Bo Young Yim

Seasonal variation of eddy kinetic energy of the North Pacific Subtropical Countercurrent simulated by an eddy‐resolving OGCM

[1] The seasonal variation of eddy kinetic energy (EKE) of the North Pacific Subtropical Countercurrent (STCC) was investigated by analyzing the data from an eddy-resolving OGCM with the horizontal resolution 1/12° in comparison with satellite altimetry data. Although the annual cycle of simulated EKE of the whole STCC domain showed agreement with satellite data, with a maximum in spring and a minimum in fall, it revealed latitudinal dependence; that is, near 19°N the EKE level is higher in summer than in winter, but it is higher in winter than in summer north of 20°N. The OGCM also reproduced two branches of the STCC, which affect the EKE variation. The variation of EKE level was shown to be closely related to the growth rate of baroclinic eddies. The simulated zonal velocity is much larger than climatological value, thus allowing much faster growth of baroclinic eddies.

Pacific Decadal Oscillation and its relation to the extratropical atmospheric variation in CMIP5

Abstract It is investigated how the simulated Pacific Decadal Oscillation (PDO) differs among various coupled general circulation models (CGCMs), and how it is related to the simulated atmospheric variation in the North Pacific. The dataset of the historical runs of the 26 CGCMs reported to the Coupled Model Intercomparison Project Phase 5 are used. It is shown that the differences in the PDO pattern among 26 CGCMs are closely related to diverse displacements of the simulated Aleutian Low (AL), that is, the PDO pattern is highly dependent on longitudinal and latitudinal position of the anomalous AL associated with the PDO (PDO-related AL; ALPDO). In addition, it is demonstrated that in the models that have the southward shift of the ALPDO, the North Pacific sea surface temperature anomalies (SSTAs) associated with the PDO tend to modulate the atmospheric circulation strongly, implying strong two-way feedback between ocean and atmosphere. Therefore, this gives relatively strong persistency of the PDO. It is also found that the PDO in these models is highly correlated with the variation of the Kuroshio Extension current, implying that ocean dynamical processes play an important role in developing SSTAs.

The Vertical Structure of Eddy Heat Transport Simulated by an Eddy-Resolving OGCM

Abstract The vertical structure of meridional eddy heat transport (EHT) of the North Pacific was investigated by analyzing the results from an eddy-resolving ocean general circulation model (OGCM) with a horizontal resolution of , while comparing with previous simulation results and observation data. In particular, the spatial and temporal variation of the effective depth of EHT He was investigated, which is defined by the depth integrated EHT (D-EHT) divided by EHT at the surface. It was found that the annual mean value of He is proportional to the eddy kinetic energy (EKE) level at the surface in general. However, its seasonal variation is controlled by the mixed layer depth (MLD) in the extratropical ocean (>20°N). Examination of the simulated eddy structures reveals that the temperature associated with mesoscale eddies is radically modified by the surface forcing in the mixed layer, while the velocity field is not, and the consequent enhanced misalignment of temperature and velocity anomalies leads to...

Ocean mixed layer processes in the Pacific Decadal Oscillation in coupled general circulation models

It is investigated how the Pacific Decadal Oscillation (PDO) is simulated differently among various coupled general circulation models (CGCMs), and how it is related to the heat budget of the simulated ocean mixed layer, which includes the surface heat flux and ocean heat transport. For this purpose the dataset of the climate of the 20th Century experiment (20C3M) from nine CGCMs reported to IPCC’s AR4 are used, while the MRI and MIROC models are examined in detail. Detailed analyses of these two CGCMs reveal that the PDO is mainly affected by ocean heat transport rather than surface heat flux, in particular in the MRI model which has a larger contribution of ocean heat transport to the heat budget. It is found that the ocean heat transport due to Ekman advection versus geostrophic advection contributes differently to the PDO in the western and central North Pacific. Specifically, the strength of PDO tends to be larger for CGCMs with a larger ocean heat transport in the region.

Role of the ocean mixed layer processes in the response of the North Pacific winter SST and MLD to global warming in CGCMs

It is investigated how the changes of winter sea surface temperature (SST) and mixed layer depth (MLD) under climate change projections are predicted differently in the North Pacific depending on the coupled general circulation models (CGCMs), and how they are related to the dynamical property of the simulated ocean mixed layer. For this purpose the dataset from eleven CGCMs reported to IPCC’s AR4 are used, while detailed analysis is given to the MRI and MIROC models. Analysis of the CGCM data reveals that the increase of SST and the decrease of MLD in response to global warming tend to be smaller for the CGCM in which the ratio of ocean heat transport (OHT) to surface heat flux (SHF), R (=|OHT/SHF|), is larger in the heat budget of the mixed layer. The negative correlation is found between the changes of OHT and SHF under global warming, which may weaken the response to global warming in the CGCM with larger R. It is also found that the models with low horizontal resolution tend to give broader western boundary currents, larger R, and the smaller changes of SST and MLD under global warming.

Multi-model attribution of upper-ocean temperature changes using an isothermal approach.

Both air-sea heat exchanges and changes in ocean advection have contributed to observed upper-ocean warming most evident in the late-twentieth century. However, it is predominantly via changes in air-sea heat fluxes that human-induced climate forcings, such as increasing greenhouse gases, and other natural factors such as volcanic aerosols, have influenced global ocean heat content. The present study builds on previous work using two different indicators of upper-ocean temperature changes for the detection of both anthropogenic and natural external climate forcings. Using simulations from phase 5 of the Coupled Model Intercomparison Project, we compare mean temperatures above a fixed isotherm with the more widely adopted approach of using a fixed depth. We present the first multi-model ensemble detection and attribution analysis using the fixed isotherm approach to robustly detect both anthropogenic and natural external influences on upper-ocean temperatures. Although contributions from multidecadal natural variability cannot be fully removed, both the large multi-model ensemble size and properties of the isotherm analysis reduce internal variability of the ocean, resulting in better observation-model comparison of temperature changes since the 1950s. We further show that the high temporal resolution afforded by the isotherm analysis is required to detect natural external influences such as volcanic cooling events in the upper-ocean because the radiative effect of volcanic forcings is short-lived.