Unusual Premonsoon Eddy and Kelvin Wave Activities in the Bay of Bengal During Indian Summer Monsoon Deficit in June 2009 and 2012
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This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg) Nanyang Technological University, Singapore. Unusual premonsoon eddy and Kelvin wave activities in the Bay of Bengal during Indian summer monsoon deficit in June 2009 and 2012 Dey, Subhra Prakash; Dash, Mihir K.; Deb, Pranab; Samanta, Dhrubajyoti; Sharma, Rashmi; Gairola, Rakesh Mohan; Kumar, Raj; Pandey, Prem Chand 2018 Dey, S. P., Dash, M. K., Deb, P., Samanta, D., Sharma, R., Gairola, R. M., ⋯ Pandey, P. C. (2018). Unusual premonsoon eddy and Kelvin wave activities in the Bay of Bengal during Indian summer monsoon deficit in June 2009 and 2012. IEEE Geoscience and Remote Sensing Letters, 15(4), 483‑487. doi:10.1109/LGRS.2018.2802543 https://hdl.handle.net/10356/106053 https://doi.org/10.1109/LGRS.2018.2802543 © 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. The published version is available at: https://doi.org/10.1109/LGRS.2018.2802543 Downloaded on 29 Sep 2021 06:11:48 SGT Unusual Premonsoon Eddy and Kelvin Wave Activities in the Bay of Bengal During Indian Summer Monsoon Deficit in June 2009 and 2012 Subhra Prakash Dey, Mihir K. Dash , Pranab Deb, Dhrubajyoti Samanta, Rashmi Sharma, Rakesh Mohan Gairola, Raj Kumar, and Prem Chand Pandey Abstract— An investigation of the eddy and coastal Kelvin in ISM season (June–September) are positively correlated wave activities in the Bay of Bengal (BoB) is carried out during with the premonsoon sea surface temperature (SST) over the premonsoon season in two years of Indian summer monsoon BoB [2]. However, the BoB in general and the northern deficit in June (2009 and 2012), occurred in the recent warming hiatus period. Using altimeter observations, our study reveals that bay, in particular, plays a significant role in the monsoon over the northern BoB cyclonic eddy kinetic energy is reduced by rainfall through the formation of the low-pressure systems [3]. 35% and 50% from the climatology during premonsoon seasons Despite the significant control of upper ocean behavior on the in 2009 and 2012, respectively, while the cyclonic eddy area is ISM rainfall, most monsoon prediction models have a poor reduced by 18% and 24%, respectively. A concurrent reduction is representation of upper ocean mixing [3]. Therefore, the study observed in the first upwelling Kelvin wave (uKW) activities in the eastern equatorial Indian Ocean as well as in the coastal of mesoscale eddies, which control the upper ocean mixing, BoB for these years. The reduction in the generation of the over the BoB is necessary. first uKW in the eastern equatorial Indian Ocean is attributed Mesoscale eddies in the ocean are the spatial structures to the westerly wind anomalies in January–March of these similar to coherent vortices of 2-D turbulence [4] and are years. Additionally, meridional wind stress anomalies during responsible for the vertical mixing, upper surface thermal March–April in these years are found to be southerly, causing anomalous coastal downwelling in the eastern rim of BoB. This structure, and transport of heat between different parts of the coastal downwelling blocks the propagation of the first uKW. The ocean. Also, they can locally affect the near-surface wind, decrease in the first uKW activities in the coastal waveguide of cloud properties, and rainfall, and thereby the larger low- the BoB reduces the radiation of upwelling Rossby waves, thereby pressure systems [5]. The coastal Kelvin wave in the eastern decreasing the cyclonic eddy activities in the northern BoB. The rim of BoB radiates its energy toward the interior bay through results from this letter could be helpful for further understanding of upper ocean mixing processes in the BoB during monsoon eddy shedding [6]. Further, using nine years of altimetry data, deficit years. Cheng et al. [7] found that eddy train originated from the coastal waveguide of BoB contributes to the intraseasonal vari- Index Terms— Altimetry, Bay of Bengal (BOB), eddy activity, Indian summer monsoon (ISM), Kelvin wave. ability of sea level and thermal properties in the central bay. During the recent warming hiatus period (1999–2013) [8], I. INTRODUCTION the upper ocean heat content in the Indian Ocean showed an increase [9]. The increase in heat content is attributed to the HE Bay of Bengal (BoB) is a semienclosed bay located strengthening of Walker circulation and a subsequent increase in the northeastern part of the Indian Ocean. Upper ocean T in heat advection toward the Indian Ocean by Indonesian processes in the BoB play a vital role in the onset and pro- throughflow [9]. An increase in rainfall is noted during the hia- gression of South Asian and Indian summer monsoon (ISM). tus period compared to prehiatus time over the western Pacific Sea surface warming in the central BoB during premonsoon and the western Indian Ocean, while a decrease is evident over season (March–May) triggers the onset of South Asian mon- the east Asia [8]. This hiatus period has observed two severe soon [1]. The frequency and lifetime of low-pressure areas June ISM deficit years, in 2009 and 2012, with more than Manuscript received November 18, 2017; revised January 3, 2018; accepted 20% rainfall deficiency (see Table S1 in the Supplementary January 28, 2018. Date of publication February 22, 2018; date of current Material). version March 23, 2018. This work was supported in part by the Space Applications Centre, ISRO, India. (Corresponding author: Mihir K. Dash.) This letter investigates the premonsoon eddy activity in tan- S. P. Dey, M. K. Dash, and P. C. Pandey are with the Centre for Oceans, dem with Kelvin wave activities using altimetric observations Rivers, Atmosphere and Land Sciences, IIT Kharagpur, Kharagpur 721302, over the BoB in two June ISM deficit years (2009 and 2012). India (e-mail: [email protected]). P. Deb is with the Climatic Research Unit, University of East Anglia, The possible physical mechanisms behind such anomalous Norwich NR4 7TJ, U.K. eddy and Kelvin wave activities are also explored by analyzing D. Samanta is with the Asian School of the Environment, Nanyang the local winds over the BoB and the remote winds over the Technological University, Singapore 639798. ( , ) R. Sharma, R. M. Gairola, and R. Kumar are with the Atmospheric and equatorial Indian Ocean (EIO; 5°S–5°N 40°E–100°E ). Oceanic Science Group, Space Applications Centre, Ahmedabad 380015, India. II. DATA AND METHODOLOGY Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. We used merged and gridded daily sea level anomaly (SLA) and the geostrophic current data, from 1994 to 2015, available from the archiving, validation, and interpretation of satellite oceanographic data (AVISO, htttp://www.aviso.oceanobs.com). These products are constructed by merging multisatellite (TOPEX/Poseidon, European Remote Sensing Satellite 1/2, Jason 1, Jason 2, Envisat, etc.) data sets. These data sets are available on a Cartesian grid of 0.25° × 0.25° spatial resolution derived from a Mercator grid through linear interpolation. Also, we use daily surface wind stress and wind field, from 1994 to 2015, available from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis product. For eddy detection, we used the Okubo–Weiss method [10] that showed potentiality to detect eddies over the Mediter- ranean Sea, the Tasman Sea, and the Gulf of Alaska [11]. After the identification of cyclonic and anticyclonic eddies, their effective area is calculated separately by integrating the 2 −2 area of simply connected grid boxes inside a vortex. The area Fig. 1. Black lines represent total cyclonic EKE (in m s ) in the northern BoB (14°N−23°N; 78°E–99°E) for (a) 2009 and (b) 2012, and cyclonic eddy of the vortex is given as area (in the unit of 104 km2) over the same region for (c) 2009 and (d) 2012. The green curves represent the climatology considering the period 1994–2015. N gp The magenta lines show 95% confidence levels from the average. A = An (1) n=1 where An is the area of nth grid box, calculated considering the cyclonic EKE and eddy area during mid-April to the first earth to be spherical, and Ngp represents the number of simply week of June [Fig. 1(b) and (d)]. On average, 50% reduction connected grid boxes inside a vortex. Total surface eddy of cyclonic EKE and 24% reduction of cyclonic eddy area kinetic energy of a vortex is calculated using the following are noticed during 15 April–15 May. On the other hand, : equation the analysis of anticyclonic eddy activity (see Fig. S1 in the N gp Supplementary Material) does not show any systematic signal. = 1 2 + v2 E ui i (2) Naturally, the question arises, what causes the cyclonic eddy 2 = i 1 activities to reduce? Eddies in this region may occur due where ui and vi are the zonal and meridional components of to local wind forcing, internal turbulence, and the remote geostrophic velocity, respectively. forcing from the equator through coastal Kelvin waves and subsequently radiated Rossby waves. Due to high Reynold’s III. RESULTS AND DISCUSSIONS number (order of 108 [12]), any current in the ocean can In this section, we first examine eddy activity over the produce some turbulent eddies [13]. Since currents in the northern BoB in the premonsoon period of 2009 and 2012. bay are weak, the vortices generated due to the turbulence Then, activities of equatorial Kelvin waves and coastal Kelvin of the system are mostly negligible.