Strong phytoplankton blooms are occasionally observed around a recurvature point of typhoon tracks in the western subtropical Pacific. These are noteworthy events in subtropical regions where both nutrient concentrations and biological production are persistently low. We investigated the response of phytoplankton to typhoon passage using a numerical model with/without biogeochemical processes. The model reproduced the observed patch-like phytoplankton bloom around a recurvature point of Typhoon Keith in 1997. The strong bloom is caused by the typhoon-centered upwelling of nutrient-rich water from below the euphotic layer, which supplies the nutrients required for phytoplankton growth, resulting in higher chlorophyll-a concentrations. Biogeochemical processes then play essential roles in determining the response after the passage of typhoons in subtropical regions. Citation: Shibano, R., Y. Yamanaka, N. Okada, T. Chuda, S. Suzuki, H. Niino, and M. Toratani (2011), Responses of marine ecosystem to typhoon passages in the western subtropical North Pacific, Geophys. Res. Lett., 38, L18608, doi:10.1029/2011GL048717.

Surface heat fluxes around 2 degrees N, 138 degrees E in the tropical western Pacific have been acquired by Research Vessel (R/V) Mirai for a total of 74 days during four observational periods. Significant fractions of the sensible and latent heat fluxes are found to occur during occasional passages of precipitating cloud systems which cause large surface fluxes. Based on the radar reflectivity image and in situ precipitation observation at the R/V Mirai, the contributions of the surface heat flux enhancements due to precipitating clouds to the total accumulated surface heat fluxes throughout the observational periods are estimated to be 41 and 10 percent for the sensible and latent heat fluxes, respectively (12 percent for their sum). Furthermore, each 20 percent increase in the radar echo area ratio leads to increases in sensible and latent heat fluxes by about 11 and 30 W m(-2), respectively. The significant enhancements of the sensible heat flux are due to the combined effects of increased wind speeds and large air-sea temperature differences resulting from distinct drops in air temperature at the surface. On the other hand, the enhancements of the latent heat flux are primarily due to the increased wind speeds. This difference in the way the sensible and latent heat fluxes are enhanced by precipitating clouds leads to the differences in the contributions of these enhanced fluxes to the total accumulated fluxes as well as their frequency distributions.These results suggest that precipitating convective systems play an important role in determining the surface heat fluxes over the tropical western Pacific on temporal scales that range from the life cycle of an isolated cumulus to Madden-Julian oscillation.

This paper describes the structure and evolution of northward propagating mesoscale convective systems (MCSs) observed over the tropical western Pacific on 15 June 2005. A wedge‐shaped convective cloud area, consisting of three groups of MCSs, was generated near the equator to the north of New Guinea at around 0000 LST on 15 June, and one of the groups of MCSs propagated toward Palau (7°–8°N). Dropsonde analysis revealed that the MCSs developed at the northern edge of a cold air mass that had a horizontal scale of more than 400 km. Global objective analysis data from the Japan Meteorological Agency showed that the cold air mass observed by the dropsonde analysis originated in a land breeze from New Guinea. The cold air mass was initially formed by cold advection from the south and was maintained by cooling with the MCSs. When the initial MCSs were generated along the land‐breeze front, an easterly wave, which was accompanied by low‐level southerly wind and a low‐pressure area, was located near Palau. The cold air mass associated with the land breeze between 132° and 137°E was locally extended toward the center of the low‐pressure area. The results suggest that the easterly wave located around Palau helped locally promote northward cold advection from New Guinea, inducing the long‐distance northward propagation of MCSs generated along the land‐breeze front.

The effect of the assimilation of dropsonde data over the tropical western Pacific was investigated in an objective analysis. In June 2005, 30 dropsondes were released on four separate flight days. The data impact was assessed using the objective analysis dataset of “ALERA.” The analysis of the zonal wind field over the tropical western Pacific in ALERA revealed large errors corresponding to active convection. These errors were reduced by 1-3 m s-1 due to the assimilation of the dropsondes. The impact signal due to the assimilation of the dropsondes propagated northward and appeared significantly around Japan. The phase and group speeds of the impact signal at 700 hPa were approximately 3 m s-1 and 12 m s-1, respectively. The former speed was consistent with the mean meridional wind speed, and the latter speed roughly corresponded to the meridional group speed of Rossby waves with wavelengths of a few thousand kilometers.

More than 400 upper-air soundings at different geographical locations over the Japanese Islands for various seasons reveal that vertical fine structures of temperature and water vapor, having a venical scale of several hundreds of meters, commonly exist in the troposphere between 1 and 8 km MSL. The temperature and specific humidity fluctuations associated with the fine structures range between −0.4 and 0.4 K, and between −0.6 and 0.6 g kg−1, respectively. The specific humidity fluctuations are negatively correlated with the temperature fluctuations, and the positive (negative) peaks of the former are located right below the local maxima (minima) of buoyancy frequency. The existence of such fine structures of temperature and water vapor was also confirmed by an observation using simultaneously a radiosonde and a Raman lidar, which measures water vapor independently.

(left) Enlarged map of the Palau region. 14D.6 DIURNAL VARIATION OF PRECIPITATION OBSERVED OVER PALAU IN THE WESTERN PACIFIC Hisayuki Kubota*, Ryuichi Shirooka, Tomoki Ushiyama, Takashi Chuda, Krishna. K. Reddy, Masaki Katsumata, Kunio Yoneyama, Kazuaki Yasunaga, Qoosaku Moteki, Naoki Sato, Mikiko Fujita, and Nobuo Suginohara Institute of Observational Research for Global Change (IORGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan