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- Author or Editor: Ants Leetmaa x
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Abstract
The role of local heating in producing annual and interannual sea-surface temperature variations in the eastern tropical Pacific is studied. Removed from the eastern boundary (122°W), and off the equator, local heating plays a major role in producing annual temperature fluctuations. At the same longitudes from 10°N to 10°S interannual variations in the yearly-average temperature and the anomalous net heat input into the ocean are of the same sign and magnitude. During the 1969 and 1972 mean warmings there was increased heat input into the ocean. Closer to the eastern boundary, oceanic processes such as advection are as important as local heating. Results from a simple model incorporating local heating, offshore Ekman transports, and upwelling suggest the following scenario for the 1972–73 El Niño. During February and March 1972 enhanced local heating and reduced offshore advection were the main reasons for anomalously warm temperatures in the open ocean adjacent to Peruvian coastal waters. From April 1972 to March 1973 temperatures remained high because of offshore transport of anomalously warm inshore waters. Whether the latter were warm because of upwelling of warmer water or transport of warmer water from farther south is not clear.
Abstract
The role of local heating in producing annual and interannual sea-surface temperature variations in the eastern tropical Pacific is studied. Removed from the eastern boundary (122°W), and off the equator, local heating plays a major role in producing annual temperature fluctuations. At the same longitudes from 10°N to 10°S interannual variations in the yearly-average temperature and the anomalous net heat input into the ocean are of the same sign and magnitude. During the 1969 and 1972 mean warmings there was increased heat input into the ocean. Closer to the eastern boundary, oceanic processes such as advection are as important as local heating. Results from a simple model incorporating local heating, offshore Ekman transports, and upwelling suggest the following scenario for the 1972–73 El Niño. During February and March 1972 enhanced local heating and reduced offshore advection were the main reasons for anomalously warm temperatures in the open ocean adjacent to Peruvian coastal waters. From April 1972 to March 1973 temperatures remained high because of offshore transport of anomalously warm inshore waters. Whether the latter were warm because of upwelling of warmer water or transport of warmer water from farther south is not clear.
Abstract
Three trans-oceanic XBT sections (∼5000 km in length) and a shorter one (∼2500 km in length) were taken in the central North Atlantic in 1974 to study the distribution of the horizontal scales and amplitudes of mesoscale variability. Few features of the MODE scale (∼400 km wavelength) were observed; the most dominant features had space scales of 1000–2000 km and amplitudes of 50–100 m.
Abstract
Three trans-oceanic XBT sections (∼5000 km in length) and a shorter one (∼2500 km in length) were taken in the central North Atlantic in 1974 to study the distribution of the horizontal scales and amplitudes of mesoscale variability. Few features of the MODE scale (∼400 km wavelength) were observed; the most dominant features had space scales of 1000–2000 km and amplitudes of 50–100 m.
Abstract
Vertical profiles of current, temperature and salinity were taken in the upper ocean from 3°S to 2°N along 55°30′E in the Indian Ocean during February–June in 1975 and 1976. During both years a strong O(80 cm s−1) equatorial undercurrent was present throughout the measurement period in the vicinity of the equator. A second region of eastward flow above the thermocline was observed at 3°S. During May and June the undercurrent moved southward and merged with the southern region of eastward flow. The meridional flow field was dominated by transients that during strong events were antisymmetric about the equator and had a vertical wavelength of ∼180 m. The transient events strongly affected the zonal flow field; during strong events the undercurrent was almost eliminated. This is in contrast to the GATE observations where the undercurrent was advected back and forth across the equator.
Abstract
Vertical profiles of current, temperature and salinity were taken in the upper ocean from 3°S to 2°N along 55°30′E in the Indian Ocean during February–June in 1975 and 1976. During both years a strong O(80 cm s−1) equatorial undercurrent was present throughout the measurement period in the vicinity of the equator. A second region of eastward flow above the thermocline was observed at 3°S. During May and June the undercurrent moved southward and merged with the southern region of eastward flow. The meridional flow field was dominated by transients that during strong events were antisymmetric about the equator and had a vertical wavelength of ∼180 m. The transient events strongly affected the zonal flow field; during strong events the undercurrent was almost eliminated. This is in contrast to the GATE observations where the undercurrent was advected back and forth across the equator.
Abstract
To assess the importance of possible feedbacks between ocean and atmosphere in the Arabian Sea, we computed a preliminary heat budget for the upper ocean layer. The observed total heat loss during the Southwest Monsoon between April and August is essentially balanced by three phenomena: positive heat gain from the atmosphere, negative northward heat flux across the equator, and heat loss due to upwelling along the coasts of East Africa and Arabia. Upwelling constitutes the dominant factor and the question is discussed as to which processes replenish the cold upwelled water on a seasonal time scale.
The average annual heating rate above and beyond seasonal fluctuations is found to be 24 W m−2. This net heat input must be compensated by ocean currents. The manner in which the ocean accomplishes this remains to be clarified.
Abstract
To assess the importance of possible feedbacks between ocean and atmosphere in the Arabian Sea, we computed a preliminary heat budget for the upper ocean layer. The observed total heat loss during the Southwest Monsoon between April and August is essentially balanced by three phenomena: positive heat gain from the atmosphere, negative northward heat flux across the equator, and heat loss due to upwelling along the coasts of East Africa and Arabia. Upwelling constitutes the dominant factor and the question is discussed as to which processes replenish the cold upwelled water on a seasonal time scale.
The average annual heating rate above and beyond seasonal fluctuations is found to be 24 W m−2. This net heat input must be compensated by ocean currents. The manner in which the ocean accomplishes this remains to be clarified.
Abstract
A section along 110°W in the eastern Pacific from about 6°N to 6°S was occupied in March and June of 1981. Measurements consisted of absolute velocity profiles and CTD cuts. The large-scale structure of the subsurface zonal flow remained relatively invariant between these cruises. The Equatorial Undercurrent and North and South Equatorial Undercurrents appear as strong eastward flows, separated by westward currents. Away from the equator, comparison of currents estimated geostrophically with the direct observations indicate that the two techniques are in agreement within estimated errors except close to the surface. In the vicinity of the equator the geostrophic technique in general fails and the directly measured currents must be used. During March, within 3° of the equator from the surface to 700 m, the flow was more eastward by about 0.15 m s−1; than in June. In March, the flow and temperature fields were relatively symmetric about the equator. By June, strong asymmetries had developed. In the top 100 m, eastward flow extended from the Undercurrent to about 3°S. A strong, shallow westward flow was situated over and to the north of the Undercurrent. A shallow southward flow developed from 4°N to 2°S. Order-of-magnitude estimates suggest that this can advect westward momentum onto the equator in the top 50 m and modify the Undercurrent. Asymmetry also developed in the near-surface thermal field. In June, upwelling was primarily located south of the equator. This resulted in a cold band lying south of the equator at the core of which the flow was predominantly eastward. A strong meridional temperature gradient at the equator separated the colder water from warmer water to the north. Thee asymmetries develop presumably in response to the seasonal increase from March to June of the winds. Computations of zonal transports in various σ t -classes in the near-surface layers suggest that the bulk of the Undercurrent water does not return west on the same density surfaces, but does so in the surface layers.
Abstract
A section along 110°W in the eastern Pacific from about 6°N to 6°S was occupied in March and June of 1981. Measurements consisted of absolute velocity profiles and CTD cuts. The large-scale structure of the subsurface zonal flow remained relatively invariant between these cruises. The Equatorial Undercurrent and North and South Equatorial Undercurrents appear as strong eastward flows, separated by westward currents. Away from the equator, comparison of currents estimated geostrophically with the direct observations indicate that the two techniques are in agreement within estimated errors except close to the surface. In the vicinity of the equator the geostrophic technique in general fails and the directly measured currents must be used. During March, within 3° of the equator from the surface to 700 m, the flow was more eastward by about 0.15 m s−1; than in June. In March, the flow and temperature fields were relatively symmetric about the equator. By June, strong asymmetries had developed. In the top 100 m, eastward flow extended from the Undercurrent to about 3°S. A strong, shallow westward flow was situated over and to the north of the Undercurrent. A shallow southward flow developed from 4°N to 2°S. Order-of-magnitude estimates suggest that this can advect westward momentum onto the equator in the top 50 m and modify the Undercurrent. Asymmetry also developed in the near-surface thermal field. In June, upwelling was primarily located south of the equator. This resulted in a cold band lying south of the equator at the core of which the flow was predominantly eastward. A strong meridional temperature gradient at the equator separated the colder water from warmer water to the north. Thee asymmetries develop presumably in response to the seasonal increase from March to June of the winds. Computations of zonal transports in various σ t -classes in the near-surface layers suggest that the bulk of the Undercurrent water does not return west on the same density surfaces, but does so in the surface layers.
Abstract
During March–April 1980, a velocity and CTD transect was made in the Pacific along the equator from 110 to 180°W. The horizontal baroclinic pressure gradient was observed to be primary confined between 160 and 130°W. Direct velocity profiles between 125 and 159°W showed the equatorial undercurrent to be a continuous feature. Maximum eastward transport (per unit width) in the undercurrent was 2.5 × 108 cm2 s−1 at 150°W and decreased both westward and eastward to about 1.5 × 108 cm2 s−1 at 159°W and at 125°W. Despite these variations the maximum speeds along the transect remained ∼150 cm s−1.
Beneath the undercurrent, the velocities decreased and were of the order of 25 cm s−1. They exhibited small-scale variation in the vertical as has recently been observed at other equatorial locations. Above 1600 m, the vertical wavelength of these variations in the zonal component was ∼300 m. Small-scale features in these zonal velocities were identifiable over 10° of longitude (1000 km). The small vertical and large horizontal scale suggests that these features might be Kelvin or long Rossby waves. The meridional velocities were primarily confined to the top 1000 m and their structure differed in profiles taken 5° apart.
Abstract
During March–April 1980, a velocity and CTD transect was made in the Pacific along the equator from 110 to 180°W. The horizontal baroclinic pressure gradient was observed to be primary confined between 160 and 130°W. Direct velocity profiles between 125 and 159°W showed the equatorial undercurrent to be a continuous feature. Maximum eastward transport (per unit width) in the undercurrent was 2.5 × 108 cm2 s−1 at 150°W and decreased both westward and eastward to about 1.5 × 108 cm2 s−1 at 159°W and at 125°W. Despite these variations the maximum speeds along the transect remained ∼150 cm s−1.
Beneath the undercurrent, the velocities decreased and were of the order of 25 cm s−1. They exhibited small-scale variation in the vertical as has recently been observed at other equatorial locations. Above 1600 m, the vertical wavelength of these variations in the zonal component was ∼300 m. Small-scale features in these zonal velocities were identifiable over 10° of longitude (1000 km). The small vertical and large horizontal scale suggests that these features might be Kelvin or long Rossby waves. The meridional velocities were primarily confined to the top 1000 m and their structure differed in profiles taken 5° apart.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
A linear model is developed for the near-equatorial zone to estimate wind-driven convergences in the near-surface viscous boundary layer. Using the winds observed during EASTROPAC, an attempt is made to relate these convergences to the measured displacements of the tropical thermocline. Between 4° and 15°N, the sign of the displacements is predicted; however, the amplitude is generally underestimated. At the equator, extremely large values of the vertical eddy coefficients are necessary in order to obtain agreement between predicted and observed changes. This probably indicates that some essential physics has been neglected.
Abstract
A linear model is developed for the near-equatorial zone to estimate wind-driven convergences in the near-surface viscous boundary layer. Using the winds observed during EASTROPAC, an attempt is made to relate these convergences to the measured displacements of the tropical thermocline. Between 4° and 15°N, the sign of the displacements is predicted; however, the amplitude is generally underestimated. At the equator, extremely large values of the vertical eddy coefficients are necessary in order to obtain agreement between predicted and observed changes. This probably indicates that some essential physics has been neglected.
Abstract
Maps of sea surface temperature in the North Atlantic subtropical convergence during the 1973 MODE field experiment (and recent satellite imagery) show large meridional and zonal features on a scale of 40–400 km which are superimposed on the seasonal meridional temperature gradient. After comparing these maps with dynamic topography relative to 1500 db it is argued that these features are mainly due to advective distortion by surface currents associated with the deep baroclinic mesoscale eddy field. Wind-induced surface currents appear to have a lesser effect in generating such structure. Surface frontogenesis observed during MODE and by earlier workers in the area suggests that jet-like shallow surface density currents may be also significant in advecting and distorting the surface temperature field on scales of 10 km and less. Finally, rough calculations indicate that these advective processes of the sea surface may supply annually an amount of heat to the surface water mass of the northern Sargasso Sea which is significant compared with that lost to the atmosphere.
Abstract
Maps of sea surface temperature in the North Atlantic subtropical convergence during the 1973 MODE field experiment (and recent satellite imagery) show large meridional and zonal features on a scale of 40–400 km which are superimposed on the seasonal meridional temperature gradient. After comparing these maps with dynamic topography relative to 1500 db it is argued that these features are mainly due to advective distortion by surface currents associated with the deep baroclinic mesoscale eddy field. Wind-induced surface currents appear to have a lesser effect in generating such structure. Surface frontogenesis observed during MODE and by earlier workers in the area suggests that jet-like shallow surface density currents may be also significant in advecting and distorting the surface temperature field on scales of 10 km and less. Finally, rough calculations indicate that these advective processes of the sea surface may supply annually an amount of heat to the surface water mass of the northern Sargasso Sea which is significant compared with that lost to the atmosphere.
