Growth and Decay of the Arabian Sea Mini Warm Pool During May 2000: Observations and Simulations

ARTICLE IN PRESS

Deep-Sea Research I 56 (2009) 528–540

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Deep-Sea Research I

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Growth and decay of the Arabian Sea mini warm pool during May 2000: Observations and simulations

P.V. Hareesh Kumar a, Madhu Joshi b, K.V. Sanilkumar a, A.D. Rao b,Ã, P. Anand a, K. Anil Kumar a, C.V.K. Prasada Rao a a Naval Physical and Oceanographic Laboratory, Thrikkakara PO, Kochi, Kerala 682 021, India b Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, New Delhi 110016, India article info abstract

Article history: Oceanographic surveys were carried out in the southeastern Arabian Sea in two phases Received 9 April 2008 during 14–17 May and 23–27 May 2000 to study the evolution of the Arabian Sea mini Received in revised form warm pool. The hydrographic data collected during these experiments, satellite imagery 5 December 2008 of sea surface temperature for the corresponding period and ship drift data were utilized Accepted 21 December 2008 for this purpose. Both satellite and in situ observations indicated the presence of a mini Available online 25 December 2008 warm pool up to 14 May and its dissipation thereafter. The Arabian Sea mini warm Keywords: pool region was characterized by low-salinity Bay of Bengal water (o22 sigma-t). Arabian Sea mini warm pool This watermass was advected into the southeastern Arabian Sea by the East India Watermass Coastal Current mainly during November–January and recirculated in this region Heat flux in an anticyclonic eddy. Immediately after the dissipation of the mini warm pool, Western disturbance Numerical model this watermass could not be traced in this region. Simulation of mini warm pool Advection characteristics using the Princeton Ocean Model (POM) agreed well with in situ and satellite observations. Moreover, the model showed strengthening of southerly currents in the warm pool region during its dissipation period, which caused the disappearance of the Bay of Bengal watermass. Specific experiments with the POM revealed the significance of heat flux components, low wind speed and salinity stratification in the formation and sustenance of the mini warm pool. & 2008 Elsevier Ltd. All rights reserved.

1. Introduction sea surface temperatures (SST) in excess of 28 1C occur (Levitus and Boyer, 1994a, b; Webster and Lukas, 1992). In Large areas of warmer waters (428 1C) in the surface the South China Sea and the North Indian Ocean, the SST is layers of the tropical oceans greatly influence the atmo- above 29.5 1C during summer, but the area of these spheric systems, viz. the seasonal monsoons, depressions, warmer waters is smaller than that of the Indo-Pacific cyclones, El-Nino/La-Nina, Indian Ocean dipole and other warm pool (Levitus and Boyer, 1994a, b; Chu et al., 1998, modes of oscillations ranging from 2 to 6 and 30 to 60 1997; Wang and Wang, 2006). A notable feature of the days (Lau and Chan Paul, 1988; Webster and Lukas, 1992; southeastern Arabian Sea is the formation of warmer Godfrey et al., 1995; Saji et al., 1999). Considering their waters (SST 430 1C) during the pre-monsoon period importance, areas of warmer waters, popularly known as (March–May) compared to the Indo-Pacific warm pool warm pools, are of great interest. The largest and best (Fig. 1a). As this feature appears almost every year, several studied warm pool is the Indo-Pacific warm pool, where studies have brought out its bearing on pre-monsoon and summer monsoon climate (Seetaramayya and Master, 1984; Kershaw, 1985, 1988; Joseph, 1990; Rao and

Ã Corresponding author. Tel.: +911126591317; fax: +911126591386. Sivakumar, 1999; Joseph et al., 2003; Deepa et al., 2007). E-mail address: [email protected] (A.D. Rao). This warmer water is named the Arabian Sea mini warm

P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540 529

the formation of the mini warm pool. The formation of the mini warm pool has also been explained in terms of 25N Feb Mar 20N propagating wave phenomena (Shankar and Shetye, 1997). 15N During November–December, a downwelling Kelvin wave 10N arrives at the southern tip of the Indian Peninsula and

N) propagates northward. This wave radiates Rossby waves ° 5N westward and creates high sea level in the southeastern 0 Arabian Sea. Consequently, a large-scale anticyclonic eddy, 25N Apr May known as ‘‘Laccadive High’’, forms over the region (Bruce

Latitude ( 20N et al., 1994, 1998; Shankar and Shetye, 1997). The genesis 15N of the mini warm pool is believed to occur with the 10N 5N formation of this anticyclonic eddy. The Kelvin wave also 0 triggers the East India Coastal Current (EICC), which in 50E 60E 70E 80E 50E 60E 70E 80E turn brings low-salinity waters from the Bay of Bengal. Longitude (°E) This low-salinity water recirculates in the anticyclonic eddy and leads to the formation of highly stratified surface N layers (Shenoi et al., 1999, 2005b; Sanilkumar et al., 2004; Pakistan Coast 25 500 INDIA Hareesh Kumar et al., 2005; Gopalakrishna et al., 2005; 2500 500 Murty et al., 2006). In addition, Sanilkumar et al. (2004) 50 showed that the horizontal coverage of the warm pool is controlled by the extent of low salinity waters, while 3000 Gulf of 20 its vertical extent and stratification control the intensity Khambhat of heating and thickness of warm pool. The net heat flux 100 is positive in the Arabian Sea during the pre-monsoon 3500 season because of high incoming solar radiation, clear 15 skies and weak winds (Hastenrath and Lamb, 1979a, b),

Lat 500 Ker resulting in heat accumulation in these highly stratiﬁed

150 ala Coa surface layers. e 0 The presence of low-salinity waters in the surface Laccativ 10 layers produces a barrier layer between the base of the st 500 surface-mixed layer and top of the thermocline in the Arabian 4000 mini warm pool region (Thadathil and Ghosh, 1992; Sea 2000 Durand et al., 2004; Shankar et al., 2004; Shenoi et al., 100 2004, 2005a; Murty et al., 2006). Temperature inversions 5 0 observed in this barrier layer play an active role in increasing the SST during the initial stages (November– E March) of the warm pool (Durand et al., 2004). How- 60 65 70 75 80 ever, Kurian and Vinayachandran (2007) questioned Long this hypothesis and suggested another mechanism for Fig. 1. (a) Location of the mini warm pool based on SST from Reynolds the warming during the initial stages of the warm pool. and Smith (1994). (b) Model bathymetry (m) and analysis area. Hollow According to this study, the orographic effect of the circles represent the station locations of in-situ data. Western Ghats (a chain of mountains along the southern regions of the west coast of India) reduces the wind speed pool (Seetaramayya and Master, 1984). In order to in the southeastern Arabian Sea and thereby latent heat study the oceanographic conditions during its mature loss, resulting in positive heat flux into the ocean and and decay stages, observations were carried out onboard increase of SST during winter months. Murty et al. (2006) INS Sagardhwani during May 2000 (Sanilkumar et al., suggested that the intra-seasonal variability (between a 2004). After this, extensive observational programs were 41- and 63-day period) in the oceanic and atmospheric planned and carried out during the Arabian Sea Monsoon parameters also affects the heat content in the upper Experiment (ARMEX) in the years 2002–2003 (DST, 2003). layers of the southeastern Arabian Sea. One of the main objectives of ARMEX was to understand The Arabian Sea mini warm pool is the longest lasting the mechanism responsible for the evolution and collapse warm region in the world oceans (DST, 2003). In this of the warm pool both in the ocean and in the atmosphere region, by mid March SST goes beyond 29 1C, which is (Mausam, 2005; Murty et al., 2006). the threshold for triggering convective activities in other Rao and Sivakumar (1999) examined the plausible oceans. However, convective activities over the warm pool mechanisms of gradual build up of the mini warm pool region are not sufficient to dampen the rise in SST. Studies utilizing monthly climatology of surface winds, surface based on Outgoing Longwave Radiation (OLR) and SST heat fluxes, near-surface thermohaline field, near-surface relationships between the Indian Ocean and the Pacific currents, sea levels and model simulations. Another con- Ocean revealed that the Indian Ocean basin is different temporary study (Shenoi et al., 1999) critically analysed from the Pacific Ocean (Waliser et al., 1993; Webster et al., various factors and summarized the processes that lead to 1999). In the Pacific Ocean, there is a general increase in ARTICLE IN PRESS

530 P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540 the height of convection (indicated by the reduced values and affects the rainfall in and around the Indian Ocean of OLR) with increasing SST. Contrary to this, there is no (Ashok et al., 2003; Ashok and Saji, 2007). Many attempts such relationship between SST and OLR in the Indian have been made to study the relation of Indian Ocean Ocean (Godfrey et al., 1995). This lack of correlation dipole with the Indo-Pacific warm pool (Saraswat et al., suggests that deep convection results from processes 2007). As the Arabian Sea mini warm pool is a part of the other than warm SST. Thus convection in the Indian Ocean Indo-Pacific warm pool, its interannual variability is may be more strongly tied to atmospheric dynamics than related to that of the Indian Ocean Dipole. Recently, in the Pacific Ocean. The atmosphere shows a complex Hareesh Kumar et al. (2007) reported that this mini warm structure during pre-monsoon, with the lowest 1–2 km pool influences the propagation of sound in the ocean. layer with maritime properties, i.e. warm and moist, and In spite of the importance of the mini warm pool, the 2–4 km layer of continental origin and very warm studies putting together satellite information, field mea- but dry. This could be a possible factor inhibiting the surements and modeling are extremely sparse. Such a convective activity, as this structure can cause an inver- study is carried out in the present paper utilizing Tropical sion layer (Rao, 2005). With the advance of the inter- rainfall measuring mission Microwave Imager (TMI) SST, tropical convergence zone (thermal equator) over the in situ measurements, ship drift data and a 3-D ocean warm pool region, the atmosphere becomes fully coupled circulation model for May 2000. Specific modeling studies with the ocean, and convective activities are triggered are carried out to understand the role of heat flux and the (Rao and Sikka, 2005). In most years, this advance is low-salinity plume in the southeastern Arabian Sea in the coupled with a monsoon onset vortex. Deepa et al. (2007) formation and dissipation of the mini warm pool. showed that the monsoon onset vortex forms in the shear line of the northern flank of a low-level jet at 850 hPa over the warm pool region. However, a monsoon onset vortex 2. Data and methodology does not necessarily form every year over the warm pool (Rao and Sivakumar, 1999). The research vessel INS Sagardhwani carried out It is well known that the warmer waters and their surveys in the southeastern Arabian Sea during May temporal variability greatly influence the biosphere, 2000 along two zonal transects (91N and 10.51N) with especially the coral reefs (Doval and Hansell, 2000; Abram stations every 30 nautical miles between 691E and 761E et al., 2003; Borgne et al., 2002; Sarma, 2006). Considering (Fig. 1b). The southern (Transect 1) and northern (Transect the location of this mini warm pool, there can be 2) were repeatedly covered during 14–17 May 2000 significant effect on the coral reefs and biological produc- (Phase 1, mature phase of mini warm pool) and 23–27 tion in the Laccadive Sea. Another aspect is the vertical May 2000 (Phase 2, decaying phase). At each station, mini cycle of the nutrient distribution, which can be affected by CTD (accuracies: temperature 70.01 1C, salinity 70.02, the stability/stratification in the upper layers and the pressure 70.02% of 1000 m) casts were performed to barrier layer in the subsurface layers induced by the warm collect vertical profiles of temperature and salinity. In pool. Similar to that of the mini warm pool during pre- addition, SST and surface winds obtained from Tropical monsoon, the Indian Ocean Dipole occurs during post- Rainfall Measuring Mission Microwave Imager SST and monsoon (Saji et al., 1999) in the tropical Indian Ocean QuikSCAT satellite, respectively, at 0.251 Â 0.251 grid

SST (Phase 2) SSTTMI (Phase 1) TMI SST (Phase 1) SST (Phase 2) 32 CTD CTD

30 32 29 C)

° 28 31

32 C) ° SST ( ( 31 30 CTD

30 SST 29 29 RMS = 0.4591 28 28 66 68 70 72 74 76 28 29 30 31 32 Longitude (°E) ° SSTTMI ( C)

Fig. 2. Comparison of SST obtained from TMI (SSTTMI) and in situ (SSTCTD) measurements along (A) 91N, (B) 10.51N latitude and (C) scatter diagram between SSTTMI and SSTCTD. ARTICLE IN PRESS

P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540 531 resolution (between 401E and 801E and between 101S and et al. (2001), who reported an error of 0.6 1C between TMI 251N) for May 2000 were utilized to study the basin-scale SST and BOBMEX-99 data. evolution of the mini warm pool. The daily net heat flux Numerical simulation of warm pool characteristics and solar radiation data available at 11 Â11 grid resolution was carried out using the Princeton Ocean Model (POM). (Lisan and Weller, 2007) were utilized for model simula- The role of wind, net heat flux and salinity in governing tion. In this paper, the mini warm pool is defined as the its characteristics were also studied using this model. region where SST is more than 30.25 1C in the south- The POM model (described in detail by Blumberg and eastern Arabian Sea following Sanilkumar et al. (2004). Mellor, 1987) is fitted with realistic bottom topography As the main objective of the paper is to study the from ETOPO5 of the National Geophysical Data Center. evolution and decay of the warm pool during May 2000, it The model domain extends from 51N to 24.51N along is worthwhile to compare the TMI SST data with the the southeastern Arabian Sea with an offshore extent of available ground truth measurements. The SST obtained 600 km (Fig. 1). The continental shelf is narrow (50 km) from the ground truth measurements (i.e. 1 m value from near the Pakistan coast, widens to 340 km off the Gulf of Mini CTD, 55 data points) along 91N and 10.51Nwere Khambhat and gradually tapers towards the south to utilized for this purpose. From the figure (Fig. 2A, B), it can 110 km off the Kerala coast. To reduce the topographic be seen that the TMI SST captured most of the variability, gradients, which could otherwise cause spurious along- as observed in the ground truth data. The root-mean- slope currents in a sigma coordinate model, a bilinear square differences (Fig. 2C) between the satellite and interpolation is used (Haney, 1991). More details of the ground truth SST data indicated a difference of 0.46 1C. model description are given in Rao et al. (2008). Accord- Here, it is to be noted that the satellite measures the skin ingly, there are 175 Â 250 grid points in the horizontal temperature whereas the ground truth SST was from 1 m computational plane and 26 computational levels in the depth. Probably this resulted in the departure between vertical. Resolution in the zonal direction varies from 6 to these two observations, especially during the heating 9 km, whereas it varies from 7 to 12 km in the meridional regime. The error obtained is much less than that of Bhat direction with finer resolution near the coast. In the

Fig. 3. TMI SST for May 2000. ARTICLE IN PRESS

532 P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540 vertical, a terrain following sigma coordinate is used elevation along with temperature and salinity are used as with fine resolution of 0.5 and 15 m near the surface and initial conditions for the model simulations in prognostic the bottom, respectively, while a relatively coarse grid of mode. In the simulation, mechanical forcing was done about 30 m is used at the mid-depths. The horizontal time using the daily QuikSCAT wind data and thermal forcing differencing is an explicit scheme, whereas the vertical using daily net heat flux and solar radiation data for May time differencing is an implicit scheme. It uses a mode 2000. splitting technique with barotropic (11.25 s) and baroclinic (450 s) mode. The horizontal boundary conditions over the land are implemented by a land mask, which ensured 3. Results and discussion that the velocity over the land and the normal velocity along the coastline were zero. The radiation conditions are 3.1. Satellite observations used to allow disturbances generated from the interior to travel outwards as progressive waves when they reach the To study the basin-scale variability in the surface open boundaries (Rao et al., 2005, 2008). temperature in the Arabian Sea, the 3-day running mean The initial fields of temperature and salinity for May of TMI SST for typical days of May 2000 is presented are obtained from the monthly climatological data sets (Fig. 3). During the first week of May, temperature in of World Ocean Atlas 2001 (Conkright and Boyer, 2002). excess of 30 1C was noticed in the southern part of the In the diagnostic mode, temperature and salinity are held central Arabian Sea. Here, one of the notable observations unchanged during the integration. When the wind forcing was the occurrence of the Arabian Sea mini warm pool is applied, the velocity field will respond and result in (SST 430.25 1C) from 7 May. The warming continued and establishment of a combined Ekman and geostrophic attained its peak (431 1C) in the eastern Arabian Sea on balance. After a sufficient period of the diagnostic run 14 May. During this period, i.e. 7–14 May, the observed (in our case, it is 15 days), a steady-state velocity field and winds were found to be less than 3 m/s over the entire surface elevation that are dynamically consistent with southeastern Arabian Sea (Fig. 4), a condition favourable temperature and salinity are established. This velocity and for the formation and sustenance of the mini warm pool.

Fig. 4. QuickScat winds with magnitude shaded for May 2000. ARTICLE IN PRESS

P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540 533

The progressive warming in the mini warm pool was southeastern Arabian Sea without leaving any of its arrested by 14 May by strengthening of winds (47 m/s) in signatures. This highlights the role of intense air–sea these regions (Fig. 4). In the Arabian Sea, the cross- interaction on the sustenance and dissipation of the equatorial winds were first observed on 1 May off the Arabian Sea mini warm pool. Somalia coast. With time, the cross-equatorial winds extended towards eastern longitudes and strengthened 3.2. In situ measurements to more than 7 m/s, indicating the advance of the southwest monsoon. A western disturbance was formed over The field measurements were carried out in the Jammu and Kashmir and its neighbourhood on 13 May southeastern Arabian Sea along 91N, and 10.51N in two (Indian Daily Weather Report, 2000), which created an phases, representing the warming stage (Phase 1, 14–17 offshore trough parallel to the west coast of India (Fig. 5). May) and the post-dissipation stage (Phase 2, 23–27 May) The intensification of the trough by 16 May, as evident of the mini warm pool. Therefore, the data collected were from the isobars, also contributed to the strengthening of utilized to study the characteristics of the mini warm the winds. From the ground truth observations, Sanilk- pool during its mature and decay phases. The vertical umar et al. (2004) also reported strong winds and change sections of temperature (Fig. 6) showed that the Arabian in the atmospheric condition during this period. These Sea mini warm pool was present only during Phase 1 strong winds led to a drop of surface temperature by more along Transect 1. During this phase, the warm water than 1 1C within 2 days (Fig. 3) at the warm pool core. This (SST430.25 1C) occupied the upper 25 m of the water suggested the immediate response of the mini warm pool column. At the core (between 72.51E and 741E), tempera- to the atmospheric disturbance. After 18 May, the drop ture was in excess of 30.75 1C (maximum value 431.2 1C in temperature was found to be gradual (0.1 1C/day). at 73.51E), and this isotherm extended to a depth of 8 m. By 25 May, the entire warm pool vanished from the This core temperature was much higher than that reported by Seetaramayya and Master (1984) and Shenoi 40 et al. (1999). By the time of the survey along Transect 2 (Fig. 6b), wind speed increased to more than 7 m/s (Fig. 4), 1006 1006 35 1004 1004 0830 Hrs IST (0300 Hrs GMT) resulting in the disappearance of isotherms in excess of Saturday 13 May 2000 30.25 1C. After a period of 1 week (23–24 May), the entire 1004 1002 L study region further cooled and isotherms in excess of 30 1000 1004 30 1C also disappeared (Phase 2, Fig. 6). Time-longitude 1006

N) sections (Hovmullor diagrams) of TMI SST and surface ° 25 1004 winds along Transects 1 and 2 (Fig. 7) are presented to show their time evolution with respect to Phases 1 and 2 20 of the observational program. Progressive warming was Latitude ( 1006 L observed along both transects from 7 to 14 May. During 15 1008 1003 this period, winds were westerly with speed less than 1010 3 m/s. After 14 May, wind speed increased to more than 10 1008 7 m/s and the TMI SST decreased by more than 1 1C. This

1010 situation prevailed during the period of coverage of the 5 second transect, resulting in the absence of a mini warm 60 65 70 75 80 85 90 95 100 pool. In short, the mini warm pool was present along 91N only during the ﬁrst phase of the ﬁeld measurement 40 H 1008 1008 program before the strengthening of the winds. 1006 1004 1004 0830 Hrs IST (0300 Hrs GMT) The vertical section of salinity (Fig. 6) indicated the 35 1002 Saturday 16 May 2000 presence of low-salinity ( 35) waters in the mini warm 1000 o 1002 pool region, with its maximum thickness (50 m) at 69.51E 1002 H 30 1004 10061008 1 1000 1010 and minimum (o5m) at 68 E during Phase 1. Along L 1000 Transect 2 also, the intermittent appearance of tempera- N) ° 25 1002 ture in excess of 30 1C coincided with the zones of low- 1008 1004 salinity water, but its thickness was reduced to less than 20 1006 10 m. This low-salinity water disappeared from the warm Latitude ( pool region within a period of 1 week, i.e. during Phase 2. 15 1008 Utilizing buoy measurements of National Institute of 1008 Ocean Technology, India, Jossia Joseph et al. (2005) also 10 1010 reported rapid changes in the surface salinity off the 1010 southwest coast of India. 5 60 65 70 75 80 85 90 95 100 3.3. Water masses in the warm pool region Longitude (°E)

Fig. 5. Pressure chart from Indian Daily Weather Report for 16 and 22 Many studies have indicated that one of the mechan- May 2000. isms for the formation of the mini warm pool is the ARTICLE IN PRESS

534 P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540

Along 10.5°N Along 10.5°N 0 0

25 25

50 50

75 75 Along 9°N Along 9°N 0 (°C) 0 Depth (m) Depth (m) 30.75 36.0 25 30.25 25 35.5 29.00 35.0 50 27.00 50 25.00 34.5 23.00 34.0 75 21.00 75 67 70 73 76 70 73 76 67 70 73 76 70 73 76 ° Longitude (°E) Longitude ( E)

Fig. 6. Depth–longitude sections of temperature and salinity along 91N and 10.51N during (a) Phase 1 and (b) Phase 2 of the ﬁeld measurement program.

10m/s May 29 29

25 25 ° 21 21 ( C) (m/s) 31 30.75 17 17 30.5 10 30.25 30 Days 13 8 13 29 28 9 6 9

5 4 5 2 May 1 1 66 68 70 72 74 76 66 68 70 72 74 76 64 66 68 70 72 74 76 64 66 68 70 72 74 76 Longitude (°E) Longitude (°E)

Fig. 7. Hovmullor diagram along 91N and 10.51N of (A) QuikScat winds and (B) TMI SST: dots represent the station locations of in situ data.

32 32 C) C) ° 1 ° 2 28 22 22

24 Temperature ( Temperature Temperature ( Temperature 24 22 24 28 20 34 35 36 34 35 36 34 35 36 Salinity (PSU) Salinity (PSU) Within Warm Pool Outside the Warm Pool In the warm pool core

Fig. 8. T–S diagrams (a) within the warm pool, (b) during Phase 1 and (c) during phase 2 of the ﬁeld measurement program. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) ARTICLE IN PRESS

P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540 535

Fig. 9. Monthly averages of ship drift currents (11 Â11 grid) from October–March.

Fig. 10. Model SST with net heat ﬂux during May 2000. ARTICLE IN PRESS

536 P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540 occurrence of low-salinity waters in the surface layers prominent watermasses, viz. the Indian Equatorial Water of the southeastern Arabian Sea (Rao and Sivakumar, with sigma-t of 23 and the Arabian Sea High Salinity 1999; Sanilkumar et al., 2004; Gopalakrishana et al., 2005; Water between 23 and 24 sigma-t levels, were identified Shenoi et al., 2004; Murty et al., 2006). To identify the based on the characteristics described in the earlier characteristics of this watermass, a T–S diagram utilizing studies (Rochford, 1964; Kumar and Prasad, 1999; Schott the temperature and salinity data within the mini warm and McCreary, 2001; Shenoi et al., 2005b). Among them, pool region is shown (Fig. 8a). The analysis clearly brought the first one was found around the mini warm pool and out the presence of a watermass having sigma-t less than the latter one underneath the mini warm pool. Interest- 22 in this region, which is the well-known Bay of Bengal ingly, outside the mini warm pool, the BBW was not Water (BBW, Rochford, 1964). It is important to note that observed even during its mature phase (Fig. 8b, red dots). this is the only watermass present in this mini warm pool. Moreover, the BBW disappeared from both the observa- Further, to identify the other watermasses around the tional transects during the decay phase (Fig. 8c). However, mini warm pool, T–S diagrams were prepared utilizing the the Indian equatorial and Arabian Sea High Salinity entire temperature and salinity data collected during both Waters were present throughout the observational period. the phases of the field measurements (Fig. 8b and c). In To investigate the probable period of the inflow of the the figure, blue and red dots indicate the T–S values with above watermasses, monthly mean ship drift currents in and outside the mini warm pool during its mature obtained from JNODC and NODC (1900–1993) for the (Fig. 8b) and decay (Fig. 8c) phases, respectively. relevant region in each 11 Â11 latitude/longitude grid from Surrounding the mini warm pool (Fig. 8b), two more October to March are presented in Fig. 9. From the figure,

50 cms-1 20 1 May 7 May 11 May 13 May

5 20 14 May 15 May 16 May 17 May

E) 15 °

Latitude ( 10

5 20 18 May 20 May 23 May 26 May

5 70 75 70 75 70 75 70 75 Longitude (°E)

Fig. 11. Simulated surface currents for May 2000. ARTICLE IN PRESS

P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540 537 it can be seen that the East India Coastal Current turns cessation of the inflow of this watermass. This longer around Sri Lanka and enters the southeastern Arabian Sea residence time of BBW in the southeastern Arabian Sea in November. It is well known that this current carries is made possible only by trapping/recirculation in an low-salinity BBW to the Arabian Sea. It is to be noted that eddy. Earlier studies have already reported the formation the North Equatorial Current forms only in December, and of such an anti-cyclonic eddy in this region (Bruce et al., thereafter this current merges with the East India Coastal 1994; Shankar and Shetye, 1997). Current. By February, the East India Coastal Current in the Bay of Bengal becomes northerly and forms the western boundary current of the Bay of Bengal (Shetye et al., 1993; 3.4. Numerical experiments Sanilkumar et al., 1997). Therefore, the EICC separates from the North Equatorial Current during February and Numerical experiments were performed using the ceases the inflow of the BBW into the southeastern POM to simulate the evolution of the mini warm pool Arabian Sea. Thus, the observations revealed that the during May 2000 and to understand the underlying southeastern Arabian Sea receives BBW only during 3 mechanisms. Daily averaged values of simulated SST in months, i.e. November–January. However, the Indian the southeastern Arabian Sea are presented in Fig. 10. The Equatorial Water (Levitus and Boyer, 1994a, b) can con- results clearly indicate progressive warming in the surface tinue to flow into this region until the formation of the layers from 1 May, as the mini warm pool was found very Southwest Monsoon Current as it is supported by the close to the coast. Later, it is intensified and extended presence of the North Equatorial Current (Fig. 9). Our towards the west beyond 661E. Maximum temperature analysis clearly indicated that the BBW occupies the core in this region was noticed on 14 May, and there- of the mini warm pool even during May, well after the after the temperature decreased. Both the ground truth

Fig. 12. SST simulated for May 2000 without heat ﬂux. ARTICLE IN PRESS

538 P.V. Hareesh Kumar et al. / Deep-Sea Research I 56 (2009) 528–540 measurements and TMI SST also supported the maxima progressive cooling of about 0.5 1C was noticed in on 14 May and cooling thereafter. The associated currents the entire study region. On the other hand, observations simulated by the model (Fig. 11) were southerly, which is (Figs. 3 and 6) and simulations with heat flux terms in agreement with the ship-drift data (Cutler and Swallow, (Fig. 10) showed an increase of 1.25 1C during the heating 1984). It is also to be noted that the currents intensified on regime (up to 14 May) and decrease of 1 1C during 15 May as a response of advance of the southwesterly the cooling regime (14–26 May). It is to be noted that wind system. The strengthening of currents coincided continuous cooling was also simulated off the southern tip with the disappearance of the low-salinity water from of India even in the absence of heat flux. This suggests the the mini warm pool region, as observed during the field role of internal ocean dynamics in the observed cooling in measurements (Fig. 6). these regions. Many studies have attributed the cooling off Another notable observation (Fig. 10) is the pro- the southwest coast of India during this period to coastal nounced cooling around the southern tip of India. The upwelling (Sharma, 1966; Johannessen et al., 1981; Rao field measurements along 91N and 10.51N latitudes and Madhu, 2008; Rao et al., 2008). clearly indicate upsloping of isotherms towards the coast The ground truth measurements revealed a significant and cooling in the coastal region (Fig. 6) attributable relation between the low-salinity stratification and the to upwelling. Therefore, we conclude that cold water location of the mini warm pool (Fig. 6). Hence, an simulated off the southern tip of India is upwelling experiment was carried out with the POM to study the induced (Rao and Madhu, 2008; Rao et al., 2008). role of salinity in the evolution of the mini warm pool In order to study the influence of heat flux on the (Fig. 13). Since the average salinity between the surface evolution of the mini warm pool, the POM model was run layers and the base of the Arabian Sea High Salinity Water without heat flux components (Fig. 12). In this case, a is 36, the model was run with a uniform salinity of

Fig. 13. SST simulated for May 2000 with constant salinity of 36. ARTICLE IN PRESS

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36 over the entire domain. In this experiment, though the are gratefully acknowledged. The support provided progressive growth of the mini warm pool was clearly by the Commanding officer and other officers of the INS seen, its extent and rate of warming were significantly Sagardhwani are also gratefully acknowledged. lower than the observations (Fig. 6) and simulation with actual salinity values (Fig. 9). For example, on 13 May, a References 31 1C isotherm appeared (Fig. 9, with actual values in the model) in the southeastern Arabian Sea, whereas Abram, N.J., Gagan, M.K., McCulloch, M.T., Chappell, J., Hantoro, W.S., this isotherm was absent on the same day when uniform 2003. Coral reef death during the 1997 Indian Ocean Dipole linked to salinity was considered in the model (Fig. 12). The Indonesian wildfires. Science 301, 952–955. Ashok, K., Guan, Z., Yamagata, T., 2003. 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