Wind shear

About: Wind shear is a research topic. Over the lifetime, 8023 publications have been published within this topic receiving 185373 citations.

The western boundary current of the seasonal subtropical gyre in the Bay of Bengal

Abstract: Hydrographic data collected during March–April 1991 show the presence of a poleward current along the western boundary of the Bay of Bengal north of about 10° N carrying warmer waters of southern origin. The inshore side of the current was marked by cooler, more saline waters brought to the surface due to the presence of the current which transported approximately 10 × 106 m3/s. The hydrography is suggestive of many of the features that have been associated with the western boundary currents of the subtropical gyres of the world oceans: a recirculation zone, waves, eddies, etc. These features, however, were not satisfactorily resolved in the data. Using available climatologies of monthly mean ship drifts, seasonal hydrography, and monthly mean wind stress, we propose that the poleward current is the western boundary current of a seasonal anticyclonic subtropical gyre which forms in the Bay during January, is best developed during March-April, and decays by June. The gyre and the western boundary current are unique because of their seasonal character. The pattern of circulation leading to formation and decay of the gyre is reproduced reasonably well in the computation of the monthly mean barotropic transport induced by the curl of wind stress, which has a well-defined annual cycle due to the monsoons and which is conducive to the formation of an anticyclonic gyre only during the months of January-May. The pattern of circulation due to baroclinic transport induced by the wind stress curl, however, is not known at present, and this makes it difficult to conclude unequivocally that the wind stress curl over the bay is the sole mechanism to force the gyre.

A new evaluation of the wind stress coefficient over water surfaces

Abstract: An analysis of data from numerous investigators, as well as information obtained directly by the authors, indicates that a large portion of the difficulties encountered in the past in establishing a relationship between the wind stress coefficient C10 and the wind velocity U10 can be attributed to computationally induced scatter of the data points. However, plots of the shear velocity u* against U10 reveal clear trends which show that three regions exist in the development of the wind shear stress: (1) a lower region in which the wind waves have not begun to break, and for which C10 is approximately constant; (2) a transitional region, after the onset of breakers, for which C10 varies nonlinearly with U10 and (3) a limiting region for which C10 tends again toward a constant value, and corresponds to a condition of breaker saturation. A single general equation to express C10 as a function of U10 is proposed, which agrees with the above findings. It is shown that in contrast with the perception of previous investigators, Charnock's coefficient α = z0g/u*2, where z0 is the roughness length and g is the acceleration due to gravity, is not constant anywhere in the range of wind velocities 0 < U10 < 40 m/s. Finally, the data indicate that the wind flow boundary in each one of the three regions described above can be classified as having ‘low roughness,’ ‘transitional,’ and ‘high roughness,’ respectively.

Subtidal circulation over the northern California shelf

Abstract: The circulation over the shelf and upper slope off northern California, between 38°N and 42°N, was observed using moored current and temperature recorders deployed as part of the Northern California Coastal Circulation Study (NCCCS), from March 1987 through October 1989. The results of this study provide a larger-scale view of the wind-driven circulation than that described through the 1981–1982 Coastal Ocean Dynamics Experiment (CODE), particularly with regard to alongshore and temporal variations. From an improved description of the frequency structure of wind and current, a very low frequency (VLF) signal is identified in the observed currents at frequencies below those typical of wind forcing. The majority of this VLF variance appears to be accounted for by persistent flow events associated with the presence of mesoscale circulation in the adjacent ocean. The longer duration of the NCCCS also allows an improved description of the seasonality of flow regimes off northern California. Three oceanic seasons are identified: an upwelling season (April-July), a relaxation season (August-November), and a storm season (December-March). Alongshore variations in the strength of upwelling, in the strength of the alongshore flow, both near-surface and undercurrent, and in water temperature not only are a function of latitude, as is the wind, but they also correspond to location relative to promontories, notably Cape Mendocino. Immediately south of Cape Mendocino, the near-surface flow exhibits an equatorward minimum and a temperature minimum, whereas the undercurrent exhibits a poleward maximum. Conversely, at the moorings immediately north of the cape, temperatures are a maximum and the undercurrent exhibits a minimum. The maximum in near-surface temperature relates to a minimum in upwelling; no significant correlation was found between local wind and current immediately north of Cape Mendocino. This upwelling minimum and the upwelling maximum south of the cape were also observed as persistent sea surface temperature patterns in satellite imagery.

Modulation of North Pacific Tropical Cyclone Activity by Three Phases of ENSO

Abstract: Tropical Pacific Ocean warming has been separated into two modes based on the spatial distribution of the maximum sea surface temperature (SST) anomaly: an east Pacific warming (EPW) and a central Pacific warming (CPW). When combined with east Pacific cooling (EPC), these three regimes are shown to have different impacts on tropical cyclone (TC) activity over the North Pacific by differential modulation of both local thermodynamic factors and large-scale circulation patterns. In EPW years, the genesis and the track density of TCs tend to be enhanced over the southeastern part and suppressed in the northwestern part of the western Pacific by strong westerly wind shear. The extension of the monsoon trough and the weak wind shear over the central Pacific increases the likelihood of TC activity to the east of the climatological mean TC genesis location. In CPW years, the TC activity is shifted to the west and is extended through the northwestern part of the western Pacific. The westward shifting of C...

Temporal Changes of the Primary Circulation in Tropical Cyclones.

Abstract: More than 900 radial profiles of in situ aircraft observations collected in 19 Atlantic hurricanes and tropical storms over 13 years confirm that the usual mechanism of tropical cyclone intensification involves contracting maxima of the axisymmetric swirling wind. Radar shows that annuli of convective echoes accompany the wind maxima. These features, called convective rings exist and move inward because latent heat released in the rings leads to descent, adiabatic warming, and rapid isobaric height falls in the area they enclose. The radial change in rate of isobaric height fall is concentrated at the inner edge of the wind maximum, causing the gradient wind to increase there and the maximum to contract. Vigorous convection organized in rings invariably causes well defined, inward moving wind maxima, but when convection is weak, the rings are also weak or even absent. In this case, the swirling wind may be nearly constant with radius and change slowly in time. Hurricanes that have a single, vigor...