Kentaro Ando

RAMA: The research moored array for African-Asian-Australian monsoon analysis and prediction

The Indian Ocean is unique among the three tropical ocean basins in that it is blocked at 25°N by the Asian landmass. Seasonal heating and cooling of the land sets the stage for dramatic monsoon wind reversals, strong ocean–atmosphere interactions, and intense seasonal rains over the Indian subcontinent, Southeast Asia, East Africa, and Australia. Recurrence of these monsoon rains is critical to agricultural production that supports a third of the worlds population. The Indian Ocean also remotely influences the evolution of El Nino–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), North American weather, and hurricane activity. Despite its importance in the regional and global climate system though, the Indian Ocean is the most poorly observed and least well understood of the three tropical oceans. n nThis article describes the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA), a new observational network designed to address outstanding scientific questions related to Indian Ocean variability and the monsoons. RAMA is a multinationally supported element of the Indian Ocean Observing System (IndOOS), a combination of complementary satellite and in situ measurement platforms for climate research and forecasting. The article discusses the scientific rationale, design criteria, and implementation of the array. Initial RAMA data are presented to illustrate how they contribute to improved documentation and understanding of phenomena in the region. Applications of the data for societal benefit are also described.

Variability of surface layer hydrography in the tropical Pacific Ocean

The purpose of this study is to examine the variation of ocean surface layer hydrography on interannual timescales in the tropical Pacific Ocean using conductivity-temperature-depth measurements from 1976 to 1994. We demonstrate that associated with interannual variations in atmospheric forcing, there were distinct changes in mixed layer temperature, salinity, depth, and barrier layer thickness between normal, El Nino, and La Nina time periods. During El Nino a warm, fresh mixed layer accompanied by an underlying barrier layer develops in the central and eastern Pacific in association with increased precipitation and reduced trade wind forcing. Conversely, during La Nina, when unusually cold conditions prevail in the central and eastern equatorial Pacific and the warm pool is confined to the far western Pacific, a thick barrier layer is found west of 160°E. Statistically, we demonstrate that barrier layers occur with increasing frequency as mixed layer temperatures increase from 20°C to 30°C, suggesting that barrier layer thickness may be one of the factors affecting surface temperatures. The physical mechanism underlying this relationship is likely to be related to the reduced efficiency of vertical turbulent mixing in cooling the surface via entrainment of thermocline water when the barrier layer is thick. Changes in mixed layer temperature, on the other hand, can affect precipitation and therefore mixed-layer salinity, leading to the possibility of feedbacks on interannual timescales involving the mixed-layer temperature balance and the hydrologic cycle over the ocean. The extent to which such feedbacks, if operative, may influence the detailed evolution of large-scale, lower-frequency variability in the tropical Pacific needs to be critically assessed in the context of coupled ocean-atmosphere models.

Mismo field experiment in the equatorial Indian Ocean

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 observatio...

Observed correlation of surface salinity, temperature and barrier layer at the eastern edge of the western Pacific warm pool

[1]xa0Recent theory suggests that ocean-atmosphere interactions in the western Pacific warm pool are of fundamental importance to interannual variations associated with El Nino and the Southern Oscillation (ENSO). The warm pool encompasses the highest mean sea surface temperatures (SSTs) in the world ocean, intense atmospheric deep convection and heavy rainfall, and the formation of thick salt-stratified barrier layers that help to sustain the high SSTs. This study shows that the eastern edge of the warm pool is characterized by a strong zonal salinity front throughout 2002–2004. The analysis also indicates a tighter empirical relationship than previously observed between the eastern edge of the warm pool, high SSTs, the presence of barrier layers, and the fetch of westerly wind bursts. These results suggest that such a frontal region is a critical in controlling ocean-atmosphere interactions in the western Pacific warm pool and highlight the importance of the upper ocean salinity in climate variability.

Characteristics of the convergence zone at the eastern edge of the Pacific warm pool

[1]xa0The characteristics of the convergence zone at the eastern edge of the equatorial Pacific warm pool are studied using a compilation of in-situ current and salinity measurements during the period 1992–2001. The displacement of the convergence zone is observed, for the first time, as far west as 140°E in the far western Pacific, mainly during La Nina periods, and near 140°W in the central Pacific during the 1997–98 El Nino. The convergence zone may be associated with a salinity front dividing the fresh waters of the warm pool from the salty waters upwelled in the central equatorial Pacific. Despite a zonal displacement ranging over about one fifth of the equatorial circumference of the earth, the characteristics of the main parameters involved in the air-sea interactions are nearly constant on each side of the convergence zone/salinity front. These results suggest that coupled models used for El Nino research and forecasting should be able to reproduce these important features.

Drift Characteristics of a Moored Conductivity–Temperature–Depth Sensor and Correction of Salinity Data

Abstract The temperature and conductivity drift (time change of the characteristics) of moored SBE37IM conductivity and temperature (CT) sensors was investigated by pre- and postdeployment calibration of the Triangle TransOcean Buoy Network (TRITON). This buoy network comprises the western portion of the basinwide (Tropical Atmosphere Ocean) TAO/TRITON buoy array, which monitors phenomena such as El Nino and contributes to forecasting climate change. Over the time of deployment the drift of the temperature sensors was very small, within 3 mK of the postdeployment calibration data. The drift of the conductivity sensors was more significant. After 1 yr of mooring, conductivity drift observed in the shallowest layer (1.5–100 m) was positive and 0.010 S m−1 [equivalent to 0.065 (PSS-78) at 30°C and 6 S m−1; here, 1 S is 1 Ω−1] at 6 S m −1 on average. Drift observed in the thermocline layer (125–200 m) was also positive and 0.0053 S m−1 [0.034 (PSS-78)] at 6 S m−1 on average. Conversely, the drift of conductiv...

Stability of Temperature and Conductivity Sensors of Argo Profiling Floats

After recalibration of the temperature and conductivity sensors of three Argo profiling floats recovered after operations for four to nine months, the results indicate that the floats basically showed no significant drift, either in temperature or salinity, and adequately fulfilled the accuracy requirement of the Argo project (0.005°C for temperature and 0.01 psu for salinity). Only the third float showed a significant offset in salinity of about −0.02 psu, as expected from comparison between the float data and the shipboard conductivity-temperature-depth data. This offset was caused by the operational error of the PROVOR-type float, in which the surface water was pumped immediately after the launch, fouling the conductivity sensor cell.

Pausing of the ENSO Cycle : A Case Study from 1998 to 2002

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 Nina to the 2002 El Nino. 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 Nina to El Nino. Previous studies have described this slow transition as a pausing period of the ENSO cycle; that is, La Nina lingers and El Nino 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 Nina to El Nino. In contrast, a cooling anomaly of the horizontal heat advection reduces the warming anomaly and slows down the transition fr...

Short‐term upper‐ocean variability in the central equatorial Indian Ocean during 2006 Indian Ocean Dipole event

[1]xa0Intensive observations, using an array of surface and subsurface moored buoys, are conducted around 80.5°E in the equatorial Indian Ocean during October/November 2006. An intriguing data set of atmospheric and oceanic variables during a peak phase of a positive Indian Ocean Dipole is obtained. The ocean observation data shows relatively shallow thermocline, which intensifies with time during the one-month period, and eastward subsurface zonal flow under westward flowing surface current, generating unusually strong vertical shear above the thermocline. Intraseasonal meridional current variability is also observed. Upper-ocean volume budget analysis indicates that a strong upwelling event, larger than 10 m/day, and associated upward movement of the isotherms below the thermocline occur for a few days in early November. These observed data demonstrates unusual conditions of the upper ocean during boreal autumn in 2006.

Upper-Ocean Salinity Variability in the Tropical Pacific: Case Study for Quasi-Decadal Shift during the 2000s Using TRITON Buoys and Argo Floats

AbstractUpper-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 Nino–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...