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Keisuke Mizuno and Warren B. White

Abstract

Individual, seasonal, 300 m temperature maps were constructed over the Kuroshio Current System from 130°E to 170°W, for a 4-year period from summer 1976 through spring 1980, using TRANSPAC XBT data and JODC temperature/depth data. Quasi-stationary meanders in the Kuroshic Current System occurred at 137°C (i.e., Kuroshio Meander), at 144°E and 150°E (i.e., lee-wave meanders), and near 160°E (i.e., meander over the Shatsky Rise). A composite of the paths of the Kuroshio (i.e., the 12°C isotherm) from the individual seasonal maps, and the total variance map, finds nodes (i.e., minima) and anti-nodes (i.e., maxima) of variability to have existed along the mean Kuroshio path. The anti-nodes coincided with the location of the quasi-stationary meanders, the nodes in between. Zonal propagation of temperature anomalies accounted for 20–30% of the total interannual variance. These temperature anomalies propagated eastward at 0.5–1.5 cm s−1 in the region 140°–155°E, and westward at −1 to −2 cm s−1 in the region 155°E–175°W. In addition to this wave propagation, 31% of the interannual variance in temperature could be explained by two empirical standing-wave modes. Within these two modes, spatial coherency in variability existed between the Kuroshio Meander, the two lee-wave meanders east of Japan and the meander over the Shatsky Rise. Both spatial patterns of variability fluctuated with a 1-year decorrelation time scale, with maximum interannual variability occurring in fall/winter and minimum interannual variability in spring/summer.

In the latter part of the 4-year period (1979–80), the Kuroshio Meander became weak and the Kuroshio Extension was displaced southward, from 36–37°N during the first 2 years to 34°N during the latter two years. Associated with these large scale changes, the quasi-stationary meander pattern in the Kuroshio Extension became unstable, associated with increased eddy activity and ring production. In fact, ring production doubled, i.e., from 5 per year to 10 rings per year, from what it was during the previous 3 years. Prior to this regimal change, the Kuroshio Extension bifurcated near the Shatsky Rise (160°E) with a secondary branch of the Kuroshio Extension extending northeastward along the Shatsky Rise to 40°N, where it turned east, and with the main branch extending eastward along 36°N. After the regional change, this bifurcation occurred much farther to the west near 150°E.

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Motoki Nagura, Kentaro Ando, and Keisuke Mizuno

Abstract

The heat balance of the surface mixed layer is analyzed at the eastern equatorial Pacific Ocean (0°, 140°W) in order to examine the transition from the 1998 La Niña to the 2002 El Niño. The data used are observations from the Tropical Atmosphere Ocean/Triangle Trans-Ocean Buoy Network (TAO/TRITON). Results show that interannual variation of eddy heat flux due to tropical instability waves slows the transition from La Niña to El Niño. Previous studies have described this slow transition as a pausing period of the ENSO cycle; that is, La Niña lingers and El Niño does not immediately appear despite a deepened thermocline. Heat balance analysis shows that the vertical heat advection anomaly and surface heat flux anomaly warm the mixed layer from 1999 to 2002. These warming anomalies cause the rise of the mixed layer temperature anomaly in the transition from La Niña to El Niño. In contrast, a cooling anomaly of the horizontal heat advection reduces the warming anomaly and slows down the transition from La Niña to El Niño. In horizontal heat advection terms, the eddy heat flux anomaly significantly contributes to the cooling anomaly associated with weakened variability in the 14–50-day-period band, that is, weakened tropical instability waves. During the transition from La Niña to El Niño, the meridional shear between the South Equatorial Current (SEC) and North Equatorial Counter Current is weakened because of the eastward current anomaly at the equator (i.e., weakened SEC) associated with relaxing trade winds. Weakened shear would suppress tropical instability waves. The results presented here suggest that the synoptic-scale processes work effectively at the basin scale to slow down the transition from La Niña to El Niño.

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Iwao Ueki, Nobuhiro Fujii, Yukio Masumoto, and Keisuke Mizuno

Abstract

For the purpose of climate research and forecasting the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) in the Indian Ocean has been planned. Development of RAMA has been gradually accelerated in recent years as a multinational effort. To promote RAMA the authors have developed a small size buoy system, which uses the slack-line mooring method, intended for the easy handling of maintenance on a relatively small vessel. The authors have also conducted a field experiment of the simultaneous deployment of new slack-line mooring and conventional taut-line mooring in the eastern Indian Ocean. This paper describes the performance of the newly developed buoy system, especially the data consistency against the taut-line mooring system, which is usually used for a tropical moored buoy array. Although the slack-line mooring method has the advantage of downsizing the total mooring system, it also has the disadvantage of having relatively large vertical shifts of installed sensors produced by a large migration of the surface buoy. To offset this disadvantage to a certain extent, a data reconstruction method has been developed and evaluated. Through the data comparison between both mooring systems, it is confirmed that the reconstructed data of the newly developed buoy can basically capture the same features as that observed with a conventional taut-line mooring system. The maximum mean difference of −0.16°C and the maximum root-mean-square (RMS) difference of 0.58°C for temperature appeared within the thermocline layer, whereas the maximum mean difference of 0.02 and the maximum RMS difference of 0.09 for salinity appeared within the mixed layer. Considering a distance of 8 n mi between the two moorings, these values are acceptable for regarding that the two moorings can observe same feature. Results of this study support the introduction of various types of mooring systems for a multinational approach of RAMA and contribute to the further progress of RAMA, climate research, and forecasting.

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Takuya Hasegawa, Kentaro Ando, Iwao Ueki, Keisuke Mizuno, and Shigeki Hosoda

Abstract

Upper-ocean salinity variation in the tropical Pacific is investigated during the 2000s, when Triangle Trans-Ocean Buoy Network (TRITON) buoys and Argo floats were deployed and more salinity data were observed than in previous periods. This study focuses on upper-ocean salinity variability during the warming period of El Niño–Southern Oscillation (ENSO)-like quasi-decadal (QD)-scale sea surface temperature anomalies over the central equatorial Pacific (January 2002–December 2005; hereafter “warm QD phase”). It is shown that strong negative salinity anomalies occur in the western tropical Pacific and the off-equatorial Pacific in the upper ocean at depths less than 80 m, showing a horseshoe-like pattern centered at the western tropical Pacific during the warm QD phase. TRITON mooring buoy data in the western equatorial Pacific show that low-salinity and high-temperature water could be transported eastward from the western equatorial Pacific to the central equatorial Pacific during the warm QD phase. Similar patterns, but with the opposite sign of salinity anomalies, appear in the cold QD phase during January 2007–December 2009 with negative sea surface temperature anomalies over the central equatorial Pacific. It is suggested that effects from zonal salinity advection and precipitation could contribute to the generation of the salinity variations in the western equatorial Pacific for QD phases during the 2000s. On the other hand, the contribution of meridional salinity advection is much less than that of zonal salinity advection. In addition, El Niño Modoki and La Niña events could affect salinity changes for warm and cold QD phases via interannual-scale zonal salinity advection variations in the western equatorial Pacific during the 2000s.

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Kunio Yoneyama, Yukio Masumoto, Yoshifumi Kuroda, Masaki Katsumata, Keisuke Mizuno, Yukari N. Takayabu, Masanori Yoshizaki, Ali Shareef, Yasushi Fujiyoshi, Michael J. McPhaden, V. S. N. Murty, Ryuichi Shirooka, Kazuaki Yasunaga, Hiroyuki Yamada, Naoki Sato, Tomoki Ushiyama, Qoosaku Moteki, Ayako Seiki, Mikiko Fujita, Kentaro Ando, Hideaki Hase, Iwao Ueki, Takanori Horii, Chie Yokoyama, and Tomoki Miyakawa

The Mirai Indian Ocean cruise for the Study of the Madden-Julian oscillation (MJO)-convection Onset (MISMO) was a field experiment that took place in the central equatorial Indian Ocean during October–December 2006, using the research vessel Mirai, a moored buoy array, and landbased sites at the Maldive Islands. The aim of MISMO was to capture atmospheric and oceanic features in the equatorial Indian Ocean when convection in the MJO was initiated. This article describes details of the experiment as well as some selected early results.

Intensive observations using Doppler radar, radiosonde, surface meteorological measurements, and other instruments were conducted at 0°, 80.5°E, after deploying an array of surface and subsurface moorings around this site. The Mirai stayed within this buoy array area from 24 October through 25 November. After a period of stationary observations, underway meteorological measurements were continued from the Maldives to the eastern Indian Ocean in early December.

All observations were collected during an El Nino and a positive Indian Ocean Dipole (IOD) event, which tended to suppress convection in the western Pacific and eastern Indian Ocean in throughout much of November 2006. However, as the IOD began to wane in mid-November, an abrupt change from westerly to easterly took place in upper tropospheric winds in the MISMO study region. By late November and early December, deep convection developed over the central Indian Ocean and eastward movement of large-scale cloud systems were observed. This article describes these variations in detail and how they advance our understanding of the onset of tropical deep convection on intraseasonal time scales.

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