Cyclone-induced mixing does not cool SST in the post-monsoon north Bay of Bengal

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ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 9: 1 6 (28) Published online 17 December 27 in Wiley InterScience (www.interscience.wiley.com) DOI: 1.12/asl.

T in the post-monsoon north Bay of Bengal Debasis Se

1 ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 9: 1 6 (28) Published online 17 December 27 in Wiley InterScience ( DOI: 1.12/asl.162 Cyclone-induced mixing does not cool SST in the post-monsoon north Bay of Bengal Debasis Sengupta,* Bharath Raj Goddalehundi and D. S. Anitha Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India *Correspondence to: Debasis Sengupta, Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, 5612 India. dsen@caos.iisc.ernet.in Received: 15 March 27 Revised: 4 September 27 Accepted: 19 October 27 Abstract High-resolution data from the TRMM satellite shows that sea surface temperature (SST) cools by 3 C under the tracks of pre-monsoon tropical cyclones in the north Indian Ocean. However, even the strongest post-monsoon cyclones do not cool the open north Bay of Bengal. In this region, a shallow layer of freshwater from river runoff and monsoon rain caps a deep warm layer. Therefore, storm-induced mixing is not deep, and it entrains warm subsurface water. It is possible that the hydrography of the post-monsoon north Bay favours intense cyclones. Copyright 27 Royal Meteorological Society Supplementary electronic material for this paper is available in Wiley InterScience at interscience.wiley.com/jpages/ x/suppmat/ Keywords: cyclone cooling; upper ocean processes; Bay of Bengal 1. Introduction Evaporation due to high winds over warm water sustains the thermodynamic cycle of a tropical cyclone (TC) (Holland, 1997; Emanuel, 23). Deepening of the ocean-mixed layer cools sea surface temperature (SST) by several degrees Celsius under the track of intense TCs (Price, 1981; Wentz et al., 2; Lin et al., 23). SST cooling is thought to inhibit cyclone intensification, preventing the storm from attaining its potential intensity (Emanuel, 1999). If SST does not cool because the upper ocean has a deep warm water layer, a TC could intensify rapidly and sustain its intensity longer (Shay et al., 2; Lin et al., 25). Gradual warming of the subsurface ocean, and reduced SST cooling by storm-induced mixing, may have contributed to increased intensities and lifetimes of TCs since the 197s (Emanuel, 25). North Indian Ocean (nio) SST has warmed by.5 C between and , while the proportion of (Saffir-Simpson) category 4 and 5 cyclones in this basin has increased significantly (Webster et al., 25). The possibility of SST-intensity feedback is of practical importance for Bay of Bengal (BoB) cyclones, which have killed over 35 people since 197 (De et al., 25). However, there exists no systematic study of the response of the ocean to nio cyclones. Here we study cyclone-induced SST drop using a 3-day.25 by.25 Tropical Rainfall Measuring Mission (TRMM) TMI (version 3) SST (Wentz et al., 2). The history and tracks of all 14 TCs (windspeed exceeds 32 ms 1 ) in the BoB and the Arabian Sea (AS) during are based on Climate Diagnostics Bulletins of the Indian Meteorological Department (IMD), and data from the Joint Typhoon Warning Centre (JTWC; There are two cyclone seasons in the nio: April early June (pre-monsoon) and late September December (post-monsoon). We find that the post-monsoon north BoB responds quite differently to cyclones than the pre-monsoon AS or BoB (illustrated in Figures 1 and 2): SST cooling along the track of pre-monsoon TCs is about 3 C (Figure 1(a)), whereas cooling due to post-monsoon TCs in the north Bay is 1 C (Figure 1(b)), broadly consistent with previous case studies (e.g., Murty, 1983; Rao, 1987; Subrahmanyam et al., 25). This result is explained by making simple estimates of mixing, based on thermistor chain data from a moored buoy in the eastern AS in June 1998, and Conductivity-Temperature-Depth (CTD) data from ORV Sagar Kanya cruise SK197 in the north BoB in October SST cooling under pre-monsoon and post-monsoon cyclones The spatial pattern of net SST cooling due to pre-monsoon cyclones (Figure 2(a) (d)) and postmonsoon north Bay cyclones (Figure 2(e) (h)) is shown by the difference in pre-storm and post-storm SST. Cooling generally represents maximum prestorm minus post-storm SST. We use 3-day data because daily and pass-by-pass TMI SST have large gaps due to heavy rain (Wentz et al., 2) and occasional short-lived, localised SST spikes near rain pixels. Day-by-day evolution of SST for selected storms is shown in Supplementary Figure S1. Our main findings are based on the study of all 14 cyclones during , although we present results for 8 cyclones that have a lifetime of at least 2 days over the ocean. The June 1998 cyclone in the eastern AS developed over very warm water, with SST over 31 C. Copyright 27 Royal Meteorological Society

2 2 D. Sengupta, B. R. Goddalehundi and D. S. Anitha Figure 1. SST cooling due to tropical cyclones in the (a) pre-monsoon Arabian Sea and Bay of Bengal and (b) post-monsoon north Bay of Bengal. Three-day TMI SST is averaged over a 1 -wide strip along the track of each cyclone; start dates of the TCs (left bottom in each panel) have been moved so that periods of SST cooling coincide. Average SST cooling is shown in black. Figure 2. SST cooling due to tropical cyclones in the pre-monsoon Arabian Sea and Bay of Bengal (top, panels a d), and post-monsoon north Bay of Bengal (bottom, panels e h). Dots mark storm location every 12 h (from JTWC). Net SST cooling (blue shades) is the difference between pre-storm and post-storm SST (dates on top), e.g. 4 9 Jun 1998 denotes 4 June 1998 SST minus 9 June 1998 SST. The location of mooring DS1 is marked by the red cross in panel (a). SST cooling directly under the TC and to the right of the track was abrupt; the maximum SST drop exceeded 5 C on 7 June at 15.3 N 68.4 E (Figure 2(a)). The SST drop was small ahead of the storm centre, as well as a few hundred kilometres away. The cyclone left a distinct cool wake (Figures 1, S1) that took several days to disappear. The 1998 Indian summer monsoon onset followed this TC, so the ocean did not return to its warm pre-monsoon state. Copyright 27 Royal Meteorological Society Atmos. Sci. Let. 9: 1 6 (28) DOI: 1.12/asl

3 Cyclone mixing does not cool Bay of Bengal SST 3 The magnitude of maximum SST cooling due to the May 21 AS cyclone (Figure 2(b)) is comparable to the June 1998 case, but SST cooling is more extensive. The pre-monsoon BoB cyclone of 23 developed on 11 May over warm water (SST>31 C). SST cools everywhere except in the northwest Bay; from 9 to 21 May, it remains warmer than 3 C along the east coast of India (Figures 2(c), S1); the maximum SST drop is 5 C in the central Bay, where the storm moves slowly. The May 24 cyclone in the north Bay developed relatively close to land on 18 May 18 UTC, and made landfall in Myanmar on 19 May 12 UTC. Net cooling of 1.5 C or more was confined to the north-eastern Bay, with maximum SST drop exceeding 2.5 C (Figure 2(d)). The near absence of distinct SST cooling along the cyclone track is characteristic of the post-monsoon north BoB. The two TCs of November 1998 cause small SST cooling of.5 1 C over the north-west Bay (Figure 2(e), (f)). TC 4B developed on 16 October 1999 and intensified within 24 h to category 4, at 17.7 N, 85.3 E, UTC on 17 October, when it lay 2 km from the coast. The large-scale prestorm SST was about 29.5 C (Figure S1). 4B leads to little SST cooling (.5 1 C) over the open ocean, but causes a drop of 2 Cina1 by 2 region off the continental shelf (Figures 2(g), S1). The Orissa supercyclone (TC 5B) formed as a low pressure system in the South China Sea, crossed into the Andaman Sea (eastern BoB), and developed into a TC early on 27 October 1999 at 16 N, 92 E. Largescale pre-storm Bay SST was about 29 C (Figure S1). Within 3 h, the TC intensified from category 2 to 5, with sustained maximum winds of >6 ms 1. When it reached category 5 on 28 October, 5B lay 25 km from land; it made landfall near the port town of Paradip about 24 h later (12 UTC on 29 October), and remained at TC strength until early 3 October. The pattern of SST cooling due to 5B (Figures 2(h), S1) has similarities with 4B: In the open ocean, there is no distinct cooling under the track, but widespread cooling of.5 1 C; near the continental slope, the cyclone leads to abrupt cooling by over 2 C over a limited area (Figures 2(g), S1). Comparison of the premonsoon May 24 (Figures 1, 2(d)) and May 1998 (not shown) cyclones in the north BoB with postmonsoon cyclones (Figures 1, 2(e) (h)) suggests that the nature and magnitude of SST drop depends on the season. At the point of closest approach, the June 1998 AS cyclone passed about 2 km west of moored buoy DS1 (15.32 N, E; Figure 2(a)). DS1 carried a current meter with temperature (T ) and salinity (S ) sensors at 2.2 m depth, and thermistors at several depths between 5 and 125 m. The evolution of temperature at selected depths (Figure 3) shows warm pre-storm SST and strong thermal stratification (about 4 C) between 2 and 6 m due to spring warming of the eastern AS (Sengupta et al., 22). The isothermal layer (daily mean SST minus 1 C) deepens from 1 2 m to about 6 m (Figure 3), and SST cools by about 3 C in response to the storm. SST warming by 1.2 C over the next 1 days (Figure 1(a)) is associated with gradual, weak restratification of the upper ocean. A persistent.5 C temperature inversion after 6 June is possibly due to evaporative cooling in the presence of haline stratification (as suggested by the 2.2 m S; not shown), and/or subsurface lateral advection. Maximum buoy windspeed at 3 m height is 21 ms 1, and maximum surface current is 1.4 ms 1, with pronounced inertial oscillations up to 16 June (not shown). Storm-induced mixing increases the potential energy of the upper ocean. The total increase in potential energy per unit area, PE tot, can be estimated from the DS1 density profiles prior to the storm and at the time the isothermal layer is deepest: ρ i (Z)(D Z)gdz + PE tot = ρ f (z)(d Z)gdz (1) where ρ is density, and subscripts i and f denote initial (pre-storm) and final (post-storm) profiles. We assume a depth D below which there is no mixing, i.e. the final and initial profiles are identical at all depths greater than D. In the absence of subsurface salinity data, we assume constant S, and use the observed temperature in the upper 8 m (Figure 3) to obtain a PE tot of about 1 J m 2. This is an underestimate because the upper ocean must be stratified by S as well the mixed layer is distinctly shallower than the isothermal layer after 6 June (Figure 3), i.e. a barrier layer is present. Although temperature is influenced by processes other than vertical mixing, such as cooling by turbulent heat fluxes and advection, we find that PE tot is not sensitive (changes less than 1%) to the exact choice of pre-storm and post-storm dates. Shenoi et al. (22) use climatological T and S data to calculate the potential energy PE required to uniformly mix the upper 5 m of the tropical Indian Figure 3. Temperature evolution at selected depths (in metres) on eastern AS mooring DS1, June 1998, based on 3-hourly thermistor data. Copyright 27 Royal Meteorological Society Atmos. Sci. Let. 9: 1 6 (28) DOI: 1.12/asl

4 4 D. Sengupta, B. R. Goddalehundi and D. S. Anitha ocean in different seasons PE for the eastern AS in April May is about 25 J m 2, while for the salinitystratified post-monsoon north BoB it is 5 J m 2. Our estimate of PE tot is larger, mainly because the observed pre-storm stratification in early June 1998 is stronger than climatological April May values, and we have assumed that mixing extends to 6 m (Figure 3). 3. Small post-monsoon cooling in north Bay of Bengal The total freshwater input to the BoB north of 14 N during the summer monsoon (June September) is about 35 km 3, 125 km 3 from rainfall and 18 km 3 of runoff from eight major rivers (see estimates in Sengupta et al., 26). Shallow pools of lowsaline water appear in summer; climatological surface salinity is lowest in the post-monsoon season (Boyer et al., 22). On ORV Sagar Kanya cruise SK197 in October 23 in the north Bay, we found surface salinity of 19 33, with a localised fresh pool in the northwest (25 by 2 km with salinity <28; Figure 4(a)). Station data within the pool show one or more intense haloclines in the upper ocean (e.g. station 16, Figure 4(b)). Area-average profiles in the far north Bay (see below) have a salinity difference of 6 psu between 1 and 4 m, while the isothermal layer (SST-1 C, or T > 28 C) is almost 5 m deep. There is no available in situ data from the north Bay during the actual passage of a TC. We assume that the October 23 hydrography represents typical upper ocean conditions encountered by post-monsoon TCs. SST cooling can be estimated under the assumption that total mass, heat, and salt content of the water column is unchanged under vertical mixing; the column potential energy increases because subsurface water is entrained into the mixed layer. The contribution of turbulent heat fluxes or advection to the observed cooling cannot be estimated from available data. Given pre-storm vertical profiles of temperature, salinity, and density, post-storm profiles are found from ρ i (Z)dz = ρ i (Z)S i (Z)dz = ρ i (Z)T i (Z)C p dz = ρ f (z)dz (2) ρ f (z)s f (Z)dz (3) ρ f (z)t f (Z)C p dz (4) where g is gravity and C p is the specific heat. Since D is not known, we generate final profiles of T, S, and ρ for all D between d i and 1 m, where d i stands for initial mixed layer depth (MLD). In all solutions (i.e. final profiles), T and S are uniform from the surface to the final MLD d i, and vary linearly with depth between d f and D. The system of Equations (2 4) is underdetermined (we have three equations and four unknowns: D, d f, SST, and SSS after mixing); a potential energy criterion is used to close the system we pick the subset of solutions for which final minus initial potential energy lies within 1% of the prescribed PE tot (Equation 1). This gives a range of plausible post-storm MLDs and SST drops. The area-averaged pre-storm MLD in the far north Bay is 5 1 m (Figure 5). Guided by the June 1998 AS data, we assume that PE tot due to a post-monsoon TC is 9 11 J m 2. The estimated post-storm MLD is then 4 45 m, and storm-induced mixing cools SST by.1 C. Directly under an intense TC, PE tot can be larger than 1 J m 2 with PE tot = 2 J m 2, post-storm MLD is equal to Figure 4. (a) October 23 salinity at 5 m depth from cruise SK197; triangles mark stations 4 and 16. (b) T (temperature) and S (salinity) profiles at station 16 (inside the freshwater pool; blue) and station 4 (outside the pool; red). Copyright 27 Royal Meteorological Society Atmos. Sci. Let. 9: 1 6 (28) DOI: 1.12/asl

5 Cyclone mixing does not cool Bay of Bengal SST 5 Figure 5. Average pre-storm profiles (black) from cruise SK197, October 23 of (a) T and (b) S, based on all CTD stations located in E, N, to the right of the Orissa supercyclone track. Typical post-storm profiles (red and blue) generated from Equations (2) (4), for PE tot of 1 and 2 J m 2 (labelled1k,2k;seetext) m, and SST cooling is equal to.15.4 C (Figure 5). For the same (area-averaged) stratification, SST cooling of 2 C would require post-storm MLD to be almost 1 m such deep mixing could only be achieved if PE tot were about 6 J m 2. We examine sensitivity to pre-storm hydrographic structure by using the observed T and S profiles at stations 4 (outside the fresh pool, near-isothermal layer with T > 28 C to 3 m) and 16 (in the pool, deep thermocline, isothermal layer 7 m). For PE tot = 2 J m 2, SST cooling at station 4 is 1.9 C (Supplementary Figure S2), similar to that observed off the shelf (Figures 2, S1). At station 16, there is a temperature inversion of about 1 C (Pankajakshan et al. (22)), possibly due to evaporative cooling of the shallow mixed layer by dry, cool winds from land. Storm-induced mixing actually warms SST by about.7 C (Figure S2) because sub-mixed layer water is warmer than SST down to 5 m depth (Figure 4(b)). 4. Concluding remarks The observations and simple estimates presented here suggest that: SST cooling by pre-monsoon TCs in the AS and south BoB is large because of strong thermal stratification below a relatively shallow and very warm mixed layer (Sengupta et al., 22). SST cooling of the open north Bay by post-monsoon TCs is small mainly because of the presence of a deep isothermal (or warm) subsurface layer underlying a shallow fresh layer, i.e. a barrier layer. The barrier layer is absent or shallow in April May in the BoB, but is 2 5 m deep along the northern and western boundary of the Bay in the post-monsoon season (de Boyer Montegut et al., 27; Pankajakshan et al., 27). Strong near-surface haline stratification resists storm-induced mixing; if mixing does break through the halocline, subsurface warm water is entrained into the mixed layer. Thus, the observed.5 1 C open ocean cooling in the post-monsoon north BoB is likely to be mainly due to evaporation. In the presence of a temperature inversion in the upper ocean, mixing can actually warm SST. Sensitivity tests show that the magnitude and sign of mixing-induced SST change under the cyclone track depends on the net supply of potential energy to the upper ocean, as well as the details of local hydrographic structure. The 2 C SST cooling as the storm approaches the coast (Figures 2, S1) suggests a hydrographic structure similar to station 4 (Figures 4, S2), with no barrier layer. Similarly, test calculations with observed pre-monsoon BoB profiles suggest storminduced SST cooling of 2 C or more, consistent with our findings. We note that the main results about seasonality and geographic dependence of SST cooling are generally valid for all the 14 AS and BoB TCs examined. A deep warm layer underlying a fresh plume is associated with the equatorward post-monsoon East India Coastal Current (Shetye et al., 1996; Pankajakshan et al., 27). Ocean models suggest that relaxation of monsoon winds, and downwelling waves from the equatorial Indian Ocean, deepen the thermocline off the east coast of India (Schott and McCreary, 21). A distinct thermodynamic mechanism can also Copyright 27 Royal Meteorological Society Atmos. Sci. Let. 9: 1 6 (28) DOI: 1.12/asl

6 6 D. Sengupta, B. R. Goddalehundi and D. S. Anitha warm the sub-mixed layer in this season: the sky is clear over the north Bay, and shortwave radiative flux through the base of the shallow mixed layer is large (6 8 Wm 2, see Sengupta and Ravichandran, 21; Sengupta et al., 21). The sub-mixed layer, unaffected by evaporative and longwave cooling, warms rapidly. Solar penetration could also contribute to the formation of temperature inversions such as that at station 16 (Figure S2; Shetye et al., 1996). It is possible that basin-scale dynamics, as well as freshwater from monsoon rain and river runoff, influence the intensity of BoB cyclones. Supplementary Material Supplementary electronic material for this paper is available in Wiley InterScience at: suppmat/ Acknowledgements DS thanks P. V. Joseph, R. R. Rao, S. R. Kalsi, S. K. Behera, D. V. Bhaskar Rao and V. S. N. Murty for discussions; the Captain, crew, and engineers on ORV Sagar Kanya for help with observations. We acknowledge the National Institute of Ocean Technology, Chennai for buoy data, and NASA/Remote Sensing Systems for TMI data. This work is supported by the Department of Ocean Development, New Delhi. References Boyer TP, Stephens C, Antonov JI, Conkright ME, Locarnini RA, O Brien TD, Garcia HE. 22. World Ocean Atlas 21, Vol 2: Salinity, NOAA Atlas NESDIS. US Government Printing Office: Washington, DC; 176 pp. De US, Dube RK, Prakasa-Rao GS. 25. Extreme weather events over India in the last 1 years. Journal of the Indian Geophysical Union 9: de Boyer Montegut C, Mignot J, Lazar A, Cravatte S. 27. Control of salinity on the mixed layer depth in the world ocean:1. General description. Journal of Geophysical Research 112: C611, Doi:1.129/26JC3953. Emanuel KA Thermodynamic control of hurricane intensity. Nature 41: Emanuel KA. 23. Tropical cyclones. Annual Review of Earth and Planetary Sciences 31: Emanuel KA. 25. Increasing destructiveness of tropical cyclones over the past 3 years. Nature 436: Holland GJ The maximum potential intensity of tropical cyclones. Journal of the Atmospheric Sciences 54(21): Lin II, Liu WT, Wu CC, Wong GTF, Hu C, Chen Z, Liang W-D, Yang Y, Liu K-K. 23. New evidence for enhanced ocean primary production triggered by tropical cyclone. Geophysical Research Letters 3(13): DOI:1.129/23GL17141; Lin II, Wu CC, Emanuel KA, Lee IH, Wu CR, Pun IF. 25. The interaction of Supertyphoon Maemi (23) with a warm ocean eddy. Monthly Weather Review 133: Murty VSN The lowering of sea-surface temperature in the east central Arabian Sea associated with a cyclone. Mahasagar: Bulletin of the National Institute of Oceanography 16: Pankajakshan T, Gopalakrishna VV, Muraleedharan PM, Reddy GV, Araligidad N, Shenoy S. 22. Surface layer temperature inversion in the Bay of Bengal. Deep-Sea Research I 49(1): Pankajakshan T, Muraleedharan PM, Rao RR, Somayajulu YK, Reddy GV, Revichandran C. 27. Observed seasonal variability of barrier layer in the Bay of Bengal. Journal of Geophysical Research 112: C29, Doi:1.129/26JC3651. Price JF Upper Ocean response to a Hurricane. Journal of Physical Oceanography 11: Rao RR Further analysis on the thermal response of the upper Bay of Bengal to the forcing of pre-monsoon cyclonic storm and summer monsoonal onset during MONEX-79. Mausam 38: Schott FA, McCreary JP. 21. The monsoon circulation of the Indian Ocean. Progress in Oceanography 51: Sengupta D, Ravichandran M. 21. Oscillations of Bay of Bengal sea surface temperature during the 1998 summer monsoon. Geophysical Research Letters 28(1): Sengupta D, Goswami BN, Senan R. 21. Coherent intraseasonal oscillations of Ocean and atmosphere during the Asian summer monsoon. Geophysical Research Letters 28(21): , DOI:1.129/22GL1534. Sengupta D, Ray PK, Bhat GS. 22. Spring warming of the eastern Arabian Sea and Bay of Bengal from Buoy data. Geophysical Research Letters 29(15): 1734, DOI:1.129/22GL1534. Sengupta D, Bharathraj GN, Shenoi SSC. 26. Surface freshwater from Bay of Bengal runoff and Indonesian throughflow in the tropical Indian Ocean. Geophysical Research Letters 33: L2269, DOI:1.129/26GL Shay LK, Goni GJ, Black PG. 2. Effects of a warm oceanic feature on hurricane opal. Monthly Weather Review 128: Shenoi SSC, Shankar D, Shetye SR. 22. Differences in heat budgets of the near surface Arabian Sea and Bay of Bengal: implications for the summer monsoon. Journal of Geophysical Research 17(C6): 352, DOI:1.129/2JC679. Shetye SR, Gouveia AD, Shankar D, Shenoi SSC, Vinayachandran PN, Sundar D, Michael GS, Nampoothiri G Hydrography and circulation in the western Bay of Bengal during the northeast monsoon. Journal of Geophysical Research 11: 14,11 14,25. Subrahmanyam B, Murty VSN, Sharp RJ, O Brien JJ. 25. Air-sea coupling during the Tropical Cyclones in the Indian Ocean: a case study using satellite observations. Pure and Applied Geophysics 162: Webster PJ, Holland GJ, Curry JA, Chang HR. 25. 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First results from a new observational system over the Indian seas

PERSPECTIVES ON OCEAN RESEARCH IN INDIA First results from a new observational system over the Indian seas K. Premkumar, M. Ravichandran, S. R. Kalsi*, Debasis Sengupta** and Sulochana Gadgil**, National