Toru Miyama

A Regional Ocean–Atmosphere Model for Eastern Pacific Climate: Toward Reducing Tropical Biases*

Abstract The tropical Pacific Ocean is a climatically important region, home to El Nino and the Southern Oscillation. The simulation of its climate remains a challenge for global coupled ocean–atmosphere models, which suffer large biases especially in reproducing the observed meridional asymmetry across the equator in sea surface temperature (SST) and rainfall. A basin ocean general circulation model is coupled with a full-physics regional atmospheric model to study eastern Pacific climate processes. The regional ocean–atmosphere model (ROAM) reproduces salient features of eastern Pacific climate, including a northward-displaced intertropical convergence zone (ITCZ) collocated with a zonal band of high SST, a low-cloud deck in the southeastern tropical Pacific, the equatorial cold tongue, and its annual cycle. The simulated low-cloud deck experiences significant seasonal variations in vertical structure and cloudiness; cloud becomes decoupled and separated from the surface mixed layer by a stable layer in...

Study of seasonal transport variations in the Indonesian seas

Seasonal transport variations between the Pacific and Indian Oceans via the Indonesian seas were studied by the Euler-Lagrangian method. The velocity field was calculated with a fairly high resolution robust diagnostic model. The model well reproduces the features of seasonal variations in the Indonesian seas. The total volume transport of the Indonesian throughflow is 20±3 Sv (1 Sv = 106 m3 s−1), the maximum being from boreal spring to boreal summer and the minimum in boreal winter. The values are similar to those of previous general circulation models with a wide Indonesian passage despite resolution of the presence of the many small islands in the Indonesian seas. Although a large portion of the net transport is contained in the upper layer, deep transport below 1000-m depth is about 5 Sv. This value corresponds to approximately 25% of the total transport, which means that disregard of the deep transport leads to underestimation of the volume transport of the throughflow. Tracking of numerous labeled particles in the calculated velocity field clarified the sources and pathways of the Indonesian throughflow. The major route is a western one through both the Makassar and Lombok Straits. Most of the North Pacific water supplied from the Mindanao Current passes along this route, entering the Indian Ocean within several months with almost no loss of its properties (intense vertical mixing around the Lombok sill reported by observations could not be reproduced in our model). In contrast, South Pacific water takes the eastern route into the eastern Indonesian seas and subsequently mixes with waters from the North Pacific and Indian Oceans in the Banda Sea, which means that it has a long travel time (at least a few years). Water taking the eastern route therefore loses its original properties before arriving in the Indian Ocean. The transport processes also are significantly affected by seasonal variations in equatorial circulation in the western Pacific. In the surface layer, North Pacific water is vigorously supplied to the western route only from boreal spring to summer in association with the linkage between the current flowing through the Makassar Strait and the Mindanao Current. In other seasons, because the Mindanao Current is strongly linked with the North Equatorial Countercurrent and the New Guinea Coastal Current primarily by northeasterly monsoonal winds, its upper water flows back to the Pacific Ocean. In the subsurface layer, a pronounced inflow of Mindanao Current water into the western route occurs from boreal winter to spring, when the subsurface link between that current and the Equatorial Undercurrent tends to weaken. In the deep, the quasi-steady transport of Pacific water into the Indian Ocean via the eastern route is fed by the westward deep current in the equatorial Pacific.

Influences of Atlantic Climate Change on the Tropical Pacific via the Central American Isthmus

Recent global coupled model experiments suggest that the atmospheric bridge across Central America is a key conduit for Atlantic climate change to affect the tropical Pacific. A high-resolution regional ocean– atmosphere model (ROAM) of the eastern tropical Pacific is used to investigate key processes of this conduit by examining the response to a sea surface temperature (SST) cooling over the North Atlantic. The Atlantic cooling increases sea level pressure, driving northeasterly wind anomalies across the Isthmus of Panama year-round. While the atmospheric response is most pronounced during boreal summer/fall when the tropical North Atlantic is warm and conducive to deep convection, the Pacific SST response is strongest in winter/spring when the climatological northeast trade winds prevail across the isthmus. During winter, the northeasterly cross-isthmus winds intensify in response to the Atlantic cooling, reducing the SST in the Gulf of Panama by cold and dry advection from the Atlantic and by enhancing surface turbulent heat flux and mixing. This Gulf of Panama cooling reaches the equator and is amplified by the Bjerknes feedback during boreal spring. The equatorial anomalies of SST and zonal winds dissipate quickly in early summer as the seasonal development of the cold tongue increases the stratification of the atmospheric boundary layer and shields the surface from the Atlantic influence that propagates into the Pacific as tropospheric Rossby waves. The climatological winds over the far eastern Pacific warm pool turn southwesterly in summer/fall, superimposed on which the anomalous northesterlies induce a weak SST warming there. The ROAM results are compared with global model water-hosing runs to shed light on intermodel consistency and differences in response to the shutdown of the Atlantic meridional overturning circulation. Implications for interpreting paleoclimate changes such as Heinrich events are discussed. The results presented here also aid in understanding phenomena in the present climate such as the Central American midsummer drought and Atlantic multidecadal oscillation.

Oceanic fronts and jets around Japan: a review

This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.

What Maintains the SST Front North of the Eastern Pacific Equatorial Cold Tongue

Abstract A coupled ocean–atmosphere regional model suggests a mechanism for formation of a sharp sea surface temperature (SST) front north of the equator in the eastern Pacific Ocean in boreal summer and fall. Meridional convergence of Ekman transport at 5°N is forced by eastward turning of the southeasterly cross-equatorial wind, but the SST front forms considerably south of the maximum Ekman convergence. Geostrophic equatorward flow at 3°N in the lower half of the isothermally mixed layer enhances mixed layer convergence. Cold water is upwelled on or south of the equator and is advected poleward by mean mixed layer flow and by eddies. The mixed layer current convergence in the north confines the cold advection, so the SST front stays close to the equator. Warm advection from the north and cold advection from the south strengthen the front. In the Southern Hemisphere, a continuous southwestward current advects cold water far from the upwelling core. The cold tongue is warmed by the net surface flux, whic...

Dynamics of Biweekly Oscillations in the Equatorial Indian Ocean

Variability of the wind field over the equatorial Indian Ocean is spread throughout the intraseasonal (10–60 day) band. In contrast, variability of the near-surface field in the eastern, equatorial ocean is concentrated at biweekly frequencies and is largely composed of Yanai waves. The excitation of this biweekly variability is investigated using an oceanic GCM and both analytic and numerical versions of a linear, continuously stratified (LCS) model in which solutions are represented as expansions in baroclinic modes. Solutions are forced by Quick Scatterometer (QuikSCAT) winds (the model control runs) and by idealized winds having the form of a propagating wave with frequency and wavenumber kw. The GCM and LCS control runs are remarkably similar in the biweekly band, indicating that the dynamics of biweekly variability are fundamentally linear and wind driven. The biweekly response is composed of local (nonradiating) and remote (Yanai wave) parts, with the former spread roughly uniformly along the equator and the latter strengthening to the east. Test runs to the numerical models separately forced by the x and y components of the QuikSCAT winds demonstrate that both forcings contribute to the biweekly signal, the response forced by y being somewhat stronger. Without mixing, the analytic spectrum for Yanai waves forced by idealized winds has a narrowband (resonant) response for each baroclinic mode: Spectral peaks occur whenever the wavenumber of the Yanai wave for mode n is sufficiently close to kw and they shift from biweekly to lower frequencies with increasing modenumber n. With mixing, the higher-order modes are damped so that the largest ocean response is restricted to Yanai waves in the biweekly band. Thus, in the LCS model, resonance and mixing act together to account for the ocean’s favoring the biweekly band. Because of the GCM’s complexity, it cannot be confirmed that vertical mixing also damps its higher-order modes; other possible processes are nonlinear interactions with near-surface currents, and the model’s low vertical resolution below the thermocline. Test runs to the LCS model show that Yanai waves from several modes superpose to form a beam (wave packet) that carries energy downward as well as eastward. Reflections of such beams from the near-surface pycnocline and bottom act to maintain near-surface energy levels, accounting for the eastward intensification of the near-surface, equatorial field in the control runs.

Vertical mixing in the ocean and its impact on the coupled ocean-atmosphere system in the eastern tropical Pacific.

Abstract The zonal and meridional asymmetries in the eastern tropical Pacific (the eastern equatorial cold tongue and the northern intertropical convergence zone) are key aspects of the region that are strongly influenced by ocean–atmosphere interactions. Here the authors investigate the impact of vertical mixing in the ocean on these asymmetries, employing a coupled ocean–atmosphere regional model. Results highlight the need to study the impact of processes such as vertical mixing in the context of the coupled system. Changes to the vertical mixing in the ocean are found to produce large changes in the state of the system, which include changes to the surface properties of the ocean, the ocean currents, the surface wind field, and clouds and precipitation in the atmosphere. Much of the strength of the impact is through interactions between the ocean and atmosphere. Increasing ocean mixing has an opposite effect on the zonal and meridional asymmetries. The zonal asymmetry is increased (i.e., a colder east...

Effect of shallow cumulus convection on the eastern Pacific climate in a coupled model

This work has been funded by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) as category 7 of the RR2002 Project, by JAMSTEC, and by the United States National Oceanic and Atmospheric Administration (NOAA).

Impacts of a warming marginal sea on torrential rainfall organized under the Asian summer monsoon.

Monsoonal airflow from the tropics triggers torrential rainfall over coastal regions of East Asia in summer, bringing flooding situations into areas of growing population and industries. However, impacts of rapid seasonal warming of the shallow East China Sea ECS and its pronounced future warming upon extreme summertime rainfall have not been explored. Here we show through cloudresolving atmospheric model simulations that observational tendency for torrential rainfall events over western Japan to occur most frequently in July cannot be reproduced without the rapid seasonal warming of ECS. The simulations also suggest that the future ECS warming will increase precipitation substantially in such an extreme event as observed in midJuly 2012 and also the likelihood of such an event occurring in June. A need is thus urged for reducing uncertainties in future temperature projections over ECS and other marginal seas for better projections of extreme summertime rainfall in the surrounding areas.

A striking early-summer event of a convective rainband persistent along the warm Kuroshio in the East China Sea

ABSTRACT A narrow, well-defined rainband persisted over the East China Sea on 19–20 May 2010, well separated from the Baiu/Meiyu front to its north. The rainband formed along the Kuroshio, leading us to the hypothesis that its high sea-surface temperature (SST) helped organise and maintain convective precipitation within the warm, moist surface southerlies. This hypothesis is verified through a pair of experiments with a regional atmospheric model. An experiment where high-resolution SST is prescribed as the lower-boundary condition is successful in reproducing the observed rainband. The reproduction is, however, unsuccessful in the other experiment where the narrow band of SST maxima along the Kuroshio has been artificially eliminated by smoothing. These experiments demonstrate that the high SST along the Kuroshio was of critical importance in organising the convective rainband separated from the Baiu/Meiyu front, thus presenting evidence that a mid-latitude western boundary current can influence the overlying atmosphere. Additional experiments suggest that the orography of Taiwan can also contribute positively to the organisation of the rainband by enhancing the convergence of the surface southerlies over the warm Kuroshio.