Genesis of the northeast Brazil upper-tropospheric cyclonic vortex: a primitive equation barotropic instability study

The primitive equation barotropic unstable linear normal modes are computed using an eigenvalue approach for daily latitudinal profiles of zonal flow in the upper-tropospheric layer of 100-350 hPa before and after formation of cyclonic vortices during January 1993 and November 2001 off the coast of northeast Brazil. The wave kinetic energy equation for μ- and ν-motion is presented. Equations are derived to isolate the contribution of divergence and other dynamical processes in the movement and growth of unstable modes. Numerical accuracy and physical nature of unstable modes are tested. In a short span of 2-3 days, prior to formation of vortices, a progressive and a sharp intensification of the basic flow shear zone and its barotropic instability are seen with time. The horizontal structure, momentum transport, and zonal and meridional scales of the most unstable normalized wave are obtained and compared with the vortex extracted from the 200-hPa observed winds using a bandpass smoother. A close agreement is found between them. It is shown that the zonal and meridional scales of the preferred wave are related to the length scale of the shear zone. The wave is confined to the shear zone and its maximum amplitude is located at the latitude of maximum β - μyy. The role of divergence in the movement and growth of the wave is investigated. The energetics of the unstable wave μ- and ν-motion is computed, and it is inferred that the energy source for the growth of wave μ- (ν-) motion is the energy conversion (work done by pressure force), which lies in the shear zone. It is emphasized that a deeper insight regarding the genesis of the cyclonic vortex can be gained on the basis of stability analysis of daily observed zonal flow profiles, which may not be possible using idealized or mean profiles. An explanation for nonmanifestation of the instability in the monthly mean flow is provided

Quasi-stationary waves in the Southern Hemisphere during El Niño and La Niña events

Characteristics of quasi-stationary (QS) waves in the Southern Hemisphere are discussed using 49 years (1950–1998) of NCEP/NCAR reanalysis data. A comparison between the stationary wave amplitudes and phases between the recent data (1979–1998) and the entire 49 years data showed that the differences are not large and the 49 years data can be used for the study. Using the 49 years of data it is found that the amplitude of QS wave 1 has two maxima in the upper atmosphere, one at 30°S and the other at 55°S. QS waves 2 and 3 have much less amplitude. Monthly variation of the amplitude of QS wave 1 shows that it is highest in October, particularly in the upper troposphere and stratosphere. </p><p style="line-height: 20px;"> To examine the QS wave propagation Plumb's methodology is used. A comparison of Eliassen-Palm fluxes for El Niño and La Niña events showed that during El Niño events there is a stronger upward and equatorward propagation of QS waves, particularly in the austral spring. Higher upward propagation indicates higher energy transport. A clear wave train can be identified at 300hPa in all the seasons except in summer. The horizontal component of wave activity flux in the El Niño composite seems to be a Rossby wave propagating along a Rossby wave guide, at first poleward until it reaches its turning latitude in the Southern Hemisphere midlatitudes, then equatorward in the vicinity of South America. The position of the center of positive anomalies in the austral spring in the El Niño years over the southeast Pacific, near South America, favors the occurrence of blocking highs in this region. This agrees with a recent numerical study by Renwick and Revell (1999).<br><br> <b>Key words.</b> Meteorology and atmospheric dynamics (climatology; general circulation; ocean-atmosphere interactions

An observational study of the evolution of the atmospheric boundary-layer over Cabo Frio, Brazil

The effect of coastal upwelling on the evolution of the atmospheric boundary layer (ABL) in Cabo Frio (Brazil) is investigated. For this purpose, radiosounding data collected in two experiments made during the austral summer (upwelling case) and austral winter (no upwelling case) are analysed. The results show that during the austral summer, cold waters that crop up near the Cabo Frio coast favour the formation of an atmospheric stable layer, which persists during the upwelling episode. Due to the low SSTs, the descending branch of the sea-breeze circulation is located close to the coast, inhibiting the development of a mixed layer mainly during the day. At night, with the reduction of the land-sea thermal contrast the descending motion is weaker, allowing a vertical mixing. The stable ABL favours the formation of a low level jet, which may also contribute to the development of a nocturnal atmospheric mixed layer. During the austral winter, due to the higher SSTs observed near the coast, the ABL is less stable compared with that in the austral summer. Due to warming, a mixed layer is observed during the day. The observed vertical profiles of the zonal winds show that the easterlies at low levels are stronger in the austral summer, indicating that the upwelling modulates the sea-breeze signal, thus confirming model simulations

Population parameters and the relationships between environmental factors and abundance of the Acetes americanus shrimp (Dendrobranchiata: Sergestidae) near a coastal upwelling region of Brazil

AbstractThe population dynamics of Acetes americanus was investigated, focusing on the sex ratio, individual growth, longevity, recruitment and relationship between abundance and environmental factors in the region of Macaé, strongly influenced by coastal upwelling. Otter trawl net samplings were performed from July 2010 to June 2011 at two points (5 m and 15 m). Nearly 19,500 specimens, predominantly females (77.15%), were captured. Their sizes, larger than that of males, indicated sexual dimorphism. Shrimps at lower latitudes present larger sizes and longer longevity than those from higher latitudes. This difference is probably due to low temperatures and high primary productivity. Though no statistical correlation was found between abundance and environmental factors, the species was more abundant in temperatures closer to 20.0º C and in months with high chlorophyll-a levels. Due to the peculiar characteristics of this region, A. americanusshowed greater differences in size and longevity than individuals sampled in other studies undertaken in the continental shelf of Southeast Brazil

Transport of potential vorticity and Eliassen-Palm fluxes for two contrasting years: 1995–1996 (La Niña) and 1997–1998 (El Niño)

Potential vorticity transport (PV)-Eliassen and Palm (EP) cross sections are studied for two contrasting years: 1995–1996 (La Niña) and 1997–1998 (El Niño). The results show that the largest differences in PV transport-EP fluxes between El Niño and La Niña events occur in winter in both hemispheres, but the changes are higher in the Northern Hemisphere. PV transport-EP fluxes are stronger in both July 1997 and January 1998 than in July 1995 and January 1996, respectively, indicating stronger baroclinic activity in the El Niño year compared to the La Niña year. The changes in PV transport seem to be due mainly to the changes in eddy heat fluxes. Due to the increase in the wind shear the Eady growth rate is stronger in the 1997–1998 El Niño compared to 1995–1996 La Niña. Consequently, the zonal winds are stronger in the El Niño event, although the location of the jet streams is almost the same in both the contrasting years. However, in the Southern Hemisphere, there are two regions of maxima Eady growth rate in July 1997 and a double jet is observed while a single jet is seen in July 1995 associated with only one maximum of Eady growth rate. In the case of summer, there is little difference in PV transport-EP fluxes between the two contrasting years in the hemispheres, although they are slightly higher in the El Niño event than in the La Niña event.<br><br><b>Key words. </b>Meteorology and atmospheric dynamics (climatology; general circulation

The effect of coastal upwelling on the sea-breeze circulation at Cabo Frio, Brazil: a numerical experiment

The effect of coastal upwelling on sea-breeze circulation in Cabo Frio (Brazil) and the feedback of sea-breeze on the upwelling signal in this region are investigated. In order to study the effect of coastal upwelling on sea-breeze a non-linear, three-dimensional, primitive equation atmospheric model is employed. The model considers only dry air and employs boundary layer formulation. The surface temperature is determined by a forcing function applied to the Earth's surface. In order to investigate the seasonal variations of the circulation, numerical experiments considering three-month means are conducted: January-February-March (JFM), April-May-June (AMJ), July-August-September (JAS) and October-November-December (OND). The model results show that the sea-breeze is most intense near the coast at all the seasons. The sea-breeze is stronger in OND and JFM, when the upwelling occurs, and weaker in AMJ and JAS, when there is no upwelling. Numerical simulations also show that when the upwelling occurs the sea-breeze develops and attains maximum intensity earlier than when it does not occur. Observations show a similar behavior. In order to verify the effect of the sea-breeze surface wind on the upwelling, a two-layer finite element ocean model is also implemented. The results of simulations using this model, forced by the wind generated in the sea-breeze model, show that the sea-breeze effectively enhances the upwelling signal.Key words. Meteorology and atmospheric dynamics (mesoscale meteorology; ocean-atmosphere interactions) · Oceanography (numerical modeling