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1.
《Ocean Modelling》2009,26(3-4):120-131
Observations of the western Arabian Sea over the last decade have revealed a rich filamentary eddy structure, with large horizontal SST gradients in the ocean, developing in response to the southwest monsoon winds. This summertime oceanic condition triggers an intense mesoscale coupled interaction, whose overall influence on the longer-term properties of this ocean remains uncertain. In this study, a high-resolution regional coupled model is employed to explore this feedback effect on the long-term dynamical and thermodynamical structure of the ocean.The observed relationship between the near-surface winds and mesoscale SSTs generate Ekman pumping velocities at the scale of the cold filaments, whose magnitude is the order of 1 m/day in both the model and observations. This additional Ekman-driven velocity, induced by the wind-eddy interaction, accounts for approximately 10–20% of oceanic vertical velocity of the cold filaments. This implies that Ekman pumping arising from the mesoscale coupled feedback makes a non-trivial contribution to the vertical structure of the upper ocean and the evolution of mesoscale eddies, with obvious implications for marine ecosystem and biogeochemical variability.Furthermore, SST features associated with cold filaments substantially reduce the latent heat loss. The long-term latent heat flux change due to eddies in the model is approximately 10–15 W/m2 over the cold filaments, which is consistent with previous estimates based on short-term in situ measurements. Given the shallow mixed layer, this additional surface heat flux warms the cold filament at the rate of 0.3–0.4 °C/month over a season with strong eddy activity, and 0.1–0.2 °C/month over the 12-year mean, rendering overall low-frequency modulation of SST feasible. This long-term mixed layer heating by the surface flux is approximately ±10% of the lateral heat flux by the eddies, yet it can be comparable to the vertical heat flux. Potential dynamic and thermodynamic impacts of this observed air–sea interaction on the monsoons and regional climate are yet to be quantified given the strong correlation between the Somalia upwelling SST and the Indian summer monsoons.  相似文献   

2.
The transition zone of the Canary Current upwelling region   总被引：2，自引：0，他引：2  
Like all the major upwelling regions, the Canary Current is characterised by intense mesoscale structure in the transition zone between the cool, nutrient-rich waters of the coastal upwelling regime and the warmer, oligotrophic waters of the open ocean. The Canary Island archipelago, which straddles the transition, introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and Trade winds. The combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability. On the basis of historical data and a series of interdisciplinary field studies, the principal features of the region are described. These include a prominent upwelling filament originating near 28°N off the African coast, cyclonic and anti-cyclonic eddies downstream of the archipelago, and warm wake regions protected from the Trade winds by the high volcanic peaks of the islands. The filament is shown to be a recurrent feature, apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone. Its role in the transport and exchange of biogenic material, including fish larvae, is considered. Strong cyclonic eddies, observed throughout the year, drift slowly southwestward from Gran Canaria. One sampled in late summer was characterised by large vertical isopycnal displacements, apparent surface divergence and strong upwelling, producing a fourfold increase in chlorophyll concentrations over background values. Such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen. The lee region of Gran Canaria is shown to be a location of strong pycnocline deformation resulting from Ekman pumping on the wind shear boundaries, which may contribute to the eddy formation process.  相似文献   

3.
Mesoscale eddies and tropical instability waves in the eastern tropical Pacific, first revealed by satellite infrared imagery, play an important role in the dynamics and biology of the region, and in the transfer of mass, energy, heat, and biological constituents from the shelf to the deep ocean and across the equatorial currents.From boreal late autumn to early spring, four to 18 cyclonic or anticyclonic eddies are formed off the coastal region between southern Mexico and Panama. The anticyclonic gyres, which tend to be larger and last longer than the cyclonic ones, are the best studied: they typically are 180–500 km in diameter, depress the pycnocline from 60 to 145 m at the eddy center, have swirl speeds in excess of 1 m s−1, migrate west at velocities ranging from 11 to 19 cm s−1 (with a slight southward component), and maintain a height signature of up to 30 cm. The primary generating agents for these eddies are the strong, intermittent wind jets that blow across the isthmus of Tehuantepec in Mexico, the lake district in Nicaragua and Costa Rica, and the Panama canal. Other proposed eddy-generating mechanisms are the conservation of vorticity as the North Equatorial Counter Current (NECC) turns north on reaching America, and the instability of coastally trapped waves/currents.Tropical Instability Waves (TIWs) are perturbations in the SST fronts on either side of the equatorial cold tongue. They produce SST variations on the order of 1–2 °C, have periods of 20–40 days, wavelengths of 1000–2000 km, phase speeds of around 0.5 m s−1 and propagate westward both north and south of the Equator. The Tropical Instability Vortices (TIVs) are a train of westward-propagating anticyclonic eddies associated with the TIWs. They exhibit eddy currents exceeding 1.3 m s−1, a westward phase propagation speed between 30 and 40 km d−1, a signature above the pycnocline, and eastward energy propagation. Like the TIWs, they result from the latitudinal barotropically unstable shear between the South Equatorial Current (SEC) and the NECC with a potential secondary source of energy from baroclinic instability of the vertical shear with the Equatorial Undercurrent (EUC).This review of mesoscale processes is part of a comprehensive review of the oceanography of the eastern tropical Pacific Ocean.  相似文献   

4.
This study investigated the seasonal variation in the atmospheric response to oceanic mesoscale eddies in the North Pacific Subtropical Countercurrent (STCC) and its mechanism, based on satellite altimetric and reanalysis datasets. Although mesoscale eddy in the study area is more active in summer, the sea surface temperature (SST) anomaly associated with mesoscale eddies is more intense and dipolar in winter, which is largely due to the larger background SST gradient. Similarly, the impact of the oceanic eddy on sea surface wind speed and heat flux is strongest in winter, whereas its effect on precipitation rate is more significant in summer. The study revealed that the SST gradient in STCC could impact the atmosphere layer by up to 800 hPa (900 hPa) in boreal winter (summer) through the dominant vertical mixing mechanism. Moreover, the intensity of the SST gradient causes such seasonal variation in mesoscale air-sea coupling in the study region. In brief, a stronger (weaker) background SST gradient field in wintertime (summertime) leads to a larger (smaller) eddy-induced SST anomaly, thus differently impacting atmosphere instability and transitional kinetic energy flux over oceanic eddies, leading to seasonal variation in mesoscale air-sea coupling intensity.  相似文献   

5.
Submesoscale activity in the upper ocean has received intense studies through simulations and observations in the last decade, but in the eddy-active South China Sea (SCS) the fine-scale dynamical processes of submesoscale behaviors and their potential impacts have not been well understood. This study focuses on the elongated filaments of an eddy field in the northern SCS and investigates submesoscale-enhanced vertical motions and the underlying mechanism using satellite-derived observations and a high-resolution (~500 m) simulation. The satellite images show that the elongated highly productive stripes with a typical lateral scale of ~25 km and associated filaments are frequently observed at the periphery of mesoscale eddies. The diagnostic results based on the 500 m-resolution realistic simulation indicate that these submesoscale filaments are characterized by cross-filament vertical secondary circulations with an increased vertical velocity reaching O(100 m/d) due to submesoscale instabilities. The vertical advections of secondary circulations drive a restratified vertical buoyancy flux along filament zones and induce a vertical heat flux up to 110 W/m2. This result implies a significant submesoscale-enhanced vertical exchange between the ocean surface and interior in the filaments. Frontogenesis that acts to sharpen the lateral buoyancy gradients is detected to be conducive to driving submesoscale instabilities and enhancing secondary circulations through increasing the filament baroclinicity. The further analysis indicates that the filament frontogenesis detected in this study is not only derived from mesoscale straining of the eddy, but also effectively induced by the subsequent submesoscale straining due to ageostrophic convergence. In this context, these submesoscale filaments and associated frontogenetic processes can provide a potential interpretation for the vertical nutrient supply for phytoplankton growth in the high-productive stripes within the mesoscale eddy, as well as enhanced vertical heat transport.  相似文献   

6.
The mean available potential energy released by baroclinic instability into the meso-scale eddy field has to be dissipated in some way and Tandon and Garrett [Tandon, A., Garrett, C., 1996. On a recent parameterization of mesoscale eddies. J. Phys. Oceanogr. 26 (3), 406–416] suggested that this dissipation could ultimately involve irreversible mixing of buoyancy by molecular processes at the small-scale end of the turbulence cascade. We revisit this idea and argue that the presence of dissipation within the thermocline automatically requires that a component of the eddy flux associated with meso-scale eddies must be associated with irreversible mixing of buoyancy within the thermocline. We offer a parameterisation of the implied diapycnal diffusivity based on (i) the dissipation rate for eddy kinetic energy given by the meso-scale eddy closure of Eden and Greatbatch [Eden, C., Greatbatch, R.J., 2008. Towards a meso-scale eddy closure. Ocean Modell. 20, 223–239.] and (ii) a fixed mixing efficiency. The implied eddy-induced diapycnal diffusivity (κ) is implemented in a coarse resolution model of the North Atlantic. In contrast to the vertical diffusivity given by a standard vertical mixing scheme, large lateral inhomogeneities can be found for κ in the interior of the ocean. In general, κ is large, i.e. up to o(10) cm2/s, near the western boundaries and almost vanishing in the interior of the ocean.  相似文献   

7.
An ocean model was used to examine whether the scatterometer winds can improve the model performance both dynamically and thermodynamically. Comparisons were done using QuikSCAT and NCEP2 winds for both the mean and variability from 2000 to 2004. The comparisons showed that the model forced by QuikSCAT winds gives more realistic mean SST, 20 °C isotherm depth (Z20), and latent heat flux than NCEP2 winds do. Sensitivity experiments indicated that QuikSCAT mean wind stress is important for the improved mean SST, Z20, and latent heat release to the atmosphere in the eastern Pacific. QuikSCAT wind speed, through its effect on the turbulent heat fluxes, is most important for the mean SST in the western Pacific. Finally, there were comparable correlations with observations of both SST and Z20 on the intra-seasonal time scale between the model forced with QuikSCAT winds and the model forced with NCEP2 winds.  相似文献   

8.
Northern Norwegian shelf regions are highly productive, supporting fisheries rich in commercially important species such as cod, herring and capelin. It has been long recognized that the mesoscale jets, meanders and eddies associated with interactions between the North Atlantic Current, Norwegian Coastal Current and regional bottom topographic features such as troughs, banks and shelfbreaks play important roles in transporting and retaining zooplankton. To investigate zooplankton distributions and their correspondence with the physical fields, three large-scale surveys with mesoscale resolutions on physical and biological fields were conducted in northern Norwegian shelf regions between latitudes 68°15′N and 70°15′N in springs of 2000–2002. Survey results provide insights into the relationships between zooplankton distributions and the physical features such as fronts, the Norwegian Coastal Current and eddies related to topographic features. The physical and biological data are integrated and analyzed focusing on water types, estimation of geostrophic currents from direct current measurements, along-shelf transport of zooplankton, and retention of zooplankton by the mesoscale meander–eddy over a typical bank area on the shelf. The estimated mean transport in the upper 100 m on the shelf in the survey region is approximately 6.4×103 tonnes wet weight day−1 northward. High zooplankton abundances were found over both Malangsgrunnen and Sveinsgrunnen banks. The specific accumulation rate from northward–southward transport in the upper 100 m over Malangsgrunnen was approximately 0.08 day−1, while variable currents with an offshore gradient of zooplankton abundance over Sveinsgrunnen implies an offshore dispersion of coastal-originated zooplankton cohort.  相似文献   

9.
Accumulating evidence points to the importance of mesoscale eddies in supplying nutrients to surface waters in oligotrophic gyres. However, the nature of the biological response and its evolution over time has yet to be elucidated. Changes in mesozooplankton community composition due to eddy perturbation also could affect biogeochemical cycling. Over the course of two summers we sampled seven eddies in the Sargasso Sea. We focused on and followed a post-phytoplankton bloom cyclonic eddy (C1) in 2004 and a blooming mode-water anticyclonic eddy (A4) in 2005. We collected zooplankton in all eddies using a Multiple Opening and Closing Net Environmental Sampling System (MOCNESS) and quantified biomass (>0.15 mm, in five size fractions) from 0 to 700 m over nine discrete depth intervals. Zooplankton biomass (>0.5 mm) in the upper 150 m was similarly enhanced at night for the periphery of C1 and the center of A4 at 0.514 g m−2 and 0.533 g m−2, respectively, compared to outside (0.183 g m−2 outside C1 and 0.197 g m−2 outside A4). Despite minimal chlorophyll a enhancement and dominance by picoplankton in C1, zooplankton biomass increased most for the largest size class (>5 mm). Gut fluorescence for euphausiids and large copepods was also elevated on the C1 periphery. In A4, peak biomass occurred at eddy center coincident with peak primary production, but was highly variable (changing by >3-fold) over time, perhaps resulting from the dense, but patchy distribution of diatom chains in this region. Shifts in zooplankton community composition and abundance were reflected in enhancement of fecal pellet production and active transport by diel vertical migration in eddies. Inside C1 the flux of zooplankton fecal pellets at 150 m in June 2004 was 1.5-fold higher than outside the eddy, accounting for 9% of total particulate organic carbon (POC) flux. The flux of fecal pellets (mostly from copepods) increased through the summer in eddy A4, matching concurrent increases in zooplankton <2 mm in length, and accounting for up to 12% of total POC flux. Active carbon transport by vertically migrating zooplankton was 37% higher on the periphery of C1 and 74% higher at the center of A4 compared to the summer mean at the Bermuda Atlantic Time-series Study (BATS) station. Despite contrasting responses by the phytoplankton community to cyclonic and mode-water eddies, mesozooplankton biomass was similarly enhanced, possibly due to differential physical and biological aggregation mechanisms, and resulted in important zooplankton-mediated changes in mesoscale biogeochemistry.  相似文献   

10.
Mesoscale eddies may enhance primary production (PP) in the open ocean by bringing nutrient-rich deep waters into the euphotic zone, potentially leading to increased transport of particles to depth. This hypothesis remains controversial, however, due to a paucity of direct particle export measurements. In this study, we investigated particle dynamics using 234Th–238U disequilibria within a mesoscale cold-core eddy, Cyclone Opal, which formed in the lee of the Hawaiian Islands. 234Th samples were collected along two transects across Cyclone Opal as well as during a time-series within the eddy core during a decaying diatom bloom. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO2) fluxes at 150 m varied spatially and temporally within the eddy and strongly depended on the 234Th model formulation used (e.g., steady state versus non-steady state, inclusion of upwelling, etc.). Particle fluxes estimated from a steady state model assuming an upwelling rate of 2 m day−1 yielded the best fit to sediment-trap data. These 234Th-derived particle fluxes ranged from 332±14 to 1719±53 μmol C m−2 day−1, 27±3 to 114±12 μmol N m−2 day−1, and 33±20 to 309±73 μmol Si m−2 day−1. Although PP rates within Cyclone Opal were elevated by a factor of 2–3, PC and PN fluxes were the same, within error, inside and outside of Cyclone Opal. The ratio of PC export to PP remained surprisingly low at <0.03 and similar to those measured in surrounding waters. In contrast, bSiO2 fluxes within the eddy core were three times higher. Detailed analyses of 234Th depth profiles consistently showed excess 234Th at 100–175 m, associated with the remineralization and possible accumulation of suspended and dissolved organic matter from the surface. We suggest that strong microzooplankton grazing facilitated particulate organic matter recycling and resulted in the export of empty diatom frustules. Thus, while eddies may increase PP, they do not necessarily increase PC and PN export to deep waters. This may be a general characteristic of wind-driven cyclonic eddies of the North Pacific Subtropical Gyre and suggests that eddies may preferentially act as a silica pump, thereby playing an important role in promoting silicic-acid limitation in the region.  相似文献   

11.
Mesoscale eddies are important suppliers of nutrients to the surface waters of oligotrophic gyres, but little is known about the biological response, particularly that of higher trophic levels, to these physical perturbations. During the summers of 2004 and 2005, we followed the development of a cyclonic eddy and an anti-cyclonic mode-water eddy in the Sargasso Sea. Zooplankton (>150 μm) were collected across both eddies in 9 discrete depth intervals between 0 and 700 m. Comparison of the abundance of major taxa of mesozooplankton in the upper 150 m at eddy center and outside the eddies (day and night) indicated that the cyclone and mode-water eddy supported similar mesozooplankton communities, with several taxa significantly higher in abundance inside than outside the eddies, when compared with the Bermuda Atlantic Time-series Study site as representative of mean conditions. In both eddies copepod peak abundance occurred in the 50-100 m depth interval, coincident with the chlorophyll a maximum, suggesting elevated food concentration in the eddies may have influenced zooplankton vertical distribution. The two eddies differed in the strength of diel vertical migration of zooplankton, as indicated by the ratio of night:day abundance in the epipelagic zone, which was higher at the center of the mode-water eddy for most taxa. Over the sampling interval of 1-2 months, abundance of the three most common taxa (copepods, chaetognaths, and ostracods) decreased in the cyclone and increased in the mode-water eddy. This further supports previous findings that over the sampling period the cyclone was in a decay phase, while the mode-water eddy was sustaining nutrient fluxes and high phytoplankton concentrations. A more detailed analysis of community structure in the mode-water eddy indicated the 0-700 m integrated abundance of doliolids was significantly higher inside the mode-water eddy than outside. The presence of a mesopelagic (200-700 m) layer of lepadid barnacle cyprids in this eddy highlights the potential of eddies to transport and disperse biota. We conclude that when compared with average ambient conditions (as measured at BATS), mesoscale eddies can influence zooplankton behavior and alter zooplankton community structure which can affect food-web interactions and biogeochemical cycling in the open ocean.  相似文献   

12.
连续台风对海表温度和海表高度的影响   总被引：1，自引：0，他引：1  
利用多卫星观测资料,分析了2008年9月3个连续台风前后的海表温度(SST)和海表高度距平(SSHA)的时空变化特征,并探讨了影响其变化的主要因子。结果表明:(1)3个台风引起了强烈的上升流(1×10-5~150×10-5 m/s),海表显著降温(1~6 ℃),海表高度也有不同程度降低(10~50 cm);(2)台风引起的SST最大降温中心与SSHA负值或中尺度冷涡的区域中心十分吻合,同时台风使得先前存在的海洋中尺度冷涡得到加强;(3)同一区域台风对SST影响程度大小受台风的强度、移动速度以及台风对海面强迫时间等因素控制;(4)在原先SSHA为正值的海域,3个台风连续强迫下使得局地洋面形成一个SSHA为负值的中尺度涡,这与单一"打转"台风强迫海洋生成中尺度涡的现象不同。因此,对于西北太平洋海域而言,频发的台风在中尺度涡生消演变过程中的影响应不容忽视。  相似文献   

13.
Lagrangian time series of dimethylsulfide (DMS) concentrations from a cyclonic and an anticyclonic eddy in the Sargasso Sea were used in conjunction with measured DMS loss rates and a model of vertical mixing to estimate gross DMS production in the upper 60 m during summer 2004. Loss terms included biological consumption, photolysis, and ventilation to the atmosphere. The time- and depth (0–60 m)-averaged gross DMS production was estimated to be 0.73±0.09 nM d−1 in the cyclonic eddy and 0.90±0.15 nM d−1 in the anticyclonic eddy, with respective DMS replacement times of 5±1 and 6±1 d. The higher estimated rate of gross production and lower measured loss rate constants in the anticyclonic eddy were equally responsible for this eddy's 50% higher DMS inventory (0–60 m). When normalized to chlorophyll and total dimethylsulfoniopropionate (DMSP), estimated gross production in the anticyclonic eddy was about twice that in the cyclonic eddy, consistent with the greater fraction of phytoplankton that were DMSP producers in the anticyclonic eddy. Higher rates of gross production were estimated below the mixed layer, contributing to the subsurface DMS maximum found in both eddies. In both eddies, gas exchange, microbial consumption, and photolysis were roughly equal DMS loss terms in the surface mixed layer (0.2–0.4 nM d−1). Vertical mixing was a substantial source of DMS to the surface mixed layer in both eddies (0.2–0.3 nM d−1) owing to the relatively high DMS concentrations below the mixed layer. Estimated net biological DMS production rates (gross production minus microbial consumption) in the mixed layer were substantially lower (by almost a factor of 3) than those estimated in a previous study of the Sargasso Sea, which may explain the relatively low mixed-layer DMS concentrations found here during July 2004 (3 nM) compared to previous summers (4–6 nM).  相似文献   

14.
In this study, we use existing observational datasets to evaluate 20th century climate simulations of the tropical Pacific. The emphasis of our work is decadal variability of the shallow meridional overturning circulation, which links the tropical and subtropical Pacific Ocean. In observations, this circulation is characterized by equatorward geostrophic volume transport convergence in the interior ocean pycnocline across 9°N and 9°S. Historical hydrographic data indicate that there has been a decreasing trend in this convergence over the period 1953–2001 of about 11 Sverdrup (1 Sv = 106 m3 s−1), with maximum decade-to-decade variations of 7–11 Sv. The transport time series is highly anti-correlated with sea surface temperature (SST) anomalies in the central and eastern tropical Pacific, implying that variations in meridional overturning circulation are directly linked to decadal variability and trends in tropical SST. These relationships are explored in 18 model simulations of 20th century climate from 14 state-of-the-art coupled climate models. Significant correlation exists between meridional volume transport convergence and tropical SST in the majority of the models over the last half century. However, the magnitude of transport variability on decadal time scales in the models is underestimated while at the same time modeled SST variations are more sensitive to that transport variability than in the observations. The effects of the meridional overturning circulation on SST trends in most the models is less clear. Most models show no trend in meridional transport convergence and underestimate the trend in eastern tropical Pacific SST. The eddy permitting MIROCH model is the only model that reasonably reproduces the observed trends in transport convergence, tropical Pacific SST, and SST gradient along the equator over the last half century. If the observed trends and those simulated in the MIROCH model are ultimately related to greenhouse gas forcing, these results suggest that the Bjerknes feedback, by affecting pycnocline transport convergences, may enhance warming that arises from anthropogenic forcing in the eastern tropical Pacific.  相似文献   

15.
Physical forcing plays a major role in determining biological processes in the ocean across the full spectrum of spatial and temporal scales. Variability of biological production in the Bay of Bengal (BoB) based on basin-scale and mesoscale physical processes is presented using hydrographic data collected during the peak summer monsoon in July–August, 2003. Three different and spatially varying physical processes were identified in the upper 300 m: (I) anticyclonic warm gyre offshore in the southern Bay; (II) a cyclonic eddy in the northern Bay; and (III) an upwelling region adjacent to the southern coast. In the warm gyre (>28.8 °C), the low salinity (33.5) surface waters contained low concentrations of nutrients. These warm surface waters extended below the euphotic zone, which resulted in an oligotrophic environment with low surface chlorophyll a (0.12 mg m−3), low surface primary production (2.55 mg C m−3 day−1) and low zooplankton biovolume (0.14 ml m−3). In the cyclonic eddy, the elevated isopycnals raised the nutricline upto the surface (NO3–N > 8.2 μM, PO4–P > 0.8 μM, SiO4–Si > 3.5 μM). Despite the system being highly eutrophic, response in the biological activity was low. In the upwelling zone, although the nutrient concentrations were lower compared to the cyclonic eddy, the surface phytoplankton biomass and production were high (Chl a – 0.25 mg m−3, PP – 9.23 mg C m−3 day−1), and mesozooplankton biovolume (1.12 ml m−3) was rich. Normally in oligotrophic, open ocean ecosystems, primary production is based on ‘regenerated’ nutrients, but during episodic events like eddies the ‘production’ switches over to ‘new production’. The switching over from ‘regenerated production’ to ‘new production’ in the open ocean (cyclonic eddy) and establishment of a new phytoplankton community will take longer than in the coastal system (upwelling). Despite the functioning of a cyclonic eddy and upwelling being divergent (transporting of nutrients from deeper waters to surface), the utilization of nutrients leading to enhanced biological production and its transfer to upper trophic levels in the upwelling region imply that the energy transfer from primary production to secondary production (mesozooplankton) is more efficient than in the cyclonic eddy of the open ocean. The results suggest that basin-scale and mesoscale processes influence the abundance and spatial heterogeneity of plankton populations across a wide spatial scale in the BoB. The multifaceted effects of these physical processes on primary productivity thus play a prominent role in structuring of zooplankton communities and could consecutively affect the recruitment of pelagic fisheries.  相似文献   

16.
During winter, south of the Indo-Sri Lanka Channel (ISLC), the observed sea-surface temperature (SST) distribution shows a distinct mini-cold pool (MCP) with relatively cooler waters (SST<28 °C). All the available satellite and in-situ measurements are utilized to characterize and explain the mechanisms that govern the evolution of the observed MCP. During December–January, the northeasterly surface winds blow through the ISLC manifesting a patch of strong winds in the south with peak intensity of about 10 m/s, enhance surface turbulent heat losses and drive near-surface vertical mixing resulting in the observed cooling. The vertical temperature profiles in this region also show cooling and deepening of the near-surface isothermal layer from November to January. This cooling occurs episodically on an intra-seasonal time scale with a typical periodicity of 8–15 days and is stronger when the surface winds intensify, surface net heat losses are larger and the near-surface circulation is more pronounced. The cooling episodes varied in number, intensity, duration and spatial extent in each winter during 1998–2006. The cooler surface waters from this MCP flow initially southwestward and are then topographically steered northwestward by the Maldives Island Chain. The resultant near-surface circulation also appears to strengthen the amplitude of the near-surface thermal inversions observed in the SouthEastern Arabian Sea (SEAS).  相似文献   

17.
In the winter Kuroshio Extension region, the atmospheric response to oceanic eddies is studied using reanalysis and satellite data. The detected eddies in this region are mostly under the force of northwesterly wind, with the sea surface temperature (SST) anomaly located within the eddy. By examining the patterns of surface wind divergence, three types of atmospheric response are identified. The first type, which occupies 60%, is characterized by significant sea surface wind convergence and divergence at the edge and a vertical secondary circulation (SC) aloft, supporting the “vertical momentum mixing mechanism”. The SCs on anticyclonic eddies (AEs) can reach up to 300 hPa, but those on cyclonic eddies (CEs) are limited to 700 hPa. This can be explained by analyzing vertical eddy heat transport: When northwesterly wind passes the warmer center of an AE, it is from the cold to warm sea surface, resulting in stronger evaporation and convection, triggering stronger upward velocity and moist static heat flux. For the cases of CEs, the wind blows from warm to cold, which means less instability and less evaporation, resulting in weaker SCs. The second type, which occupies 10%, is characterized by divergence and a sea level pressure anomaly in the center, supported by the “pressure adjustment mechanism”. The other 30% are mostly weak eddies, and the atmospheric variation aloft is unrelated to the SST anomaly. Our work provides evidence for the different atmospheric responses over oceanic eddies and explains why SCs over AEs are much stronger than those over CEs by vertical heat flux analysis.  相似文献   

18.
为研究帕里西维拉海盆区上层水体中悬浮体分布特征,对2003年冬季在该海区现场测得的上层水体悬浮体质量浓度数据进行详细分析。结果表明,调查期间研究区悬浮体质量浓度在水平上表现出西高东低的特征,垂向上的差异较小。现场大面站调查和同期卫星高度计数据显示,调查期间在研究区海域同时存在冷涡和暖涡活动,其中冷涡活动区位于研究区西部,暖涡活动区位于东部。分层分析表明,调查区中尺度涡活动对悬浮体分布有明显的控制效应:在冷涡活动区内,深层的富营养盐水体上涌,使冷涡内部的浮游生物量增多,导致冷涡活动区内的悬浮体质量浓度明显高于周围;在暖涡活动区内,因上层的寡营养盐水体下沉,使暖涡区的浮游生物量减少,导致悬浮体质量浓度明显低于周围。受洋流和中尺度涡活动的影响,调查海区西北部岛礁的物质供应也是导致研究区悬浮体分布呈现西高东低格局的重要因素。  相似文献   

19.
Altimeter data and output from the HYbrid Coordinate Ocean Model global assimilation run are used to study the seasonal variation of eddy shedding from the Kuroshio intrusion in the Luzon Strait. The results suggest that most eddy shedding events occur from December through March, and no eddy shedding event occurs in June, September, or October. About a month before eddy shedding, the Kuroshio intrusion extends into the South China Sea and a closed anticyclonic eddy appears inside the Kuroshio loop which then detaches from the Kuroshio intrusion. Anticyclonic eddies detached from December through February move westward at a speed of about 0.1 m s−1 after shedding, whereas eddies detached in other months either stay at the place of origin or move westward at a very slow speed (less than 0.06 m s−1). The HYCOM outputs and QuikSCAT wind data clearly show that the seasonal variation of eddy shedding is influenced by the monsoon winds. A comparison between eddy volume and integrated Ekman transport indicates that, once the integrated Ekman transport exceeds 2 × 1012 m3 (which roughly corresponds to the volume of an eddy), the Kuroshio intrusion expands and an eddy shedding event occurs within 1 month. We infer that the Ekman drift of the northeasterly monsoon pushes the Kuroshio intrusion into the SCS, creates a net westward transport into the Strait, and leads to an eddy detachment from the Kuroshio.  相似文献   

20.
Sea surface salinity (SSS) data in the Atlantic Ocean is investigated between 50°N and 30°S based on data collected mostly during the period 1977–2002. Monthly mapping of SSS is done to extract the large-scale variability. This mapped variability indicates fairly long (seasonal) time scales outside the equatorial region. The spatial scales of the seasonal anomalies are regional, but not basin-wide (typically 500–1000 km). These seasonal SSS anomalies are found to respond with a 1–2 month lag to freshwater flux anomalies at the air–sea interface or to the horizontal Ekman advection. This relation presents a seasonal cycle in the northern subtropics and north-east Atlantic indicating that the late-boreal spring/summer season is less active than the boreal winter/early-spring season in forcing the seasonal SSS variability. In the north-eastern mid-latitude Atlantic, SSS is positively correlated to SST, with SSS slightly lagging SST. There are noticeable long-lasting larger-scale signals overlaid on this regional variability. Part of it is related to known climate signals, for example ENSO and NAO. A linear trend is present during the first half of the period in some parts of the basin (usually towards increasing salinities, at least between 20°N and 45°N). Based on a linear regression analysis, these signals combined can locally represent up to 20% of SSS variance (in particular near 30°N/60°W or 40°N/10–30°W), but usually represent less than 10% of the variance.  相似文献   

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