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Ants Leetmaa

Abstract

A simple example of the steady motion of a rotating, stratified fluid is studied. The solution which is uniformly valid for all values of the stratification, σsδ = vαgDΔT/(κf 2 L 2), is presented. The transitions in the dynamics from the homogeneous limit to strong stratification are illustrated in detail. The motion is driven by a stress. Consequently, Ekman suction is weaker than in cases where the driving force is a moving boundary, and Ekman layers are important until a stratification of O(1) at which point they combine with Lineykin layers to form the thermal equivalent of the Stewartson E½ layer.

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Ants Leetmaa

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.

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Ants Leetmaa

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.

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Walter Düing and Ants Leetmaa

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.

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Michiko Masutani and Ants Leetmaa

Abstract

The link between El Niño and the California wintertime rainfall has been reported in various studies. During the winter of 1994/95, warm sea surface temperature anomalies (SSTAs) were observed in the central Pacific, and widespread significant flooding occurred in California during January 1995 and March 1995. However, the El Niño–Southern Oscillation alone cannot explain the flooding. In March 1995 California suffered flooding after the warm SSTA over the central Pacific had weakened considerably. During November and December, in spite of El Niño conditions, California was not flooded, and more than two standard deviations above normal SSTA in the North Pacific were observed. A possible link between midlatitude warm SSTA and the timing of the onset of flooding is suspected within the seasonal forecasting community.

The climate condition during the northern winter of 1994/95 is described using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data. Diagnostics show the typical El Niño pattern in the seasonal mean and the link between the position of the jet exit and the flooding over California on the intraseasonal timescale.

The relationship among California floods, the Pacific jet, tropical rainfall, and SSTA is inferred from results of general circulation model (GCM) experiments with various SSTAs. The results show that the rainfall over California is associated with an eastward extension of the Pacific jet, which itself is associated with enhanced tropical convection over the warm SSTA in the central Pacific. The GCM experiments also show that rainfall over the Indian Ocean can contribute to the weakening of the Pacific jet and to dryness over California. The GCM experiments did not show significant impact of North Pacific SSTA, either upon the Pacific jet or upon rainfall over California. The agreement with diagnostics results is discussed. GCM experiments suggest the link between the tropical intraseasonal oscillation (TIO) and the flooding in March in California, since there is a strong TIO component in rainfall over the Indian Ocean.

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Ming Ji and Ants Leetmaa

Abstract

In this study, the authors compare skills of forecasts of tropical Pacific sea surface temperatures from the National Centers for Environmental Prediction (NCEP) coupled general circulation model that were initiated using different sets of ocean initial conditions. These were produced with and without assimilation of observed subsurface upper-ocean temperature data from expendable bathythermographs (XBTs) and from the Tropical Ocean Global Atmosphere–Tropical Atmosphere Ocean (TOGA–TAO) buoys.

These experiments show that assimilation of observed subsurface temperature data in the determining of the initial conditions, especially for summer and fall starts, results in significantly improved forecasts for the NCEP coupled model. The assimilation compensates for errors in the forcing fields and inadequate physical parameterizations in the ocean model. Furthermore, additional skill improvements, over that provided by XBT assimilation, result from assimilation of subsurface temperature data collected by the TOGA–TAO buoys. This is a consequence of the current predominance of TAO data in the tropical Pacific in recent years.

Results suggest that in the presence of erroneous wind forcing and inadequate physical parameterizations in the ocean model ocean data assimilation can improve ocean initialization and thus can improve the skill of the forecasts. However, the need for assimilation can create imbalances between the mean states of the oceanic initial conditions and the coupled model. These imbalances and errors in the coupled model can be significant limiting factors to forecast skill, especially for forecasts initiated in the northern winter. These limiting factors cannot be avoided by using data assimilation and must be corrected by improving the models and the forcing fields.

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Ants Leetmaa and Henry Stommel

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.

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Ants Leetmaa and Christopher S. Welch

Abstract

No abstract available.

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Ants Leetmaa and Robert L. Molinari

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 σt-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.

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P. Webb DeWitt and Ants Leetmaa

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.

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