scholarly journals A Framework for Interpreting Regularized State Estimation

2014 ◽  
Vol 142 (1) ◽  
pp. 386-400 ◽  
Author(s):  
Nozomi Sugiura ◽  
Shuhei Masuda ◽  
Yosuke Fujii ◽  
Masafumi Kamachi ◽  
Yoichi Ishikawa ◽  
...  
Keyword(s):  
State Estimation ◽  
General Circulation ◽  
Optimization Problem ◽  
Chaotic Systems ◽  
Initial Conditions ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Vertical Diffusion ◽  
Reference Time

Abstract Four-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.

scholarly journals Influence of XBT Temperature Bias on Decadal Climate Prediction with a Coupled Climate Model

2011 ◽  
Vol 24 (20) ◽  
pp. 5303-5308 ◽  
Author(s):  
Sayaka Yasunaka ◽  
Masayoshi Ishii ◽  
Masahide Kimoto ◽  
Takashi Mochizuki ◽  
Hideo Shiogama
Keyword(s):  
General Circulation ◽  
Climate Model ◽  
Initial Conditions ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Climate Prediction ◽  
Temperature Bias ◽  
Subsurface Ocean ◽  
Decadal Climate ◽  
Decadal Climate Prediction

Abstract The influence of the expendable bathythermograph (XBT) depth bias correction on decadal climate prediction is presented by using a coupled atmosphere–ocean general circulation model called the Model for Interdisciplinary Research on Climate 3 (MIROC3). The global mean subsurface ocean temperatures that were simulated by the model with the prescribed anthropogenic and natural forcing are consistent with bias-corrected observations from the mid-1960s onward, but not with uncorrected observations. The latter is reflected by biases in subsurface ocean temperatures, particularly along thermoclines in the tropics and subtropics. When the correction is not applied to XBT observations, these biases are retained in data assimilation results for the model’s initial conditions. Hindcasting past Pacific decadal oscillations (PDOs) is more successful in the experiment with the bias-corrected observations than that without the correction. Improvement of skill in predicting 5-yr mean vertically average ocean subsurface temperature is also seen in the tropical and the central North Pacific where PDO-related signals appear large.

scholarly journals Long-Term Evolution of the Caspian Sea Thermohaline Properties Reconstructed in an Eddy-Resolving OGCM

2018 ◽  
Author(s):  
Gleb S. Dyakonov ◽  
Rashit A. Ibrayev
Keyword(s):  
Caspian Sea ◽  
General Circulation ◽  
Decadal Variability ◽  
Global Climate ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Atmospheric Conditions ◽  
The Caspian Sea ◽  
Error Accumulation

Abstract. The decadal variability of the Caspian Sea thermohaline properties is investigated by means of a high-resolution ocean general circulation model including sea ice thermodynamics and air-sea interaction, forced by prescribed realistic atmospheric conditions and riverine runoff. The model describes synoptic, seasonal and climatic variations of the sea thermohaline structure, water balance and level height. A reconstruction experiment was conducted for the period of 1961–2001, covering a major regime shift in the global climate of 1976–1978, which allows to investigate the Caspian Sea response to such significant episodes of climate change. The long-term trends in the sea circulation patterns are considered with an assessment of the influence of model error accumulation.

scholarly journals Long-term evolution of Caspian Sea thermohaline properties reconstructed in an eddy-resolving ocean general circulation model

Ocean Science ◽  
2019 ◽  
Vol 15 (3) ◽  
pp. 527-541 ◽  
Author(s):  
Gleb S. Dyakonov ◽  
Rashit A. Ibrayev
Keyword(s):  
Sea Level ◽  
General Circulation Model ◽  
Caspian Sea ◽  
General Circulation ◽  
Global Climate ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Atmospheric Conditions ◽  
Ocean General Circulation

Abstract. Decadal variability in Caspian Sea thermohaline properties is investigated using a high-resolution ocean general circulation model including sea ice thermodynamics and air–sea interaction forced by prescribed realistic atmospheric conditions and riverine runoff. The model describes synoptic, seasonal and climatic variations of sea thermohaline structure, water balance, and sea level. A reconstruction experiment was conducted for the period of 1961–2001, covering a major regime shift in the global climate during 1976–1978, which allowed for an investigation of the Caspian Sea response to such significant episodes of climate variability. The model reproduced sea level evolution reasonably well despite the fact that many factors (such as possible seabed changes and insufficiently explored underground water infiltration) were not taken into account in the numerical reconstruction. This supports the hypothesis relating rapid Caspian Sea level rise in 1978–1995 with global climate change, which caused variation in local atmospheric conditions and riverine discharge reflected in the external forcing data used, as is shown in the paper. Other effects of the climatic shift are investigated, including a decrease in salinity in the active layer, strengthening of its stratification and corresponding diminishing of convection. It is also demonstrated that water exchange between the three Caspian basins (northern, middle and southern) plays a crucial role in the formation of their thermohaline regime. The reconstructed long-term trends in seawater salinity (general downtrend after 1978), temperature (overall increase) and density (general downtrend) are studied, including an assessment of the influence of main surface circulation patterns and model error accumulation.

scholarly journals The South China Sea Throughflow Retrieved from Climatological Data*

2009 ◽  
Vol 39 (3) ◽  
pp. 753-767 ◽  
Author(s):  
Max Yaremchuk ◽  
Julian McCreary ◽  
Zuojun Yu ◽  
Ryo Furue
Keyword(s):  
South China Sea ◽  
General Circulation Model ◽  
South China ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Luzon Strait ◽  
The South China Sea ◽  
China Sea ◽  
Ocean General Circulation

Abstract The salinity distribution in the South China Sea (SCS) has a pronounced subsurface maximum from 150–220 m throughout the year. This feature can only be maintained by the existence of a mean flow through the SCS, consisting of a net inflow of salty North Pacific tropical water through the Luzon Strait and outflow through the Mindoro, Karimata, and Taiwan Straits. Using an inverse modeling approach, the authors show that the magnitude and space–time variations of the SCS thermohaline structure, particularly for the salinity maximum, allow a quantitative estimate of the SCS throughflow and its distribution among the three outflow straits. Results from the inversion are compared with available observations and output from a 50-yr simulation of a highly resolved ocean general circulation model. The annual-mean Luzon Strait transport is found to be 2.4 ± 0.6 Sv (Sv ≡ 106 m3 s−1). This inflow is balanced by the outflows from the Karimata (0.3 ± 0.5 Sv), Mindoro (1.5 ± 0.4), and Taiwan (0.6 ± 0.5 Sv) Straits. Results of the inversion suggest that the Karimata transport tends to be overestimated in numerical models. The Mindoro Strait provides the only passage from the SCS deeper than 100 m, and half of the SCS throughflow (1.2 ± 0.3 Sv) exits the basin below 100 m in the Mindoro Strait, a result that is consistent with a climatological run of a 0.1° global ocean general circulation model.

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