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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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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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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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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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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 Christopher S. Welch

Abstract

No abstract available.

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Vernon E. Kousky and Ants Leetmaa

Abstract

The evolution of oceanic and atmospheric anomaly fields for the period prior to and during the 1986–87 El Niño/Southem Oscillation (ENSO) is presented. A comparison is made between the 1986–87 ENSO and other warm episodes that occurred during the last 20 yr. In addition, for the first time, an ocean general circulation model was run in a real-time diagnostic mode. The model output provided detailed information about the evolution of subsurface features throughout the Pacific basin.

A slow trend towards warm episode (ENSO) conditions in the Pacific was evident throughout the period 1985–86 in certain atmospheric and oceanic variables. Atmospheric and oceanic fields changed much more rapidly during late 1986 as enhanced atmospheric convection developed in the equatorial Pacific near the date line. Thermocline depths rapidly increased (decreased) in the eastern (western) equatorial Pacific as low-level westerlies developed in the western portion of the basin. A remote response to those westerlies was felt along the west coast of South America in early 1987 as sea surface temperatures (SSTs) increased 3°–5°C above normal. Conditions remained anomalous in the tropical Pacific throughout 1987, but began a rapid return towards normal late in the year as low-level easterlies increased in strength. By the northern spring 1988, below normal SSTs were observed throughout the equatorial Pacific east of the date line.

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Ants Leetmaa and Peter F. Spain

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.

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