Kochi–Kavaratti XBT transect from September to February during 2002–07.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 7.
Fig. 7.

(a) Typical thermal structure from GODAS in the SEAS region showing the thermal inversions. (b) Thermal structure in the upper 100 m averaged in the SEAS region.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 8
Fig. 8

a. Evolution of QuikSCAT monthly mean climatology of wind speed during (left) September–February and the wind speed anomaly during (right) September 2005–February 2006. (b) Evolution of monthly mean climatology of NCEP net surface heat flux during (left) September–February (note that the color scales are not at regular intervals) and net heat flux anomaly during (right) September 2005–February 2006.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 9.
Fig. 9.

Upper-ocean heat budget terms derived from GODAS in the upper 0–35 m for (a) 2003–04 and (b) 2005–06.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 10.
Fig. 10.

Hovmöller plot of sea surface salinity (psu) along the Koch–Kavaratti XBT transect. Black dots indicate station locations.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 11.
Fig. 11.

Comparison of Argo float (ID 2900530 and 2900091) vertical salinity profiles for W56 (black solid lines) in the Lakshadweep Sea with corresponding climatology (dashed line) and available mean monthly salinity profiles (blue lines for 2004; red lines for 2005; pink lines for 2006; green line for 2007).

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 12.
Fig. 12.

Comparison of (a) Argo float (ID 2900530 and 2900091) vertical temperature profiles for W56 in the Lakshadweep Sea (floats locations shown Fig. 1) and corresponding climatology (dashed lines) and (b) individual XBT profiles (light-shaded lines in the left panel) and climatology (dark lines in the left panel) and station locations (right panel) during October–December 2005 in the Lakshadweep Sea.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 13.
Fig. 13.

Comparison of river discharge (May–November) for the rivers Mahanadi and Godavary.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 14.
Fig. 14.

(a) Areas (Box A and Box B) considered for comparison of total rainfall (mm) during June 2005–February 2006. (b) Histograms showing the total rainfall (June–February) over (i) Box A and (ii) Box B for the years 2002–06.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 15.
Fig. 15.

Monthly mean near-surface currents (Ekman and geostrophic): (arrows represent current vectors and the background color represents current speed) derived from QuikSCAT winds and AVISO T/P merged anomalies for September–February during 2002–06.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 16.
Fig. 16.

(a) Salt budget terms (salinity tendency, horizontal and vertical advection) derived from GODAS in the upper 35 m in the SEAS region and (b) freshwater flux.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Fig. 17.
Fig. 17.

Time series of SST differences between the Boxes B and A during the winter seasons of 2002–07.

Citation: Journal of Physical Oceanography 39, 5; 10.1175/2008JPO3879.1

Table 1.

Characteristics of temperature inversions in the Lakshadweep Sea during 2002–06.

Table 1.

* National Institute of Oceanography Contribution Number 4512.

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