摘要:

AbstractTurbulence, quantified as the rate of dissipation of turbulent kinetic energy (ε), was measured with 1400 temperature‐gradient microstructure profiles obtained concurrently with time series measurements of temperature and current profiles, meteorology, and lake‐atmosphere fluxes using eddy covariance in a 4 km2temperate lake during fall cooling. Winds varied from near calm to 5 m s−1but reached 10 m s−1during three storm events. Near‐surface values of ε were typically on the order of 10−8to 10−7m2s−3and reached 10−5m2s−3during windy periods. Above a depth equal to |LMO|, the Monin‐Obukhov length scale, turbulence was dominated by wind shear and dissipation followed neutral law of the wall scaling augmented by buoyancy flux during cooling. During cooling, εz = 0.56u*w3/kz + 0.77 JB0and during heating εz = 0.6u*w3/kz, whereu*wis the water friction velocity computed from wind shear stress, k is von Karman's constant, z is depth, and JB0is surface buoyancy flux. Below a depth equal to |LMO| during cooling, dissipation was uniform with depth and controlled by buoyancy flux. Departures from similarity scaling enabled identification of additional processes that moderate near‐surface turbulence including mixed layer deepening at the onset of cooling, high‐frequency internal waves when the diurnal thermocline was adjacent to the air‐water interface, and horizontal advection caused by differential cooling. The similarity scaling enables prediction of near‐surface ε as required for estimating the gas transfer coefficient using the surface renewal model and

ISSN:0148-0227    

DOI:10.1002/2014JC010135

年代:2014

数据来源: WILEY

摘要:

AbstractSea level extremes in the Caribbean Sea are analyzed on the basis of hourly records from 13 tide gauges. The largest sea level extreme observed is 83 cm at Port Spain. The largest nontidal residual in the records is 76 cm, forced by a category 5 hurricane. Storm surges in the Caribbean are primarily caused by tropical storms and stationary cold fronts intruding the basin. However, the seasonal signal and mesoscale eddies also contribute to the creation of extremes. The five stations that have more than 20 years of data show significant trends in the extremes suggesting that flooding events are expected to become more frequent in the future. The observed trends in extremes are caused by mean sea level rise. There is no evidence of secular changes in the storm activity. Sea level return periods have also been estimated. In the south Colombian Basin, where large hurricane‐induced surges are rare, stable estimates can be obtained with 30 years of data or more. For the north of the basin, where large hurricane‐induced surges are more frequent, at least 40 years of data are required. This suggests that the present data set is not sufficiently long for robust estimates of return periods. ENSO variability correlates with the nontidal extremes, indicating a reduction of the storm activity during positive ENSO events. The period with the highest extremes is around October, when the various sea level contributors' maxima coinc

ISSN:0148-0227    

DOI:10.1002/2014JC009929

年代:2014

数据来源: WILEY

摘要:

AbstractConstraining dynamical systems with new information from ocean measurements, including observations of sea surface salinity (SSS) from Aquarius and SMOS, requires careful consideration of data errors that are used to determine the importance of constraints in the optimization. Here such errors are derived by comparing satellite SSS observations from Aquarius and SMOS with ocean model output and in situ data. The associated data error variance maps have a complex spatial pattern, ranging from less than 0.05 in the open ocean to 1–2 (units of salinity variance) along the coasts and high latitude regions. Comparing the data‐model misfits to the data errors indicates that the Aquarius and SMOS constraints could potentially affect estimated SSS values in several ocean regions, including most tropical latitudes. In reference to the Aquarius error budget, derived errors are less than the total allocation errors for the Aquarius mission accuracy requirements in low and midlatitudes, but exceed allocation errors in high latitu

ISSN:0148-0227    

DOI:10.1002/2014JC009906

年代:2014

数据来源: WILEY

摘要:

AbstractRecently it has been shown that anticyclonic eddies are generated in the Comoros Basin contesting the long‐held notion of a single large anticyclonic cell, the Comoros Gyre. Limited knowledge exists about the mesoscale activity within the basin, a potential key source of variability for the Mozambique Channel and subsequently the Agulhas Current. In this paper an automated eddy tracking scheme, applied to satellite altimetry data and a high‐resolution model simulation, is used to determine the characteristics of the anticyclonic eddies generated in the Comoros Basin. The generation and characteristics of cyclonic eddies are also investigated. The eddy tracking scheme revealed that anticyclonic eddies are primarily generated west of the tip of Madagascar due to barotropic instabilities whereas cyclonic eddies are mainly generated along the northwest coast of Madagascar as a result of baroclinic instabilities. Anticyclonic eddies, with a mean lifespan of about 3 months, reside in the basin for half their lifespan before propagating into the Mozambique Channel. On the other hand, the majority of cyclonic eddies, with a similar mean lifespan, dissipate within the basin. Initially, the anticyclones, with translation speeds of 6–8 km d−1and mean radii of 80–100 km, follow the trajectory of the North East Madagascar Current and turn south upon reaching the African coast. The cyclonic eddies tend to be smaller (∼60 km) and have slower translation speeds (2.5–3.5 km d−1) than their anticyclon

ISSN:0148-0227    

DOI:10.1002/2014JC010008

年代:2014

数据来源: WILEY

摘要:

AbstractZonal geostrophic velocity fields above 1975 dbar have been estimated for the Southern Ocean from 2004 into 2011 based on sea surface topography observed by Jason altimetry and temperature/salinity measured by Argo autonomous floats. The velocity at 1000 dbar estimated with the method has been compared to Argo drift trajectory at the same pressure level available from the Asia Pacific Data Research Center (APDRC). The inferred velocities agree with those from the Argo drift within the estimated sampling error of the latter, but have fewer gaps in space and time. The velocity has also been integrated from depth to surface to determine the mean and time‐variable zonal geostrophic transport in the Southern Ocean between 29.5°S and 58.5°S, primarily in the South Atlantic and South Indian Ocean basins, due to limitations in coverage of Argo. Analysis shows errors can be reduced by>70% by averaging gridded results over wide areas. Zonal transport averaged over the entire Indian Ocean basin shows a significant correlation with the Antarctic Oscillation (AAO) at low frequencies: transport is higher than normal during a positive phase of the AAO, and lower during the negative ph

ISSN:0148-0227    

DOI:10.1002/2014JC009853

年代:2014

数据来源: WILEY

摘要:

AbstractSingular Evolutive Extended Kalman (SEEK) filter has been used to assimilate Aquarius‐derived sea surface salinity (SSS) in a near‐global ocean general circulation model (OGCM). Advanced Very High Resolution Radiometer (AVHRR)‐derived sea surface temperature (SST) has also been assimilated in conjunction. The primary aim of the study is to investigate the improvement in simulation of global ocean surface currents as a result of this assimilation. The route of empirical orthogonal function (EOF) analysis has been taken for an efficient assessment of this impact separately in the space and time domains and satellite‐derived surface current has been used as a benchmark. As expected, the assimilation has been found to impart significant positive impact in both the domains. Also, joint assimilation of SSS and SST has been found to be better than standalone SSS assimilation. These results have been further corroborated by a comparison with buoy‐derived surface currents. Further emphasis has been laid on the simulation of Wyrtki and monsoon jets in the equatorial Indian Ocean, because of their importance in the climate of this region and again it has been found that assimilation guides the simulation toward realism in both the cases. Finally, impact on the SSS and SST fronts and their zonal displacements in the western Pacific has been investigated and here again the assimilation has led to an improvement in simulation of these

ISSN:0148-0227    

DOI:10.1002/2014JC009984

年代:2014

数据来源: WILEY

摘要:

AbstractData collected from different platforms in the Cretan Sea during the 2000s decade present evidence of gradually increasing salinity in the intermediate and deep intermediate layers after the middle of the decade. The observed gradual salt transport toward the deeper layers indicates contributions of dense water masses formed in various Aegean Sea subbasins. The accumulation of these saline and dense water masses in the Cretan Sea finally led to outflow from both Cretan Straits, with density greater than typical Levantine/Cretan Intermediate water but not dense enough to penetrate into the deep layers of the Eastern Mediterranean. We name this outflowing water mass as dense Cretan Intermediate Water (dCIW). A retrospective analysis of in situ data and literature references during the last four decades shows that similar events have occurred in the past in two occasions: (a) in the 1970s and (b) during the Eastern Mediterranean Transient (EMT) onset (1987–1991). We argue that these salinity‐driven Aegean outflows are mostly attributed to recurrent changes of the Eastern Mediterranean upper thermohaline circulation that create favorable dense water formation conditions in the Aegean Sea through salinity preconditioning. We identify these phenomena as “EMT‐like” events and argue that in these cases internal thermohaline mechanisms dominate over atmospheric forcing in dense water production. However, intense atmospheric forcing over an already salinity preconditioned basin is indispensable for creating massive deep water outflow from the Cretan Sea, such as the

ISSN:0148-0227    

DOI:10.1002/2014JC009937

年代:2014

数据来源: WILEY

摘要:

AbstractThe uncertainty of Aquarius sea surface salinity (SSS) retrieved under rain is assessed. Rain not only has instantaneous impact on SSS but also interferes with the microwave remote sensing signals, making the task to retrieve SSS under rainy conditions difficult. A rain correction model is developed based on analysis of the L‐band radiometer/scatterometer residual signals after accounting for roughness due to wind and flat surface emissivity. The combined active passive algorithm is used to retrieve SSS in parallel with (CAP_RC) or without rain correction (CAP). The CAP bias against individual ARGO floats increases with rain rate with slope of −0.14 PSU per mm h−1, which reduced to near zero in CAP_RC. On the global monthly basis, CAP_RC is about 0.03 PSU higher than CAP. RMSD against ARGO is slightly smaller for CAP_RC than CAP. Regional biases are examined in areas with frequent rain events. As expected, results show that ΔSSS (CAP_RC‐CAP) is highly correlated with the seasonal precipitation pattern, reaching about 0.2–0.3 PSU under heavy rain. However, ΔSSS shows no correlation with the difference pattern between ARGO and CAP or CAP_RC. This, along with regional analyses, suggests that the difference between ARGO and Aquarius' SSS is likely caused by the different spatial and temporal sampling, in addition to near surface stratification depicted by radiometer and ARGO at different depths. The effect of ΔSSS on water cycle in terms of mixed‐layer salt storage tendency is about 10% in areas where evaporation‐minus‐precipitation is the dominant process driving the variability of nea

ISSN:0148-0227    

DOI:10.1002/2014JC009834

年代:2014

数据来源: WILEY

摘要:

AbstractGlobal observations of quasi‐zonal jet‐like structures have recently been reported in estimates of upper ocean circulation. To date, these observations have come primarily from float‐derived and altimeter‐derived estimates of zonal velocity. Here, we explore the existence of similar structures in the ocean using satellite‐derived estimates of sea surface temperature (SST) from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR‐E). Applying an ocean front detection algorithm globally to microwave measurements of SST, we find that repeated ocean fronts occur along quasi‐zonal bands in a multiyear (2002–2011) average of detections. Such a pattern is also observed in SST gradient magnitude. Composite analyses of SST, sea surface height (SSH), and upper ocean temperatures from Argo profiling floats suggest repeated fronts in the subtropics occur as a result of neighboring anticyclonic and cyclonic eddies. Horizontal advection in the presence of a background temperature gradient likely plays a role as evidenced by the tilt of temperature anomalies with depth. High gradient events found within the bands are observed to propagate westward with speed comparable to mesoscale eddies and we estimate these events explain 20% of the observed variance in SST gradient magnitude (2002–2011). In a final analysis, we regress the decay of the bands with averaging period and observe mild‐to‐strong persistence throughout much of the World Ocean. These findings support the view that propagating eddies help give rise to the bands. Whether or not eddies follow preferred pa

ISSN:0148-0227    

DOI:10.1002/2014JC010088

年代:2014

数据来源: WILEY

摘要:

AbstractMesoscale eddy effects on the subduction of North Pacific mode waters are investigated by comparing observations and ocean general circulation models where eddies are either parameterized or resolved. The eddy‐resolving models produce results closer to observations than the noneddy‐resolving model. There are large discrepancies in subduction patterns between eddy‐resolving and noneddy‐resolving models. In the noneddy‐resolving model, subduction on a given isopycnal is limited to the cross point between the mixed layer depth (MLD) front and the outcrop line whereas in eddy‐resolving models and observations, subduction takes place in a broader, zonally elongated band within the deep mixed layer region. Mesoscale eddies significantly enhance the total subduction rate, helping create remarkable peaks in the volume histogram that correspond to North Pacific subtropical mode water (STMW) and central mode water (CMW). Eddy‐enhanced subduction preferentially occurs south of the winter mean outcrop. With an anticyclonic eddy to the west and a cyclonic eddy to the east, the outcrop line meanders south, and the thermocline/MLD shoals eastward. As eddies propagate westward, the MLD shoals, shielding the water of low potential vorticity from the atmosphere. The southward eddy flow then carries the subducted water mass into the thermocline. The eddy subduction processes revealed here have important implications for designing field observations and imp

ISSN:0148-0227    

DOI:10.1002/2014JC009861

年代:2014

数据来源: WILEY