An ITCZ-like convergence zone over the Indian Ocean in boreal late autumn

Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L10811, doi:10.1029/2006gl028341, 2007 An ITCZ-like convergence zone over the Indian Ocean in boreal late autumn N. Sato, 1 K. Yoneyama, 1 M. Katsumata, 1 R. Shirooka, 1 and Y. N. Takayabu 2,3 Received 2 October 2006; revised 2 April 2007; accepted 19 April 2007; published 26 May 2007. [1] We examined the convective activity over the tropical Indian Ocean in boreal autumn using satellite-observation data. In November and December, we detected an intertropical convergence zone-like (ITCZ-like) precipitation zone between Maldives and Somalia that is unobserved in the other periods of the year. It accompanied a high sea-surface temperature (SST) north of the equator, and a cold tongue near the equator, similar to the ITCZs over the Pacific and Atlantic Oceans. However, the formation mechanism of the SST anomalies over the Indian Ocean is not the same as that in the Pacific and Atlantic Oceans. The high-sst region forms in early November corresponding to weak sea-surface wind during the monsoon transition. The low SST along the equator appears, associated with the upwelling of the subsurface water. Citation: Sato, N., K. Yoneyama, M. Katsumata, R. Shirooka, and Y. N. Takayabu (2007), An ITCZ-like convergence zone over the Indian Ocean in boreal late autumn, Geophys. Res. Lett., 34, L10811, doi:10.1029/2006gl028341. 1. Introduction [2] The ITCZ is a convection zone over the tropical ocean. Many previous studies have been conducted on the ITCZ over the Pacific and Atlantic Oceans [e.g., Waliser and Gautier, 1993]. According to these studies, the ITCZ is more active on the northern side of the equator over the Pacific and Atlantic Oceans. This asymmetricity was examined by Mitchell and Wallace [1992] in terms of the annual cycle. Previous studies do not recognize the northward shift of the oceanic convergence zone near the equator over the Indian Ocean. The convergence zone is clearer south of the equator almost throughout the year. However, Zhang [2001] found a convective zone on the northern side in November only, and regarded this as a part of the double ITCZs. Zhang [2001] investigated the formation mechanism of the double ITCZs over the Pacific Ocean and revealed that the local SST minimum at the equator is necessary for the double ITCZ. However, the appearance of the ITCZ north of the equator in the Indian Ocean was not well examined. [3] Xie and Philander [1994] suggested that the ITCZ shifts northward due to the wind-evaporation-sst (WES) 1 Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan. 2 Center for Climate System Research, University of Tokyo, Kashiwa, Japan. 3 Also at Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan. Copyright 2007 by the American Geophysical Union. 0094-8276/07/2006GL028341$05.00 feedback. They believe the asymmetricity of the ITCZ is related to the warmer SST on the northern side of the equator, and the cooler one on the southern side. Once the warmer SST reaches the northern side, the northward surface-pressure gradient is created. The easterly wind is decelerated (accelerated) through the balance of the pressure-gradient force and the Coriolis force, resulting in warming (cooling) of the sea surface through suppressed (enhanced) evaporation in the Northern (Southern) Hemisphere. The anomalous latent heat flux associated with the wind-speed difference maintains and strengthens the SST anomalies across the equator. [4] According to Murakami and Matsumoto [1994], a westerly wind prevails in the lower troposphere over the North Indian Ocean as part of the Southeast Asian summer monsoon (SEAM) during boreal summer, which retreats in October. Corresponding to the withdrawal of the summer monsoon, the low-level westerly wind weakens in autumn over the North Indian Ocean. The lower wind speed in boreal autumn may cause relatively high SST over the North Indian Ocean. [5] In general, more precipitation is observed south of the equator than north over the tropical Indian Ocean. However, if the relatively warm SST is maintained north of the equator in boreal autumn, it may cause an ITCZ-like convergence zone on the northern side. The northern part of the double ITCZs was not examined sufficiently over the tropical Indian Ocean. In the present study, we first verified that an ITCZ-like convergence zone appears over the North Indian Ocean in boreal autumn. We then examined the airsea interaction contributing to the convergence zone by analyzing satellite-observation data. 2. Data [6] We first investigated the horizontal distribution of convective activity over the Indian Ocean using Special Sensor Microwave/Imager (SSM/I) rain rate data. The precipitations averaged from 2000 to 2005 for November 1 to 15, 16 to 30, and December 1 to 15 were examined. We focused on the convergence zone appearing in late November. We also confirmed the existence of the precipitation zone using the Tropical Rainfall Measuring Mission (TRMM) near-surface rain data. We used the PR Heating (PRH) grid dat [Kodama et al., 2005]. [7] Corresponding to the convergence zone, we then examined the TRMM Microwave Imager (TMI) sea surface temperature (SST) data for the same periods. [8] We also analyzed the latitude-time section of rain rate and atmospheric water vapor from the SSM/I data and TMI SST at 55 to 70 E to examine their annual cycles. When we investigated the latitude-time sections, we used 11-day L10811-157- 1of6

Figure 1. Rain rate averaged for (top) November 1 to 15, (middle) November 16 to 30, and (bottom) December 1 to 15, from 2000 to 2005. The contour interval is 0.1 mm/hr. running means to remove short-range variations. Similarly, the QuikSCAT sea-surface wind data were also examined. Here, we analyzed the latitude-time sections of scalar wind speed and zonal wind. [9] Furthermore, we analyzed the time-zonal section of TMI SST at 2.5 S to2.5 N to examine the SST variation along the equator. Here, we also used 11-day running means. The 20 100 day filtered values were indicated together, in order to show the intraseasonal-scale variability. Figure 2. SST averaged for November 16 to 30, from 2000 to 2005. The contour interval is 0.2 C. 2of6-158-

Figure 3. Time-meridional sections of (top) rain rate and (bottom) SST at 55 to 75 E averaged for 2000 to 2005. The contour intervals are 0.1 mm/hr for rain rate, and 0.2 C for SST. [10] In recent years, the vertical profiles for sea-water temperature and salinity are automatically obtained over all the oceans in 10-day intervals using the Argo profiling floats [Argo Science Team, 2001]. In the present study, realtime quality-controlled data from Argo floats deployed in the Indian Ocean were examined. We compared the vertical profiles of temperature and salinity for October and November from 2003 to 2005, averaged over 2.5 S to 2.5 N, 55 to 70 E. The T-S diagrams were also examined. 3. Results [11] Figure 1 plots the rain rate for November 1 to 15 (top), 16 to 30 (middle), and December 1 to 15 (bottom). In Figure 1 (middle and bottom), we see a precipitation zone around 5 N between 50 and 80 E. The maximum value reaches 200 to 300 mm per month. Another precipitation belt is detected on the southern side of the equator. Both correspond to the double ITCZs over the Indian Ocean found by Zhang [2001]. The double ITCZs can be identified in most of the years (five out of six), even if we analyze data for each year. The northern part of them is detected in all of the six years. The precipitation zone is identified also in the TRMM near-surface rainfall data (not displayed). The precipitation zone on the northern side is not identified in the other seasons of the year, although that on the southern side is detected in the annual mean field (not displayed). [12] Figure 2 depicts the TMI SST for November 16 to 30. The SST is higher than 29 C over the subtropical western North Indian Ocean. The high SST is observed also in early November (not shown). It precedes the convection zone in Figure 1 (middle and bottom) by a half month. However, a low-sst region appears near the equator in November, where the cold tongue extends eastward along the equator from Africa. [13] Figure 3 indicates the time-meridional sections of rain rate measured by SSM/I (top), and TMI SST (bottom), averaged for 55 to 70 E. We confirmed the existence of the precipitation zone from late November to early December in Figure 3 (top). Corresponding to the precipitation zone, maxima of water vapor and SST are identified. The water vapor has already increased in middle November (not shown). The water-vapor maximum precedes the appearance of the precipitation zone. In addition, the water-vapor maximum follows the peak of the warm SST in early to middle November (Figure 3, bottom). These results are consistent with the hypothesis that the high SST causes the increase of water vapor, resulting in the precipitation peak. However, a minimum of SST is observed in 5 S to 2.5 N in November. [14] In addition, Figure 4 illustrates the time-meridional sections of the zonal component of the QuikSCAT seasurface wind (top) and the scalar mean of the sea-surface wind speed (bottom), averaged for 55 to 70 E. Consistent with the results demonstrated by Matsumoto [1992], the sign of zonal wind changes over the North Indian Ocean during October and November, corresponding to the monsoon transition. Associated with the changes in the zonal wind, the scalar wind speed is low over the western North Indian Ocean in late October and early November (Figure 4). The SST increases during this period in Figure 3 (bottom). [15] From Figure 5 (left), we understand that the low SST at the equator extends from west to east, and the SST decreases between 55 to 90 E in November. Moreover, the eastward propagation of the intraseasonal-scale signal is clear in November in the right panel. Although another low SST is observed after late December as a part of the long-range seasonal march (Figure 3), the low SST in 3of6-159-

Figure 4. Time-meridional sections of (top) sea-surface wind speed and (bottom) zonal wind at 55 to 70 E averaged for 2000 to 2005. The contour interval is 1 m/s. Negative contours are dotted. November seems to be separated from that after late December. [16] Figure 6 illustrates vertical profiles of the temperatures and salinities obtained by Argo floats in October (thick line) and November (thin line), averaged over 55 to 70 E near the equator. The number of samples is 96 (101) in October (November). By comparing both temperature profiles, we can infer that the water in the subsurface layer is lifted upward. The changes in salinity are consistent with those in temperature. This upward shift of the profile corresponds to the cooling of near-surface water. Vertical mixing and adiabatic cooling do not largely contribute to the low SST, since the mixed layer becomes shallow. Figure 7 depicts the T-S diagrams for 55 to 70 E in October (thick line) and November (thin line), and that for 40 to 55 E in October (dotted line). The T-S diagram does not largely differ between October and November 55 to 70 E, compared with the differences between 55 to 70 E (solid lines) and 40 to 55 E (dotted line). It is suggested that horizontal advection from the west is not a predominant process contributing to the decrease of SST. 4. Discussion [17] In the present study, the existence of a zonallyelongated precipitation zone reported by Zhang [2001] was confirmed over the western North Indian Ocean in Figure 5. (left) Time-zonal sections of SST and (right) 20 100 day filtered SST at 2.5 S to 2.5 N averaged for 2000 to 2005. The contour interval is 0.2 C for Figure 5 (left) and 0.1 C for Figure 5 (right). Positive values are shaded in Figure 5 (right). 4of6-160-

Figure 6. Averaged vertical profiles of sea-water temperature (solid line) and salinity (dotted line) in 2.5 Sto2.5 N, 55 to 70 E in October (thick line), and November (thin line) from 2003 to 2005. the mean field during late autumn. The present analyses revealed that the convergence zone is accompanied by high SST north of the equator. Furthermore, a cold tongue is identified near the equator, as displayed in Figure 2. These surface conditions are consistent with the necessary conditions for the formation of double ITCZs mentioned by Zhang [2001]. [18] However, the sea-surface wind field associated with the ITCZ-like convergence zone over the western Indian Ocean is quite different from those over the Pacific and Atlantic Oceans. The northern part of the double ITCZs over the western Indian Ocean corresponds to the higher SST north of the equator in late autumn when the convection of the ITCZs is active (Figure 2). The SST increases when the precipitation is suppressed and the wind speed is low during the westerly to easterly monsoon transition in middle autumn (Figure 4). [19] In contrast, the low-sst region seems to extend from west to east in November (Figures 2 and 5), when a strong westerly wind and an eastward current called Wyrtki Jet are observed [Wyrtki, 1973]. It appears that the eastward advection of clod sea-surface water contributes to the low- SST. However, according to the T-S diagrams (Figure 7), the effect of the advection from the west is not predominant. O Brien and Hurlburt [1974] demonstrated that the westerly sea-surface wind excites not only the eastward current along the equator, but also the upwelling of subsurface water in the western part of the basin. In Figure 6, the low- SST actually accompanies the upward shift of the subsurface water. However, the T-S diagram does not largely change during this period (Figure 7). The low SST is supposed to be brought by the upwelling of the subsurface water. The upwelling has a local maximum near the thermocline (100 db), suggesting that it mainly corresponds to the wave propagation as the first baroclinic mode. It is inferred that the upwelling, eastward current, and the resultant low SST are caused as a dynamic response of the ocean basin to the westerly wind. In the Pacific and Atlantic Oceans, the cold tongue forms associated with the equatorial upwelling corresponding to the zonally-uniform easterly wind without the effects of the coasts. Therefore, the formation mechanism of the cold tongue is different from that in the Pacific and Atlantic Oceans. [20] It has been argued that the meridional location of the ITCZ is maintained by the WES feedback in the climatological Pacific and Atlantic Oceans [Xie and Philander, 1994], and in the equatorially asymmetric SST pattern that forms in boreal winter and spring in the Indian Ocean [Kawamura et al., 2001]. Easterly wind in the tropics is a necessary condition for the WES feedback. However, the precipitation zone analyzed here does not accompany easterly wind over the equator. At this point, the formation mechanism of the precipitation zone between Maldives and Somalia is not the same as the WES feedback. [21] The southward evolution of the winter-monsoon easterly is halted in November (Figure 4). The westerly wind caused by the convective activity over the ITCZ-like convergence zone may block the onset of the easterly wind regime during this period, resulting in persistent, weak wind speed. [22] Twin tropical cyclones may be observed over the western North Pacific and the eastern Indian Ocean [e.g., Ferreira et al., 1996]. In general, the double ITCZ-like precipitation pattern may be derived as a statistic of twin tropical cyclones. However, the cyclones are less frequently observed over the western Indian Ocean than the eastern Indian Ocean. [23] The eastward-propagating intraseasonal-scale disturbances are observed in boreal winter [e.g., Zhang, 2005]. It appears that the ITCZ-like convergence zone over the western Indian Ocean does not directly correspond to such intraseasonal disturbances, since it does not propagate eastward. [24] The weak wind speed just north of the equator is also observed in boreal spring corresponding to the other monsoon transition (Figure 4). However, water vapor is less before April (not shown), resulting in weak convective activity. Moreover, after the Asian summer monsoon onset in April, the larger-scale monsoon dynamics are predominant. Therefore, the convection region is recognized as a part of the monsoon system. The ITCZ-like precipitation zone does not appear in boreal spring (not displayed). 5. Conclusions [25] We identified an ITCZ-like convergence zone over the western Indian Ocean from late November to early December using satellite observation data. It is not observed in other periods of the year, and accompanies a high SST Figure 7. T-S diagrams for the averaged vertical profiles in 2.5 S to2.5 N, 55 to 70 E in October (thick line) and November (thin line), and that in 2.5 S to2.5 N, 40 to 55 E in October (dotted line) from 2003 to 2005. 5of6-161-

north of the equator, and a cold tongue near the equator. The SST north of the equator increases corresponding to the suppressed precipitation and the weak sea-surface wind during the monsoon transition from October to November. However, the low SST at the equator appears associated with the upwelling of subsurface water as a dynamic response to the westerly sea-surface wind along the equator. [26] Acknowledgments. M. Yoshizaki of the Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology (IORGC/JAMSTEC) gave us useful advice and encouragement. N. Shikama, E. Oka, and K. Sato of IORGC/JAMSTEC kindly helped us obtain and utilize the Argo data. Y.-M. Kodama of the Department of Earth and Environment Science, Hirosaki University, kindly helped us obtain and utilize the TRMM data. The authors also thank the anonymous reviewers for their constructive comments. The SSM/I, TMI, and QuikSCAT data were provided by Remote Sensing Systems (RSS) (available at http://www.ssmi.com/). It was produced by RSS under the sponsorship of the Ocean Vector Winds Science Team at the National Aeronautics and Space Administration (NASA). The TRMM data were provided by NASA and the Japan Aerospace Exploration Agency Earth Observation Research and Application Center (JAXA-EORC). The Argo data were provided by IORGC/JAMSTEC (available at ftp://ftp.jamstec. go.jp/pub/argo/). The GFD-DENNOU Library was utilized for drawing the figures. References Argo Science Team (2001), Argo: The global array of profiling floats, in Observing the Oceans in the 21st Century, edited by C. J. Koblinsky and N. P. Smith, pp. 248 258, GODAE Proj. Off., Melbourne, Aust. Ferreira, R. N., W. H. Schubert, and J. J. Hack (1996), Dynamical aspects of twin tropical cyclones associated with the Madden-Julian Oscillation, J. Atmos. Sci., 53, 929 945. Kawamura, R., T. Matsumura, and S. Iizuka (2001), Role of equatorially asymmetric sea surface temperature anomalies in the Indian Ocean in the Asian summer monsoon and El Niño-Southern Oscillation coupling, J. Geophys. Res., 106, 4681 4693. Kodama, Y.-M., A. Ohta, M. Katsumata, S. Mori, S. Satoh, and H. Ueda (2005), Seasonal transition of predominant precipitation type and lightning activity over tropical monsoon areas derived from TRMM observations, Geophys. Res. Lett., 32, L14710, doi:10.1029/2005gl022986. Matsumoto, J. (1992), The seasonal changes in Asian and Australian monsoon regions, J. Meteorol. Soc. Jpn., 70, 257 273. Mitchell, T. P., and J. M. Wallace (1992), The annual cycle in equatorial convection and sea surface temperature, J. Clim., 5, 1140 1156. Murakami, T., and J. Matsumoto (1994), Summer monsoon over the Asian continent and western North Pacific, J. Meteorol. Soc. Jpn., 72, 719 745. O Brien, J. J., and H. E. Hurlburt (1974), Equatorial jet in the Indian Ocean: Theory, Science, 184, 1075 1077. Waliser, D. E., and C. Gautier (1993), A satellite-derived climatology of the ITCZ, J. Clim., 6, 2162 2174. Wyrtki, K. (1973), An equatorial jet in the Indian Ocean, Science, 181, 262 264. Xie, S.-P., and S. G. H. Philander (1994), A coupled ocean-atmosphere model of relevance to the ITCZ in the eastern Pacific, Tellus, Ser. A, 46, 340 350. Zhang, C. (2001), Double ITCZs, J. Geophy. Res., 106, 11,785 11,792. Zhang, C. (2005), Madden-Julian Oscillation, Rev. Geophys., 43, RG2003, doi:10.1029/2004rg000158. M. Katsumata, N. Sato, R. Shirooka, and K. Yoneyama, Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. (naoki@jamstec.go.jp) Y. N. Takayabu, Center for Climate System Research, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8568, Japan. 6of6-162-