Tomonori Matsuura

The Indian Ocean SST dipole simulated in a coupled general circulation model

We are successful in simulating the recently discovered ocean-atmosphere coupled phenomenon called the Indian Ocean Dipole for the first time, using a coupled general circulation model without flux correction. During the analyzed 50 years of model integration, the anomalous climate events have appeared 8 times over the Indian Ocean (IO). They are characterized by the cooling of the sea surface temperature (SST) in the southeastern tropical IO and the warming of the SST in the western tropical IO, associated with the anomalous easterly winds along the equator. The spatial pattern of the anomalous SST shows an east-west dipole mode (DM) structure that is similar to the recent reports. The simulated DM events are independent of the El Nino simulated in the same model. The heat budget analysis shows that the tropical air-sea interaction, which is strongly influenced by ocean dynamics, is crucial in generating the model DM events.

Role of equatorially asymmetric sea surface temperature anomalies in the Indian Ocean in the Asian summer monsoon and El Niño‐Southern Oscillation coupling

Using an ocean general circulation model forced by daily mean wind stresses and heat fluxes derived from the bulk formulation with the National Centers for Environmental Protection/National Center for Atmospheric Research reanalysis, we examined possible roles of the tropical Indian Ocean in the Asian summer monsoon and El Nino-Southern Oscillation (ENSO) coupling. A dominant precursory signal of anomalous monsoon circulation relevant to ENSO is the presence of latitudinally asymmetric anomalies of tropical convection and sea surface temperature (SST) near the equator in the preceding spring. The model results show that the equatorial asymmetry of surface latent heat flux plays a vital role in generating the equatorially asymmetric SST in the preceding spring, which implies that a positive wind-evaporation-SST (WES) feedback proposed by Xie and Philander [1994] is crucially responsible for generation and maintenance of those equatorial asymmetries. The westward extension of anomalous convection from the vicinity of the Philippines into the northern Indian Ocean from the preceding winter to spring, which has a signal of ENSO, can be viewed as a developing process of WES. Persistence of the WES feedback regime affects land surface hydrologic processes in the Asian continent through a Rossby wave response to anomalous convective heating. It is anticipated that the WES mode in the tropical Indian Ocean serves as a bridge between the ENSO prevailing in the preceding winter and the anomalous summer monsoon.

On the Evolution of Nonlinear Planetary Eddies Larger than the Radius of Deformation

Abstract The properties of a new equation governing the evolution of planetary eddies larger than the radius of deformation are numerically investigated. Two types of dynamical balances showing remarkable solitary behavior are found. The first is the balance between the weak dispersion due to the planetary beta-effect and the weak nonlinearity due to the continuity equation. Only anticyclonic eddies are extremely long-lived due to this balance. The second is the balance between weak lateral advection due to a particular westward flow and weak planetary dispersion. The collision experiment shows robustness of the two-dimensional solitary eddy, suggesting the existence of a two-dimensional soliton of the latter type. Also discussed is the relevance of our results to the evolution of the anticyclonic eddies off the Pacific coast of Central America reported by Stumpf and Legeckis (1977).

A mechanism of the onset of the Australian summer monsoon

[1] An onset mechanism of the Australian summer monsoon that incorporates possible air-sea feedback processes is investigated using the National Centers for Environmental Prediction /National Center for Atmospheric Research daily reanalysis data aided by an ocean general circulation model. Rapid intensification of land-ocean thermal contrast during the premonsoon period results in a well-organized continental-scale shallow vertical circulation over the Australian continent. The shallow vertical circulation is dynamically coupled both with a thermally induced low at the lower level below 850 hPa and a thermal high at 600–700 hPa level. Intensified low-level westerly anomalies and increased solar radiation in less cloudy air induced by the subsidence in the periphery of the Australian thermal low results in increasing sea surface temperature (SST) along the northern coast of Australia. The thermal high concurrent with the shallow vertical circulation leads to dry intrusion into the layer at � 700 hPa over the Arafura Sea and Coral Sea through the horizontal and vertical advective processes. A combination of the SST increase and the dry intrusion creates a more convectively unstable condition. When convective instability is intensified while subsidence suppresses convection, the arrival of large-scale disturbances with ascending motion (such as the Madden-Julian oscillation) at the domain where the instability is enhanced triggers deep cumulus convection, implying the onset of the monsoon. The onset mechanism proposed in this study may apply not only to the Australian monsoon but also to other monsoon systems that have continental masses in the subtropics. INDEX TERMS: 1625 Global Change: Geomorphology and weathering (1824, 1886); 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 3374 Meteorology and Atmospheric Dynamics: Tropical meteorology; KEYWORDS: Australian summer monsoon, onset, convective instability, thermal low, dry intrusion

An Experimental and Numerical Study of the Internal Wave Generation by Tide—Topography Interaction

Abstract A stratified fluid response to barotropic oscillatory now over a large-amplitude obstacle is examined on the basis of the results of laboratory and numerical experiments. It is demonstrated that, when the obstacle height is fixed relative to the water depth (δ), the type of fluid response is dependent on two dimensional parameters, that is, the maximum internal Froude number at the top of the obstacle (Frm) and the oscillatory period normalized to the time interval an internal wave travels over the horizontal length scale of an obstacle (Td). For the parameter range 0.5 ≤ Frm ≤ 1.75 and 1.5 ≤ Td ≤ 2.5, a detailed comparison is made between the results of laboratory and numerical experiments and shown to be in very good agreement. First and second mode internal waves are specifically identified over the leeside slope of the obstacle. When the value of Frmis greater than one, in particular, internal waves of large amplitude occur because the elementary waves converge at the vicinity of the critical...

The evolution of frontal-geostrophic vortices in a two-layer ocean

Abstract The properties of large amplitude vortices are numerically investigated using a set of two-layer primitive equations. Numerical experiments are systematically conducted for the f and β plants, quasigeostrophic (small amplitude) and frontal-geostrophic (large amplitude) dynamics, cyclone and anticyclone, and vortex sizes comparable to the radius of deformation and larger. In particular, the evolution and migration of frontal-geostrophic vortices are discussed with reference to the evolution equations in a two-layer ocean. It is shown that the anticyclonic vortex with large amplitude displacement evolves differently from the quasigeostrophic vortex and that it is extremely long-lived. The evolution of anticyclonic vortices, in which interfacial displacement is large and the size is greater than the radius of deformation, is different from that of cyclonic vortices. While the shapes of anticyclonic vortices change from circle to ellipse because of instability, the large amplitude effect of interfaci...

Typhoon and ENSO simulation using a high-resolution coupled GCM

The relationship between the character of tropical storm (TS) activity in the northwestern equatorial Pacific (NWEP) and the El Nino-Southern Oscillation (ENSO) is investigated from the viewpoint of how TS behavior varies interannually. A high-resolution coupled general circulation model (CGCM) capable of simultaneously reproducing both TSs and ENSOs has been developed to study the role of air-sea interaction in linking TS activity with ENSO. Analysis of 15 years simulation and observations shows that TS frequency decreases slightly off the Philippine Islands (120°E-150°E, 5°N-20°N) and that the location of their generation shifts toward the east during El Nino years. This results in inactive convection in the NWEP during El Nino years due to an increase in sea level pressure over ocean areas.

Seasonal Variability of Near-Surface Heat Budget of Selected Oceanic Areas in the North Tropical Indian Ocean

The results obtained from an Ocean General Circulation Model (OGCM), the Modular Ocean Model 2.2, forced with the National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis data, and observational data have been utilized to document the climatological seasonal cycle of the upper ocean response in the Tropical Indian Ocean. We address the various roles played by the net surface heat flux and the local and remote ocean dynamics for the seasonal variation of near-surface heat budget in the Tropical Indian Ocean. The investigation is based in seven selected boxes in the Arabian Sea, Bay of Bengal and the Equatorial Indian Ocean. The changes of basin-wide heat budget of ocean process in the Arabian Sea and the Western Equatorial Indian Ocean show an annual cycle, whereas those in the Bay of Bengal and the Eastern Equatorial Indian Ocean show a semi-annual cycle. The time tendency of heat budget in the Arabian Sea depends on both the net surface heat flux and ocean dynamics while on the other hand, that in the Bay of Bengal depends mainly on the net surface flux. However, it has been found that the changes of heat budget are very different between western and eastern regional sea areas in the Arabian Sea and the Bay of Bengal, respectively. This difference depends on seasonal variations of the different local wind forcing and the different ocean dynamics associated with ocean eddies and Kelvin and Rossby waves in each regional sea areas. We also discuss the comparison and the connection for the seasonal variation of near-surface heat budget among their regional sea areas.

A numerical study of a viscous flow past a right circular cylinder on a β-plane

Abstract The viscous flow past a right circular cylinder on a β-plane is numerically studied. The basic flow, unbounded laterally, is assumed to be uniform. The explored parameter space is Ro<O(1), Ek≪O(1), ∼O(1) and δ=2, where Ro is the Rossby number, Ek the Ekman number, the beta parameter, and δ the aspect ratio. Under these conditions, the quasigeostrophic potential vorticity equation is integrated numerically. It is found that an almost exact correspondence exists for steady flows between the numerical solutions and the flow-patterns observed in the laboratory experiments of Boyer and Davies (1982). For a prograde flow, in addition to the upstream influence, the accelerated jet extends to the rear of the_ cylinder because of the existence of Rossby lee-waves. This tendency increases as increases. The separation points shift toward the rear of the cylinder with increasing . As a result the size of the standing eddy is reduced. In addition, the total drag coefficient Cd increases as increases. For a re...

Observations of an intrusion of a warmer and less saline water mass into a bay

Abstract Detailed hydrographic observations were made in Wakasa Bay, Japan, in August 1979 as the first of a series on the topic of bay intrusions. An anomalous water tongue, evident at a depth of 50 m, was observed to move eastward into the bay at a speed of about 10 km day −1 . The width of the tongue was about 20 to 30 km in agreement with the Rossby internal radius of deformation. The results of current meter measurements and the observed temporal and spatial evolution of the temperature field near the front of the anomalous water tongue have suggested that ageostrophic cross-frontal motion, in balance with the along-front acceleration, may be important in our understanding of intrusion processes.