Hiroyasu Hasumi

Developments in ocean climate modelling

This paper presents some research developments in primitive equation ocean models which could impact nthe ocean component of realistic global coupled climate models aimed at large-scale, low frequency climate nsimulations and predictions. It is written primarily to an audience of modellers concerned with the ocean ncomponent of climate models, although not necessarily experts in the design and implementation of ocean nmodel algorithms.

A Series of Zonal Jets Embedded in the Broad Zonal Flows in the Pacific Obtained in Eddy-Permitting Ocean General Circulation Models

Abstract A series of zonal currents in the Pacific Ocean is investigated using eddy-permitting ocean general circulation models. The zonal currents in the subsurface are classified into two parts: one is a series of broad zonal flows that has the meridional pattern slanting poleward with increasing depth and the other is finescale zonal jets with the meridional scale of 3°–5° formed in each broad zonal flow. The basic pattern for the broad zonal flows is similar between the coarse-resolution model and the eddy-permitting model and is thought to be the response to the wind forcing. A part of the zonal jets embedded in each zonal flow is explained by the anomalous local wind forcing. Most of them, however, seem to be mainly created by the rectification of turbulent processes on a β plane (the Rhines effect), and zonal jets in this study have common features with the zonally elongated flows obtained in previous modeling studies conducted in idealized basins. The position of zonal jets is not stable when the ...

Deep Pacific Circulation Controlled by Vertical Diffusivity at the Lower Thermocline Depths

Abstract Deep Pacific circulation is investigated by using a World Ocean model with depth-dependent vertical diffusivity. Vertical diffusivity estimated from observations, 0.1 × 10−4 m2 s−1 for the upper layer and 3.0 × 10−4 m2 s−1 for the bottom layer, is adopted. Comparison is made between cases with different vertical diffusivity at middepths. With larger vertical diffusivity at middepths, the deep Pacific circulation becomes stronger. This is due to enhanced heat exchange between the thermocline water and the deep water through more intense diffusion at middepths. The water below the thermocline is warmed and that at the thermocline is cooled for the whole basin. The warmed deep water leads to larger heat loss through the sea surface, causing the enhanced deep-water formation in the deep-water formation region. On the other hand, the cooled thermocline water leads to larger heat gain through the sea surface where the thermocline water outcrops, counterbalancing the larger heat loss in the deep-water f...

Sensitivity of the Global Thermohaline Circulation to Interbasin Freshwater Transport by the Atmosphere and the Bering Strait Throughflow

Abstract Sensitivity of the global thermohaline circulation to interbasin freshwater transport by the atmosphere and the Bering Strait throughflow is investigated by using a free-surface, coarse-resolution ocean general circulation model. The model is run by prescribing freshwater flux at the sea surface without restoring the sea surface salinity to climatology in order that effects of salinity advection are properly represented. Comparison of experiments with the open and closed Bering Strait shows that the throughflow reduces the intensity of the Atlantic deep circulation by ∼17%, while minimally affecting the Pacific deep circulation. Increase in the atmospheric freshwater transport from the Atlantic to the Pacific intensifies both the Atlantic deep circulation and the Bering Strait throughflow. On the other hand, changes in the throughflow transport under a fixed amount of atmospheric interbasin freshwater transport are found to have a minor impact on the global thermohaline circulation. This insensit...

Far‐reaching effects of the Hawaiian Islands in the CCSR/NIES/FRCGC high‐resolution climate model

[1]xa0For the first time, using a high-resolution atmosphere-ocean coupled general circulation model (CGCM), we succeed in reproducing the far-reaching effects of the Hawaiian Islands, recently showed by satellite observations. The model reproduces the distributions of sea surface temperature (SST), surface winds and cloud liquid water (CLW) in the wake of the Hawaiian Islands. It is revealed that these distributions are caused by the Hawaiian Lee Counter Current (HLCC) and that this current is driven by the wind-curls induced by the orographic effect of the islands, as suggested from an observational study. It is also shown that wind changes around the Hawaiian Islands can further affect the speed of the North Equatorial Current (NEC) and SST over the current, and intra-annual variability in CLW to the west of the islands is governed, not only by SST but also by wind speed.

Intensification of the Atlantic Deep Circulation by the Canadian Archipelago Throughflow

Abstract Low-salinity water export through the Canadian Archipelago is one of the main components of the freshwater budget in the Arctic Ocean. Nevertheless, the Canadian Archipelago is closed in most global ocean models. How it is that deep-water formation at high latitudes of the Northern Hemisphere depends on the opening and closing of the Canadian Archipelago is investigated. An ice–ocean coupled model, whose horizontal resolution is 1°, is used without restoring surface salinity to observed data. When the Canadian Archipelago is open, the Atlantic deep circulation strengthens by 21%. This enhancement is caused by intensification of deep-water formation in the northern North Atlantic Ocean. Surface salinity in these regions is affected by the East Greenland Current, which flows from the Fram Strait and increases its salinity when the Canadian Archipelago is opened. The low-salinity flow through the Canadian Archipelago affects surface salinity only in the western part of the Labrador Sea. A cyclonic c...

Effects of surface freshwater flux induced by sea ice transport on the global thermohaline circulation

[1]xa0Effects of surface freshwater flux induced by sea ice formation and melting on the thermohaline circulation are investigated by using a sea-ice-ocean coupled general circulation model forced by monthly climatology. Restoring to the observed sea surface salinity is not employed in order to evaluate precisely the sea ice effects. Because of the models improper representation of interaction between sea ice and the Northern Hemisphere deep convection, the discussion is focused on the effect of sea ice in the Southern Hemisphere. In the control case, deep water formation around Antarctica occurs under compact sea ice cover, where positive annual mean sea ice production is an essential factor to induce deep convection. When sea ice motion is turned off, deep water formation is maintained primarily by thermal destabilization of water columns, as annual mean sea ice production is almost zero everywhere. Consequently, the deep ocean around Antarctica is warmer and less saline compared with that for the control case, and the Atlantic bottom circulation is weakened by 16%. Northward salt transport from the Southern Ocean in the bottom layer also decreases. The amount of the decrease is greater by 1 order of magnitude than that of the surface salt input associated with sea ice. In a case where the freshwater flux at the ice-ocean interface is turned off, results are similar to the case without sea ice transport, although surface heat and momentum fluxes significantly differ between them. This suggests that the influence of sea ice on freshwater flux is more important than on heat and momentum fluxes in affecting the global thermohaline circulation.

Effects of Freshwater Forcing on the Atlantic Deep Circulation: A Study with an OGCM Forced by Two Different Surface Freshwater Flux Datasets

Abstract Numerical experiments are conducted using a sea ice–coupled ocean general circulation model (OGCM) forced by two different freshwater flux datasets. These two datasets are the National Centers for Environmental Prediction– National Center for Atmospheric Research (NCEP–NCAR) reanalysis and the European Centre for Medium-Range Weather Forecasts (ECMWF)-based climatological datasets, which are widely used to force OGCMs. It is found that the strength of the simulated Atlantic deep circulation considerably differs between the two experiments. To explain the resulting difference, these two freshwater fluxes are compared and additional experiments are carried out, focusing on the difference at northern high and midlatitudes, at low latitudes, and in the Southern Ocean, separately. An examination of these experiments shows that the difference in the simulated Atlantic deep circulation comes mainly from the difference in the river runoff data, especially at the northern high latitudes. Although the amou...

Comparison of Two Classical Advection Schemes in a General Circulation Model

Abstract The authors compare two classical advection schemes, the centered difference and weighted upcurrent, for coarse-resolution OGCMs, using an idealized ocean basin and a realistic World Ocean topography. For the idealized basin, three experiments are run, one with 12 vertical levels and the centered difference scheme, one with 12 levels and the weighted upcurrent scheme, and the other with 800 levels and the centered scheme. The last experiment perfectly satisfies the grid Peclet number stability criterion and is regarded as the “true solution.” Comparison of the coarse vertical resolution experiments with the true solution indicates 1) that with the centered scheme, when strong vertical motion crosses a strong stratification, false density values are created in the coarse resolution model and this leads to false convective adjustment, which transports those false density values downward; and 2) that because of computational diffusion, the weighted upcurrent scheme leads to a less dense deep water w...

Arctic sea ice response to wind stress variations

[1]xa0Timescale of sea ice response to wind stress variations and the mechanisms controlling the timescale are investigated by a coupled sea ice–ocean model. Wind stress variations account for a significant part of the changes of sea ice volume in the Arctic Ocean over the last several decades. The changes of sea ice volume associated with wind stress are mainly induced by changes of sea ice outflow from the Arctic Ocean. While outflow immediately responds to wind stress variations, responses of sea ice volume lag behind the changes of the outflow by several years. For example, when the model is driven by interannually varying wind stress with the other atmospheric forcing components given by climatology, sea ice outflow abruptly decreases in 1996 and remains almost constant for the following several years. However, the responding increase of sea ice volume is gradual and continues for several years. In order to clarify the timescale of sea ice response to an abrupt change of wind stress and the mechanisms controlling the timescale, the model is forced by two typical wind stress fields, which cause small and large outflow. Equilibrium annual mean ice thickness averaged over the Arctic Ocean is different by about 50 cm between these two wind-forcing fields. Starting from the equilibrium state obtained under one of these two fields, wind-forcing is switched to the other. Adjustment time of ice thickness is then defined as the year when annual mean ice thickness is adjusted to new equilibrium by 90% of the difference between the two equilibria. The timescale of the sea ice response and the mechanisms controlling the timescale are found to be different depending on whether the outflow increases or decreases. When the outflow increases, the timescale depends on the advection time of the sea ice flowing toward the exits. When the outflow decreases, the timescale depends on the thermodynamic growth rate of sea ice. The change in the late 1990s corresponds to the latter case.