Sea Surface Wind Field Research Articles

African climate variability in the satellite era

Interannual fluctuations in rainfall and ocean-atmosphere fields over and around Africa were studied in the satellite era of 1979–2007 using singular value decomposition. The leading modes of rainfall variability in GPCP satellite-gauge merged fields include a leading mode over central Africa, two modes of marine origin in the Gulf of Guinea and Eastern Africa, and two sub-tropical modes over the Sahel and Southern Africa. This differs from earlier gauge-based studies that tend to isolate three leading modes over western, eastern, and southern Africa. In the sea-surface temperature, sea-level pressure and upper wind fields, ENSO signals dominate the leading modes. However, for the low-level wind field, a trough circulation over the southeast Atlantic – Kalahari is the leading mode. It demonstrates predictive potential when cross-correlated with rainfall at 6- to 12-month lead time. Based on continuous filtered data, the value of various indices and the predictability of different zones are examined. The Sahel achieves the highest rank followed by the Congo and southern zones in the next tier. The Guinea and East African rains, which peak in the March-to-May season, appear least predictable. The seasonal rainfall is shown to modulate economic growth rate, and multi-variate predictive algorithms are tested at 6-month lead time.

Identification and removal of block ambiguities in sea surface wind field with scatterometers

The solution of the current wind retrieval algorithm for scatterometers has several wind vector ambiguities, due to the bi-harmonic relationship between normalized backscattering cross section and the relative wind direction and the existence of the measurement error. In order to remove the ambiguities for a unique wind field, a circular median filter approach (CMF) is usually adopted. But under the condition for clustering distribution of the false ambiguities in some local areas, the CMF fails and thus engenders block ambiguities, which degrade the precision of the retrieved wind field. For such a situation, a technique of identification and removal of the block ambiguities is presented to further optimize the retrieved wind field after CMF. It is demonstrated in experiment that this technique can identify and remove most of the block ambiguities.

Responses of summertime extreme wave heights to local climate variations in the East China Sea

We detected the responses of summertime extreme wave heights (Htop10, average of the highest 10% of significant wave heights in June, July and August) to local climate variations in the East China Sea by applying an empirical orthogonal function analysis to Htop10 derived from the WAVEWATCH‐III wave model driven by 6 hourly sea surface wind fields from ERA‐40 reanalysis over the period 1958–2002. Decreases in Htop10 in the northern East China Sea (Yellow Sea) correspond to attenuation of the East Asian Summer Monsoon, while increases in the south are primarily due to enhancement of tropical cyclone activities in the western North Pacific.

ENVISAT ASARデータを用いた洋上の高解像度風速分布推定

The estimation of the high resolution wind speed distribution is required for the potential evaluation for the wind power generation over the sea. In this paper an algorithm is presented which enables high-resolution sea surface wind fields to be retrieved from the advanced synthetic aperture radar (ASAR) data acquired by European remote sensing satellite ENVISAT. The wind speeds are compared with the normalized radar cross section (NRCS) around the coast of Amami-Oshima Island. As a result, there is a high correlation (r=0.97) in NRCS derived from ASAR and the in-situ sea surface wind speed. The estimation RMSE of the ASAR wind speed is about 1.2 m/s. Thus, if the ENVISAT ASAR data is used, we can estimate the sea surface wind speed distribution with a resolution of about 125m.

An analysis of the characteristics of chlorophyll in the Sulu Sea

The seasonal and spatial variations of chlorophyll concentrations, Sea Surface Temperature (SST), wind fields and wind-induced Ekman pumping in the Sulu Sea are investigated using a set of new remote sensing measurements from October 1997 to December 2004. The results show the seasonality of chlorophyll, wind fields and SST and reveal the phytoplankton blooming events in the Sulu Sea basin during the northeast monsoon season. In summer, chlorophyll concentrations were relatively low (< 0.2 mg/m 3) and distributed uniformly throughout the basin with a narrow belt of high chlorophyll concentrations along the coastal waters, particularly the coasts of Borneo and of the Sulu Archipelago. In winter, chlorophyll concentrations increased (> 0.2 mg/m 3) throughout the entire basin, and phytoplankton bloomed southward to the central basin, while chlorophyll concentrations reached high levels (1 mg/m 3) in the center of the blooms. One peak was observed during the northeast monsoon season each year. SSTs have significant negative correlations with chlorophyll concentrations; i.e., high and uniformly distributed in summer but lower with an obvious tongue of cold waters southward to the central basin in winter. The seasonal variation of chlorophyll concentrations and SST distribution were associated with the seasonally reversing monsoon. The winter phytoplankton blooming and the tongue of the cold waters were correlated to the vertical upwelling cold and nutrient-rich waters drawn by the northeast wind, with the center of the blooms and the location of cold tongues coinciding with the maximum of the wind speeds and the Ekman pumping velocities.

Influence of orographically steered winds on Mutsu Bay surface currents

Effects of spatially dependent sea surface wind field on currents in Mutsu Bay, which is located at the northern end of Japanese Honshu Island, are investigated using winds derived from synthetic aperture radar (SAR) images and a numerical model. A characteristic wind pattern over the bay was evidenced from analysis of 118 SAR images and coincided with in situ observations. Wind is topographically steered with easterly winds entering the bay through the terrestrial gap and stronger wind blowing over the central water toward its mouth. Nearshore winds are weaker due to terrestrial blockages. Using the Princeton Ocean Model, we investigated currents forced by the observed spatially dependent wind field. The predicted current pattern agrees well with available observations. For a uniform wind field of equal magnitude and average direction, the circulation pattern departs from observations demonstrating that vorticity input due to spatially dependent wind stress is essential in generation of the wind‐driven current in Mutsu Bay.

Mediterranean wave climate variability and its links with NAO and Indian Monsoon

This study examines the variability of the monthly average significant wave height (SWH) field in the Mediterranean Sea, in the period 1958–2001. The analysed data are provided by simulations carried out using the WAM model (WAMDI group, 1988) forced by the wind fields of the ERA-40 (ECMWF Re-Analysis). Comparison with buoy observations, satellite data, and simulations forced by higher resolution wind fields shows that, though results underestimate the actual SWH, they provide a reliable representation of its real space and time variability. Principal component analysis (PCA) shows that the annual cycle is characterised by two main empirical orthogonal functions (EOF) patterns. Most inter-monthly variability is associated with the first EOF, whose positive/negative phase is due to the action of Mistral/Etesian wind regimes. The second EOF is related to the action of southerly winds (Libeccio and Sirocco). The annual cycle presents two main seasons, winter and summer characterised, the first, by the prevalence of eastwards and southeastwards propagating waves all over the basin, and the second, by high southwards propagating waves in the Aegean Sea and Levantin Basin. Spring and fall are transitional seasons, characterised by northwards and northeastwards propagating waves, associated to an intense meridional atmospheric circulation, and by attenuation and amplification, respectively, of the action of Mistral. These wave field variability patterns are associated with consistent sea level pressure (SLP) and surface wind field structures. The intensity of the SWH field shows large inter-annual and inter-decadal variability and a statistically significant decreasing trend of mean winter values. The winter average SWH is anti-correlated with the winter NAO (North Atlantic Oscillation) index, which shows a correspondingly increasing trend. During summer, a minor component of the wave field inter-annual variability (associated to the second EOF) presents a statistically significant correlation with the Indian Monsoon reflecting its influence on the meridional Mediterranean circulation. However, the SLP patterns associated with the SWH inter-annual variability reveal structures different from NAO and Monsoon circulation. In fact, wave field variability is conditioned by regional storminess in combination with the effect of fetch. The latter is likely to be the most important. Therefore, the inter-annual variability of the mean SWH is associated to SLP patterns, which present their most intense features above or close to Mediterranean region, where they are most effective for wave generation.

Recent increase in summertime extreme wave heights in the western North Pacific

Recent increase in summertime extreme wave heights (H10, June–August average of the highest 10% of significant wave heights at each month) in the western North Pacific is found by applying an empirical orthogonal function analysis to H10 derived from 3rd–generation wave model forced by 6‐hourly sea surface wind fields of NCEP‐NCAR reanalysis over the period 1980–2004. The leading principal component of H10 demonstrates that the highest four years of H10 during 1980–2004 occurred in the last 10 years. It is found that the recent increase in H10 corresponds to the recent increase in total duration of intense tropical cyclones (ITC, with central pressure below 980 hPa) in the western North Pacific. Time series of the total duration of ITC reveals that the highest three years of the total duration of ITC during the last three decades occurred in the last 10 years.

Observing the coupling effect between warm pool and ?rain pool? in the Pacific Ocean

Traditionally, the tropical zone is known as the “heat reservoir” of the ocean and the “firebox” of the atmosphere. The western equatorial Pacific has been identified as both the warmest portion of the heat reservoir, named “warm pool” (WP), and the hottest portion of the firebox where a huge amount of precipitation-induced latent-heat release is accumulated. The latter mirrors a fact that the western tropical Pacific is also the wettest area on the globe, termed “rain pool” (RP), where the maximum annual precipitation is observed. The accumulation of continuous satellite data has reached a point that decade-long simultaneous observations of many important geophysical parameters have become available in recent years. One such example is the availability of a concurrent dataset of sea surface temperature, oceanic precipitation, and sea surface wind field for 1993–2002 derived from NOAA/AVHRR (Advanced Very High Resolution Radiometer), TOPEX/TMR (TOPEX Microwave Radiometer), and ERS-1,2/QuikSCAT, respectively. In the present study, this dataset is used to demonstrate and investigate the coupling and covarying effects of the Pacific WP and RP, leading to a number of interesting findings on their structural similarity, locational shift, phase lag, and evolutional coherency in association with the development of and the vacillation between El Niño and La Niña events.

Observing the coupling effect between warm pool and “rain pool” in the Pacific Ocean

Traditionally, the tropical zone is known as the “heat reservoir” of the ocean and the “firebox” of the atmosphere. The western equatorial Pacific has been identified as both the warmest portion of the heat reservoir, named “warm pool” (WP), and the hottest portion of the firebox where a huge amount of precipitation-induced latent-heat release is accumulated. The latter mirrors a fact that the western tropical Pacific is also the wettest area on the globe, termed “rain pool” (RP), where the maximum annual precipitation is observed. The accumulation of continuous satellite data has reached a point that decade-long simultaneous observations of many important geophysical parameters have become available in recent years. One such example is the availability of a concurrent dataset of sea surface temperature, oceanic precipitation, and sea surface wind field for 1993–2002 derived from NOAA/AVHRR (Advanced Very High Resolution Radiometer), TOPEX/TMR (TOPEX Microwave Radiometer), and ERS-1,2/QuikSCAT, respectively. In the present study, this dataset is used to demonstrate and investigate the coupling and covarying effects of the Pacific WP and RP, leading to a number of interesting findings on their structural similarity, locational shift, phase lag, and evolutional coherency in association with the development of and the vacillation between El Niño and La Niña events.

USING SATELLITE TECHNOLOGY IMPROVE CHILEAN PURSEINE FISHING FLEET

A work of gathering historical fishing (catch, fishing effort, CPUE, characteristics and operation of purseiner fleet) and satellite (NOAA, SeaStar and SeaWind) database of northern Chile, and fishing and oceanographic in situ sampling for validation purpose is made. With this information and the use of geographical information system (GIS) the distribution of pelagic resources and the associated environmental conditions (sea surface temperature, thermal gradients, chlorophyll concentration and wind fields) are analysed in order to develop a probable fishing ground (PFG) index prediction model. A PFG model of anchovy (Engraulins ringens) is operative for the northern Chile. The impact of the use of PFG charts was evaluated analyzing the operational efficiency of the involved purse seiner fleet. An availability index, independent from the present fishery activity, is estimated from the PFG index and images that depend on historical and present environmental conditions and historical catches. The management measures that assure the resources sustainability are present

Design and calibration of the seawinds scatterometer

The SeaWinds Scatterometer is a Ku-band Earth orbiting remote sensing radar. It has a 1 m dish antenna shared by two beams with respective nadir look angles of 40 and 46 deg, scanning azimuthally to provide greater than 90% daily coverage of the Earth at an altitude of 800 km. The first sensor was launched in 1999 and produces sea surface wind field to 2 m/s accuracy at 25 km resolution. The design and calibration of the SeaWinds radar is described here.

C-band scatterometer and wind-field retrieval

The wind scatterometer is a microwave active sensor capable of simultaneously measuring the normalized radar cross section (σo) of the sea surface with high radiometric accuracy at different azimuth angles. The scattering process governing the electromagnetic wave - sea surface interaction allows these measurements to be related to the sea surface wind field. Estimation of the wind field from σo measurements, however, requires significant processing. This paper is a comparative study of four objective functions within the framework of a new wind field retrieval procedure and shows that the retrieval procedures based on the classical L2 norm objective function, i.e., the Euclidean objective function, and the L2 norm objective function as modified by Stoffelen perform best.

1899-1995年の間の海面気圧を用いた北太平洋上海上風の再現

Sea surface wind (SSW) fields north of 20°N over the North Pacific are reconstructed using sea level pressure (SLP) fields (Trenberth's Corrected Yearly-Monthly Northern Hemisphere SLP, version 010.1) for the period of 97 years from 1899 to 1995.The reconstruction method is purely empirical as follows. First, since the mean height of anemometers installed on ships in the 1980s is approximately 35m, values of monthly mean SSW in the 1980s computed using winds of the Comprehensive Ocean-Atmosphere Data Set (COADS) are adjusted to those at the standard height of 10m. Then, monthly climatologies of SSW and of SLP fields averaged for the ten years from 1981 through 1990 are computed. Next, monthly anomalies of both SSW and SLP fields during the ten years are calculated. Under the assumption of a geostrophic wind balance, geostrophic wind (GW) anomalies are computed using monthly SLP anomalies. Based on comparison between GW and COADS SSW anomalies, two adjustment factors of the reduction coefficient and correction angle are simultaneously estimated for each of four seasons using a neutral regression analysis. Adopting these adjustment factors for GW anomaly fields, corrected wind anomalies are obtained. Finally, the reconstructed SSW fields are obtained as the sum of corrected wind anomalies and ten-year monthly climatologies. When we regard height-corrected COADS SSW fields as the reference winds, root-mean-square differences of the reconstructed SSW fields can be estimated to be less than 1m s-1 in magnitude, and less than 15 degrees in wind direction for the winds greater than 2m s-1. Comparison of the reconstructed SSWs with NCEP reanalysis SSWs is also made and shows that NCEP reanalysis SSWs are systematically 8% greater in magnitude than the reconstructed SSWs throughout the comparison period from 1950 to 1995.Time series of magnitudes of the reconstructed SSWs averaged in the wintertime mid-latitude westerlies region shows interannual and decadal/multidecadal time scale variations: from mid-1930s to mid-1940s, westerlies was remarkably strengthened compared with the rest of the period.

Basin-Scale, High-Wavenumber Sea Surface Wind Fields from a Multiresolution Analysis of Scatterometer Data

Abstract A numerical technique sensitive to both spectral and spatial aspects of sea surface wind measurements is introduced to transform the irregularly sampled satellite-based scatterometer data into regularly gridded wind fields. To capture the prevailing wavenumber characteristics (power-law dependence) of sea surface wind vector components, wavelet coefficients are computed from the scatterometer measurements along the satellite tracks. The statistics of the wavelet coefficients are then used to simulate high-resolution wind components over the off-track regions where scatterometer data are not available. Using this technique, daily wind fields with controlled spectral features have been produced by combining the low-wavenumber wind fields from ECMWF analyses with the high-wavenumber measurements from the ERS-1 scatterometer. The resulting surface wind fields thus reflect nearly all available measurements affecting surface wind, including the synoptic surface pressure. The new surface wind forces a b...

Numerical Simulation of Tide and Storm Surges in the Seto Inland Sea using the Princeton Ocean Model.

Tide and storm surges in the Seto Inland Sea were numerically simulated in a σ-coordinate system using the Princeton Ocean Model (POM). For tide simulation, we obtained good results compared to the astronomical tide, particularly overall amplitude and phase features. For storm surge simulation, results from the model show sea surface elevation time series similar to that observed at each station. Several differences also arise, however. The discrepancy is attributable to the sea surface wind field over a wide area and the lack of sufficient resolution. More important ingredients are involved in reproducing the coastal region storm surge than nonlinearity or 3-dimensionality, however. Maximum sea surface elevation values for each location calculated using a 3-dimensional, stratified model were found to be up to about 10% larger than those calculated with a 2-dimensional, barotropic model. A physical explanation is proposed here. Tidal motion and storm surge are found to be almost independent in our simulations; that is, results are obtained merely by superposing the simulation driven by each independent forcing.

Climate variation and El Ni�o-Southern Oscillation events on the Great Barrier Reef: 1958 to 1987

Seasonal and inter-annual variation of several surface climate variables near the Great Barrier Reef (GBR) are described for the 30-year period, 1958–1987. Large inter-annual variability of rainfall and river flow in coastal Queensland is linked to the aperiodic influences of El Nino-Southern Oscillation events. These events also affect sea surface temperature and wind fields, though the inter-annual variability of these variables is not as large as rainfall and river flow. The major impacts on waters of the GBR appear to be greatly increased freshwater inputs, reduced surface radiation (and thus light levels) and enhanced tropical cyclone activity during anti-El Nino events. El Nino events have less effect on climate of the GBR because they tend to maintain winter-like conditions. The effects of this background of high variability in the physical environment on reef processes must be considered when examining changes in such processes, changes in climate (e.g. due to global warming) or increases in anthropogenic impacts.

Observations of planview flux patterns within convective structures of the marine atmospheric surface layer

Air/sea flux variability on horizontal scales from 50 m to several km results, in part, from the presence of coherent convective structures within the atmospheric boundary layer. The horizontal distribution of fluxes within these convective updrafts and downdrafts is, therefore, central to studies of air/sea interaction and remote sensing of sea surface wind and wave fields. This study derives these flux patterns from observations of the Marine Atmospheric Surface Layer (MASL).

Circulation mechanisms related to northeast Brazil rainfall anomalies

This study explores the role of the upper ocean for circulation anomalies in the overlying atmosphere, with focus on the late boreal winter and extreme climatic events in northeast Brazil. Data sources comprise rainfall records in the Nordeste, surface ship observations of the sea surface temperature (SST), pressure, and wind fields (COADS, period 1948–1990), satellite observations of tropical convection (HRC, 1971–1988), and upper air analyses of the European Centre for Medium Range Weather Forecasts (ECMWF, 1980–1991), all for the tropical Atlantic sector. Interhemispheric SST gradients are most strongly associated with north‐south contrasts in pressure and the meridional wind component, whereas the inverse SST‐pressure relations in situ are less close. Enhanced northward temperature increase in the tropical Atlantic is accompanied by steeper meridional pressure gradient and accelerated southerly wind component, which is representative of a northward displaced Intertropical Convergence Zone (ITCZ), with the latter in turn leading to drought in northeast Brazil. Interhemispheric contrasts in the 850/1000 mbar layer mean temperature are similar to those of SST and account for most of the meridional surface pressure gradient. During wet as compared to dry years in northeast Brazil, the tropical North Atlantic is cooler and the South Atlantic warmer, and accordingly the lower tropospheric thickness is reduced to the north but inflated to the south of the equator, resulting in increased/reduced surface pressure over the North/South Atlantic, as well as reduced southerly surface wind, stronger subsidence over the outer tropics of the northern hemisphere and intensified ascending motion and convective activity over the Nordeste, related to a southward displaced near‐equatorial convection belt. The surface and upper air evidence thus indicates the ways in which the interhemispheric SST gradients exert a hydrostatic control on the lower tropospheric thickness pattern, and thus force the south‐north surface pressure gradients and the surface meridional wind component, and hence modulate the latitude position of the ITCZ and Nordeste rainfall.

A Model El Ni&amp;ntilde–Southern Oscillation

A coupled atmosphere-ocean model is developed and used to study the ENSO (El Ni&ntilde/Southern Oscillation) phenomenon. With no anomalous external forcing, the coupled model reproduces certain key features of the observed phenomenon. including the recurrence of warm events at irregular intervals with a preference for three to four years. It is shown that the mean sea surface temperature, wind and ocean current fields determine the characteristic spatial structure of ENSO anomalies. The tendency for phase-locking of anomalies is explained in terms of a variation in coupling strength associated with the annual cycle in the mean fields. Sensitivity studies reveal that both the amplitude and the time of scale of the oscillation are sensitive to several parameters that affect the strength of the atmosphere–ocean coupling. Stronger coupling implies larger oscillations with a longer time scale. A critical element of the model oscilliation is the variability in the equatorial heat content of the upper ocean. Equatorial heat content increases prior to warm events and decreases sharply during the events. A theory for this variability and the associated transitions between the non-El Niño and El Niño states is presented. Implications of the model results for the prediction of El Niño events are discussed.