Shuhei Masuda

Simulated Rapid Warming of Abyssal North Pacific Waters

Warming the Deep The coldest ocean waters are located at the bottoms of the major ocean basins, and, because it takes a long time for water to sink from the surface to these regions, they are relatively isolated from the warming trends that are now occurring at shallower depths. However, warming in these deep waters has recently been observed, sooner than anticipated. Masuda et al. (p. 319, published online 24 June) performed computer simulations of ocean circulation and found that internal waves are able to transport heat rapidly from the surface waters around Antarctica to the bottom of the North Pacific, which can occur within four decades, rather than the centuries that conventional mechanisms have suggested. Computer simulations suggest a possible reason for the warming of North Pacific bottom water during the past four decades. Recent observational surveys have shown significant oceanic bottom-water warming. However, the mechanisms causing such warming remain poorly understood, and their time scales are uncertain. Here, we report computer simulations that reveal a fast teleconnection between changes in the surface air-sea heat flux off the Adélie Coast of Antarctica and the bottom-water warming in the North Pacific. In contrast to conventional estimates of a multicentennial time scale, this link is established over only four decades through the action of internal waves. Changes in the heat content of the deep ocean are thus far more sensitive to the air-sea thermal interchanges than previously considered. Our findings require a reassessment of the role of the Southern Ocean in determining the impact of atmospheric warming on deep oceanic waters.

Effects of Stratification and Bottom Topography on the Kuroshio Path Variation South of Japan. Part I: Dependence of the Path Selection on Velocity

Abstract Numerical experiments are executed using a two-layer inflow–outflow ocean model with simplified geometry to investigate the effects of stratification and bottom topography on the path variation of the Kuroshio south of Japan. In a flat-bottom ocean, the dependence of the Kuroshio path selection on its inflow velocity Vmax is basically the same as in a barotropic ocean, that is, the Kuroshio takes a straight path at low Vmax (regime I), a meandering path at high Vmax (regime II), and both paths at intermediate Vmax (regime III: multiple equilibrium state). However, the range of regime III shifts to higher Vmax by 0.10∼0.30 m s−1. Stratification causes and maintains the offshore shift of the current path south of Kyushu through the conservation of potential vorticity. As a result, a small meander stagnates southeast of Kyushu, not developing into a large meander even for higher Vmax since the vorticity supply from the coast is reduced. For the same reason, higher Vmax is needed to maintain a meande...

Effects of Stratification and Bottom Topography on the Kuroshio Path Variation South of Japan. Part II: Path Transitions in a Multiple Equilibrium Regime

Abstract The effects of stratification and bottom topography on the Kuroshio path transitions in a multiple equilibrium regime due to a short-term velocity increase are examined using a two-layer inflow–outflow model with simplified coastal geometry and bottom topography. For an imposed velocity increase on a straight-path state, the typical transition from a straight to a meandering path occurs with or without bottom topography through the same process as in a barotropic case with flat bottom. Thus, the geometrical effect of Kyushu is essential to this transition or to the formation of a small meander triggering the transition; stratification and bottom topography are somewhat secondary. Nevertheless, under the influence of stratification, a small meander significantly develops south of Kyushu during a decreasing phase of velocity through shoaling of the interface depth and triggers the transition with the amplification rate of velocity Amp of 1.5. The continental slope south of Japan prevents a small me...

Improved coupled GCM climatologies for summer monsoon onset studies over Southeast Asia

[1] To enhance accurate estimates of Asian monsoon variability by a coupled general circulation model (GCM), the bulk adjustment factors that control latent heat, sensible heat and momentum fluxes are optimized using a 4-dimensional variational data assimilation method. When using the optimized values, a coupled GCM is better able to define the summer monsoon features over Southeast Asia. In particular, the early, rapid onset is realistically simulated around the Indochina Peninsula, which is a key region for the initial stage of the Asian summer monsoon development. The spatial patterns of precipitation rate, wind velocity, and sea surface temperature are successfully reproduced. The optimized values of the bulk adjustment factors for latent heat flux become significantly correlated with the strength of the subgrid-scale disturbances primarily associated with energetic deep convective activities in the tropics, which is one of the major sources of model biases in commonly-used coarse resolution models.

Evaluation of the applicability of the Estimated State of the Global Ocean for Climate Research (ESTOC) data set

A long-term data synthesis experiment was conducted for the period 1957–2011 using a modified quasi-global four-dimensional variational data assimilation system that was originally developed to improve the representation of the deep ocean, including a unique method for anomaly assimilation. The overall characteristics of the resulting ocean state estimate, which is dynamically consistent without any artificial sources or sinks for heat and salt, are evaluated in the Pacific Ocean. It is shown that the data set better represents the comprehensive ocean state: the mean state of the water mass distribution and volume transport and components of temporal variability from the sea surface to the bottom on interannual to multidecadal timescales. This suggests that the data set can be used to examine interactions between temporal variations throughout the entire depth range and is useful for understanding ocean physics and its role in the climate system.

Multiyear climate prediction with initialization based on 4D‐Var data assimilation

An initialization relevant to interannual-to-decadal climate prediction has usually used a simple restoring approach for oceanic variables. Here we demonstrate the potential use of four-dimensional variational (4D-Var) data assimilation on the leading edge of initialization approach particularly in multiyear (5 year long) climate prediction. We perform full-field initialization rather than anomaly initialization and assimilate the atmosphere states together with the ocean states to an atmosphere-ocean coupled climate model. In particular, it is noteworthy that ensembles of multiyear hindcasts using our assimilation results as initial conditions exhibit an improved skill in hindcasting the multiyear changes of the upper ocean heat content (OHC) over the central North Pacific. The 4D-Var approach enables us to directly assimilate a time trajectory of slow changes of the Aleutian Low that are compatible with the sea surface height and the OHC. Consequently, we can estimate a coupled climate state suitable for hindcasting dynamical changes over the extratropical North Pacific as observed.

Role of the oceanic bridge in linking the 18.6 year modulation of tidal mixing and long‐term SST change in the North Pacific

The impact of the 18.6 year modulation of tidal mixing on sea surface temperature (SST) in the North Pacific is investigated in a comparative study using an ocean data synthesis system. We show that remote impact through a slow ocean response can make a significant contribution to the observed bidecadal variation in wintertime SST near the center of action of the Pacific Decadal Oscillation in the eastern Pacific. A comparative data synthesis experiment showed that the modified SST variation is amplified by bidecadal variation in the westerly wind. This relationship between SST and wind variations is consistent with an observed air-sea coupled mode in the extratropics, which suggests that a midlatitude air-sea interaction plays an important role in enhancing the climate signal of the 18.6 year modulation. This result supports the hypothesis that the 18.6 year tidal cycle influences long-term variability in climate; thus, knowledge of this cycle could contribute toward improving decadal predictions of climate.

Methods and Applications of Ocean Synthesis in Climate Research

The past 20 years have provided us with an unprecedented ability to observe, monitor, and forecast the oceans. In situ and remotely sensed ocean observations in combination with ocean general circulation models using data assimilation and state estimation methods underpin climate applications. State estimation aims to provide a dynamically consistent estimation of ocean fields, of errors of these fields, and of certain model parameters such as mixing coefficients. Conversely, data assimilation tools have been developed predominantly for ocean prediction applications and ocean reanalyses. This chapter describes approaches used by state estimation and data assimilation systems in synthesizing observations and model dynamics. We highlight some applications, including their limitations for climate research, and address the challenges ahead in relation to the ocean observing system.

A new Approach to El Niño Prediction beyond the Spring Season.

The enormous societal importance of accurate El Niño forecasts has long been recognized. Nonetheless, our predictive capabilities were once more shown to be inadequate in 2014 when an El Nino event was widely predicted by international climate centers but failed to materialize. This result highlighted the problem of the opaque spring persistence barrier, which severely restricts longer-term, accurate forecasting beyond boreal spring. Here we show that the role played by tropical seasonality in the evolution of the El Niño is changing on pentadal (five-year) to decadal timescales and thus that El Niño predictions beyond boreal spring will inevitably be uncertain if this change is neglected. To address this problem, our new coupled climate simulation incorporates these long-term influences directly and generates accurate hindcasts for the 7 major historical El Niños. The error value between predicted and observed sea surface temperature (SST) in a specific tropical region (5°N–5°S and 170°–120°W) can consequently be reduced by 0.6 Kelvin for one-year predictions. This correction is substantial since an “El Niño” is confirmed when the SST anomaly becomes greater than +0.5 Kelvin. Our 2014 forecast is in line with the observed development of the tropical climate.

Multidecadal change in the dissolved inorganic carbon in a long‐term ocean state estimation

By using a 4-dimensional variational data assimilation system capable of estimating physical and biogeochemical variables for the global ocean, we investigated multi-decadal changes in the dissolved inorganic carbon (DIC) in the ocean. The system was newly constructed with a pelagic ecosystem model and an oceanic general circulation model to integrate available ocean observations obtained with a wide range of observation tools. The distribution of estimated DIC was by and large consistent with previous reports. We validated the changes in DIC along the World Ocean Circulation Experiment (WOCE) Hydrographic Program sections. The correlation coefficients of the modeled versus observed decadal difference patterns of DIC ranged from 0.25 to 0.51 in the Pacific Ocean, from 0.36 to 0.62 in the Atlantic Ocean, and from 0.23 to 0.57 in the Indian Ocean, and were significant at the 95% confidence level. Thus, at basin scale, the reproducibility of long-term climate change was similar. Estimation of vertical DIC fluxes in each basin showed that the fluxes changed on a multi-decadal time scale in our system. These changes were possibly due to changes in the dynamical state of CO2 absorption and to changes in ocean circulation. Our integrated dataset on the basis of a dynamically self-consistent ocean state is a promising tool for examining long-term changes in the ocean carbon cycle. This article is protected by copyright. All rights reserved.