Influence of mechanical mixing on a low summertime SST in the western North Pacific ITCZ region

GEOPHYSICAL RESEARCH LETTERS, VOL. 33,, doi:10.1029/2006gl025997, 2006 Influence of mechanical mixing on a low summertime SST in the western North Pacific ITCZ region N. Sato, 1 H. Tokinaga, 1 R. Shirooka, 1 and N. Suginohara 1 Received 8 February 2006; revised 1 June 2006; accepted 9 June 2006; published 22 July 2006. [1] A region with a low sea-surface temperature (SST) was identified in the western North Pacific (WNP) Intertropical Convergence Zone (ITCZ) or warm water pool region in boreal summer. The SST decreases by up to 0.5 C from May to the June July August (JJA) season just east of Mindanao Island. Analyses of the observed data indicated that a northeastward surface current constantly exists throughout the year, supplying cold subsurface water through the Molucca Strait. As a result, the subsurface water is colder by several degrees compared with that in the surrounding regions. The sea-surface wind is strong over this region in the JJA season. Examination of Argo float data demonstrated that the mixed layer becomes deeper from May to the JJA season. It is suggested that the strong wind causes vertical mixing between the surface and subsurface layers, resulting in the low summertime SST. Citation: Sato, N., H. Tokinaga, R. Shirooka, and N. Suginohara (2006), Influence of mechanical mixing on a low summertime SST in the western North Pacific ITCZ region, Geophys. Res. Lett., 33,, doi:10.1029/2006gl025997. 1. Introduction [2] Large-scale convective activity over the ocean east of the Philippines is sometimes referred to as the WNP summer monsoon (WNPSM) [Wang and LinHo, 2002]. This convective activity is most active corresponding to high SST over the WNPSM or the WNP ITCZ region. The seasonal cycle of atmospheric convective activity on large spatial scales has been examined by Wang and LinHo [2002]. Furthermore, Kubota et al. [2005] revealed a seasonal cycle of convective activity over the WNP ITCZ region in more detail, by using in-situ observation data from Palau. In general, large-scale convection systems are more active in the JJA season than in other seasons, associated with the low-level westerly wind. [3] However, the seasonal changes in the air-sea coupling system in the WNPSM and WNP ITCZ region have not been well examined, partly because we have not had sufficient observed data. Although the Philippine Sea is often recognized as a warm-water pool with a high SST, oceanic conditions may differ over relatively small spatial scales, since the deformation radius in the ocean is much smaller than that in the atmosphere. Moreover, the topography is complicated near the maritime continent. Localized eddies actually exist in the climatological field corresponding to such complicated geographical conditions. For example, the 1 Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan. Copyright 2006 by the American Geophysical Union. 0094-8276/06/2006GL025997 Mindanao and Halmahera eddies are already well known to oceanographers [Arruda and Nof, 2003]. However, the airsea interaction associated with such localized oceanic eddies has not yet been studied in detail. [4] In the present study, we examine the air-sea interaction over the WNP ITCZ region by using satellite observation data and in-situ measurement data provided by Argo floats [Argo Science Team, 2001]. In particular, we focus on the airsea interaction related to the Mindanao and Halmahera eddies. 2. Methods [5] Tropical Rainfall Measuring Mission Microwave Imager (TMI) SST data were used to examine the seasonal changes in SST with a resolution of 0.25 0.25 averaged from 1998 to 2005 over the WNP ITCZ region. For simplicity, the daily data sets were edited first. Missing values caused by the lack of a path width were replaced with temporally interpolated values at each grid point. We focused on the difference between the SST distribution in May and the JJA season, and confirmed that the SST is lower in the JJA season over the WNP ITCZ region. [6] The Special Sensor Microwave/Imager (SSM/I) precipitation rate data, with a resolution of 0.25 0.25, were also examined for the same periods in order to confirm that convection is relatively weak over the low SST region. [7] Sea-surface wind fields were investigated using QuikScat sea-surface wind data. We replaced the missing values with interpolated values as we did for TMI SST data. Note that we analyzed data from 2000 to 2005 for the seasurface wind, since QuikScat data was available back to July 1999. [8] The Aviso absolute dynamic topography (ADT) data obtained by satellite observation were used to infer nearsurface currents in the WNP ITCZ region. The data set was a merged satellite observation that was taken only one or two times a week. Thus, we made a daily data set, with a resolution of 2.5 2.5, using temporal interpolation, and then calculated the climatological means for May and for the JJA season. We analyzed data from 2002 to 2005 here. [9] Real-time quality-controlled data from Argo profiling floats deployed in the Pacific Ocean were examined. The data indicated the sea-water temperature and salinity at 10- day intervals [Argo Science Team, 2001]. The vertical interval is about 10 db, and the shallowest level is approximately 5 db in most cases. First, the data were used to examine the horizontal temperature distribution of near- or sub-surface water. The temperature at the 80 db level was calculated by vertical interpolation for each observed temperature profile. The 80 db temperatures were plotted for the March April May (MAM) and JJA seasons. Note that we 1of5

SATO ET AL.: WARM WATER POOL Figure 2. Precipitation rate averaged for JJA season. Figure 1. SST averaged for (top) May and (bottom) JJA season. The low-sst region is marked by a circle. analyzed data for the MAM season instead of May because we could not obtain enough data for the one-month period. Data for the years from 2003 to 2005 were used here. [10] We also investigated the vertical profiles of temperature and salinity in the surface layer using the Argo data. The vertical profiles of JJA temperature, averaged over 0 to 5 N, 5 to 10 N, and 10 to 15 N for 125 to 135 E, were compared. Next, the seasonal changes of the temperature profiles averaged over 5 to 10 N, 125 to 135 E, were also examined. Here, the changes from May to the JJA season were analyzed. T-S diagrams were also drawn for both periods, in order to confirm the effects of vertical mixing. May. However, to the south or east of Mindanao Island, the convection does not change significantly or becomes less intense from May to JJA (not shown). [13] Figure 3 depicts the QuikScat sea-surface wind during JJA. The arrows indicate the mean wind vector, and the contours and tones indicate the averaged scalar wind speed. The southwesterly wind speed is high over the lowsst region or the suppressed convection region. High wind speed is not observed in May (not shown). [14] The Aviso ADT averaged for JJA is depicted in the top plot of Figure 4. A cyclonic eddy associated with a low ADT is identified just to the east of Mindanao Island. This corresponds to the Mindanao eddy [Arruda and Nof, 2003]. Although the Halmahera eddy is not as clear as the Mindanao eddy, relatively high ADT is observed near Halmahera Island. The gradient of ADT is extremely large southeast of Mindanao Island as illustrated in Figure 4 (top). The estimated geostrophic current is 1 m/s at maximum. It is inferred that a strong northeastward current exists in this region. We can identify the large ADT gradient and the strong geostrophic current in the other seasons of the year (not shown). [15] The features related to the low-sst region depicted in Figures 1 4 can be seen in the JJA season of each year, for all or most of the years. [16] The 80 db sea-water temperatures observed by Argo floats in the JJA season are depicted in the bottom plot of 3. Results [11] The TMI SSTs averaged for May (top) and the JJA season (bottom) are shown in Figure 1. In May (top), the SST is above 29.6 C between the equator and 10 N in the WNP. The warm SST region expands northward in the JJA season (bottom). However, the SST decreases east of Mindanao Island. As a result, a low-sst region is formed in the southern part of the WNP warm pool. It is identified every year except 1998, a post-el Nin o year. [12] Figure 2 depicts the averaged precipitation rate for JJA. Convective activity is strong over the subtropical WNP in the JJA season, corresponding to the WNPSM. However, the precipitation rate is relatively lower south and east of Mindanao Island. The suppressed convection region corresponds to the low-sst region illustrated in Figure 1 (bottom). In general, convective activity over the warm pool intensifies in the JJA season compared with that in Figure 3. Sea-surface wind vector (arrows) and scalar wind speed (tones) averaged for JJA season. 2 of 5

SATO ET AL.: WARM WATER POOL Figure 5. Averaged vertical profiles of JJA sea-water temperature in 0 to 5 N (thin solid line), 5 to 10 N (thick solid line) and 10 to 15 N (dotted line), 125 to 135 E. Figure 4. (top) Absolute dynamic height averaged for JJA season and (bottom) sea-water temperature observed by Argo floats at 80 db. For the top plot, the contour interval is 10 cm. Values greater than 250 cm are shaded, and those less than 200 cm are hatched. Figure 4. Roughly speaking, the sea-water temperature is high (28 C) in the WNP warm pool. However, we detected lower temperatures, below 22 C in some locations, between 5 N and 10 N. Low near- or sub-surface water temperatures are also found in the MAM season (not shown). The cold region in the bottom plot corresponds well to the northeastward or eastward geostrophic current in the top plot. [17] Figure 5 illustrates vertical profiles of the temperatures obtained by Argo floats averaged over 0 to 5 N (thin solid line), 5 to 10 N (thick solid line), and 10 to 15 N (dotted line) for 125 to 135 E in the JJA season. The mixed layer is shallower and the subsurface water is colder for 5 to 10 N (thick solid line) compared with those in the other regions (thin solid line and dotted line). Figure 6 presents the seasonal changes in the temperature profiles averaged over 5 to 10 N, 125 to 135 E in May (thin line) and JJA (thick line). The mixed layer becomes deeper with statistical significance and becomes cold in JJA. The T-S diagram is smoothed and shifted to the low-salinity side in the JJA season (not shown). 4. Discussion [18] In Figure 1, the SST decreases east of Mindanao Island from May to the JJA season. Conversely, in the other areas of the low-latitude WNP, the SST rises in boreal summer. The seasonal change in SST in the WNP ITCZ region located east of Mindanao Island is quite different from that in the surrounding regions. The spatial distribution of precipitation suggests that convective activity is suppressed by the low SST. [19] The large gradient of ADT between the cyclonic Mindanao eddy and the anticyclonic Halmahera eddy depicted in Figure 4 (top) indicates a strong northeastward or eastward geostrophic current from the Molucca Strait. Although only the surface current is estimated here, we can infer similar currents in the surface and subsurface layers, since the baroclinicity caused by the horizontal gradient of density is not so large here. The sea water at the near- or sub-surface level is colder in the current than that in the surrounding regions (the bottom plot of Figure 4). Furthermore, cold subsurface water is most clearly observed near the Molucca Strait. The vertical temperature profiles shown in Figure 5 also confirm the clear difference in the subsurface temperature. These results demonstrate that the cold subsurface water is supplied from the Molucca Strait. A Figure 6. Averaged vertical profiles of sea-water temperature in 5 to 10 N, 125 to 135 E in May (thin solid line) and JJA (thick solid line). 3of5

SATO ET AL.: WARM WATER POOL comparison of Figure 1 (bottom) and Figure 4 (bottom) suggests that the subsurface cold water corresponds to the low SST. Although some in-situ measurements of the currents near the Philippines have been done, focusing on the bifurcation of the North Equatorial Current [e.g., Toole et al., 1990], we cannot yet obtain enough observed data in the Molucca Strait. However, the existence of the northeastward current through the Molucca Strait is confirmed in the result of the simulation using an ocean general circulation model [Masumoto et al., 2001]. [20] The strong northeastward or eastward subsurface current and the resulting cold subsurface water also exist in the other seasons (not shown). Thus, the cold subsurface water alone cannot explain why a low SST appears only in the JJA season. According to the results shown in Figure 2, the sea-surface wind is strong east of Mindanao Island in the JJA season. The strong wind can be regarded as a part of the Asian-Australian monsoon system [e.g., Matsumoto, 1992], and it appears to be localized by the complicated topography over the maritime continent. Such a strong wind is not detected in the other seasons (not shown). Moreover, the mixed layer becomes deeper and colder from May to JJA, as indicated in Figure 6. These results suggest that the strong surface wind that appears in the JJA season mixes up the cold subsurface water, resulting in the low SST. The T-S diagram is smoothed from May to JJA, which is consistent with the inference that the vertical mixing is strengthened in the JJA season (not shown). The curl of the surface wind is almost zero or anticyclonic near the center of the low-sst region (Figure 4). Therefore, it is inferred that the Ekman upwelling does not contribute to the SST decrease. Since the low-sst region is not in contact with the coast of the Philippines, it is suggested that the contribution of coastal upwelling is also small. [21] The region of subsurface cold water extends eastward from the center of the low-sst region (Figure 4). On the other hand, the strong-wind region does northeastward (Figure 3). The low SST is observed only where both of them are observed. In more detail, the low-sst region in Figure 1 (bottom) is slightly shifted southeastward compared with the surface wind maximum and the 80-db temperature minimum. This may be caused by the Ekman transport associated with the southwesterly wind. [22] In boreal summer, the SST is lower in the Arafra Sea than in the Philippine Sea. The strong surface current may transport the cold surface water from the Arafra Sea into the warm pool. However, a minimum of SST is observed off Mindanao Island, especially in June. The low-sst region then expands over the southern part of the Philippine Sea (not shown). Therefore, the formation of a low-sst region cannot be explained by only the advection of cold surface water from the Arafra Sea through the Molucca Strait. [23] Sato [2005] demonstrated that sudden SST coolings over a 10-day interval occur when the oceanic mixed layer is shallow in the WNP ITCZ region. The locations of such cooling events roughly correspond to the cold subsurface water examined in the present study. Although Sato [2005] did not discuss the formation mechanism of the shallow mixed layer which is favored by the cooling events, the shallow mixed layer related to the sudden cooling appears to also be related to the cold subsurface water supplied through the Molucca Strait. [24] Xie et al. [2003] demonstrated that the low summertime SST over the South China Sea is formed by adiabatic surface cooling related to strong surface wind and by an Ekman upwelling associated with the positive wind-stress curl. In the WNP ITCZ region, the low SST does not correspond to the cyclonic curl of the surface wind in Figure 3. Furthermore, the sea water in and near the mixed layer is not lifted up in the vertical temperature profiles shown in Figure 6. The Ekman upwelling is not a predominant process in the formation of the low-sst region. Since the latent heat flux from the sea surface is generally important to the surface heat budget in a tropical ocean, adiabatic cooling caused by strong surface wind may contribute to the low SST. However, from the authors examination of the vertical profile of sea-water temperature and the T-S diagram, adiabatic cooling does not seem to be a predominant process for the low SST. It appears that the formation mechanisms of a cold water pool are different in different regions. 5. Conclusions [25] The SST over the WNP ITCZ region just to the east of Mindanao Island is lower in boreal summer compared with the SST in May or that in the surrounding region. Convective activity is weak over this cold-sst region. Cold subsurface water is supplied almost constantly from the Molucca Sea throughout the year by the northeastward current related to the Mindanao and Halmahera eddies. The temperature of the subsurface water is lower by several degrees at maximum than that near the equator or in the central Philippine Sea at the same longitude. A strong sea-surface wind is observed in boreal summer. The mixingup of the subsurface water due to the strong wind stress causes the summertime cold SST to the east of Mindanao Island. [26] Acknowledgments. The authors would like to thank the many people who kindly assisted in this work. N. Shikama and E. Oka of the Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology (IORGC/JAMSTEC) kindly helped us obtain and utilize the Argo data. The authors would like to thank all the members of the Argo group in IORGC/JAMSTEC for their efforts in acquiring data with such high quality. They also thank the two anonymous reviewers for their comments. The TMI, SSM/I and QuikScat data were provided by Remote Sensing Systems (RSS) (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 ADT data were provided by Aviso, from their Web site at http://www.jason.oceanobs.com/. The Argo data were provided by IORGC/ JAMSTEC (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, Victoria, Australia. Arruda, W. Z., and D. Nof (2003), The Mindanao and Halmahera eddies Twin eddies induced by nonlinearities, J. Phys. Oceanogr., 33, 2815 2830. Kubota, H., R. Shirooka, T. Ushiyama, T. Chuda, S. Iwasaki, and K. Takeuchi (2005), Seasonal variations of precipitation properties associated with the monsoon over Palau in the western Pacific, J. Hydrol., 6, 518 531. Masumoto, Y., T. Kagimoto, M. Yoshida, M. Fukuda, N. Hirose, and T. Yamagata (2001), Intraseasonal eddies in the Sulawesi Sea simulated in an ocean general circulation model, Geophys. Res. Lett., 28, 1631 1634. 4of5

SATO ET AL.: WARM WATER POOL Matsumoto, J. (1992), The seasonal changes in Asian and Australian monsoon regions, J. Meteorol. Soc. Jpn., 70, 257 273. Sato, N. (2005), Influences of intraseasonal disturbances on the oceanic mixed layer in the western North Pacific ITCZ region, Geophys. Res. Lett., 32, L17601, doi:10.1029/2005gl023577. Toole, J. M., R. C. Millard, Z. Wang, and S. Pu (1990), Observations of the Pacific North Equatorial Current bifurcation at the Philippine coast, J. Phys. Oceanogr., 20, 307 318. Wang, B., and LinHo (2002), Rainy season of the Asian-Pacific summer monsoon, J. Clim., 15, 386 398. Xie, S., Q. Xie, D. Wang, and W. T. Liu (2003), Summer upwelling in the South China Sea and its role in regional climate variations, J. Geophys. Res., 108(C8), 3261, doi:10.1029/2003jc001867. N. Sato, R. Shirooka, N. Suginohara, and H. Tokinaga, Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061, Japan. (naoki@jamstec.go.jp) 5of5