Hiroaki Tatebe

Improved Climate Simulation by MIROC5: Mean States, Variability, and Climate Sensitivity

Abstract A new version of the atmosphere–ocean general circulation model cooperatively produced by the Japanese research community, known as the Model for Interdisciplinary Research on Climate (MIROC), has recently been developed. A century-long control experiment was performed using the new version (MIROC5) with the standard resolution of the T85 atmosphere and 1° ocean models. The climatological mean state and variability are then compared with observations and those in a previous version (MIROC3.2) with two different resolutions (medres, hires), coarser and finer than the resolution of MIROC5. A few aspects of the mean fields in MIROC5 are similar to or slightly worse than MIROC3.2, but otherwise the climatological features are considerably better. In particular, improvements are found in precipitation, zonal mean atmospheric fields, equatorial ocean subsurface fields, and the simulation of El Nino–Southern Oscillation. The difference between MIROC5 and the previous model is larger than that between th...

Pacific decadal oscillation hindcasts relevant to near-term climate prediction

Decadal-scale climate variations over the Pacific Ocean and its surroundings are strongly related to the so-called Pacific decadal oscillation (PDO) which is coherent with wintertime climate over North America and Asian monsoon, and have important impacts on marine ecosystems and fisheries. In a near-term climate prediction covering the period up to 2030, we require knowledge of the future state of internal variations in the climate system such as the PDO as well as the global warming signal. We perform sets of ensemble hindcast and forecast experiments using a coupled atmosphere-ocean climate model to examine the predictability of internal variations on decadal timescales, in addition to the response to external forcing due to changes in concentrations of greenhouse gases and aerosols, volcanic activity, and solar cycle variations. Our results highlight that an initialization of the upper-ocean state using historical observations is effective for successful hindcasts of the PDO and has a great impact on future predictions. Ensemble hindcasts for the 20th century demonstrate a predictive skill in the upper-ocean temperature over almost a decade, particularly around the Kuroshio-Oyashio extension (KOE) and subtropical oceanic frontal regions where the PDO signals are observed strongest. A negative tendency of the predicted PDO phase in the coming decade will enhance the rising trend in surface air-temperature (SAT) over east Asia and over the KOE region, and suppress it along the west coasts of North and South America and over the equatorial Pacific. This suppression will contribute to a slowing down of the global-mean SAT rise.

An overview of decadal climate predictability in a multi-model ensemble by climate model MIROC

Decadal climate predictability is examined in hindcast experiments by a multi-model ensemble using three versions of the coupled atmosphere-ocean model MIROC. In these hindcast experiments, initial conditions are obtained from an anomaly assimilation procedure using the observed oceanic temperature and salinity with prescribed natural and anthropogenic forcings on the basis of the historical data and future emission scenarios in the Intergovernmental Panel of Climate Change. Results of the multi-model ensemble in our hindcast experiments show that predictability of surface air temperature (SAT) anomalies on decadal timescales mostly originates from externally forced variability. Although the predictable component of internally generated variability has considerably smaller SAT variance than that of externally forced variability, ocean subsurface temperature variability has predictive skills over almost a decade, particularly in the North Pacific and the North Atlantic where dominant signals associated with Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) are observed. Initialization enhances the predictive skills of AMO and PDO indices and slightly improves those of global mean temperature anomalies. Improvement of these predictive skills in the multi-model ensemble is higher than that in a single-model ensemble.

Oyashio Southward Intrusion and Cross-Gyre Transport Related to Diapycnal Upwelling in the Okhotsk Sea

Abstract The intrusion of western boundary currents (WBC) across the wind-driven gyre boundary and associated cross-gyre transport are examined using layered numerical models with realistic topography and annual mean wind stress, with special emphasis on the Oyashio southward intrusion and North Pacific Intermediate Water (NPIW) formation. Diapycnal transport due to strong tidal mixing around the Kuril Islands in the subarctic North Pacific Ocean is modeled by source/sink thickness restoring terms in the continuity equation to form the deep pycnocline observed in the Okhotsk Sea. About 3 Sv (Sv ≡ 106 m3 s−1) of the model diapycnal upwelling occurring along the Kuril Islands induces a meridional–diapycnal overturning circulation, and the transport is consistent with the estimates from recent observations with chemical tracers, GCM results, and the present study using observed density fields with enhanced diapycnal diffusivity around the Kuril Islands. The model with the diapycnal transport reproduced the r...

Effects of the 18.6-yr Modulation of Tidal Mixing on the North Pacific Bidecadal Climate Variability in a Coupled Climate Model

AbstractDiapycnal mixing induced by tide–topography interaction, one of the essential factors maintaining the global ocean circulation and hence the global climate, is modulated by the 18.6-yr period oscillation of the lunar orbital inclination, and has therefore been hypothesized to influence bidecadal climate variability. In this study, the spatial distribution of diapycnal diffusivity together with its 18.6-yr oscillation estimated from a global tide model is incorporated into a state-of-the-art numerical coupled climate model to investigate its effects on climate variability over the North Pacific and to understand the underlying physical mechanism. It is shown that a significant sea surface temperature (SST) anomaly with a period of 18.6 years appears in the Kuroshio–Oyashio Extension region; a positive (negative) SST anomaly tends to occur during strong (weak) tidal mixing. This is first induced by anomalous horizontal circulation localized around the Kuril Straits, where enhanced modulation of tida...

Predictability of Two Types of El Niño Assessed Using an Extended Seasonal Prediction System by MIROC

AbstractPredictability of El Nino–Southern Oscillation (ENSO) is examined using ensemble hindcasts made with a seasonal prediction system based on the atmosphere and ocean general circulation model, the Model for Interdisciplinary Research on Climate, version 5 (MIROC5). Particular attention is paid to differences in predictive skill in terms of the prediction error for two prominent types of El Nino: the conventional eastern Pacific (EP) El Nino and the central Pacific (CP) El Nino, the latter having a maximum warming around the date line. Although the system adopts ocean anomaly assimilation for the initialization process, it maintains a significant ability to predict ENSO with a lead time of more than half a year. This is partly due to the fact that the system is little affected by the “spring prediction barrier,” because increases in the error have little dependence on the thermocline variability. Composite analyses of each type of El Nino reveal that, compared to EP El Ninos, the ability to predict C...

Seasonal axis migration of the upstream Kuroshio Extension associated with standing oscillations

The TOPEX/Poseidon altimeter data from October 1992 to October 1998 are analyzed to investigate the seasonal cycle of the sea surface height (SSH) in the upstream region of the Kuroshio Extension. After removing steric height changes caused by surface buoyancy fluxes, two maxima of the seasonal SSH amplitude are found around the crest and trough of the quasi-stationary meanders as standing waves, accompanied with nodes of amplitude minima. Seasonal variations are identified in the subsurface temperature at 400 m depth similar to those of SSH in amplitude and phase. Associated with the standing waves, the current follows a straight path in fall and then changes to a meandering path in spring. From a composite map of the seasonal Kuroshio Extension axes, nodes of minimum displacement are found. One node is located over the Izu Ridge, while the others are located in the areas where meridional acceleration is large. The seasonal axis migration of the upstream Kuroshio Extension is regulated by these nodes.

Control of Decadal and Bidecadal Climate Variability in the Tropical Pacific by the Off-Equatorial South Pacific Ocean

AbstractDelayed negative feedback processes determining intrinsic decadal and bidecadal time scales for the tropical variability in the Pacific are investigated based on climate model experiments. By comparing a control run driven by preindustrial forcing and partial blocking runs driven by the same forcing but with ocean temperature and salinity restored to climatology in selected regions, subsurface oceanic signals of South Pacific origin are shown to precede SST variability in the Nino-3.4 region. Using a linear reduced-gravity ocean model driven only by wind stress changes and an offline tracer model, oceanic wave adjustment triggered by changes of wind stress curl in the South Pacific extratropics is suggested to be essential for the decadal component of the equatorial SST, while slower isopycnal advection of subsurface temperature anomalies from the formation region of South Pacific Eastern Subtropical Mode Water controls the bidecadal component. The intrinsic time scales of the tropical variability...

South Pacific influence on the termination of El Nino in 2014

The El Niño/Southern Oscillation (ENSO) is the dominant mode of climate variability affecting worldwide extreme weather events; therefore, improving ENSO prediction is an important issue. In this regard, a peculiar time evolution of ENSO in 2014 posed a challenge to the climate science community. Despite the observance of several precursors for a strong El Niño to develop during the summer and autumn, cold sea surface temperature (SST) anomalies appeared unexpectedly to the south of the equatorial cold tongue, which prevented development of an El Niño event in the late summer. Several hypotheses have been raised to explain the unmaterialized El Niño in 2014, but complete understanding of processes responsible for terminating this event has not yet been obtained. Here we show, using observations and extended seasonal prediction experiments with a climate model, that cold off-equatorial subsurface water in the South Pacific Ocean penetrated into the equatorial region along the slanted isopycnal surface via the mean advection, and it prevented the El Niño evolution in 2014. The negative subsurface temperature anomalies in the off-equatorial South Pacific Ocean were persistent throughout the last decade, and additional numerical simulations indicated that they contributed to the suppression of El Niño events during the 2000s.

Numerical Experiments on the Seasonal Variations of the Oyashio near the East Coast of Japan

Abstract Seasonal variations of the Oyashio near the east coast of Japan are examined using a simple layered model with a localized diapycnal transport due to the strong tidal mixing around the Kuril Islands. The modeled southernmost latitude is located at around 40°N in spring and 42°N in autumn. The southwestward Oyashio transport in the upper layers and depth-integrated transport become large (small) during winter–spring (summer–autumn). These results are in agreement with observations. The seasonal variation of the southernmost latitude is mainly caused by the baroclinic velocity anomalies off Tohoku and Hokkaido, Japan, which have been excited by the wind stress changes along the west coast of North America, the Kamchatka Peninsula, and the model northern boundary. This is different from the previous studies that emphasized the importance of direct barotropic response of the ocean to the basin-scale wind stress changes. For comparison, an additional experiment in which the diapycnal transport around ...