摘要:

Zonal wavenumber frequency spectra of sea surface temperature (SST) anomalies along the equator in the Indo‐Pacific basin for the 98 years from 1900 to 1997 and of surface temperature (ST) and sea level pressure (SLP) anomalies extending around the globe along 10°N for the 48 years from 1950 to 1997 display significant peak spectral energy density for standing and eastward propagating waves of 3–7 year periods and 120°–360° zonal wavelengths. The global standing wave is the familiar Southern Oscillation, but the global propagating wave represents a new paradigm for the El Niño‐Southern Oscillation (ENSO). Global distributions of the phase velocities for this global ENSO wave finds covarying SLP and ST anomalies propagating eastward along the mean path of the Intertropical Convergence Zone (ITCZ), with the global zonal wavenumber 1 (2) component taking ∼4 (6) years to cross the tropical Indian, Pacific, and Atlantic Oceans at a zonal average speed of 90° (60°) longitude per year. Along this path the interannual SST and SLP anomalies are directly out of phase. Since thermocline depth anomalies underneath the ITCZ in the Pacific Ocean propagate westward [White et al.1985], we view the global ENSO wave as a slow coupled SST wave trapped onto the ITCZ. Separating the global ENSO wave from the Southern Oscillation using complex empirical orthogonal function analysis finds the amplitude of the propagating wave to be half that of the standing wave, with the former (latter) accounting for one third (two thirds) of the interannual variability in Niño‐3 SST and SLP indices during the 1980s. The global ENSO wave is shown to be responsible for the eastward propagation of covarying zonal surface wind and thermocline depth anomalies across the equatorial Pacific Ocean and through this mechanism is able to influence both the phasing and intensity of El Niño. Examining the persistence of the global ENSO wave from 1900 to 1997 finds it and the intensity of El Niño in the eastern equatorial Pacific Ocean modulated by interdecadal change. Both were strong (weak or absent) during decades of global tropic

ISSN:0148-0227    

DOI:10.1029/1999JC900246

年代:2000

数据来源: WILEY

摘要:

The objective of this study is to derive estimates of the mean seasonal variations in the Pacific to Indian Ocean throughflow transport using climatological data of the temperature/salinity fields and surface wind stress. A variable grid global general circulation model (GCM) with 1/6° resolution in Indonesian seas configured with ETOP05 bottom topography is utilized to diagnose the three‐dimensional velocity field. An extensive set of sensitivity experiments is conducted to obtain estimates of the error bars. The computations show that Indonesian Throughflow (ITF) is maximum in boreal summer (13.9±1.3 Sv) and minimum in boreal winter (7.5±1.1 Sv). The annual mean ITF transport amounts to 11.5±1.1 Sv (1 Sv = 106m3s−1). These values are within the range of various estimations of ITF transport, and they agree in phase. The largest seasonal variations of the transport occur in the upper 175 m of the water column, exhibiting a 8 Sv amplitude of the seasonal cycle. This causes a substantial variability in the heat transport from the Pacific to the Indian Ocean. Its value increases from 0.4±0.1 PW in January to 1.1±0.1 PW in August (1 PW = 1015watts). In contrast to previous GCM‐based studies, the annual mean transport is found to be more evenly distributed among the three major outlets from the Indonesian seas (Lombok, Ombai, and Timor Straits). The Luzon Strait inflow gives a considerable contribution to ITF. In winter the inflow of 6.3±1.5 is distributed between the two outflows from South China Sea, which follow the routes through Karimata (4.4±0.5) and Mindoro (1.9±1.5 Sv) Straits. In summer the Karimata pathway is blocked, and the net inflow of 4.7±0.6 Sv exits through the Mindoro Strait west of southern Luzon. The total contribution of the Luzon Strait inflow to the net ITF transport varies from 85% in boreal winter to 35% in summer, with an annual average value of ∼50%. Cross‐correlation analysis of the TOPEX/Poseidon monthly mean sea surface height (SSH) anomalies and the diagnosed transport anomalies demonstrates high correlation of the mean SSH difference between the regions in western Banda Sea and north of Luzon with the heat (0.96) and volum

ISSN:0148-0227    

DOI:10.1029/2000JC900015

年代:2000

数据来源: WILEY

摘要:

We present here a 1/4° resolution near‐global barotropic tidal model designed specifically to provide more accurate tides in shallow water. The model assimilates tides derived from 4 years of TOPEX/Poseidon (T/P) altimetric data. Procedures are used that tend to preserve the small spatial scales in the tidal structure in shallow water. Data from coastal tide gauges are also assimilated into the model. The result is a tidal model that is useful in most of the shallow and deep areas of the global oceans. Some problems exist in regions where no data are available for assimilation, such as the Southern Ocean. Pelagic tide gauge comparisons show that in deep water the new model is comparable in accuracy to the best of the existing T/P‐based global tidal models. Comparisons to crossover differences in shallow water suggest improved performance. We found that more accurate accounting of the load tides in the ocean tide model using an iterative technique based on the Green's function formalism does not yield ocean tides that are significantly different from the simpler approaches used thus far by ocean tide modelers when data are assimilated. It is our hope that this tidal model will not only help advance our understanding of shallow water tidal processes around the globe but also extend the utility of altimetry to waters shallower than 10

ISSN:0148-0227    

DOI:10.1029/1999JC900314

年代:2000

数据来源: WILEY

摘要:

A regional non‐eddy‐resolving configuration of the SPEM5.1 primitive equation model is used to study the dynamics of the eastern North Atlantic. Along most of the domain perimeter, extensive open boundaries (OBs) evacuate outgoing perturbations, while the outer ocean is simulated by lateral relaxation of some of the model variables. Unlike in existing regional models, this lateral constraint only concerns the tracers and the baroclinic velocities, for which good climatological estimates exist. Along the OBs, the barotropic stream function is entirely determined by the model response to the inner dynamics, without relaxation to any questionable a priori estimate. The adjustment processes and their interaction with the open boundaries are described. The final solution is remarkably stable and quite realistic. The net transports crossing the boundaries are prescribed; we therefore investigated the impact of these parameters. We first show that the prescribed mass exchange between the basin and the rest of the Atlantic has almost no influence on the interior circulation since the western and southern lateral conditions naturally adjust to the interior dynamics. The net circulation constrained around Iceland, however, has large‐scale consequences: Through the intensity of the overflow above the Iceland‐Scotland Ridge (ISR), this parameter affects the stratification north and south of the Rockall Plateau, the subpolar dynamics, and finally the North Atlantic Current path up to the Charlie Gibbs Fracture Zone. This slowly propagating dynamical effect, already noticed in previous studies, is shown to be forced baroclinically near

ISSN:0148-0227    

DOI:10.1029/1999JC900335

年代:2000

数据来源: WILEY

摘要:

A hierarchy of sea ice rheologies is evaluated on the basis of a comprehensive set of observational data. The investigations are part of the Sea Ice Model Intercomparison Project (SIMIP). Four different sea ice rheology schemes are compared: a viscous‐plastic rheology, a cavitating‐fluid model, a compressible Newtonian fluid, and a simple free drift approach with velocity correction. The same grid, land boundaries, and forcing fields are applied to all models. As verification data, there are (1) ice thickness data from upward looking sonars (ULS), (2) ice concentration data from the passive microwave radiometers SMMR and SSM/I, (3) daily buoy drift data obtained by the International Arctic Buoy Program (IABP), and (4) satellite‐derived ice drift fields based on the 85 GHz channel of SSM/I. All models are optimized individually with respect to mean drift speed and daily drift speed statistics. The impact of ice strength on the ice cover is best revealed by the spatial pattern of ice thickness, ice drift on different timescales, daily drift speed statistics, and the drift velocities in Fram Strait. Overall, the viscous‐plastic rheology yields the most realistic simulation. In contrast, the results of the very simple free‐drift model with velocity correction clearly show large errors in simulated ice drift as well as in ice thicknesses and ice export through Fram Strait compared to observation. The compressible Newtonian fluid cannot prevent excessive ice thickness buildup in the central Arctic and overestimates the internal forces in Fram Strait. Because of the lack of shear strength, the cavitating‐fluid model shows marked differences to the statistics of observed ice drift and the observed spatial pattern of ice thickness. Comparison of required computer resources demonstrates that the additional cost for the viscous‐plastic sea ice rheology is minor compared with the atmospheric and oceanic model components in global climat

ISSN:0148-0227    

DOI:10.1029/1999JC000016

年代:2000

数据来源: WILEY

摘要:

Dissolved iron and manganese and total dissolvable iron were measured in water column samples collected from the polynya region of the southern Ross Sea during cruises in November–December 1994 (spring 1994) and December 1995 to January 1996 (summer 1995). Iron and manganese addition bottle incubation experiments were also performed during these cruises in order to assess the nutritional sufficiency of ambient iron and manganese concentrations for growth of the phytoplankton community. Generally high dissolved iron concentrations (>0.5 nM) and relatively complex iron and manganese vertical profiles were obtained during the spring 1994 cruise, compared with the summer 1995 data. Dissolved iron concentrations in the upper water column averaged 1.0 nMduring spring 1994 and 0.23 nMin summer 1995, excluding two stations where concentrations exceeding 1 nMare attributed to inputs from melting sea ice. The observed differences in the distribution of iron and manganese between spring 1994 and summer 1995 are attributed to seasonal decreases in the up welling of bottom waters and melting of sea ice, which supply these metals into the upper water column, combined with the cumulative removal of iron and manganese from the water column throughout the spring and summer, due to biological uptake, vertical export and scavenging by suspended and sinking particles. Results of the metal addition bottle incubation experiments indicate that ambient dissolved iron concentrations are adequate for phytoplankton growth requirements during the spring and early summer, when algal production is highest andPhaeocystis antarcticadominates the algal community, whereas low dissolved iron concentrations limit algal community growth later in the summer, except in the stratified, iron‐enriched waters near melting sea ice, where diatoms are able to bloom. Our observations and the inferred seasonal distributions ofP. antarcticaand diatoms in these waters suggest that iron availability and vertical mixing (i.e., irradiance) exert the primary controls on phytoplankton growth and community structure in the southern Ross Sea during the spring and sum

ISSN:0148-0227    

DOI:10.1029/2000JC000256

年代:2000

数据来源: WILEY

摘要:

Temperature, salinity, oxygen, and chlorofluorocarbon (CFC) data obtained in September 1997 define the water types ventilating the deep eastern and central basins of the Bransfield Strait. These water types are observed adjacent to Joinville Island and are clearly derived from the Weddell Sea. The eastern basin bottom water characteristics closely match that of the bottom water at 310 dbar depth ∼50 km east of Joinville Island. The eastern basin bottom water is a simple mixture of low‐salinity freezing point shelf water (65%) and relatively warm saline Weddell Deep Water (35%), with a CFC‐113: CFC‐Il ratio age of 8.5 years. The eastern basin bottom water shares a common origin with a weak salinity minimum observed at 1000 dbar within the central basin, though overflow from the central basin to the eastern basin floor may also occur. The bottom water within the central basin is colder and saltier with higher concentrations of oxygen and CFC‐Il than that of the eastern basin. The central basin bottom water with a CFC ratio age of 7.5 years is composed of same Weddell water types that form the eastern basin bottom water plus a large contribution of high‐salinity freezing point shelf water of the characteristics of water observed 10 km northeast of Joinville Island and a small amount of Pacific pycnocline water. The components (and their percentage) are Weddell Deep Water (11%), low‐salinity shelf (24%), high‐salinity shelf (60%), to Pacific

ISSN:0148-0227    

DOI:10.1029/2000JC900030

年代:2000

数据来源: WILEY

摘要:

We have monitored the temperature field within first‐year sea ice in McMurdo Sound over two winter seasons, with sufficient resolution to determine the thermal conductivity from the thermal waves propagating down through the ice. Data reduction has been accomplished by direct reference to energy conservation, relating the rate of change of the internal energy density to the divergence of the heat current density. Use of this procedure, rather than the wave attenuation predicted by the thermal diffusion equation, avoids difficulties arising from a strongly temperature dependent thermal diffusivity. The thermal conductivity is an input parameter for ice growth and climate models, and the values commonly used in the models are predicted to depend on temperature, salinity, and the volume fraction of air. The present measurements were performed at depths in the ice where the air volume is small and the salinity is nearly constant, and they permit the determination of the absolute magnitude of the thermal conductivity and its temperature dependence. The weak temperature dependence is similar to that predicted by the models in the literature, but the magnitude is smaller by ∼10% than the predicted value most commonly used in climate and sea ice models. In the first season we find an additional scatter in the results at driving temperature gradients larger than ∼10–15 °C/m. We suggest that the scatter arises from a nonlinear contribution to the heat current, possibly associated with the onset of convective motion in brine inclusions. Episodic convective events are also observed. We have further determined the growth rate of the ice and compared it with the rate explained by the heat flux from the ice‐water interface. The data show a sudden rise of growth rate, without a rise in heat flux through the ice, which coincides in time and depth with the appearance of platelet ice. Finally, we discuss the observation of radiative solar heating at depth in the ice and demonstrate that the absorption exceeds that in the ice alone; dust or algae must contribute to the

ISSN:0148-0227    

DOI:10.1029/1999JC000003

年代:2000

数据来源: WILEY

摘要:

A simple model, using concentrations of nitrate and phosphate in austral winter 1992, reveals that the Antarctic Surface Water (AASW) of the southernmost Antarctic Circumpolar Current (ACC) between the Southern ACC Front and the Weddell Front is made up of ∼90% Upper Circumpolar Deep Water (UCDW) and 10% northward flowing AASW from the Weddell Gyre. With a typical timescale of ∼1 year the upwelling velocity was calculated to be as high as 60–100 m yr−1. Knowing the composition of the surface water with respect to its sources, changes due to several processes in the surface layer were deduced for carbon dioxide, oxygen, and silicate. As the timescale of changes in the surface layer of the southern ACC is ∼1 year, this allows us to calculate changes on an annual basis without interference of short‐term variations. Balancing the contributions by upwelling, biological activity, and air‐sea exchange to the concentrations in the surface layer, the area was found to be a large sink for atmospheric oxygen of 6.0 mol m−2yr−1(53 μmol kg−1) and a small sink for atmospheric carbon dioxide of 1.0 mol m−2yr−1(9 μmol kg−1). The most important cause for the oxygen sink is the upwelling of oxygen‐poor UCDW, which surpasses the oxygen‐elevating effect of primary productivity. This large oxygen sink, in between areas to the north and south which are only a small sink or even a source, conforms with the latitudinal distribution of atmospheric oxygen. The small CO2sink is largely brought about by biological activity. The annual carbon utilization amounts to 76±22 g C m−2yr−1, which is relatively high for an open ocean region in the Antarctic. However, it supports recent estimates of primary production of the Antarctic Ocean that are higher than early published values. The annual silicate consumption was calculated to be 126±19 g Si m−2yr−1. This is considerably higher than the Southern Ocean mean in current estimates. Although the southernmost ACC may be atypical for the Southern Ocean, the current estimate for Southern Ocean silica production may well be an underestimation. The silicate to carbon utilization ratio derived here is 0.53 which aligns with investigations on Antarctic phytoplankton and thus u

ISSN:0148-0227    

DOI:10.1029/2000JC000257

年代:2000

数据来源: WILEY

摘要:

The strength, shape, orientation, mean flow interactions, and propagation characteristics of the time‐varying (eddy) flow determined from TOPEX/Poseidon altimeter data in the North Pacific are investigated using equivalent barotropic diagnostic calculations as presented byHoskins et al.[1983]. Separate calculations for low‐ and high‐pass flow fields, distinguished by periods greater than and less than 6 months, respectively, are performed and revealed a fundamental difference between the eddy characteristics of the two. The areas with largest values of surface eddy kinetic energy occur in the vicinity of the Kuroshio Extension and the subtropical front. Near the Kuroshio Extension the low‐pass surface eddy kinetic energy is greater than the high‐pass energy, but in the subtropical frontal area the high‐pass eddy flow accounts for more energy than the low‐pass flow. For both areas the values of eddy kinetic energy are larger in summer than during winter. Low‐pass eddy flow tends to be aligned zonally, and the high‐pass eddy flow is aligned meridionally. Near the subtropical front the ratio of major to minor axis length is typically ∼1.5∶1, whereas near the Kuroshio Extension that ratio increases to values near 2:1. Along the southern boundary of the subtropical gyre the high‐pass eddy flow tends to propagate eastward relative to the mean flow, tending to decrease the gyre strength. However, the low‐pass eddy flow propagates westward relative to the mean flow and tends to increase the gyre strength. An additional computation of surface heat transport is used to deduce vertical propagation tendencies of the eddy flow and indicates that the low‐pass flow field transports more heat than the high‐pass flow with the largest eddy transports occurring near the coast of Honshu. The sign of the transport indicates a preference for upward energy propagation of the eddy flow there. Many of the characteristics, such as the orientation and propagation tendencies of the eddy flow, are similar to eddy flow in the atmosphere and accentuate a similarity between the general circulation

ISSN:0148-0227    

DOI:10.1029/2000JC900016

年代:2000

数据来源: WILEY