Short-term upper-ocean variability in the central equatorial Indian Ocean during 2006 Indian Ocean Dipole event

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L14S09, doi: /2008gl033834, 2008 Short-term upper-ocean variability in the central equatorial Indian Ocean during 2006 Indian Ocean Dipole event Yukio Masumoto, 1,2 Takanori Horii, 1 Iwao Ueki, 1 Hideaki Hase, 1 Kentaro Ando, 1 and Keisuke Mizuno 1 Received 3 March 2008; revised 10 April 2008; accepted 22 April 2008; published 29 May [1] Intensive observations, using an array of surface and subsurface moored buoys, are conducted around 80.5 E in the equatorial Indian Ocean during October/November An intriguing data set of atmospheric and oceanic variables during a peak phase of a positive Indian Ocean Dipole is obtained. The ocean observation data shows relatively shallow thermocline, which intensifies with time during the one-month period, and eastward subsurface zonal flow under westward flowing surface current, generating unusually strong vertical shear above the thermocline. Intraseasonal meridional current variability is also observed. Upper-ocean volume budget analysis indicates that a strong upwelling event, larger than 10 m/day, and associated upward movement of the isotherms below the thermocline occur for a few days in early November. These observed data demonstrates unusual conditions of the upper ocean during boreal autumn in Citation: Masumoto, Y., T. Horii, I. Ueki, H. Hase, K. Ando, and K. Mizuno (2008), Short-term upper-ocean variability in the central equatorial Indian Ocean during 2006 Indian Ocean Dipole event, Geophys. Res. Lett., 35, L14S09, doi: /2008gl Introduction [2] The tropical Indian Ocean is characterized as a region of large amplitude seasonal cycle associated with Asian and Australian monsoons and of significant interannual variations generated internally and externally (for detailed descriptions, see a review article of Schott and McCreary [2001], and references therein). In addition, intraseasonal variability in the atmosphere over the tropical Indian Ocean, such as Madden-Julian Oscillation (MJO) [Madden and Julian, 1972] and biweekly wind surges [Fukutomi and Yasunari, 2005], is thought to have major impacts on the upper-ocean variability not only at the same time-scale but also at different time-scales [e.g., Rao and Yamagata, 2004]. However, our understanding of detailed responses of the upper-ocean to such atmospheric short-term variations is rather limited, especially in the Indian Ocean, due mainly to a lack of in situ observations with sufficient spatial and temporal resolutions. [3] In order to observe atmospheric conditions and variability associated with the intraseasonal disturbances and 1 Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan. 2 Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan. Copyright 2008 by the American Geophysical Union /08/2008GL resulting ocean responses in the central equatorial Indian Ocean, MISMO (MIRAI Indian Ocean cruise for the Study of the MJO-convection Onset) observation campaign was conducted near 80 E in the equatorial Indian Ocean during October-to-December season in 2006 [Yoneyama et al., 2006]. With multi-platform intensive observations in both the atmosphere and ocean, MISMO provides us invaluable data set for studies on air-sea interactions and associated variability in each component in this particular region. It has been reported that a positive Indian Ocean Dipole (IOD) phenomenon, with negative (positive) sea surface temperature (SST) anomaly in the southeastern (western) tropical Indian Ocean, was taking place during the MISMO observation period [e.g., Vinayachandran et al., 2007; Horii et al., 2008]. Although the atmospheric convective activities associated with the intraseasonal variability during the MISMO period is relatively weak due to the cool SST in the eastern tropical Indian Ocean, unusual conditions under a peak phase of the interannual climate mode are observed for the first time. Some preliminary results on the atmospheric sounding observations are reported by Yoneyama et al. [2008]. This paper describes the upperocean variability during the MISMO observations and explores strength of the upwelling using a current profiler array. 2. MISMO Ocean Observations [4] Ocean observations of MISMO consist of four upward-looking subsurface acoustic Doppler current profilers (ADCPs) and two mini-triangle Trans Ocean buoy Network (m-triton) buoys, making a diamond shape intensive observation region centered at 80.5 E on the equator (Figure 1). ADCPs measure vertical profiles of horizontal currents from the surface down to about 400 m depth, while the m-triton buoys observe vertical profiles of temperature and salinity in the upper-layer down to the depth of 500 m and 100 m, respectively, as well as surface meteorological variables for the calculation of the surface heat and freshwater fluxes. All the above variables are observed every 10 minutes during an intensive observation period from October 27 to November 22, 2006, except for one ADCP at 1.5 N, 80.5 E, whose data were not obtained due to technical problems. In addition to this moored buoy array, R/V Mirai stayed at the center of the diamond during the one month period and made frequent conductivitytemperature-depth measurements with chlorophyll-a concentration observations at three-hour interval. Due to the data quality limitations for the ADCP observations, we focus L14S09 1of5

2 Figure 1. (a) Horizontal distribution of sea surface temperature (SST) anomaly and surface wind vector anomalies averaged over a period from October 27 to November 22, NOAA Optimum Interpolation SST anomaly with reference period of and QuikScat wind anomaly relative to mean are utilized. Location of the MISMO observation array is indicated by a yellow diamond. (b) Location map of R/V Mirai and the mooring buoys for the MISMO ocean observation. our analyses on the variability to the depth range between 40 m and 300 m depths. 3. Temperature and Salinity Variations [5] Figure 1a shows horizontal distributions of SST and surface wind anomalies during the core period of the MISMO observations. There is a region of cold SST anomaly, with a minimum value less than 2 C, in the southeastern tropical Indian Ocean off the coast of Sumatra and Java Islands. Most of the western and central tropical Indian Ocean is occupied by the positive SST anomalies with a maximum value of about 1.5 C, indicating a typical SST anomaly pattern associated with IOD event [Saji et al., 1999; Vinayachandran et al., 2007]. Surface signatures of IOD in 2006 started in August and continued until December [Horii et al., 2008], and the MISMO observations were conduced near a marginal region of the positive SST anomaly with significant easterly wind anomalies during the mature phase and the beginning of the decay phase of the IOD event. [6] Since the basic features observed during the MISMO period are almost the same among the three ADCPs and also between the two m-triton buoys, unless otherwise stated, we show only the results obtained by western moorings located at 79 E on the equator. The thermocline is located almost the same depth of 110 m throughout the observation period (Figure 2b), with the relatively diffused profile at the beginning until Nov. 6, While the surface layer warms up to higher than 29 C after Nov. 6, the deeper layer tends to become cooler, an indication of tightening of the thermocline. The upward movement of the 13 C isotherm at about 230 m depth associated with this cooling trend is about 15 m per day, suggesting strong upwelling during this particular period. It is interesting to note that wave-like vertical displacements of the isotherms can be seen near the thermocline depth. [7] Upper-layer salinity variability is shown in Figure 3, together with the precipitation observed by m-triton buoy. Here we show the data from the buoy at 82 E on the equator, since the precipitation data from the buoy at 79 E is noticed to contain some errors. As in the climatological salinity field, surface salinity is lower than that in the thermocline depth. However, the vertical salinity gradient observed during the MISMO period, 2.0 psu/100 m, is about three times larger than the climatological condition, 0.6 psu/75 m, obtained from World Ocean Atlas 2001 [Boyer et al., 2002]. This strong vertical gradient is due mainly to the low salinity water (33.5 psu) at the surface, compared to the surface salinity of 34.9 psu in the climatology. The surface salinity variations are in good agreement with the local precipitation variability, suggesting the importance of the local flesh water supply to the upper-layer salinity budget. However, the horizontal advection of the low salinity water originated from the Bay of Bengal cannot be neglected because of the anomalous southwestward surface current appears during the observed period (see Figure 2). 4. Zonal and Meridional Currents Variations [8] In normal, non-iod year, westerly surface winds appear along the equator near 80 E during October/November season, and associate eastward currents appear in the surface layer above the thermocline. Contrary to the typical conditions, relatively strong easterly surface wind prevailed until Nov. 7, with oscillating disturbances at a period of one week to 10 days, and the zonal wind weakened thereafter (Figure 2a). The zonal current at the depth of 10 m responded to this local zonal wind, showing strong westward flow trapped in the surface thin layer down to at most about 50 m depth, with the maximum value of 70 cm/s at the beginning on Oct. 25 and gradually decreases to almost 0 cm/s at the end of the observation. Horii et al. [2008] reported that the westward surface flow, which is in opposite direction to the climatological condition, appeared at the same time further east at 90 E. [9] Below the surface wind-driven layer is a layer of eastward current between 50 m and 200 m depths, with the maximum current appeared at the depth of about 80 m. This 2of5

3 Figure 2. Time series of (a) QuikScat zonal (red line) and meridional (blue line) winds, (b) the vertical temperature profiles observed by m-triton on the equator at 79 E, (c) 10 m depth zonal (red line) and meridional (blue line) currents observed by m-triton on the equator at 79 E, and the time-depth sections of (d) the zonal and (e) the meridional currents observed by ADCP on the equator at 79 E. 3of5

4 Figure 3. (a) Time series of the daily mean precipitation in mm day 1 and (b) the time-depth section of salinity in the upper 100 m depth obtained from m-triton buoy at 82 E on the equator. subsurface eastward current is located at the depth of the high salinity water above the thermocline(figure 3), suggesting the zonal advection along the equator of the saline water originated from the Arabian Sea [Hase et al., 2008]. The results demonstrate that there is a strong vertical shear of the zonal current within the upper-layer shallower than the thermocline, which is located at the depth of 110 m throughout the MISMO observation (see Figure 2b). The similar zonal current structure in the upper ocean is also observed by the shipboard ADCP measurements by R/V Mirai (N. Sato et al., Surface and subsurface currents observed in the equatorial Indian Ocean in a positive IOD year, submitted to Geophysical Research Letters, 2008). [10] Large amplitude short-term variability in the meridional current, with clear upward phase propagation, is observed between 40 m and 150 m depth (Figure 2e); the maximum northward (southward) flow occurs around Nov. 6 (Oct. 27 and Nov. 20) near 100 m depth. Such intraseasonal variability in the meridional current with a typical period of two weeks is reported by Masumoto et al. [2005] in the eastern equatorial Indian Ocean. The meridional current observed by m-triton buoy at 10 m depth also demonstrates variability consistent with the deeper layer (Figure 2c). The local meridional winds, however, do not show the corresponding variability, suggesting a possibility of the equatorial mixed Rossby-gravity wave propagation, as shown by Sengupta et al. [2004] and Ogata et al. [2008]. 5. Volume Budget Analysis for Equatorial Upwelling [11] The ADCP array during the MISMO observation allows us, using the continuity equation, to estimate vertical currents averaged over a triangle area, connecting the three observation locations (see Figure 1). Figure 4 shows time series of transports through each vertical surface of the triangular prism integrated between the depths of 40 m and 270 m, together with the sum of three transport components. Large compensation of the transport across the southwestern section with that across the southeastern section is associated with the strong eastward subsurface currents, and the meridional transport change of 12 Sv during the early part of November is also contributing to the volume budget. Figure 4. Time series of transports across three vertical sections of the triangle mooring buoy array. Sum of the three components are indicated by thick grey line, indicating the horizontal convergence of the flow. 4of5

5 [12] Assuming the averaged vertical current across the top surface of the prism at 40 m depth is zero, strong upwelling of 12.5 m/day is estimated at 270 m depth around Nov. 4, Since the upward current is expected at the depth of 40 m due to local easterly wind forcing, the above value may give a lower estimate for the upwelling event. This magnitude of the upwelling is consistent with the upward movement of the 13 C isotherm shown in Figure 2b, and is several times stronger than the strength of the typical equatorial upwelling in the equatorial Pacific Ocean [e.g., Halpern et al., 1989; Helber and Weisberg, 2001; Meinen et al., 2001]. Sudden increase of subsurface chlorophyll-a concentration around Nov. 3, 2006 (not shown) also supports the existence of the strong upwelling event during the MISMO observation. 6. Summary [13] Preliminary results from MISMO ocean observations reveal the unique conditions in the central equatorial Indian Ocean during the height of the Indian Ocean Dipole event in New findings are tightening of the thermocline with warming of the surface layer and cooling of the layer below the thermocline; large vertical salinity gradient, with strong association with the precipitation at the surface; large vertical shear of the zonal currents above the thermocline; strong short-term variability in the meridional current in the upper 150 m depth; and strong upwelling event of >10 m/day during early November. [14] Mechanisms responsible for the above unusual conditions and variability in the upper-layer during the IOD event require further analyses using the observed data together with the high-resolution numerical model simulations. In addition, the present results suggest the importance of the equatorial waves not only on the horizontal current fields but also on the vertical component near the equator. For better understanding of the equatorial wave dynamics, an array of the velocity observations as well as the temperature and salinity measurements are highly desirable. [15] Acknowledgments. This study is supported by the Japan EOS (Earth Observation System) Promotion Program sponsored by Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. References Boyer, T. P., C. Stephens, J. I. Antonov, M. E. Conkright, R. A. Locarnini, T. D. O Brien, and H. E. Garcia (2002), World Ocean Atlas 2001, vol. 2, Salinity, NOAA Atlas NESDIS 50, 165 pp., NOAA, Silver Spring, Md. Fukutomi, Y., and T. Yasunari (2005), Southerly surges on the submonthly timescales over the eastern Indian Ocean during the Southern Hemisphere winter, Mon. Weather Rev., 133, Halpern, D., R. A. Knox, D. S. Luther, and S. G. H. Philander (1989), Estimates of equatorial upwelling between 140 and 110 W during 1984, J. Geophys. Res., 94, Hase, H., Y. Masumoto, Y. Kuroda, and K. Mizuno (2008), Semiannual variability in temperature and salinity observed by Triangle Trans-Ocean Buoy Network (TRITON) buoys in the eastern tropical Indian Ocean, J. Geophys. Res., 113, C01016, doi: /2006jc Helber, R. W., and R. H. Weisberg (2001), Equatorial upwelling in the western Pacific warm pool, J. Geophys. Res., 106, Horii, T., H. Hase, I. Ueki, and Y. Masumoto (2008), Oceanic precondition and evolution of the 2006 Indian Ocean dipole, Geophys. Res. Lett., 35, L03607, doi: /2007gl Madden, R. A., and P. R. Julian (1972), Description of global-scale circulation cells in the tropics with a day period, J. Atmos. Sci., 29, Masumoto, Y., H. Hase, Y. Kuroda, H. Matsuura, and K. Takeuchi (2005), Intraseasonal variability in the upper layer currents observed in the eastern equatorial Indian Ocean, Geophys. Res. Lett., 32, L02607, doi: /2004gl Meinen, C. S., M. J. McPhaden, and G. C. Johnson (2001), Vertical velocities and transports in the equatorial Pacific during , J. Phys. Oceanogr., 31, Ogata, T., H. Sasaki, V. S. N. Murty, M. Sarma, and Y. Masumoto (2008), Intraseasonal meridional current variability in the eastern equatorial Indian Ocean, J. Geophys. Res., doi: /2007jc004331, in press. Rao, S. A., and T. Yamagata (2004), Abrupt termination of Indian Ocean dipole events in response to intraseasonal disturbances, Geophys. Res. Lett., 31, L19306, doi: /2004gl Saji,N.H.,B.N.Goswami,P.N.Vinayachandran,andT.Yamagata (1999), A dipole mode in the tropical Indian Ocean, Nature, 401, Schott, F. A., and J. P. McCreary (2001), The monsoon circulation of the Indian Ocean, Prog. Oceanogr., 51, Sengupta, D., R. Senan, V. S. N. Murty, and V. Fernando (2004), A biweekly mode in the equatorial Indian Ocean, J. Geophys. Res., 109, C10003, doi: /2004jc Vinayachandran, P. N., J. Kurian, and C. P. Neema (2007), Indian Ocean response to anomalous conditions in 2006, Geophys. Res. Lett., 34, L15602, doi: /2007gl Yoneyama, K., Y. Masumoto, Y. Kuroda, M. Katsumata, and K. Mizuno (2006), MISMO: Mirai Indian Ocean cruise for the study of the MJOconvection onset, CLIVAR Exchanges, 39, Yoneyama, K., M. Fujita, N. Sato, M. Fujiwara, Y. Inai, and F. Hasebe (2008), Correction for radiation dry bias found in RS92 radiosonde data during the MISMO field experiment, Sci. Online Lett. Atmos., 4, 13 16, doi: /sola K. Ando, H. Hase, T. Horii, Y. Masumoto, K. Mizuno, and I. Ueki, Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa, , Japan. (masumoto@jamstec.go.jp) 5of5