Comparison of surface wind stress characteristics over the Tropical Atlantic (10°N–40°S) in fields derived from the UWM/COADS, NCEP/NCAR and QuikSCAT datasets

A comparison of monthly wind stress derived from winds of NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis and UWM/COADS (The University of Wisconsin-Milwaukee/Comprehensive Ocean-Atmosphere Data Set) dataset (1950–1993), and of NCEP/NCAR reanalysis and satellite-based QuikSCAT dataset (2000–2006), is made over the South Atlantic (10°N–40°S). On a mean seasonal scale, the comparison shows that these three wind stress datasets have qualitatively similar patterns. Quantitatively, in general, from about the equator to 20°S in the mid-Atlantic the wind stress values are stronger in NCEP/NCAR data than those in UWM/COADS data. On the other hand, in the Intertropical Convergence Zone (ITCZ) area the wind stress values in NCEP/NCAR data are slightly weaker than those in UWM/COADS data. In the South Atlantic, between 20° S–40°S, the QuikSCAT dataset presents complex circulation structures which are not present in NCEP/NCAR and UWM/COADS data. The wind stress is used in a numerical ocean model to simulate ocean currents, which are compared to a drifting-buoy observed climatology. The modeled South Equatorial Current agrees better with observations between March–May and June–August. Between December–February, the South Equatorial Current from UWM/COADS and QuikSCAT experiments is stronger and more developed than that from NCEP/NCAR experiment. The Brazil Current, in turn, is better represented in the QuikSCAT experiment. Comparison of the annual migration of ITCZ at 20° and 30°W in UWM/COADS and NCEP/NCAR data sources show that the southernmost position of ITCZ at 30°W in February, March and April coincides with the rainy season in NE Brazil, while the northernmost position of ITCZ at 20°W in August coincides with the maximum rainfall of Northwest Africa.     

Seasonal variations of wind and waves over Taiwan waters

We here investigate the frequency and intensity of oscillations in oceanographic data within intraseasonal time scales using spectral analysis of surface wind and wave time-series data collected at off-island weather stations or moored buoys around Taiwan. Data from marine weather stations were used to trace atmospheric conditions, while we used buoy data to examine sea states. The spectra and wavelet scalogram of the wind fields revealed oscillations with a period of around 20–33 days, and the energy density of the wind field at the off-island stations was stronger than that at the data buoy stations. However, the wavelet scalogram of the wave height measured at the buoy stations was stronger than its associated wind field. This long-period oscillation is consistent with the wavelet scalogram of the wind field calculated from the off-island weather stations. About 20–33 day oscillations exist within intraseasonal variations, which are closely linked to the atmospheric environment and to wind and ocean wave fields. Oscillations with a period of 5–10 days are a pronounced feature over northeastern Taiwan waters during the winter season and can be interpreted as the wave pattern following synoptic weather systems.     

Preliminary results of measurements of atmospheric turbulence over the sea

We perform the experimental verification of the applicability of the theory of similarity to the wave boundary layer and the assessment of wave-induced perturbations of the air flow depending on various conditions of stratification of the atmosphere and the state of the sea. The measurements were carried out from a stationary platform located in the coastal part of the Black Sea. The experimental procedure is based on the simultaneous measurements of the profile and fluctuations of the wind speed at 5–6 levels in the 1.3–21-m layer, the elevations of the sea surface, the directions of waves and winds, and the mean gradients of temperature and humidity of air. The structure of the boundary layer in the region of measurements depends on the direction of the wind. For weak and moderate onshore winds (< 9 m/sec), the approximate balance is preserved between the production and dissipation of turbulent energy in the cases of unstable and neutral stratification. On the average, the estimates of friction velocity according to the profiles are higher than the dissipative estimates by 10% mainly due to the deficiency of dissipation near the surface. For the offshore wind, the structure of the boundary layer abruptly changes and is determined not by the local parameters but by strong turbulent eddies formed over the dry land. The intensity of low-frequency turbulent fluctuations and the gradient of wind velocity near the surface in the coastal zone are 1.5–2 times higher than for the open sea. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 3, pp. 42–61, May–June, 2007.     

赤道东印度洋和孟加拉湾障碍层厚度的年际变化及其影响机制

Interannual variability(IAV) in the barrier layer thickness(BLT) and forcing mechanisms in the eastern equatorial Indian Ocean(EEIO) and Bay of Bengal(BoB) are examined using monthly Argo data sets during 2002–2017. The BLT during November–January(NDJ) in the EEIO shows strong IAV, which is associated with the Indian Ocean dipole mode(IOD), with the IOD leading the BLT by two months. During the negative IOD phase, the westerly wind anomalies driving the downwelling Kelvin waves increase the isothermal layer depth(ILD). Moreover, the variability in the mixed layer depth(MLD) is complex. Affected by the Wyrtki jet, the MLD presents negative anomalies west of 85°E and strong positive anomalies between 85°E and 93°E. Therefore, the BLT shows positive anomalies except between 86°E and 92°E in the EEIO. Additionally, the IAV in the BLT during December–February(DJF) in the BoB is also investigated. In the eastern and northeastern BoB, the IAV in the BLT is remotely forced by equatorial zonal wind stress anomalies associated with the El Ni?o-Southern Oscillation(ENSO). In the western BoB, the regional surface wind forcing-related ENSO modulates the BLT variations.     

Investigation of the characteristics of surge phenomena in the Sea of Azov

We present the results of numerical simulation of currents and sea level for the Sea of Azov. In calculations, we use a three-dimensional nonlinear mathematical model taking into account the tangential wind stresses. We present the results of numerical analysis of the fields of currents and the amplitudes of oscillations of the sea level at the coastal stations as functions of the maximum velocity and the period of constant action of the west wind. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 1, pp. 12–25, January–February, 2008.     

Shipboard measurements of the modulation characteristics of 3.2 cm radar signals scattered by the sea surface

The results of shipboard measurements of the modulation characteristics of 3.2 cm radar signals scattered by a rough sea surface at low grazing angles are reported. The experiments were carried out from on-board a drifting research vessel in the Atlantic trade wind zone at wind speeds of 7–10 m s⁻¹ and coinciding directions of the wind and waves. Azimuthal isotropy of the modulation spectra was observed. It is emphasized that the ‘sea surface-scattered signal’ modified modulation transfer function is somewhat larger for horizontal polarization than for vertical polarization. Translated by V. Puchkin.     

Evaluation of marine surface winds observed by SeaWinds and AMSR on ADEOS-II

Marine surface winds observed by two microwave sensors, SeaWinds and Advanced Microwave Scanning Radiometer (AMSR), on the Advanced Earth Observing Satellite-II (ADEOS-II) are evaluated by comparison with off-shore moored buoy observations. The wind speed and direction observed by SeaWinds are in good agreement with buoy data with root-mean-squared (rms) differences of approximately 1 m s⁻¹ and 20°, respectively. No systematic biases depending on wind speed or cross-track wind vector cell location are discernible. The effects of oceanographic and atmospheric environments on the scatterometry are negligible. Though the wind speed observed by AMSR also showed agreement with buoy observations with rms difference of 1.27 m s⁻¹, the AMSR wind speed is systematically lower than the buoy data for wind speeds lower than 5 m s⁻¹. The AMSR wind seems to have a discontinuous trend relative to the buoy data at wind speeds of 5–6 m s⁻¹. Similar results have been obtained in an intercomparison of wind speeds globally observed by SeaWinds and AMSR on the same orbits. A global wind speed histogram of the AMSR wind shows skewed features in comparison with those of SeaWinds and European Centre for Medium-range Weather Forecasts (ECMWF) analyses.     

Modeling of the breeze circulation of waters in the Kerch Strait

The problem of the numerical analysis of currents in the Kerch Strait is studied within the framework of a linear nonstationary two-dimensional model in the nondivergent approximation. We describe the actual situation when the motion is induced by a breeze against the background of the daily average northeast wind. The breeze is specified as acting in a narrow coastal strip and can be classified as a zonal wind. The variations of circulation near the Tuzla Island are studied in detail. It is shown that the contribution of the breeze circulation to the total circulation can be significant for the explanation of the processes running in the Kerch Strait. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 6, pp. 16–27, November–December, 2007.     

A numerical calculation of the wind-driven circulation in the Gulf of St. Lawrence

The steady state surface circulation in the Gulf of St. Lawrence due to wind field forcing was calculated taking the topography into account using numerical techniques for each month of two ten year periods, 1940–49 and 1951–60. This model did not take into consideration the ice layer on the water surface and hence is not valid for the winter months. Most of the observed circulation features except the Gaspe current were reproduced satisfactorily.     

Temporal and spatial variabilities of Japan Sea surface temperature and atmospheric forcings

In this study, we used the National Centers for Environmental Prediction monthly sea surface temperature (SST) and surface air temperature (SAT) data during 1982–1994 and the National Center for Atmospheric Research surface wind stress curl data during 1982–1989 to investigate the Japan Sea SST temporal and spatial variabilities and their relations to atmospheric forcing. First, we found an asymmetry in the correlation coefficients between SST and wind stress curl, which implies that the SST variability at the scales of the order of one month is largely due to atmospheric forcing. Second, we performed three analyses on the data fields: annual mean, composite analysis to obtain the monthly anomaly relative to the annual mean, and empirical orthogonal function (EOF) analysis on the residue data relative to the summation of the annual mean and the monthly anomaly. The first EOF mode of SST accounts for 59.9% of the variance and represents the Subpolar Front. The temporal variation of the first EOF mode implies that the deep Japan Sea could be cooler in cold seasons (November–April) of 1984–1987. Third, we computed cross-correlation coefficients among various principal components and found that the atmospheric warming/cooling is the key factor causing intra-seasonal and interannual SST variabilities.     

Surface velocity time series derived from satellite altimetry data in a section across the Kuroshio southwest of Kyushu

A time series of surface geostrophic velocity is developed using satellite altimetry data during 1992–2010 for a track across the Kuroshio southeast of Kyushu, Japan. The temporal mean geostrophic velocity is estimated by combining the along-track sea level anomaly and shipboard ADCP data. This approximately 6-km resolution dataset is successful in representing the Kuroshio cross-current structure and temporal variation of the Kuroshio current-axis position during 2000–2010. The authors use this dataset to examine the winter Kuroshio path destabilization phenomenon. Its seasonal features are characterized as follows: the velocity shear on the inshore side of the Kuroshio becomes stronger and the Kuroshio path state becomes unstable from the summer to winter. This evidence is consistent with the hypothetical mechanism governing the destabilization phenomenon discussed in a previous study. Furthermore, the interannual amplitude modulation of the seasonality is examined in relation to interannual variations in the winter northerly wind over the northern Okinawa Trough and the Pacific Decadal Oscillation (PDO) index. The destabilization phenomenon appears 15 times in the period 2000–2010. Ten cases are related to local wind effects, and 7 of these are also connected with the PDO index. This is probably because the winter northerly wind over the northern Okinawa Trough is regulated by the PDO signal in interannual time-scales. Only 4 cases are related to the PDO index, but their driving mechanism remains uncertain.     

南半球热带外气候变量与两种ENSO的联系

This study uses the Climate Forecast System Reanalysis(CFSR) to investigate the responses of the Southern Hemisphere(SH) extratropical climate to two types of El Ni?o–Southern Oscillation(ENSO)—the eastern Pacific(EP) type and the central Pacific(CP) type in different seasons. The responses are denoted by the anomalies of climate variables associated with one-standard-deviation increase in the Ni?o3 or Ni?o4 index. The results show that in austral spring the differences in the ENSO-related anomaly(ERA) patterns of atmospheric circulation between the EP ENSO period(1979–1998) and CP ENSO period(1999–2010) are mainly associated with the change in the ENSO-PSA2 relationship. Such differences affect the ERA fields of surface air temperature and mixed layer temperature, and finally result in significant differences in sea-ice concentration anomalies in the Atlantic sector. In austral summer, significant correlation exists between the variations of SAM and both of the variations of Ni?o3 and Ni?o4 in 1979–1998, while the correlation between SAM and Ni?o4 disappears in 1999–2010. For all seasons, the strength of the climate ERAs depend on if there are close relationship between ENSO and the major climate variation modes of the SH extratropics. For the climate variables, the ERA patterns of surface air temperature are generally controlled by surface wind anomalies and mirrored by the mixed layer temperature anomalies. The mixed layer depth anomalies are primarily modulated by surface heat flux anomalies and occasionally by anomalous wind. There are strikingly strong anomalies of surface heat flux in the autumn of 1979–1998 related to the Ni?o3 variation, the period when there is only significant correlation between ENSO and PSA2. There are no evidence that the SH extratropical climate variability induced by Ni?o3 variations are stronger in the EP-ENSO period, and that variability induced by Ni?o4 variations are stronger in the CP-ENSO period.     

Seasonal Variation of Stratification in the Gulf of Thailand

Intensive hydrographic observations were carried out in the western part of the Gulf of Thailand and the east coastal sea of Peninsular Malaysia in September 1995 and April–May 1996. The characteristics of seasonal variation of oceanic condition in that area are discussed basis of an analysis of observed water temperature, salinity and density distributions in these cruises and NAGA cruises (Yanagi and Takao, 1998a). Stratification is most developed in March–May mainly due to large sea surface heating and weak sea surface wind, which weakened until September–October, vanishing in December–January. The horizontal distribution of bottom cold, saline and heavy water masses, which are found during the stratified season, is governed by the tidal mixing and the water depth. Water exchange between the Gulf of Thailand and the South China Sea becomes large in March–May due to a coupled effect of the intensified estuarine circulation and the Ekman transport by the southwest monsoon. This revised version was published online in August 2006 with corrections to the Cover Date.     

Simulation of surge phenomena and transformation of the admixture field in the Sea of Azov in the presence of stationary currents

Using a three-dimensional nonlinear mathematical model, we study the processes of transfer and diffusion of contaminants in the Sea of Azov in the presence of stationary currents. Changes in the sea level, surge phenomena, and the direction and velocity of stationary currents caused by winds with different maximum velocities are analyzed. We estimate the region of applicability of the linear approximation and the choice of the value of integration steps over space and time coordinates. It is shown that the growth of the maximum wind velocity increases the contaminated domains and the time of complete dispersion of the admixture. Solutions obtained in the linear approximation differ slightly from those obtained by using the nonlinear model for wind velocities up to 5 m/sec. Translated from Morskoi Gidrofizicheskii Zhurnal, No. 4, pp. 52–68, July–August, 2008.     

Thermohydrodynamics of the ocean decay of nonlinear oscillations in a bounded basin of variable depth

We study the time decay of surges of a liquid in a round shallow-water basin of variable depth. The dependence of the logarithmic decrement of oscillations on the bottom topography and wind velocity is analyzed. The role of convective acceleration and bottom friction in the formation of both the level of vertical displacement of the surface of the basin and the velocity field of horizontal wave currents is estimated. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 3–11, March–April, 2006.     

Interdecadal Variation of the Transition Zone Chlorophyll Front: A Physical-Biological Model Simulation between 1960 and 1990

The interdecadal climate variability affects marine ecosystems in both the subtropical and subarctic gyres, consequently the position of the Transition Zone Chlorophyll Front (TZCF). A three-dimensional physical-biological model has been used to study interdecadal variation of the TZCF using a retrospective analysis of a 30-year (1960–1990) model simulation. The physical-biological model is forced with the monthly mean heat flux and surface wind stress from the COADS. The modeled winter mixed layer depth (MLD) shows the largest increase between 30°N and 40°N in the central North Pacific, with a value of 40–60% higher during 1979–90 relative to 1964–75 values. The winter Ekman pumping velocity difference between 1979–90 and 1964–75 shows the largest increase located between 30°N and 45°N in the central and eastern North Pacific. The modeled winter surface nitrate difference between 1979–90 and 1964–75 shows increase in the latitudinal band between 30°N and 45°N from the west to the east (135°E–135°W), the modeled nitrate concentration is about 10 to 50% higher during the period of 1979–90 relative to 1964–75 values depending upon locations. The increase in the winter surface nitrate concentration during 1979-90 is caused by a combination of the winter MLD increase and the winter Ekman pumping enhancement. The modeled nitrate concentration increase after 1976–77 enhances primary productivity in the central North Pacific. Enhanced primary productivity after the 1976–77 climatic shift contributes higher phytoplankton biomass and therefore elevates chlorophyll level in the central North Pacific. Increase in the modeled chlorophyll expand the chlorophyll transitional zone and push the TZCF equatorward. This revised version was published online in August 2006 with corrections to the Cover Date.     

Low-frequency variations of the Gulf-Stream transport: description and mechanisms

On the basis of the contemporary array of oceanographic and hydrometeorological data, we compute the characteristics of variations of the Gulf-Stream transport in 1950–2004. The role played by the low-frequency oscillations of vorticity of the wind field and turbulent heat fluxes in the North Atlantic in the formation of the analyzed variations is estimated. We reveal a significant (on a 5% confidence level) positive linear trend of the monthly average Gulf-Stream transport manifested in the increase in the Gulf-Stream transport by 13 Sv for the investigated period. On the basis of the established estimates, we make a conclusion that about a quarter of the interannual variations of the Gulf-Stream transport is caused by the low-frequency oscillations of vorticity of the wind field in the Subtropical Atlantic. Moreover, the Gulf-Stream transport is delayed relative to the wind oscillations by about 2 yr. An important role in the changes in the Gulf-Stream transport is played by the response of the system of west boundary currents to the quasiperiodic action of turbulent heat fluxes on the surface of the ocean connected with the North-Atlantic Oscillation. The intensification of turbulent heat fluxes in the Northern Subpolar Cyclonic Gyre and their weakening in the north part of the Subtropical Anticyclonic Gyre are accompanied by the intensification of the Gulf Stream observed after 3–5 yr. The anomalies of turbulent heat fluxes of the opposite sign are followed by weakening of the Gulf Stream also after a period of 3–5 yr. We also mention a potentially important role played the Pacific decadal oscillation in maintaining the decadal variations of the intensity of Gulf Stream. The influence of this oscillation on the Gulf-Stream transport is realized both via the changes in the wind field in different phases of oscillations and due to its influence on the heat exchange of the ocean with the atmosphere.     

On the wind conditions of a shallow as a factor of the formation of satellite images of the Sea (Using the example of the South Caspian Sea)

Based on the scatterometer QuickScat wind data and the water-leaving radiances from the ocean color scanner SeaWiFS data for 2000–2004, we composed a pair of mean annual images of a testing site in the South Caspian Sea under conditions of west-bound and east-bound winds corresponding to the offset and onset state of the shallows in the east of the testing site. It was established that both the “offset” and “onset” radiances grow shorewards, but the former becomes about twice as large as the latter in the middle of the shallows with 10–15 m of water, while their difference tends to zero at the seaward and coastal boundaries of the shallow. These and other findings are hypothetically attributable to the inclination of the sea floor of the shallows, due to which the surfacing of bottom sediments resuspended by the drift current takes less time under the “offset” wind conditions than under the “onset” ones. The study’s results indicate that the effect of the bottom sediments resuspension upon the structure of the images of the marine shallows is universal in character and needs to be taken into account to improve the bio-optical algorithms for estimating the admixtures in the thickness of the shallows from the satellite data.     

Typical fields of wind waves and swells in the Northern Pacific

Using all of the atmospheric patterns classified by Polyakova A.M., we accomplished calculations of the surface wind fields over the North Pacific for wind waves observable 6, 12, 18, and 24 hours after a storm and the swell waves observable 24, 48, and 72 hours after a storm. The considerable extension of the ocean creates quasi-unlimited speeding up of the wind waves during continuous strong winds (over 20 meters per second). This determines the presence of wide areas of highly developed wind waves with a 5% probability that the wave heights will exceed the 10–12 meter estimates. The swell waves decay faster: their height reduces by half in 24 h, while, in 72 h, they achieve the background level of the ocean’s swell waves.