ISSN:0148-0227    

DOI:10.1029/2002JC001669

年代:2002

数据来源: WILEY

摘要:

A study is presented of modulations induced by a dominant long surface wave (LW) in the coupled airflow–short wind‐generated waves (SW) system. The modulation of SWs in the gravity range results from their interaction with the LW orbital velocity and their interaction with the wind stress modulated by the LW. The modulation of waves in the capillary range is mainly due to the modulation of the rate of generation of parasitic capillaries by steep short gravity waves. The variation of the wind surface stress is described by the dynamics of the turbulent airflow over the LW with the surface roughness varying along its profile. The variation of the surface roughness is caused by the variation of the form drag supported by modulated SWs. In turn, the LW‐induced variation of the surface stress affects the SW modulation. This provides a feedback in the coupled airflow–SW system in the presence of the LW. The model results show that the amplitude of the surface roughness modulation can be large. In terms of the modulation transfer function (MTF), it can reach values of 10–20. The modulation of the form drag, which causes the modulation of the surface roughness, comes mainly from short breaking waves strongly modulated by dominant waves. The modulation of the surface roughness considerably affects the dynamics of the airflow over the LW and thus the LW wind growth rate. Models of the airflow above waves assuming a constant roughness parameter underestimate the growth rate parameter approximately two to three times as compared to the measured values. The present study shows that when the variation in the surface roughness is accounted for, the growth rate parameter increases roughly twice, which to a large extent reduces the discrepancy with mea

ISSN:0148-0227    

DOI:10.1029/2001JC001251

年代:2002

数据来源: WILEY

摘要:

Sea surface salinity (SSS) and sea surface temperature (SST) in the western Pacific warm pool (130–180°E; 10°N–10°S) are analyzed for the period 1992–2000 taking advantage of complementary data from the ship of opportunity program and the Tropical Atmosphere‐Ocean (TAO)‐Triangle Trans‐Ocean Buoy Network (TRITON) array of moored buoys. Covariability of these variables with surface wind stress, surface zonal currents, evaporation, precipitation, and barrier layer thickness is also examined. These fields all go through large oscillations related to the El Niño Southern Oscillation (ENSO) cycle, most notably during the record breaking 1997–1998 El Niño and subsequent strong 1998–2000 La Niña. East of about 160°E, during El Niño, precipitation minus evaporation increases in the equatorial band, in conjunction with anomalous increases in westerly winds, eastward surface currents, SST, and decreases in SSS. Opposite tendencies are evident during La Niña. Peak to peak 2°N–2°S averaged variations reached as much as 1.2 m s−1for zonal currents and 1.5 practical salinity units (psu) for SSS. West of about 160°E, SST cools during El Niño and warms during La Niña, opposite to what occurs further east. To understand these SST tendencies west of 160°E, a proxy indicator for barrier layer formation is developed in terms of changes in the zonal gradient of SSS (∂S/∂x). Zonal SSS gradients have been shown in modeling studies to be related to barrier layer formation via subduction driven by converging zonal currents in the vicinity of the salinity front at the eastern edge of the warm pool. Correlation between changes in ∂S/∂xand changes in SST a few degrees longitude to the west is significantly nonzero, consistent with the idea that increased barrier layer thickness is related to warmer SSTs during periods of westward surface flow associated with La Niña, and vice versa during El Niño. Direct evidence of barrier layer thickness variations in

ISSN:0148-0227    

DOI:10.1029/2001JC000862

年代:2002

数据来源: WILEY

摘要:

The so‐called indirect method of estimating the kinetic energy dissipation rate, a key parameter in environmental fluid mechanics, involves fitting the observed spectrum to the theoretical Batchelor spectrum. This requires the statistically nonstationary temperature gradient profiles to be split onto statistically stationary segments. This comparative study of segmentation algorithms uses synthetic temperature gradient series and the temperature gradient profiles that are measured in an inland lake. The results of segmentation based on autoregressive (AR) models and wavelet analysis are compared. The bias in estimating the Batchelor spectrum from nonstationary segments, which have changes in the spectral shape or the spectral magnitude, is demonstrated. The estimated spectra of the resulting stationary segments should be stationary in both the shape and the magnitude of the spectra. A modified AR‐based test and a proposed wavelet‐based test are sensitive to changes in both the spectral shape and the spectral magnitude. An empirical segmentation technique sensitive to changes in the spectral shape only is not recommended fo

ISSN:0148-0227    

DOI:10.1029/2001JC001009

年代:2002

数据来源: WILEY

摘要:

The scatterometer ocean wind retrieval process produces several possible solutions or ambiguities at each point, requiring a separate ambiguity selection step to infer a unique wind vector field. An ambiguity selection error occurs when the selected wind vector is not the closest ambiguity to the true wind. The current ambiguity selection routine for SeaWinds is ad hoc, but performs well under most circumstances. Factors such as instrument noise and rain can also cause the estimated wind flow to deviate from the true wind. A quality assurance (QA) analysis is performed to assess the ambiguity selection effectiveness and noise level of the retrieved wind using a low‐order wind field model. The wind field model is data‐driven and shown to be rather insensitive to the training data set. The QA analysis demonstrates that the SeaWinds ambiguity selection process is at least 95% effective. Ambiguity selection errors are correlated with storms and rain corruption. A subjective analysis on a set of cyclonic storm passes confirms that the wind retrieval is somewhat less effective in storm regi

ISSN:0148-0227    

DOI:10.1029/2002JC001330

年代:2002

数据来源: WILEY

摘要:

Observations of the surface wind field off the U.S. West Coast were made from satellite by the Sea Surface Microwave/Imager (SSM/I) during June 1994 and June 1996. The coast bends inward 28° at 40° N latitude. Northerly wind speeds are high within a region that extends far south and offshore of this bend, bounded on the east by the California coast. On average, the high‐speed wind zone extends ∼250 km from shore and 800 km alongcoast. Long coast‐perpendicular aircraft transects show that moving onshore, the depth of the MABL decreases, the northerly wind speed increases, and the Froude number increases. A network of oceanographic buoys records the surface wind speed, direction, and pressure. From six buoy pressure measurements, the geostrophic wind vector was computed to represent the synoptic flow over the Oregon‐northern California coastal waters. The satellite surface wind fields were composited by the direction of the buoy geostrophic background wind. The predominant direction is 210°–180°. A high‐speed region extends south of the bend for directions in the 0°–180° range (moving counterclockwise), which corresponds to a downcoast component of the pressure gradient. For directions between 180° and 0°, flow splits at the bend and the region of highest winds is found to its north and offshore. A high‐speed region south of the bend was present in 65% of the satellite wind maps as well as in the June averages. The flow's response to coastal promontories such as Point Arena is nested within this broad zone of fast, thin, li

ISSN:0148-0227    

DOI:10.1029/2001JC000807

年代:2002

数据来源: WILEY

摘要:

A Fourier decomposition of theWorld Ocean Atlas 1998(WOA98) fields of climatological monthly mean salinity is used to describe the annual cycle of sea surface salinity (SSS). Global data sets of river runoff and evaporation minus precipitation (E−P) are used to put the annual cycle of SSS in the context of the global climate system. Most of the world ocean is shown to have an annual cycle of less than 0.3 on the practical salinity scale (PSS). Areas with an annual cycle larger than 0.3 include the northern Indian Ocean, the tropical Pacific and Atlantic, and the northern North Atlanti

ISSN:0148-0227    

DOI:10.1029/2001JC000829

年代:2002

数据来源: WILEY

摘要:

Multiple regression analysis is used here to construct statistical prediction models for the El Niño/Southern Oscillation (ENSO) to explore the potential impact of monitoring Pacific Ocean sea surface salinity (SSS) on prediction of equatorial Pacific sea surface temperature (SST). This study, one of the firsts focusing on the direct role of SSS in ENSO predictions, is motivated by proposed missions for remote sensing of SSS. A forward stepwise method is used to extract significant predictors of the Niño‐3 SST index from observed monthly anomalies of tropical SST, SSS, sea level, freshwater flux, and components of the wind stress. The results indicate that SSS monitoring would have small impact on the statistical nowcast (reconstruction) of ENSO but a potential role in the 6–12 month forecasts. Correlation maps show two regions of high correlation: an equatorial region (between 170°E and 160°W) and an off‐equatorial region (between 170°E and 140°W and 5°S and 20°S). Short lag correlations display the negative relationship between the warm phase of ENSO and the negative equatorial SSS anomalies related with the increase of local rainfall. Such an equatorial negative correlation coexists with an area of positive correlations off the equator. The region with positive correlations moves eastward as the lag increases, reaching the geographical limit of the SSS observations at 6 months lag. The region of negative correlation moves northward and becomes weaker as the lag increases (it is nonsignificant for 9 months lag). For lags longer than 9 months, significant positive correlations are found south of the equator (5°S–10°S). At these lags, positive salinity anomalies have the potential to modify the subsurface stratification of the western Pacific as they are subducted westward. Thus, the availability of continuous remotely sensed SSS data might add considerably to ENSO predictions at longer lead times as a result of SSS‐induced changes in the subsu

ISSN:0148-0227    

DOI:10.1029/2001JC000834

年代:2002

数据来源: WILEY

摘要:

We use quarterly full water depth occupations of a station at the center of the Santa Barbara Basin to estimate the timing, frequency, and magnitude of basin renewal events over the period 1986–2000. Of 58 occupations we have identified 11 events (in 11 of the 15 years) that are highly suggestive of recent bottom water renewal, with periods of annual flushing interspersed with relatively stagnant periods. All but one of these events occurred in winter or spring, with the spring events having the largest changes in bottom water properties (cooling and oxygenation). The event recorded in April 1998, during a period of rapid transition from El Niño‐influenced to La Niña‐influenced local conditions, revealed the potential magnitude of a full basin flushing: an increase in bottom water oxygen content of more than 0.4 mL/L and a cooling of nearly half a degree over 2 months. Water property profiles and near‐surface circulation patterns suggest that sufficiently dense water at the western sill, combined with strong flow into the basin from the northwest, may be key prerequisites to renewal. Interannual variability in the strength, duration, and timing of coastal upwelling and in regional circulation and hydrography, forced largely by the local impact of El Niño events, modulates the flushing potential of the basin. The Santa Barbara Basin apparently does not get flushed every year, and renewal events are not restricted to winter‐spring. Since the frequency and intensity of basin renewal events can greatly impact local denitrification rates, benthic community structure, and underlying sedimentary processes, we recommend that paleoceanographic interpretations of the local sediment record take account of this seasonally and interannually varying cycle of bottom w

ISSN:0148-0227    

DOI:10.1029/2001JC001291

年代:2002

数据来源: WILEY

摘要:

A description of the near‐surface circulation and its properties is the result of the analysis of a drifting buoy data set in the eastern North Atlantic between the Iberian Peninsula, the Azores, and the Canary Islands. World Ocean Circulation Experiment–Tropical Ocean–Global Atmosphere experiment drifters equipped with holey sock drogues centered at 15 m depth collected a total of 14.4 years of data. The drifters sampled a rather inhomogeneous velocity field with a weak mean flow regime and eddies of different scales. They meandered southward everywhere in the study region, except in the Iberian coastal transition zone north of 41°N where they headed northward. The near‐surface mean velocity field obtained from the drifter data set shows all important mean currents, including the poleward Portugal Coastal Countercurrent during the fall, winter, and early spring off western and northern Iberia, the southward Portugal Coastal Current, the slow offshore southward flow of the Portugal Current during the whole year, the southwestward Canary Current, and the eastward Azores Current, which extends to the vicinity of the African coast near the Gulf of Cadiz. Maps of the eddy kinetic energy field were obtained from the drifters and from satellite altimetry. It provides the largest part of the total kinetic energy. The rate of dispersion is estimated from the Lagrangian statistics of the drifting buoys. The dispersion of the drifters in the study region is well modeled by a simple description of eddy diffusion assuming homogeneous turbulence. Ensemble mean diffusivitiesKand the Langrangian integral length scales and timescales (LandT) were obtained for the zonal and meridional directions. The sea surface temperature measured along the drifter trajectories is used to produce estimates of the eddy diffusivity, which is compared with the diffusivity estimates obtained from the theory of Taylor. The eddy diffusivity is found to be approximately proportional to the eddy kinetic energy. Discrete eddies and meanders were observed using drifters and altimetry in order to map and describe their geographical distribution and characteristics in the eastern North

ISSN:0148-0227    

DOI:10.1029/2000JC000345

年代:2002

数据来源: WILEY