Yukio Masumoto

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. This 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.

Interannual subsurface variability in the tropical Indian Ocean with a special emphasis on the Indian Ocean Dipole

Interannual variability in the subsurface tropical Indian Ocean (TIO) is studied using three independent data sets: satellite derived sea-level data, an ocean general circulation model simulation, and in situ upper-ocean temperature data. It is found that significant interannual variability in the TIO is confined to the north of 15°S. Unlike the Pacific Ocean, the dominant modes of interannual variability in the Indian Ocean do not show co-variability between the surface and the subsurface. In contrast to the sea-surface temperature variability dominated by the El Nino and Southern Oscillation, subsurface variability is governed by the Indian Ocean Dipole in the TIO. The dominant mode of the interannual variability in the subsurface TIO is characterized by a dipole. Its evolution is controlled by equatorial ocean dynamics forced by zonal winds in the equatorial region. The subsurface dipole provides the delayed time required to reverse the phase of the surface dipole in the following year. The second dominant mode of interannual variability shows the interesting quasi-biennial tendency. It is found that the turnabout of the phase of the subsurface dipole leads to the quasi-biennial behavior of the TIO. Analysis of in situ subsurface temperature data substantiates this finding.

Forced Rossby waves in the southern tropical Indian Ocean

Seasonal and interannual variation of the upper southern tropical Indian Ocean (STIO) is described by harmonic and empirical orthogonal function (EOF) analysis of the depth of the 20°C isotherm (D20) derived from expendable bathythermograph (XBT) data and from an ocean general circulation model (OGCM). The harmonic analysis shows a band of large annual amplitude between 8° and 20°S extending across the STIO with a steady westward propagation in both the XBT and model data. The generation of the annual wave is discussed in terms of Ekman pumping and the westward propagation of long, nondispersive, baroclinic Rossby waves. The Ekman pumping on a large scale over the open ocean strongly modifies waves radiating from the eastern boundary and generates much of the structure in the amplitude and the phase of the annual signal in the STIO. The EOF analysis shows strong interannual variation of the anomaly of D20 on two XBT lines covering the western and eastern sides of the basin. Large interannual anomalies are observed in a band between 6° and 14°S on the western side, with deep D20 in 1988, 1991, and 1994 and shallow D20 in 1990 and 1992. On the eastern side, large interannual anomalies are observed close to the coast of Java, and sign is opposite to that on the western side, suggesting an oscillation in the zonal tilt of the thermocline across the STIO. Those interannual variations of D20 are also well simulated in the model. The generation of the interannual anomalies in the model are also due mainly to the wind stress curl integrated along the Rossby wave trajectories in the STIO.

An Eddy-Resolving Hindcast Simulation of the Quasiglobal Ocean from 1950 to 2003 on the Earth Simulator

An eddy-resolving hindcast experiment forced by daily mean atmospheric reanalysis data covering the second half of the twentieth century was completed successfully on the Earth Simulator. The domain covers quasiglobal from 75°S to 75°N excluding arctic regions, with horizontal resolution of 0.1° and 54° vertical levels. Encouraged by high performance of the preceding spin-up integration in capturing the time-mean and transient eddy fields of the world oceans, the hindcast run is executed to see how well the observed variations in the low- and midlatitude regions spanning from intraseasonal to decadal timescales are reproduced in the simulation. Our report presented here covers, among others, the El Nino and the Indian Ocean Dipole events, the Pacific and the Pan-Atlantic decadal oscillations, and the intraseasonal variations in the equatorial Pacific and Indian Oceans, which are represented well in the hindcast simulation, comparing with the observations. The simulated variations in not only the surface but also subsurface layers are compared with observations, for example, the decadal subsurface temperature change with narrow structures in the Kuroshio Extension region. Furthermore, we focus on the improved aspects of the hindcast simulation over the spin-up run, possibly brought about by realistic high-frequency daily mean forcing.

Indian Ocean warming modulates Pacific climate change

It has been widely believed that the tropical Pacific trade winds weakened in the last century and would further decrease under a warmer climate in the 21st century. Recent high-quality observations, however, suggest that the tropical Pacific winds have actually strengthened in the past two decades. Precise causes of the recent Pacific climate shift are uncertain. Here we explore how the enhanced tropical Indian Ocean warming in recent decades favors stronger trade winds in the western Pacific via the atmosphere and hence is likely to have contributed to the La Niña-like state (with enhanced east–west Walker circulation) through the Pacific ocean–atmosphere interactions. Further analysis, based on 163 climate model simulations with centennial historical and projected external radiative forcing, suggests that the Indian Ocean warming relative to the Pacific’s could play an important role in modulating the Pacific climate changes in the 20th and 21st centuries.

Interaction between El Niño and Extreme Indian Ocean Dipole

Abstract Climate variability in the tropical Indo-Pacific sector has undergone dramatic changes under global ocean warming. Extreme Indian Ocean dipole (IOD) events occurred repeatedly in recent decades with an unprecedented series of three consecutive episodes during 2006–08, causing vast climate and socioeconomic effects worldwide and weakening the historic El Nino–Indian monsoon relationship. Major attention has been paid to the El Nino influence on the Indian Ocean, but how the IOD influences El Nino and its predictability remained an important issue to be understood. On the basis of various forecast experiments activating and suppressing air–sea coupling in the individual tropical ocean basins using a state-of-the-art coupled ocean–atmosphere model with demonstrated predictive capability, the present study shows that the extreme IOD plays a key role in driving the 1994 pseudo–El Nino, in contrast with traditional El Nino theory. The pseudo–El Nino is more frequently observed in recent decades, coinci...

Response of the western tropical Pacific to the Asian winter monsoon: The generation of the Mindanao Dome

Abstract We have investigated the evolution of the Mindanao Dome off the Philippine coast using the GFDL ocean model. It is found that the models Mindanao Dome evolves in late fall due to local upwelling when a positive curl associated with the northeast Asian winter monsoon increases over the region. It expands eastward with a recirculation composed of the North Equatorial Current in the north, the Mindanao Current in the west, and the North Equatorial Countercurrent in the south. After reaching a maximum in winter, it begins to decay in spring due to an intrusion of downwelling long Rossby waves excited in winter by the northeast trade winds farther eastward near 160°E, as well as a retreat of the local positive wind-stress curl. Further control runs demonstrate that the variation of the models Mindanao Dome is almost perfectly determined by the change of the wind field in the western Pacific west of the date line. A possible link between the Asian winter monsoon and the seawater temperature anomaly i...

Intrusion of the Southwest Monsoon Current into the Bay of Bengal

The general eastward flow in the north Indian Ocean during summer, which is called the Southwest Monsoon Current (SMC), flows eastward south of India, turns around Sri Lanka, and enters the Bay of Bengal. The intrusion of the SMC into the Bay of Bengal is studied using expendable bathythermograph (XBT) observations along the shipping route between Sri Lanka and Malaca Strait, TOPEX/POSEIDON sea surface height anomalies, and an ocean general circulation model. The intrusion appears first as a broad northward shallow (confined to the upper 200 m) flow in the central part of the Bay of Bengal during May. As the summer season advances, it moves westward, intensifies, and becomes narrow. The mean seasonal (May-September) transport of the SMC into the Bay of Bengal is about 10 Sv. The zonal variation of the geostrophic velocity across 6°N calculated using the XBT data compares well with that from TOPEX/POSEIDON altimeter data. However, the SMC in the XBT data is faster (40 cm s−1) than in the altimeter data or the numerical simulation (25 cm s−1). Harmonic analysis of the depth of 20°C isotherm together with a simple forced Rossby wave model demonstrates that the SMC east of Sri Lanka is forced by both Ekman pumping in the Bay of Bengal and Rossby wave radiation associated with the spring Wyrtki [1973] jet in the equatorial Indian Ocean. The initial intrusion of the SMC into the Bay of Bengal is attributed to this Rossby wave radiation from the eastern boundary.

Increased frequency of extreme Indian Ocean Dipole events due to greenhouse warming

The Indian Ocean dipole is a prominent mode of coupled ocean–atmosphere variability, affecting the lives of millions of people in Indian Ocean rim countries. In its positive phase, sea surface temperatures are lower than normal off the Sumatra–Java coast, but higher in the western tropical Indian Ocean. During the extreme positive-IOD (pIOD) events of 1961, 1994 and 1997, the eastern cooling strengthened and extended westward along the equatorial Indian Ocean through strong reversal of both the mean westerly winds and the associated eastward-flowing upper ocean currents. This created anomalously dry conditions from the eastern to the central Indian Ocean along the Equator and atmospheric convergence farther west, leading to catastrophic floods in eastern tropical African countries but devastating droughts in eastern Indian Ocean rim countries. Despite these serious consequences, the response of pIOD events to greenhouse warming is unknown. Here, using an ensemble of climate models forced by a scenario of high greenhouse gas emissions (Representative Concentration Pathway 8.5), we project that the frequency of extreme pIOD events will increase by almost a factor of three, from one event every 17.3 years over the twentieth century to one event every 6.3 years over the twenty-first century. We find that a mean state change—with weakening of both equatorial westerly winds and eastward oceanic currents in association with a faster warming in the western than the eastern equatorial Indian Ocean—facilitates more frequent occurrences of wind and oceanic current reversal. This leads to more frequent extreme pIOD events, suggesting an increasing frequency of extreme climate and weather events in regions affected by the pIOD.

Equatorial Atlantic variability and its relation to mean state biases in CMIP5

Coupled general circulation model (GCM) simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed with respect to their performance in the equatorial Atlantic. In terms of the mean state, 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial SST gradient whose sign is opposite to observations. Westerly surface wind biases in boreal spring play an important role in the reversed SST gradient by deepening the thermocline in the eastern equatorial Atlantic and thus reducing upwelling efficiency and SST cooling in the following months. Both magnitude and seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that improvements have only been modest. The weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SST. They are related to both continental convection and the latitudinal position of the intertropical convergence zone (ITCZ). Particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability. The dependence of equatorial easterlies on ITCZ latitude shows a marked asymmetry. From the equator to 15°N, the equatorial easterlies intensify approximately linearly with ITCZ latitude. When the ITCZ is south of the equator, on the other hand, the equatorial easterlies are uniformly weak. Despite serious mean state biases, several models are able to capture some aspects of the equatorial mode of interannual SST variability, including amplitude, pattern, phase locking to boreal summer, and duration of events. The latitudinal position of the boreal spring ITCZ, through its influence on equatorial surface winds, appears to play an important role in initiating warm events.