Nozomi Sugiura

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.

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.

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.

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.

Synchronization of coupled stick-slip oscillators

Abstract. A rationale is provided for the emergence of synchronization in a system of coupled oscillators in a stick-slip motion. The single oscillator has a limit cycle in a region of the state space for each parameter set beyond the supercritical Hopf bifurcation. The two-oscillator system that has similar weakly coupled oscillators exhibits synchronization in a parameter range. The synchronization has an anti-phase nature for an identical pair. However, it tends to be more in-phase for a non-identical pair with a rather weak coupling. A system of three identical oscillators (1, 2, and 3) coupled in a line (with two springs k12=k23) exhibits synchronization with two of them (1 and 2 or 2 and 3) being nearly in-phase. These collective behaviours are systematically estimated using the phase reduction method.

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.

A Framework for Interpreting Regularized State Estimation

AbstractFour-dimensional variational data assimilation (4D-Var) on a seasonal-to-interdecadal time scale under the existence of unstable modes can be viewed as an optimization problem of synchronized, coupled chaotic systems. The problem is tackled by adjusting initial conditions to bring all stable modes closer to observations and by using a continuous guide to direct unstable modes toward a reference time series. This interpretation provides a consistent and effective procedure for solving problems of long-term state estimation. By applying this approach to an ocean general circulation model with a parameterized vertical diffusion procedure, it is demonstrated that tangent linear and adjoint models in this framework should have no unstable modes and hence be suitable for tracking persistent signals. This methodology is widely applicable to extend the assimilation period in 4D-Var.

Improving Computational Efficiency of 4D-VAR System for Global Ocean Circulation Study

4D-VAR is an important tool for Ocean Circulation Study. But it costs huge computational burden. To improve the computational efficiency preconditioning and parallelization were applied. We applied a simple preconditoning for conjugate gradient method which was effective for quick convergence. And the parallelization of component codes by MPI coding was effective for the speed-up of each iteration cycle. As adjoint calculation needs huge information from forward calculation, some efficiency treatment of the data was also applied. With all of these considerations we have constructed a 4D-VAR assimilation system on Earth Simulator for global ocean circulation study at higher resolution.

The Onsager–Machlup functional for data assimilation

Abstract. When taking the model error into account in data assimilation, one needs to evaluate the prior distribution represented by the Onsager–Machlup functional. Through numerical experiments, this study clarifies how the prior distribution should be incorporated into cost functions for discrete-time estimation problems. Consistent with previous theoretical studies, the divergence of the drift term is essential in weak-constraint 4D-Var (w4D-Var), but it is not necessary in Markov chain Monte Carlo with the Euler scheme. Although the former property may cause difficulties when implementing w4D-Var in large systems, this paper proposes a new technique for estimating the divergence term and its derivative.