摘要:

Using a three‐dimensional multilevel numerical model, we examine the distribution of the wind‐induced near‐inertial internal wave energy in the North Pacific. Energetic low vertical mode near‐inertial internal waves are excited at 30°–45°N in the western and central North Pacific by traveling midlatitude storms during winter and at 10°–30°N in the western North Pacific by tropical cyclones during fall. Thus excited internal waves propagate equatorward down to 5°–15°N, where their frequencies are twice the local inertial frequencies. Parametric subharmonic instability can then transfer their energy across the local internal wave vertical wavenumber spectrum to small dissipation scales. The calculated results show that low vertical mode double‐inertial frequency internal waves are very weak at the times and locations of previous microstructure measurements, which suggests that the observed value of diapycnal diffusivity of ∼10−5m2s−1, an order of magnitude lower than required to satisfy the large‐scale advective‐diffusive balance of the thermohaline circula

ISSN:0148-0227    

DOI:10.1029/2000JC900019

年代:2000

数据来源: WILEY

摘要:

Spatial and temporal structures of interdecadal variability in the Pacific Ocean are investigated using results from an atmosphere‐ocean coupled general circulation model (AOGCM). The model shows a basin‐wide spatial pattern of the principal sea surface temperature (SST) variability similar to the observed one. Both interdecadal and interannual temporal structures of the SST variability agree well between the observation and the AOGCM. On the other hand, a slab ocean model coupled to the same atmospheric model as the AOGCM fails to simulate the observed temporal structure. Therefore the timescale of the coupled variability is associated with dynamical processes in the ocean. A distinct interdecadal mode of the coupled atmosphere‐upper ocean temperature variability is found in the AOGCM, with a spatiotemporal structure coherent with the SST variability. The mode accompanies an El Niño‐Southern Oscillation (ENSO)‐like spatial pattern of SST and the surface wind and behaves like a delayed oscillator in ENSO. A wedge‐shaped anomaly pattern of the upper thermocline temperature is formed in the eastern Pacific, and its northern subtropical signal propagates westward, enhanced by a subtropical wind forcing at the central basin. Arrival of the subtropical signal at the western Pacific around 20°N switches the anomaly of subsurface temperature in the equatorial region through anomalous oceanic heat transport along the western boundary. The travel time of the trans‐Pacific signal in the subtropics appears to be responsible for the timescale of this mode. The AOGCM successfully simulated the second mode of SST with a major variation in the midlatitude North Pacific as in the observed SST. In the upper ocean heat content we found another distinct mode, which is characterized by a midlatitude‐subtropics dipole pattern migrating around the North Pacific subtropical gyre. However, the associated SST variation of this mode shows a poor correspondence in the dominant interdecadal modes for

ISSN:0148-0227    

DOI:10.1029/2000JC900034

年代:2000

数据来源: WILEY

摘要:

Internally generated variability in the subtropical gyre is studied as a possible mechanism for the observed interannual to decadal variability in subtropical mode water formation. An isopycnic ocean model with idealized geometry and forcing which mimics the North Atlantic subtropical gyre is used for this purpose. The horizontal resolution is sufficiently high and the friction and diffusion sufficiently low for the flow to become barotropically and baroclinically unstable. Two modes of low‐frequency variability are found. Both modes consist of westward propagating thickness anomalies. The anomalies have a first baroclinic modal structure with a maximum amplitude at the thermocline. One mode has a timescale of 8 years and a basin wide spatial scale; the other has a timescale of 4.5 years and a smaller spatial scale. The modes appear to be damped when the diffusion is high. In that case, the 8‐year mode can be excited by a spatially coherent stochastic wind stress. The evolution of the modes is determined by an interaction between the mean flow and the low‐frequency variability itself. The modes are instabilities of the mean flow determined by the basic stratification. It appears that coupling to the atmosphere and a parameterization of surface mixing are necessary for the low‐frequency variability to appear in the mixed layer. The coupling and surface mixing do not play a role in generating the modes. It is concluded that these internally generated modes may play a role in the observed variability in mode water fo

ISSN:0148-0227    

DOI:10.1029/2000JC900041

年代:2000

数据来源: WILEY

摘要:

TOPEX/Poseidon altimeter measurements along 16 ground tracks in the South China Sea from November 1992 to October 1997 were used to observe the seasonal variability of sea surface height (SSH). In winter (November, December, January, and February) the SSH images are generally characterized by a sea surface tilting downward toward the east. High sea levels on the western side of the sea have a little monthly change, and a low (minus) sea level peak is centered in the northeastern deep basin. In spring (March and April), a single high sea level peak (HP) centered at 14°N, 114°E becomes a dominant feature. In summer (May, June, July, and August) the SSH images show a sea surface tilting downward toward the west. The HP moves from 14°N, 114°E in May to the northwest of Luzon Island in July. Low sea levels are centered in the offshore of the northern and southern Vietnamese coast. In fall (September and October) the appearance of an eastward low sea level jet in the western basin between 12° and 14°N constitutes a major feature of the SSH images. These results are comparable with previous results and drifter measure

ISSN:0148-0227    

DOI:10.1029/2000JC900001

年代:2000

数据来源: WILEY

摘要:

A moving tropical cyclone is an intense localized source of surface wind stress and wind stress curl that produces a significant response in the ocean environment, especially in the ocean thermal structure, the upper ocean currents, and the sea surface elevation. Such a response has been well identified in the open‐ocean region, but not in the coastal ocean region. In this study we use the Princeton Ocean Model with 20 km horizontal resolution and 23 sigma levels conforming to a realistic bottom topography to identify the response of the South China Sea to Tropical Cyclone Ernie 1996. Results show strong similarities in the responses between open ocean and coastal regions, including near‐surface strong asymmetric response such as divergent currents with near‐inertial oscillations, significant sea surface temperature cooling, biase to the right of the storm track, sea surface depressions in the wake of the storm, and subsurface intense upwelling and cooling at the base of the mixed layer to the right of the storm track. The unique features of the SCS response to Ernie are also disc

ISSN:0148-0227    

DOI:10.1029/2000JC900035

年代:2000

数据来源: WILEY

摘要:

The TOPEX/Poseidon (T/P) altimetry reveals quasi‐biennial (QB) variability in the southern Japan Sea. Sea surface height anomalies of a biennial nature are most energetic in the Yamato Basin, the southeastern most part of the Japan Sea, where they can be as large as 20 cm and extend for 100–200 km. On the basis of the in situ measurements of Maizuru Marine Observatory, the 2–3 year variations are associated with thermohaline anomalies in the upper 300 m layer. The local QB oscillation is studied with a reduced gravity model of the Japan Sea. The model is forced by European Centre for Medium‐Range Weather Forecasting daily wind and seasonal inflows/outflows through the three major straits of the sea. The model sensitivity experiments suggest that the QB variability can be associated with the sea's response to wind forcing of particular years, more specifically, 1992 and 1996, and to some extent, 1994. An approximate Kaiman filter is employed for assimilation of the T/P altimeter data into the reduced gravity model. It filters out observational noise and intraseasonal sea level variability and allows the model to dynamically interpolate T/P observations. The results of the assimilation indicate that the QB anomalies are strongest at 37.5°N, 134.5°E and propagate west‐northwest with a speed of ∼0.01 m s‐1, contributing to variations of the Tsushi

ISSN:0148-0227    

DOI:10.1029/2000JC900046

年代:2000

数据来源: WILEY

摘要:

The space‐time variation of phytoplankton pigments in the western Intra‐Americas Sea (IAS), in the vicinity of the island of Cuba, is examined using digital images obtained with the Coastal Zone Color Scanner sensor flown aboard the Nimbus 7 satellite from 1978 to 1986. The results are compared to historical in situ hydrographic observations. A marked seasonality in pigment concentration was observed in waters around Cuba, with an average of 0.07 mg m−3in summer (April‐September) and 0.13 mg m−3during winter (October–March). The range of variation in pigment concentration was larger in the Gulf of Mexico relative to the western Caribbean Sea. We identified four biogeographical areas on the basis of groups of pixels with similar patterns of time variability. These are area I: southwest of Cuba, Yucatan Channel, and Florida Strait; area II: central Gulf of Mexico; area III: east of Cuba; and area IV: central Caribbean Sea, south of Jamaica and Hispaniola. Two major meteorological events led to anomalies in the seasonal cycle of pigment concentrations. During El Niño‐Southern Oscillation (ENSO) of 1982–1983, positive anomalies were observed in the pigment concentration in the western IAS during winter months. This was associated with intense mixing of the water column by higher‐frequency and stronger winds associated with cold fronts. ENSO 1982–1983 therefore had a fertilizing effect on the IAS region. Another positive anomaly was observed in 1980–1981, a non‐ENSO period that featured higher hurricane and extratropic

ISSN:0148-0227    

DOI:10.1029/2000JC900017

年代:2000

数据来源: WILEY

摘要:

For a considerable coverage the energy balance of and ice formation by leads in sea ice in the Weddell Sea are evaluated on the basis of data obtained from drifting buoys for the winter periods from 1986 to 1994 and by using a kinematic‐thermodynamic sea ice model. The net heat flux is defined as the sum total of radiative and turbulent fluxes. For thin ice the net turbulent flux is 3–4 times the net radiative flux. The contribution of the net heat flux through open and refrozen leads to the total net heat flux through sea ice is twice as large as the area contribution of open and refrozen leads to the total area covered with sea ice. In the eastern and central parts of the Weddell Sea, leads contribute some 30% to the total energy flux from the ocean to the atmosphere. This flux increases from 10–15 W m−2in the eastern and central Weddell Sea regions to 30 W m−2in the western part of the Weddell Sea, where leads contribute more than 80% of the total net energy transfer. The increase is mainly due to the colder and windier atmosphere in connection with the higher variability of the ice motion in the diurnal and semidiurnal band in the western Weddell Sea. The contribution of leads in winter ice formation exceeds 50% in the whole of the Weddell Sea. Monthly area‐weighted ice growth is 10–15 cm in the east and up to 30 cm over the western continental shelf region. In the western part of the Weddell Sea, tidal and inertial motions in the diurnal and semidiurnal bands enhance lead formation, and thus contribute 7% to total net heat flux, 12% to ice formation, and 23% to the salt mass released during ice growth. The results are used to assess quantitatively the importance of leads to the interaction of the ocean, the sea ice, and t

ISSN:0148-0227    

DOI:10.1029/2000JC900050

年代:2000

数据来源: WILEY

摘要:

A one‐dimensional (vertical) numerical model of currents, mixing, frazil ice concentration, and suspended sediment concentration has been developed and applied in the shallow southeastern Kara Sea. The objective of the calculations is to determine whether conditions suitable for turbid ice formation can occur during times of rapid cooling and wind‐ and wave‐induced sediment resuspension. Although the model uses a simplistic approach to ice particles and neglects ice‐sediment interactions, the results for low‐stratification, shallow (∼20‐m) freeze‐up conditions indicate that the coconcentrations of frazil ice and suspended sediment in the water column are similar to observed concentrations of sediment in turbid ice. This suggests that wave‐induced sediment resuspension is a viable mechanism for turbid ice formation, and enrichment mechanisms proposed to explain the high concentrations of sediment in turbid ice relative to sediment concentrations in underlying water may not be necessary in energetic conditions. However, salinity stratification found near the Ob' and Yenisey Rivers damps mixing between ice‐laden surface water and sediment‐laden bottom water and probably limits incorporation of resuspended sediment into turbid ice until prolonged or repeated wind events mix away the stratification. Sensitivity analyses indicate that shallow (≤20 m), unstratified waters with fine bottom sediment (settling speeds of ∼1 mm s−1or less) and long open water fetches (>25 km) are ideal

ISSN:0148-0227    

DOI:10.1029/2000JC900037

年代:2000

数据来源: WILEY

摘要:

Observations from sonar data have suggested enhanced melting of thick, ridged ice relative to level ice. There are several mechanisms that may account for this intensified melting. In this paper, we examine the effects of two‐dimensional (2‐D) heat conduction and enlarged basal surface area due to the sloping sides of the keel on heat conduction and melt rates. The cross section of the 2‐D ridge is taken to be an isosceles triangle with a rounded crest. This is roughly the shape observed and allows a convenient numerical representation in polar coordinates. For comparison, ridges of similar shape are also represented as a collection of 1‐D columns of varying thickness, similar to what is implicit in typical ice thickness distribution models. The results show that 2‐D ridges inhibit the heat conduction compared to 1‐D ridges owing to the dominating effect of weaker temperature gradients. The slope of the keel is the dominant factor in determining the temperature gradient. A size distribution of 2‐D ridges reduces heat transfer to the atmosphere by 3 W m−2compared to a similar distribution of 1‐D ridges. Over an annual cycle, basal ablation along the keel is insignificant for 2‐D ridges with small slopes, whereas ridges with large slopes show ablation rates determined by the ice‐ocean heat flux. These melt rates imply a transition from a triangular to a more rounded shape. The 1‐D ridge geometry is not adequate to simulate the net melting at the keel base over an annual cycle. Melt rates are calculated along the ridge keel and for level ice over a 40 day period for comparison with observations. For 1‐D ridges, all ice thicker than 5 m melts more slowly than the corresponding level ice. The inclusion of 2‐D heat conduction increases the amount of ablation in the thicker ice relative to the 5 m ice, especially for ridges with larger slopes. However, this increase explains only a small fraction of the enhanced basal melting seen in the observations. These results suggest that other mechanisms are important in determining the

ISSN:0148-0227    

DOI:10.1029/1999JC000026

年代:2000

数据来源: WILEY