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A. E. Gill

Abstract

Bathythermograph data from the equatorial Pacific are used to study changes in upper ocean thermal structure during the period 1971–73 using data averaged over 2- or 3-month segments. This gives reasonable descriptions of the changes in two regions—the central Pacific (particularly in a section crossing the equator at 170–160°W) and along the eastern boundary. The behavior in the two regions is quite distinct, e.g., in the central Pacific, thermocline depth anomalies do not correlate well with surface temperature anomalies, but they do along the eastern boundary. The first baroclinic mode can be used to describe changes in vertical structure in the central Pacific, but the major changes near the eastern boundary are much more like those associated with the second baroclinic mode.

Subsurface changes along the eastern boundary are extremely large, e.g., the 14°C isotherm at the equator plunged from the 100 m level in January–February 1972 to the 250 m level in May–June. It did not get back to the 100 m level until the second half of 1973. The very large excursions are mainly within 10° of the equator, but the isotherm depth anomaly often had the same sign over the whole section from 20°S to 30°N.

Near the eastern boundary, the surface was nearly always anomalously warm when the thermocline was anomalously deep, but in the central Pacific there was not a strong relationship between surface temperatures and isotherm depth. In fact, the very large positive temperature anomaly found at 170–160°W during the September–November 1972 season occurred when the 20°C isotherm was shallower than normal.

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A. E. Gill

Abstract

This paper has two main parts. First, it is shown that if the dynamics is simplified by assuming a one-mode, low-frequency (or long-wave) model, the whole field of anomalous motion in the tropical region can he deduced from the time series of sea-level anomaly at the eastern boundary. This is because the amplitudes of all the equatorial waves are proportional to sea level at this longitude, and the amplitudes at other longitudes can he deduced by integrating along characteristics using the method of Gill and Clarke (1974). The integration proceeds forward in time for the planetary waves and backward in time for the Kelvin wave. The calculation requires a knowledge of wind stress anomalies, although the wind effect only becomes significant after integration to large distances from the eastern boundary. The western boundary is irrelevant to the calculations. The technique is applied to estimate zonal surface current anomalies in the equatorial Pacific for the period 1971-73 spanning a major El Niño. Mean zonal surface current anomalies within ±5° of the equator are deduced to have reached their largest values (of ∼0.5 m s−1) near the date line in early 1972, when they were eastward, and in early 1973, when they were westward.

The second part of the paper uses gm reconstructed anomaly field of the first part to examine anomalies in the heat balance of the surface layer of the equatorial Pacific. The striking result here is that surface temperature anomalies in the central Pacific are well reproduced in both amplitude and timing it caused solely by horizontal advection by the zonal current anomaly. Other mechanisms give much smaller amplitude and the wrong timing. The resultant picture is that warm anomalies in the central Pacific result from huge anomalous eastward movements of warm water from the west Pacific.

The heat balance near the eastern boundary is also discussed, and two possible mechanisms seem capable of reproducing a warm anomaly with the correct strength at the right time. One is anomalous poleward advection of warm water along the coast. The other is upwelling of anomalously warm water near the coast and the spreading of this warm anomaly by advection with the mean flow.

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A. E. Gill

Abstract

The Rossby adjustment process in the wake of a storm is studied with a view to finding, within the context of linear theory, how wave dispersion in both the vertical and meridional directions spreads energy which is initially confined entirely to the mixed layer. Comparisons are made between three cases 1) a periodic storm on the f-plane (where dispersion is purely vertical); 2) a bounded storm on the f-plane; and 3) a bounded storm centered 2700 km from the equator on an equatorial beta-plane. Particular attention is paid to the initial rate of loss of energy from the mixed layer, and some simple formulas which work very well in the cases studied are derived. These show that the rate of loss goes up when the mixed-layer depth is increased, and also that the rate scales as the square of the wavenumber. Values of the rates are sufficient to provide a major source of energy for internal waves below the mixed layer.

The often-observed tendency for phase lines to propagate upward is found in all cases, but the analysis also shows possible shortcomings in the way observations are interpreted. Some attention is also paid to the closer of the ocean floor on the results. Other observed properties of near-inertial waves which are found in the model studies are the following 1) intermittency; 2) storm effects on currents are largest just below the storm track; 3) the horizontal and vertical scales tend to decrease with time after the storm has passed; 4) vertical group propagation on scales comparable with the mixed layer is very slow, and 5) some tendency for bottom intensification is found. Another result is that beta-dispersion can be quite important, and some effect are transmitted to the equator quite rapidly (typically two weeks).

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A. E. Gill and E. H. Schumann

Abstract

Currents [O(0.1 m sec−1)] and sea-level variations [O(0.1 m)] on the continental shelf can be produced when atmospheric disturbances pass by. Since the atmospheric systems have length scales large compared with the width of the shelf, a boundary-layer approximation can be made to the equations governing the local response to these forcing systems. The low-frequency response can be expressed as a sum of modes (continental shelf waves) and the amplitude of each mode is found to satisfy a first-order wave equation which can readily be solved. In a case where some details are worked out, the second mode's contribution to sea-level changes is only 25% of the first mode's and the third mode's contribution only 8%. Thus, given appropriate meteorological information, sea level changes can easily be calculated, and a prediction system is theoretically possible.

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A. E. Gill and E. H. Schumann

Abstract

Calculations are made of the changes in the structure of an inertial current which can be induced by slow changes in the topography of the continental shelf and slope along which it flows. The particular case of a uniform potential vorticity current over a shelf of uniform slope shows that smooth transitions from subcritical to supercritical flow can occur at a minimum in the shelf width. Long-wave disturbances travel away from such a point. Upstream there is a tendency for a countercurrent to occur at the coast, while downstream there is a tendency for cold water to outcrop on the inshore side of a front. Both these features occur along the path of the Agulhas Current.

A method developed for calculating the speed of long-wave disturbances in a flow with a given potential vorticity distribution is applied to sections of the Agulhas Current about 150 km apart. In this distance the shelf width is reduced, and a calculation using a current model with two active layers shows the second mode is very close to critical at Port Edward. This result supports the notion that shelf topography can hydraulically control an inertia] boundary current.

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G. R. Bigg and A. E. Gill

Abstract

The long period response of sea level on the eastern boundary of the Pacific separates, for linear dynamics, into a remotely forced part mainly due to zonal winds along the equator to the west and a locally driven part where sea level slopes to balance the alongshore wind. Examination of the annual component of sea level on the eastern boundary indicates that the locally forced part dominates, whereas the remotely forced part plays a major role at semiannual and interannual periods.

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M. J. McPhaden and A. E. Gill

Abstract

We develop a linear, reduced-gravity model with two active layers above a deep, resting layer to examine the scattering of equatorial Kelvin waves from meridional submarine ridges. Model ridges, idealized as infinitely long in the meridional direction and infinitesimally thin in the zonal direction, completely obstruct flow in the lower active layer. The equatorial long-wave approximation is made, which restricts the class of motions considered to nondispersive Kelvin and Rossby waves in each of two internal modes. Thus, coastal and topographically trapped phenomena are filtered out, but variability far from the ridge is accurately modeled.

The scattering of Kelvin wave energy depends on two parameters, r = H 0/H 1 and γ = (ρ1 − ρ0)/(ρ2 − ρ1), where H 0 and H 1 are the equilibrium thicknesses of the upper and lower active layers, respectively, and ρ0≲ρ1≲ρ2 are the layer densities. Incident first internal mode Kelvin waves are little affected by a deep ridge (i.e., for large r) and strongly reflected by a shallow ridge (i.e., for small r). Second internal mode Kelvin waves behave in an opposite sense, being more strongly reflected by a deep ridge for example. Strong stratification typical of the tropics, corresponding to large &gamma, decouples the near surface from the deep ocean, enhancing the transmissivity of the first mode and the reflectivity of the second mode.

Potentially observable topographic effects in the wake of low-mode Kelvin fronts include enhanced eddying in the far field west of ridges, enhanced vertical shear of zonal flows over and east of ridges, and changes in the depth and intensity of the thermocline across ridges. Weak eddying may also be generated to the east of ridges in the form of boundary trapped currents.

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Roxana C. Wajsowicz and A. E. Gill

Abstract

The early stages of the adjustment of an ocean toward equilibrium is examined using an ocean general circulation model. The initial state is one with uniform meridional potential temperature gradients yielding density gradients representative of those observed in the North Atlantic. The main adjustment in the time examined here (the first few months) is by coastal Kelvin waves. The first baroclinic mode dominates, so the behavior proves to be very similar to that in a shallow-water model. Also, the dynamics is locally close to that for an f-plane, but account must be taken of the way f varies with latitude. The essential result of the coastal adjustment is to reduce the temperature gradients around the perimeter of the basin, especially at the level where the first mode has the maximum effect.

This study also examines the way this adjustment process is distorted in models due to the constraints imposed by computer limitations, namely in a model with too coarse a horizontal resolution and an artificially high lateral friction needed for computational stability. In the present case, the distortions can be quantitatively assessed and understood, and it is felt there are lessons to be learned about the use of general circulation models.

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George E. Bowker, Dale A. Gillette, Gilles Bergametti, Béatrice Marticorena, and David K. Heist

Abstract

Within areas of the Chihuahuan Desert dominated by honey mesquite bushes (Prosopis glandulosa), soil erosion causes open eroded patches and the formation of large coppice dunes. The airflow patterns around the dunes and through the open areas are correlated with sand flux and erosion. This study uses wind velocity simulations from the Quick Urban and Industrial Complex (QUIC) model in combination with a sand flux parameterization to simulate sand fluxes for each of eight storms occurring in the springs of 2003 and 2004. Total sand fluxes based on the sum of all the sand collectors located within the study domain were usually within 50% of the measured values for each of the storms, with simulations for individual sand collectors also often within 50% of the measured values. Simulated fluxes based on two different sand flux parameterizations were generally within 10% of each other, differing substantially only when the sand flux was low (near the threshold velocity). Good agreement between the field observations with a Sensit instrument and QUIC simulations for the same location and time series suggests that QUIC could be used to predict the spatial and temporal variation of sand flux patterns for a domain.

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S. T. Martin, P. Artaxo, L. Machado, A. O. Manzi, R. A. F. Souza, C. Schumacher, J. Wang, T. Biscaro, J. Brito, A. Calheiros, K. Jardine, A. Medeiros, B. Portela, S. S. de Sá, K. Adachi, A. C. Aiken, R. Albrecht, L. Alexander, M. O. Andreae, H. M. J. Barbosa, P. Buseck, D. Chand, J. M. Comstock, D. A. Day, M. Dubey, J. Fan, J. Fast, G. Fisch, E. Fortner, S. Giangrande, M. Gilles, A. H. Goldstein, A. Guenther, J. Hubbe, M. Jensen, J. L. Jimenez, F. N. Keutsch, S. Kim, C. Kuang, A. Laskin, K. McKinney, F. Mei, M. Miller, R. Nascimento, T. Pauliquevis, M. Pekour, J. Peres, T. Petäjä, C. Pöhlker, U. Pöschl, L. Rizzo, B. Schmid, J. E. Shilling, M. A. Silva Dias, J. N. Smith, J. M. Tomlinson, J. Tóta, and M. Wendisch

Abstract

The Observations and Modeling of the Green Ocean Amazon 2014–2015 (GoAmazon2014/5) experiment took place around the urban region of Manaus in central Amazonia across 2 years. The urban pollution plume was used to study the susceptibility of gases, aerosols, clouds, and rainfall to human activities in a tropical environment. Many aspects of air quality, weather, terrestrial ecosystems, and climate work differently in the tropics than in the more thoroughly studied temperate regions of Earth. GoAmazon2014/5, a cooperative project of Brazil, Germany, and the United States, employed an unparalleled suite of measurements at nine ground sites and on board two aircraft to investigate the flow of background air into Manaus, the emissions into the air over the city, and the advection of the pollution downwind of the city. Herein, to visualize this train of processes and its effects, observations aboard a low-flying aircraft are presented. Comparative measurements within and adjacent to the plume followed the emissions of biogenic volatile organic carbon compounds (BVOCs) from the tropical forest, their transformations by the atmospheric oxidant cycle, alterations of this cycle by the influence of the pollutants, transformations of the chemical products into aerosol particles, the relationship of these particles to cloud condensation nuclei (CCN) activity, and the differences in cloud properties and rainfall for background compared to polluted conditions. The observations of the GoAmazon2014/5 experiment illustrate how the hydrologic cycle, radiation balance, and carbon recycling may be affected by present-day as well as future economic development and pollution over the Amazonian tropical forest.

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