Pacific Ocean General Circulation Model Research Articles

Meridional shift of the Kuroshio Extension induced by response of recirculation gyre to decadal wind variations

Abstract The meridional shift of the Kuroshio Extension to decadal wind variations is examined by analyzing the hindcast simulation of a high-resolution ( 1 / 12 ∘ × 1 / 12 ∘ ) North Pacific ocean general circulation model (OGCM) and by conducting a series of idealized OGCM experiments. The meridional shift of the Kuroshio Extension in the hindcast is consistent with observations and former studies. Using a series of sensitivity experiments in idealized settings, the following possible mechanism for this shift is proposed. The baroclinic Rossby waves that arise in the northern half of the subtropical gyre cannot directly enter the Kuroshio–Oyashio Extension region due to the strong Kuroshio Extension jet. When the Rossby waves accompanying the negative (positive) sea-surface height anomalies propagate westward and reach the region to the east of the recirculation gyre, the difference in thermocline depth between the recirculation gyre and the region to its east becomes larger (smaller), resulting in stronger (weaker) baroclinic instabilities. These stronger (weaker) instabilities cause a shallower (deeper) thermocline in the recirculation gyre. To compensate for this shrinking (stretching), the recirculation gyre migrates southward (northward), leading to the southward (northward) shift of the Kuroshio Extension.

Decadal variability of the Subtropical Front of the western North Pacific in an eddy‐resolving ocean general circulation model

We examined decadal variability of the Subtropical Front (STF) of the western North Pacific by using a North Pacific ocean general circulation model (OGCM), comparing the results of three simulations with different horizontal resolutions. In the long‐term mean fields, the eddy‐resolving model (10 km) was able to simulate the distributions of the STF and the associated Subtropical Countercurrent (STCC) between 20°N and 30°N better than either the non‐eddy‐resolving model (100 km) or the eddy‐permitting model (20 km) because it simulated the Kuroshio recirculation gyre more realistically. The simulated STF intensity exhibited significant decadal‐scale variations: it was stronger in the late 1970s and weaker in the early 1990s. During these two periods, the simulated mode waters showed corresponding differences in their potential vorticity minima density and paths. We also investigated the relationship between the decadal‐scale STF variability and atmospheric forcing. The results suggest that the decadal‐scale STF variability can be largely explained by changes in the mode waters formed in the western North Pacific and advected to north of the STF by the subtropical gyre in response to a change in surface westerly winds that occurred in the mid‐1970s.

Preliminary Results from the Coupled Atmosphere-Ocean Regional Climate Model at the Meteorological Research Institute

A ten-year integration of the Meteorological Research Institute Coupled atmosphere-ocean Regional Climate Model (MRI-CRCM) was conducted to evaluate the model’s performance in reproducing the present climate around Japan (CRCM run). MRI-CRCM couples a 20 km-mesh atmosphere Regional Climate Model (RCM20), and a North Pacific Ocean General Circulation Model, whose horizontal resolution is 1/4° (longitude) × 1/6° (latitude). Multi-nesting method is used for calculating the atmospheric part. A 60 km-mesh atmosphere Regional Climate Model (RCM60) is nested in MRI Coupled General Circulation Model, and RCM20 is nested in RCM60. To verify the effect of coupling, an atmospheric RCM run, that specified SST from a global coupled model run, was also conducted (ARCM run). Compared with observations, sea surface temperatures (SSTs) are overestimated in both runs for the winter over the Japan Sea. In the CRCM run, the SST simulation is somewhat improved and surface air temperature (1.5 m above the ground) is also lower than in the ARCM run.Japan is classified into seven regions according to climatic features. Compared with observational data, summer surface air temperatures in both runs are somewhat high in most of Japan, except for southern regions. This warm bias is reduced in the CRCM run as compared to the ARCM. The improvements in the CRCM run are especially large in northern Japan, where the differences between the runs exceed 1.5 K. The precipitation simulations are compared with radar-AMeDAS data. A typical winter pattern with much rainfall along the coast of the Japan Sea, compared to the coast of the Pacific Ocean, are well reproduced in both runs, but the simulated precipitation is overestimated on the Pacific coast in winter. Precipitation is generally overestimated in northern Japan, while underestimated in western Japan throughout the year. Precipitation is less in the CRCM than in the ARCM run, because of the reduced latent heat flux and lower SSTs. The MRI-CRCM is an effective means to reproduce the Japanese climate, but has still biases in temperature and precipitation. Further improvements are needed.

Response of North Pacific Ocean Circulation in a Kuroshio-Resolving Ocean Model to an Arctic Oscillation (AO)-Like Change in Northern Hemisphere Atmospheric Circulation Due to Greenhouse-Gas Forcing

Time-slice experiments are performed using a high-resolution North Pacific ocean general circulation model (NPOGCM) resolving the strong currents near Japan, such as the Kuroshio and the Oyashio, to investigate the effect of global warming on the North Pacific ocean circulation. The NPOGCM is forced by heat, momentum, and fresh-water fluxes obtained from a global warming projection using a global climate model (MRI-CGCM2.2). The annual mean sea-level pressure trend exhibits an annular pattern similar to the positive phase of the Arctic Oscillation in a global warming projection by MRI-CGCM2.2 based on the Intergovernmental Panel on Climate Change (IPCC) SRES A2 emission scenario. Associated with this trend, the anticyclonic atmospheric circulation is intensified over the mid-latitude North-Pacific, leading to a northward shift of the oceanic subtropical wind-driven gyre boundary, where extensions of the Kuroshio exist in MRI-CGCM2.2. Under these forcing changes, NPOGCM projects that in the future climate warm core eddies are more frequently pinched off from the Kuroshio off the eastern coast of Japan, leading to an annual mean SST rise over 5 K at its maximum, compared with the present climate. The projected annual mean sea-level rise ranges from 12 to 18 cm along the coasts of Japan, and about 40 cm over the ocean east of Japan.

Balance of Volume Transports between Horizontal Circulation and Meridional Overturn in the North Pacific Subarctic Region

Horizontal and meridional volume transports on timescales from intra-seasonal to interannual in the North Pacific subarctic region were investigated using a reanalysis dataset for 1993–2001 that was constructed from an assimilation of the TOPEX altimeter and in situ data into an eddy-permitting North Pacific ocean general circulation model. The barotropic flow is excited along east of the Emperor Seamounts by the western intensification dynamics. The volume transport of this flow compensates for that across the interior region east of the Seamounts below the summit depth of the Seamounts. The Oyashio, which is also considered as a compensation flow for the transport in the whole interior region, includes baroclinic as well as barotropic components. Baroclinic transports in the whole interior region exceed those in the western boundary region in the upper (200–1000 m) and lower (2000–5000 m) layers, and the total transport is northward (southward) in the upper (lower) layer. These excesses of the baroclinic transport are balanced by a vertical transport of the meridional overturn. The meridional overturn has a complementary relation to the basin-scale baroclinic circulation in the North Pacific subactic region.

A Model Study of Oceanic Mechanisms Affecting Equatorial Pacific Sea Surface Temperature during the 1997–98 El Niño

In this study, the processes affecting sea surface temperature variability over the 1992-98 period, encompassing the very strong 1997-98 El Nino event, are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1-2and TAO wind stresses, and climatological heat and freshwater fluxes, is first validated against observations. The model reproduces the main features of the tropical Pacific mean state, despite a weaker than observed thermal stratification, a 0.1 m s21 too strong (weak) South Equatorial Current (North Equatorial Countercurrent), and a slight underestimate of the Equatorial Undercurrent. Good agreement is found between the model dynamic height and TOPEX/Poseidon sea level variability, with correlation/rms differences of 0.80/4.7 cm on average in the 108N-108S band. The model sea surface temperature variability is a bit weak, but reproduces the main features of interannual variability during the 1992-98 period. The model compares well with the TAO current variability at the equator, with correlation/rms differences of 0.81/0.23 m s 21 for surface currents. The model therefore reproduces well the observed interannual variability, with wind stress as the only interannually varying forcing. This good agreement with observations provides confidence in the comprehensive three-dimensional circulation and thermal structure of the model. A close examination of mixed layer heat balance is thus undertaken, contrasting the mean seasonal cycle of the 1993-96 period and the 1997-98 El Nino. In the eastern Pacific, cooling by exchanges with the subsurface (vertical advection, mixing, and entrainment), the atmospheric forcing, and the eddies (mainly the tropical instability waves) are the three main contributors to the heat budget. In the central-western Pacific, the zonal advection by low-frequency currents becomes the main contributor. Westerly wind bursts (in December 1996 and March and June 1997) were found to play a decisive role in the onset of the 1997-98 El Nino. They contributed to the early warming in the eastern Pacific because the downwelling Kelvin waves that they excited diminished subsurface cooling there. But it is mainly through eastward advection of the warm pool that they generated temperature anomalies in the central Pacific. The end of El Nino can be linked to the large-scale easterly anomalies that developed in the western Pacific and spread eastward, from the end of 1997 onward. In the far-western Pacific, because of the shallower than normal thermocline, these easterlies cooled the SST by vertical processes. In the central Pacific, easterlies pushed the warm pool back to the west. In the east, they led to a shallower thermocline, which ultimately allowed subsurface cooling to resume and to quickly cool the surface layer.

A Comparison of Two Vertical-Mixing Schemes in a Pacific Ocean General Circulation Model

The upper Pacific Ocean current and temperature have been simulated by a 3-dimensional ocean general circulation model (OGCM) with two different vertical mixing schemes.

On the mechanisms in a tropical ocean-global atmosphere coupled general circulation model. Part II: interannual variability and its relation to the seasonal cycle

The thirty year simulation of the coupled global atmosphere-tropical Pacific Ocean general circulation model of the Laboratoire de Meterologie Dynamique and the Laboratoire d’Oceanographie Dynamique et de Climatologie presented in Part I is further investigated in order to understand the mechanisms of interannual variability. The model does simulate interannual events with ENSO characteristics; the dominant periodicity is quasi-biennial, though strong events are separated by four year intervals. The mechanism that is responsible for seasonal oscillations, identified in Part I, is also active in interannual variability with the difference that now the Western Pacific is dynamically involved. A warm interannual phase is associated with an equatorward shift of the ITCZ in the Western and Central Pacific. The coupling between the ITCZ and the ocean circulation is then responsible for the cooling of the equatorial subsurface by the draining mechanism. Cold subsurface temperature anomalies then propagate eastward along the mean equatorial thermocline. Upon reaching the Eastern Pacific where the mean thermocline is shallow, cold subsurface anomalies affect surface temperatures and reverse the phase of the oscillation. The preferred season for efficient eastward propagation of thermocline depth temperature anomalies is boreal autumn, when draining of equatorial waters towards higher latitudes is weaker than in spring by a factor of six. In that way, the annual cycle acts as a dam that synchronizes lower frequency oscillations.

Simulation of the 1986–1987 El Niño and 1988 La Niña events with a free surface tropical Pacific Ocean general circulation model

Observed atmospheric forcing fields over the period 1984–1989 force a free surface tropical Pacific Ocean general circulation model. Numerical simulation of the 1986–1987 El Niño and 1988 La Niña events is presented in the paper. Some quantitative comparisons between model time series and corresponding observations of sea level, and upper ocean current and temperature are made to verify the model performance. Diagnostic analyses of heat balance and available energy budget are given as well. The space‐time evolution of various model variables demonstrates that the model produces interannual variations with reasonable success. Beginning in mid‐1986, westerly wind over the western equatorial Pacific drives strong eastward surface currents which accomplish the massive transfer of warm surface water. The strong westerly wind in late 1986 excites the pronounced equatorial Kelvin waves, which propagate eastward toward the eastern and coastal Pacific where they depress the thermocline and raise sea level twice, and increase sea surface temperature. The eastern Pacific warming occurs primarily from the diminished cooling contribution of vertical advection, whereas in the central Pacific, eastward advection by anomalous zonal flows is the principal mechanism. The El Niño conditions in the eastern Pacific disappear in mid‐1987, whereas they remain in the central and western Pacific until early 1988. Subsequently, the tropical Pacific Ocean rebounds to significant La Niña conditions. Available energy (AE) has a good phase relationship with respect to other variables characterized by warm and cold conditions. AE is anomalously high prior to a warm event, accompanying conversion from kinetic energy (KE) to available potential energy (APE). During the development of El Niño, although relaxation of trade wind reduces input of wind energy, the appearance of westerly wind in the western Pacific leads to a sharp increase in KE. This excites excessive conversion from APE to KE through work done by the buoyancy force, corresponding to out‐of‐phase fluctuation between KE and APE. In the aftermath of the warm event, APE, as well as KE and available surface energy, is depleted and is extremely low. Development of La Niña is characterized by long and slow accumulation of APE with the transformation from KE to APE.

Sea surface height variability during 1986–1988 in the tropical Pacific Ocean

Sea surface height variabilities during 1986–1988 in the tropical Pacific Ocean are described, based upon the Geosat altimeter data and a tropical Pacific Ocean general circulation model (OGCM) simulation forced with observed winds from Florida State University. Both data sets are validated against in situ sea level measurements at 12 selected sea level stations in the equatorial Pacific and were found to be in good agreement with these measurements with correlation coefficients (averaged over the 12 sea level stations in the equatorial Pacific) of 0.69 for Geosat and 0.71 for OGCM. Both the Geosat data and OGCM simulation capture significant amounts of variance of in situ measurements with standard deviations (averaged over the 12 sea level stations) of 7.7 cm for Geosat and 7.1 cm for OGCM as compared with that of 9.8 cm for in situ measurements. The Geosat altimeter data, given their uncertainties, are capable of describing sea surface height variabilities during 1986–1988 in the tropical Pacific Ocean, in particular the seasonal cycle, the 1987 El Niño and 1988 La Niña episodes, and the December 1986 equatorial Kelvin wave event. The OGCM‐simulated surface dynamic height is found sensitive to different surface wind products: The standard deviation of the surface dynamic height simulated by the European Center for Medium‐Range Weather Forecasting (ECMWF) 10‐m wind product is only 5.5 cm.

Surface heat flux parameterizations and tropical Pacific Sea surface temperature simulations

Models of the tropical Pacific Ocean routinely use bulk formulae to estimate the surface fluxes of heat into and out of the ocean. The formulae generally require atmospheric variables such as cloud cover, surface wind speed, air‐sea temperature difference, and relative humidity. Since these quantities are not provided by most tropical Pacific Ocean models, modelers have resorted to simplifying parameterizations in terms of a few primary variables. Several of these parameterizations are tested by using observed data and comparing fluxes calculated using the simplified parameterizations with those from the full bulk formula. The impact of the various parameterizations is also assessed in a tropical Pacific Ocean general circulation model by comparing model sea surface temperature (SST) with the monthly means and anomalies of observed tropical Pacific SST during a 6‐year period that included both warm and cold episodes. The different flux parameterizations yield a broad mix of results. Overall, using a constant relative humidity of 0.80 does well, while using a constant downward radiation and a constant air temperature minus SST produces large discrepancies in comparison with the full flux parameterizations. In agreement with previous studies the model runs demonstrate that SST variability on the equator is dominated by wave processes whereas off the equator the surface flux of heat plays a crucial role in determining changes in SST.

Simulation of ENSO with a Global Atmospheric GCM Coupled to a High-Resolution, Tropical Pacific Ocean GCM

A global atmospheric general circulation model (GCM) coupled to an oceanic GCM that is dynamically active only in the tropical Pacific simulates variability over a broad spectrum of frequencies even though the forcing, the annual mean incoming solar radiation, is steady. Of special interest is the simulation of a realistically irregular Southern Oscillation between warm El Niño and cold La Niña states. Its time scale is on the order of 5 years. The spatial structure is strikingly different in the eastern and western halves of the ocean basin. Sea surface temperature changes have their largest amplitude in the central and eastern tropical Pacific, but the low-frequency zonal wind fluctuations are displaced westward and are large over the western half of the basin. These zonal wind anomalies are essentially confined to the band of latitudes 10° to 10°S so that they form a jet and have considerable latitudinal shear. During El Niño the associated curl contributes to a pair of pronounced minima in thermocline depth, symmetrically about the equator in the west, near 8°N and 8°S. In the east, where the low-frequency wind forcing is at a minimum, the deepening of the thermocline in response to the winds in the west has a very different shape—an approximate Gaussian shape centered on the equator. The low-frequency sea surface temperature and zonal wind anomalies wax and wane practically in place and in phase without significant zonal phase propagation. Thermocline depth variations have phase propagation; it is eastward at a speed near 15 cm s−1 along the equator in the western half of the basin and is westward off the equator. This phase propagation, a property of the oceanic response to the quasi-periodic winds that force currents and excite a host of waves with periods near 5 years, indicates that the ocean is not in equilibrium with the forcing. In other words, the ocean-atmosphere interactions that cause El Niño to develop at a certain time are countered and, in due course, reversed by the delayed response of the ocean to earlier winds. This “delayed oscillator” mechanism that sustains interannual oscillations in the model differ in its details from that prevoiusly discussed by Schopf and Suarez and others. The latter investigators invoke an explicit role for Kelvin and Rossby waves. These waves cannot be identified in the low-frequency fluctuations of this model, but they are energetic at relatively short periods and are of vital importance to a quasi-resonant oceanic mode with a period near 7 months that is excited in the model. The similarities and differences between the results of this simulation and those with other models, especially the one described in a companion paper, are discussed.

Tropical pacific sea surface temperature variations in a coupled atmosphere-ocean general circulation model

A Pacific Ocean general circulation model (horizontal resolution of 5° in longitude and 4° in latitude) is coupled with a global atmospheric general circulation model, both developed at the Meteorological Research Institute/Japan Meteorological Agency, to study the coupled atmosphere-ocean system in the tropical Pacific Ocean. Without any flux correction in the atmosphere-ocean interaction, seasonal variation of equatorial sea surface temperature ( SST) is reasonably simulated, though interannual and intraseasonal variations in the model are smaller than the observed variations. Although cooling trend of the equatorial SST is apparent in the initial 1–2 years, SST attains to a stable state with 1–2°C cooler than the observed SST in the western equatorial Pacific. The east-west difference of the model equatorial SST is smaller than the observed one mainly because the easterly wind in the eastern equatorial Pacific is not strong enough to upwell cold water there.