MISMO Workshop

- Abstract -

Yokohama Institute for Earth Sciences / JAMSTEC, Yokohama, JAPAN

November 25 - 26, 2008

Poster Session

Lower Atmosphere Observations over the Equatorial Indian Ocean with a Ship-Borne Lower Troposphere Radar During MISMO Field Experiment

Hiroyuki HASHIGUCHI¹, Noriyuki KAWANO², Michihiro TESHIBA³, Kunio YONEYAMA⁴, and Shoichiro FUKAO^5,1 ( 1: Research Institute for Sustainable Humanosphere/Kyoto Univ., 2: Japan Aerospace Exploration Agency, 3: Weathernews Inc., 4: IORGC/JAMSTEC, 5: Fukui Univ. of Technology)

Stationary intensive observations with a research vessel MIRAI over Indian Ocean (0, 80.5E) was conducted by using a ship-borne lower troposphere radar (SB-LTR) in MISMO field experiment during October 28-November 20, 2006. Small cloud clusters passed over MIRAI a few days after sea surface temperature (SST) showed large diurnal variations. Strong updrafts were observed by SB-LTR up to 1.5 km altitude, which was higher than level of free convection (LFC), and mixing layer showed large diurnal variations. It is suggested that these large diurnal variations of the mixing layer and strong updrafts are contributed by local convective instability related to large diurnal variations of SST. Super cloud clusters related to the Madden-Julian Oscillations proceeded eastward over MIRAI during November 16-20. At the same time, horizontal winds showed a large scale convergence, specific humidity and equivalent black body temperature (TBB) made similar time-height variations, and tropopause heights also jumped about 1 km up, where a breaking phase of a Kelvin wave was recognized. Strong updrafts and strong mixing layer were also observed up to 1.5 km altitude by using SB-LTR. However, there were small diurnal variations of the updraft, the mixing layer, SST, TBB, specific humidity, and LFC, which were quite different from the results in the small cloud clusters, and seemed to be correlated by the large scale convergence.

Microphysical structures of stratiform clouds associated with the MJO observed during MISMO project

Kenji SUZUKI¹, Shunsuke SHIGETO¹, Takumi KOGA¹, Kazue MORINAGA¹, and Kunio YONEYAMA² (1: Yamaguchi University, 2: IORGC/JAMSTEC)

In this study, microphysical structures of stratiform clouds associated with the Madden-Julian Oscillation (MJO) over the equatorial Indian Ocean were investigated using videosondes. Videosonde observations were conducted as part of the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO) from October 16 to November 27, 2006. We had launched six videosondes, which give us images of precipitation particles are acquired by a CCD camera, into the stratiform clouds developed over the equatorial Indian Ocean. Before and behind the onset of deep convection in the MJO after mid-November, the vertical precipitation particle distributions were greatly different. Compared with that before the onset of MJO, the number concentrations of ice crystal were greater, and the mean diameters of ice crystal were smaller. It was also found that The backward trajectory analysis shows that the air mass from the northern hemisphere (continental) after the onset of the MJO was dominant, while the air mass from the southern hemisphere (maritime) before the MJO. This suggested that a large-scale circulation might greatly influence such an ice crystal formation process in the stratiform clouds.

Lidar Observations of Tropospheric Aerosols and Clouds in the Indian Ocean

Atsushi SHIMIZU¹, Ichiro MATSUI¹ and Nobuo SUGIMOTO¹ (1: National Institute for Environmental Studies)

A Mie-scattering lidar system with two wavelengths and depolarization measurement capability had been operated continuously during October and December of 2006 in the equatorial Indian Ocean. The lidar measured vertical distribution of tropospheric aerosols, and detected bottom and top heights of ice/water clouds. Probability density of detected cloud bottom height had significant peak near 4.6km altitude, and the clouds were consist of water droplets because depolarization ratio was low in these clouds. On the other hand, highly depolarized signal was detected in the clouds located above 11km. Though detecting cirrus was difficult due to attenuation of laser beam by lower/middle clouds, maximum height of cirrus was 17km in the beginning of November. Aerosols were concentrated near the sea surface (below 2 km), and the backscatter coefficient of aerosols slightly correlated with the wind velocity at the sea surface. This relation indicates that the most aerosols in this region was wind blown sea salt which was generated locally, and there were week influence of outflow from the continent. Averaged aerosol optical depth at 532 nm was 0.02 with the assumption of fixed lidar ratio (25sr).

Cirrus observations in the Tropical Tropopause Layer over the western Pacific

Masatomo FUJIWARA (Hokkaido University)

A polarization lidar was continuously operated aboard the research vessel Mirai in the tropical western Pacific over three northern winters: at 2.0N, 138.0E during November and December 2001, at 2.0N, 138.5E during November and December 2002, and at 7.5N, 134.0E during　 December 2004 and January 2005. Intensive radiosonde soundings were made from the vessel at 3-hour intervals during all three campaigns. The mechanisms that underlie the observed variations in cirrus in the tropical tropopause layer (TTL) are discussed from the viewpoint of large-scale dynamics and transport.

During the 2001 campaign, the tropopause region was cold, but the TTL was often clear, with only some subvisual cirrus. Potential vorticity data and trajectories show that the TTL during this period was strongly affected by dry air transport from the northern midlatitude lower stratosphere. During the 2002 campaign, a packet of large-amplitude equatorial Kelvin waves was the primary control on the generation and disappearance of cirrus in the TTL. During the 2004-2005 campaign, a cold phase of large-scale waves resulted in cirrus generation in the TTL in late December of 2004, similar to that observed during the 2002 campaign. Outflow from the South Pacific Convergence Zone (SPCZ) caused optically thick cirrus in the TTL, particularly during early January of 2005, when we also observed regular diurnal variations in cirrus development within the TTL, that is, apparent sedimentation during the nighttime. Temperature variations associated with diurnal tides and diurnal variations in convective activity within the SPCZ were possibly associated with the observed variations in cirrus.

In the equatorial western Pacific, equatorial Kelvin waves are the important dynamical process that controls cirrus variations in the TTL. Dry-air horizontal transport from the midlatitude lower stratosphere and wet-air vertical transport near the tropical convergence regions should also be considered in fully explaining the cirrus observations in the TTL.

Equatorial waves and disturbances around the Tropical Tropopause Layer observed over the Indian Ocean during the MISMO field experiment

Junko SUZUKI¹, Kunio YONEYAMA¹, Ryuichi SHIROOKA¹, Masanori YOSHIZAKI¹, Masatomo FUJIWARA², Youichi. INAI², Fumio HASEBE², and Atsushi SHIMIZU³ (1: IORGC/JAMSTEC, 2: Hokkaido Univ., 3: National Institute for Environmental Studies)

Many observational studies of the tropical atmosphere have led to detailed analyses of waves and oscillations around the Tropical Tropopause Layer (TTL). Among them, the equatorial Kelvin wave is supposed to play critical roles in tropical stratosphere-troposphere exchange (STE) of ozone and in dehydrating air entering the lower stratosphere from the upper troposphere. Though the Kelvin wave around the TTL is most active around 80-120°E during the northern winter [Suzuki and Shiotani, 2008], the observations are rare in this region except in Indonesia. In October-December 2006, the field experiment MISMO was conducted a total of 329 Vaisala RS92 radiosondes and continuously operated the polarization lidar in the equatorial Indian Ocean centered at 0°, 80.5°E using the research vessel “Mirai”; observations using the Meteolabor “Snow White” chilled-mirror dew/front-point hygrometers and ozonesonde were also conducted 15 times during the same period.

The zonal wind and temperature anomalies shows the space-time characteristics of Kelvin waves around 16- 20 km described by period ~2 weeks and vertical wavelength ~5 km. Disturbances likely due to breaking Kelvin waves occur around 17 km on November 1, and then the low relative humidity signal is downward for ~5 days from the cold point tropopause to the middle troposphere. From the result of analyzing the lidar data, the cirrus cloud between the TTL and the upper troposphere is not found during the low humidity descending. The meridional wind anomaly clearly shows the eastward propagating wave around 20 km in the lower stratosphere; the characteristic of the wave corresponds to the eastward gravity wave frequently observed around Indonesia.

Diurnal variations in precipitable water observed by shipborne GPS over the tropical Indian Ocean

Kazuaki YASUNAGA (IORGC/JAMSTEC)

The present paper investigates the relationship between the skin sea surface temperature (SSTskin) and precipitable water (PW) observed over the tropical Indian Ocean. PW is derived from shipborne Global Positioning System (GPS). Composite diurnal variations indicate that the increase of PW and radar echo coverage (rainfall) in the daytime corresponds to the large SSTskin rise during the undisturbed period (The PW increase is statistically significant at 90 % level). The surface fluxes calculated using the bulk flux algorithm are too small to account for the observed increase of PW, while the bulk flux agrees with the directly measured eddy flux.

The oceanic Response to the Madden--Julian Oscillation and ENSO

Ayako SEIKI¹, Yukari N. TAKAYABU^{2, 1}, Kunio YONEYAMA¹, Naoki SATO¹,and Masanori YOSHIZAKI¹ (1: IORGC/JAMSTEC, 2: CCSR/Univ. of Tokyo)

Oceanic responses to the Madden--Julian Oscillation (MJO) are compared under different El Niño--Southern Oscillation (ENSO) phases. During the pre-El Niño phases, when westerly wind bursts frequently occur over the equatorial western and central Pacific, downwelling Kelvin waves are excited and propagate in the eastern Pacific, resulting in gradual sea surface temperature (SST) warming. These waves respond to the strong and widely extended westerly winds to the west of the date line accompanying the MJO. On the other hand, during the other (non-pre-El Niño) phases, there are clear upwelling Kelvin waves that respond to the dominant easterly winds preceding the MJO convections, which result in gradual SST cooling. These results suggest an ocean--atmosphere feedback process in which oceanic responses to the MJO and their impact on SST vary depending on the fluctuations of MJO wind structures, which are also influenced by ENSO.

Seasonal Variation of the Seychelles Dome

Takaaki YOKOI¹, Tomoki TOZUKA¹, and Toshio YAMAGATA¹ (1: The University of Tokyo)

Using an ocean general circulation model (OGCM), seasonal variation of the Seychelles Dome (SD), an oceanic thermal dome located in the southwestern Indian Ocean, is investigated. Its seasonal variation is dominated by a remarkable semiannual cycle due to local Ekman upwelling. This semiannual nature is explained by different contributions of two components of the Ekman pumping: one term proportional to the planetary beta and the zonal wind stress and the other term proportional to the wind stress curl. The former is determined by the seasonal change in the zonal component of the wind stress vector above the SD. It is associated with the Indian monsoon and causes downwelling (upwelling) during boreal summer (boreal winter). The latter, of which major contribution comes from the meridional gradient of the zonal wind stress, also shows a clear annual cycle with strong upwelling during boreal summer and fall. However, it remains almost constant for five months from June to October even though the zonal wind stress itself varies significantly during this period. The above overall feature is due to the unique location of the SD; it is located between two regions: one is dominated by the seasonal variation in wind stress due to the Indian monsoon, and the other is dominated by the southeasterly trade winds that prevail throughout a year.

Existence and interannual variations of a low-frequency (55-100day) MJO mode in Austral Summer - Role of oceanic diurnal warm layers.

Takeshi IZUMO^{1, 2}, Sébastien MASSON², Jérôme VIALARD², Gurvan MADEC³, Clément de BOYER MONTÉGUT¹, Jing-Jia LUO¹, Swadhin K. BEHERA¹, Keiko TAKAHASHI⁴ and Toshio YAMAGATA^{5, 1} (1: FRCGC/JAMSTEC, 2:LOCEAN, 3: LODYC, 4: ESC/JAMSTEC, 5: The University of Tokyo)

The Madden-Julian Oscillation is the main component of the intraseasonal variability of the convection in the tropics. Previous work had already suggested that the timescale of the MJO increased in Austral summer while going from the equator to ~8°S in the Indian Ocean. Here we suggest, based on observational records, that there are two intraseasonal modes of variability of the convection in Austral Summer. One is higher frequency (30-50 day) and more symmetric with respect to the equator, while the other is asymmetric (shifted to the south) and more low-frequency (55-100 days). This southern mode propagates slower and is more reactive to the presence of oceanic warm layers. Both modes have independent interannual variations. The lower-frequency mode is modulated at IOD timescales by the presence, during the boreal winter following a negative IOD, of a shallower oceanic mixed layer, less atmospheric convection, weaker winds and more oceanic warm layers over the thermocline ridge of the southern Indian Ocean.

Water property changes observed along WHP lines I03 and I04 between 1995 and 2003

Katsuro KATSUMATA (IORGC/JAMSTEC)

As part of the WOCE (World Ocean Circulation Experiment) Hydrographic Programme (WHP), nominally zonal lines across the subtropical gyres in the southern hemisphere oceans were occupied from 1992 to 1995. The focus in this presentation is on the lines I03 and I04 (along 20° the Indian Ocean), which was observed in April to June 1995 and revisited in December 2003 to January 2004.

Along the south lines, Subantarctic Mode Water was found warmer and saltier to the west of 95°E (e.g., by 0.2°C and 0.05 psu on gamma=26.8 neutral surfaces) and cooler and fresher to the east (e.g., a difference of -0.2°C and -0.05 psu on gamma=26.8 neutral surfaces), but the changes in the Intermediate waters were masked by the meridional meandering/movements of the South Equatorial Current.

Changes at intermidiate depths in the Indian Ocean using repeat hydrography and Argo data

Shinya KOUKETSU (IORGC/JAMSTEC)

We compare temperatures and salinities in World Ocean Atlas 1994 (WOA94) and Argo optimal interpolation gridded data sets in the Indian Ocean. Since property differences on neutral surfaces between these data sets are similar to differences between WOCE hydrographic programme (WHP) and their revisit sections, property differences between two gridded data sets may reveal not differences of observational points in two data sets but decadal changes in the Indian Ocean. Salinity of Antarctic Intermediate Water decrease from WOA94 to the Argo gridded data sets and it is suggested that these decreases distributed from 20°S to 50°S. Along 10°S, negative differences from WOA94 to Argo gridded data sets were found from 70°E to 120°E in the density range between 25.5 and 27.0 and these negative differences were persistently detected from 2003 to 2007. These negative differences may rely on the strength of the Indonesian Through Flow. However, it is not clear that the negative changes are not due to the difference of observational points or methods in these two data sets. We will be able to observe the detail of these property changes around 10°S in the repeat hydrography observation in 2009.

Chemical tracers in the Indian Ocean: results from WHP repeat hydrography in the Indian, South Atlantic, and South Pacific Oceans in 2003/04.

Yuichiro KUMAMOTO¹, Michio AOYAMA², Ken’ ichi SASAKI^{3, 1}, Akihiko MURATA¹, Shuichi WATANABE^{3, 1}, and Masao FUKASAWA¹ (1: IORGC/JAMSTEC, 2: Meteorological Research Institute, 3: MIO/JAMSTEC)

In 2003/04, we measured concentration of chemical tracers, including radiocarbon, ¹³⁷Cs, and chlorofluorocarbons (CFCs) during revisit cruises of WOCE Hydrographic Programme (WHP) I03 in the Indian Ocean (at approximately 20°S), WHP A10 in the South Atlantic (at approximately 30°S), and WHP P06 in the South Pacific Oceans (at approximately 32°S), known as the Blue Earth Global Expedition 2003 (BEAGLE2003). The zonal-mean inventory of bomb-produced radiocarbon and CFCs were lowest in the Indian Ocean (I03 line) among the three basins, which was due to a sampling bias because the I03 line in the Indian Ocean was along 20°S more equatorward than the other two lines along approximately 30°S in the South Atlantic and South Pacific Oceans. This meridional variation is caused by that the bomb radiocarbon and CFCs have been accumulated in the subtropical zone due to formation and subduction of Subantarctic Mode Water and Antarctic Intermediate Water. On the other hand, the I03 line in the Indian Ocean had the largest zonal inventory of bomb-produced ¹³⁷Cs, implying that the ¹³⁷Cs distribution in the Indian Ocean has not been determined by the thermocline ventilation within the Indian Ocean but by the inter-basin water exchange between the Indian and North Pacific Oceans. The inter-basin water exchange between the two basins had been also suggested by observational data of tritium and 90Sr in the Indian Ocean.