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Dongxiao Zhang
,
Michael J. McPhaden
, and
William E. Johns

Abstract

This study determines the mean pathways and volume transports in the pycnocline and surface layer for water flowing between the subtropical and tropical Atlantic Ocean, using potential vorticity, salinity, geostrophic flow maps on isopycnal surfaces, and surface drifter velocities. In both hemispheres, subducted salinity maximum waters flow into the Tropics in the pycnocline along both interior and western boundary pathways. The North Atlantic ventilating trajectories are confined to densities between about 23.2 and 26.0 σ θ , and only about 2 Sv (Sv ≡ 106 m3 s–1) of water reaches the Tropics through the interior pathway, whereas the western boundary contributes about 3 Sv to the equatorward thermocline flow. Flow on shallower surfaces of this density range originates from the central Atlantic near 40°W between 12° and 16°N whereas flow on the deeper surfaces originates from near 20°W just off the coast of Africa at higher latitudes. The pathways skirt around the potential vorticity barrier located under the intertropical convergence zone and reach their westernmost location at about 10°N. In the South Atlantic, about 10 Sv of thermocline water reaches the equator through the combination of interior (4 Sv) and western boundary (6 Sv) routes in a slightly higher density range than in the North Atlantic. Similar to the North Atlantic, the shallower layers originate in the central part of the basin (along 10°–30°W at 10°–15°S) and the deeper layers originate at higher latitudes from the eastern part of the basin. However, the ventilation pathways are spread over a much wider interior window in the Southern Hemisphere than in the Northern Hemisphere that at 6°S extends from 10°W to the western boundary. The equatorward convergent flows in the thermocline upwell into the surface layer and return to the subtropics through surface poleward divergence. As much as 70% of the tropical Atlantic upwelling into the surface layer is associated with these subtropical circulation cells, with the remainder contributed by the warm return flow of the large-scale thermohaline overturning circulation.

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Ricardo M. Domingues
,
William E. Johns
, and
Christopher S. Meinen

Abstract

In this study, mechanisms causing year-to-year changes in the Florida Current seasonality are investigated using controlled realistic numerical experiments designed to isolate the western boundary responses to westward-propagating open ocean signals. The experiments reveal two distinct processes by which westward-propagating signals can modulate the phase of the Florida Current variability, which we refer to as the “direct” and “indirect” response mechanisms. The direct response mechanism involves a two-stage response to open ocean anticyclonic eddies characterized by the direct influence of Rossby wave barotropic anomalies and baroclinic wall jets that propagate through Northwest Providence Channel. In the indirect response mechanism, open ocean signals act as small perturbations to the stochastic Gulf Stream variability downstream, which are then transmitted upstream to the Florida Straits through baroclinic coastally trapped signals that can rapidly travel along the U.S. East Coast. Experiments indicate that westward-propagating eddies play a key role in modulating the phase of the Florida Current variability, but not the amplitude, which is determined by its intrinsic variability in our simulations. Results from this study further suggest that the Antilles Current may act as a semipermeable barrier to incoming signals, favoring the interaction through the indirect response mechanism. The mechanisms reported here can be potentially linked to year-to-year changes in the seasonality of the Atlantic meridional overturning circulation and may also be present in other western boundary current systems.

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Erik van Sebille
,
Lisa M. Beal
, and
William E. Johns

Abstract

The advective transit time of temperature–salinity anomalies from the Agulhas region to the regions of deep convection in the North Atlantic Ocean is an important time scale in climate, because it has been linked to variability in the Atlantic meridional overturning circulation. Studying this transit time scale is difficult, because most observational and high-resolution model data are too short for assessment of the global circulation on decadal to centennial time scales. Here, results are presented from a technique to obtain thousands of “supertrajectories” of any required length using a Monte Carlo simulation. These supertrajectories allow analysis of the circulation patterns and time scales based on Lagrangian data: in this case, observational surface drifter trajectories from the Global Drifter Program and Lagrangian data from the high-resolution OGCM for the Earth Simulator (OFES). The observational supertrajectories can only be used to study the two-dimensional (2D) surface flow, whereas the numerical supertrajectories can be used to study the full three-dimensional circulation. Results for the surface circulation indicate that the supertrajectories starting in the Agulhas Current and ending in the North Atlantic take at least 4 yr and most complete the journey in 30–40 yr. This time scale is, largely because of convergence and subduction in the subtropical gyres, longer than the 10–25 yr it takes the 3D numerical supertrajectories to complete the journey.

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David M. Fratantoni
,
William E. Johns
,
Tamara L. Townsend
, and
Harley E. Hurlburt

Abstract

An eddy-resolving numerical ocean circulation model is used to investigate the pathways of low-latitude intergyre mass transport associated with the upper limb of the Atlantic meridional overturning cell (MOC). Numerical experiments with and without applied wind stress and an imposed MOC exhibit significant differences in intergyre transport, western boundary current intensity, and mesoscale ring production. The character of interaction between low-latitude wind- and overturning-driven circulation systems is found to be predominantly a linear superposition in the annual mean, even though nonlinearity in the form of diapycnal transport is essential to some segments of the mean pathway. Within a mesoscale band of 10–100 day period, significant nonlinear enhancement of near-surface variability is observed. In a realistically forced model experiment, a 14 Sv upper-ocean MOC return flow is partitioned among three pathways connecting the equatorial and tropical wind-driven gyres. A frictional western boundary current with both surface and intermediate depth components is the dominant pathway and accounts for 6.8 Sv of intergyre transport. A diapycnal pathway involving wind-forced equatorial upwelling and interior Ekman transport is responsible for 4.2 Sv. Translating North Brazil Current rings contribute approximately 3.0 Sv of intergyre transport.

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JoséL. Chávez
,
Christopher M. U. Neale
,
Lawrence E. Hipps
,
John H. Prueger
, and
William P. Kustas

Abstract

In an effort to better evaluate distributed airborne remotely sensed sensible and latent heat flux estimates, two heat flux source area (footprint) models were applied to the imagery, and their pixel weighting/integrating functionality was investigated through statistical analysis. Soil heat flux and sensible heat flux models were calibrated. The latent heat flux was determined as a residual from the energy balance equation. The resulting raster images were integrated using the 2D footprints and were compared to eddy covariance energy balance flux measurements. The results show latent heat flux estimates (adjusted for closure) with errors of (mean ± std dev) −9.2 ± 39.4 W m−2, sensible heat flux estimate errors of 9.4 ± 28.3 W m−2, net radiation error of −4.8 ± 20.7 W m−2, and soil heat flux error of −0.5 ± 24.5 W m−2. This good agreement with measured values indicates that the adopted methodology for estimating the energy balance components, using high-resolution airborne multispectral imagery, is appropriate for modeling latent heat fluxes. The method worked well for the unstable atmospheric conditions of the study. The footprint weighting/integration models tested indicate that they perform better than simple pixel averages upwind from the flux stations. In particular the flux source area model (footprint) seemed to better integrate the resulting heat flux image pixels. It is suggested that future studies test the methodology for heterogeneous surfaces under stable atmospheric conditions.

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Diane H. Portis
,
Michael P. Cellitti
,
William L. Chapman
, and
John E. Walsh

Abstract

Hourly data from 17 relatively evenly distributed stations east of the Rocky Mountains during 54 winter seasons (1948/49 through 2001/02) are used to evaluate the low-frequency variability of extreme cold air outbreaks (CAOs). The results show no overall trend in CAO frequency, despite an increase in mean temperature over the Midwest and especially upstream into the CAO formation regions of high-latitude North America. However, there are regionally based trends in the intensity of long-duration (5 day) CAOs.

Daily heat budgets from reanalysis data are also used to investigate the thermodynamic and dynamic processes involved in the evolution of a subset of the major CAOs. The cooling of the air masses can be generally traced in the heat budget analysis as the air masses track southward along the Rocky Mountains into the Midwest. The earliest cooling begins in northwestern Canada more than a week before the cold air mass reaches the Midwest. Downstream in southwestern Canada, both diabatic and advective processes contribute to the cumulative cooling of the air mass. At peak intensity over the Midwest, diabatic processes and horizontal advection cool the air mass, but warming by subsidence offsets this cooling. By contrast, to the west of the CAO track into the Midwestern United States, vertical advection by orographic lifting cumulatively cools the air in the upslope flow regime associated with the low-level airflow around a cold air mass, and this cooling is offset by diabatic warming. Diabatic processes have strong positive correlations with temperature change over all regions (especially in central Canada) except for the mountainous regions in the United States that are to the west of the track of the cold air mass. Correlations of vertical advection with horizontal advection and diabatic processes are physically consistent and give credibility to the vertical advection field.

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Carl N. Hodges
,
T. Lewis Thompson
,
John E. Groh
, and
William D. Sellers

Abstract

The University of Arizona has developed a sea water desalinization system which can economically utilize low temperature solar energy. The system consists of a horizontal plastic-covered solar collector, a packed-tower evaporator, and a finned-tube surface condenser. Incoming sea water is preheated in the surface condenser and then pumped to the solar collector where it is heated 5 to 10C. The heated sea water is pumped from the collector to the packed-tower evaporator, where a small fraction is evaporated into a circulating air stream and condensed as distilled water in the finned-tube surface condenser.

To evaluate the system a pilot plant has been constructed in cooperation with the University of Sonora at Puerto Peñasco on the Gulf of California. This plant is designed to produce between 2500 and 5000 gallons of fresh water daily.

The energy for evaporation in the system is derived from ocean water heated in the solar collector during the day. In order to allow design optimization for the entire plant the temperatures in the collector must be accurately predicted. It is shown that this can be done by a simple manipulation of the energy balance equation for the collector.

The resulting theory is applied to a number of cases involving a double glazing collector filled with 2 inches of water. Such a collector will utilize about 24 per cent of the available solar energy if the warm water in the collector in the late afternoon is flushed out and stored for nighttime use in the evaporator.

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John M. Frank
,
William J. Massman
,
Edward Swiatek
,
Herb A. Zimmerman
, and
Brent E. Ewers

Abstract

Sonic anemometry is fundamental to all eddy-covariance studies of surface energy and ecosystem carbon and water balance. Recent studies have shown that some nonorthogonal anemometers underestimate vertical wind. Here it is hypothesized that this is due to a lack of transducer and structural shadowing correction. This is tested with a replicated intercomparison experiment between orthogonal (K-probe, Applied Technologies, Inc.) and nonorthogonal (A-probe, Applied Technologies, Inc.; and CSAT3 and CSAT3V, Campbell Scientific, Inc.) anemometer designs. For each of the 12 weeks, five randomly selected and located anemometers were mounted both vertically and horizontally. Bayesian analysis was used to test differences between half-hourly anemometer measurements of the standard deviation of wind (σ u , συ, and σ w ) and temperature, turbulent kinetic energy (TKE), the ratio between vertical/horizontal TKE (VHTKE), and sensible heat flux (H). Datasets were analyzed with various applications of transducer shadow correction. Using the manufacturer’s current recommendations, orthogonal anemometers partitioned higher VHTKE and measured about 8%–9% higher σ w and ~10% higher H. This difference can be mitigated by adding shadow correction to nonorthogonal anemometers. The horizontal manipulation challenged each anemometer to measure the three dimensions consistently, which allowed for testing two hypotheses explaining the underestimate in vertical wind. While measurements were essentially unchanged when the orthogonal anemometers were mounted sideways, the nonorthogonal anemometers changed substantially and confirmed the lack of shadow correction. Considering the ubiquity of nonorthogonal anemometers, these results are consequential across flux networks and could potentially explain half of the ~20% missing energy that is typical at most flux sites.

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Darko Koračin
,
John Lewis
,
William T. Thompson
,
Clive E. Dorman
, and
Joost A. Businger

Abstract

A case of fog formation along the California coast is examined with the aid of a one-dimensional, higher-order, turbulence-closure model in conjunction with a set of myriad observations. The event is characterized by persistent along-coast winds in the marine layer, and this pattern justifies a Lagrangian approach to the study. A slab of marine layer air is tracked from the waters near the California–Oregon border to the California bight over a 2-day period. Observations indicate that the marine layer is covered by stratus cloud and comes under the influence of large-scale subsidence and progressively increasing sea surface temperature along the southbound trajectory.

It is hypothesized that cloud-top cooling and large-scale subsidence are paramount to the fog formation process. The one-dimensional model, evaluated with various observations along the Lagrangian path, is used to test the hypothesis. The principal findings of the study are 1) fog forms in response to relatively long preconditioning of the marine layer, 2) radiative cooling at the cloud top is the primary mechanism for cooling and mixing the cloud-topped marine layer, and 3) subsidence acts to strengthen the inversion above the cloud top and forces lowering of the cloud. Although the positive fluxes of sensible and latent heat at the air–sea interface are the factors that govern the onset of fog, sensitivity studies with the one-dimensional model indicate that these sensible and latent heat fluxes are of secondary importance as compared to subsidence and cloud-top cooling. Sensitivity tests also suggest that there is an optimal inversion strength favorable to fog formation and that the moisture conditions above the inversion influence fog evolution.

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Amanda H. Lynch
,
William L. Chapman
,
John E. Walsh
, and
Gunter Weller

Abstract

An Arctic region climate system model has been developed to simulate coupled interactions among the atmosphere, sea ice, ocean, and land surface of the western Arctic. The atmospheric formulation is based upon the NCAR regional climate model RegCM2, and includes the NCAR Community Climate Model Version 2 radiation scheme and the Biosphere–Atmosphere Transfer Scheme. The dynamic–thermodynamic sea ice model includes the Hibler–Flato cavitating fluid formulation and the Parkinson–Washington thermodynamic scheme linked to a mixed-layer ocean.

Arctic winter and summer simulations have been performed at a 63 km resolution, driven at the boundaries by analyses compiled at the European Centre for Medium-Range Weather Forecasts. While the general spatial patterns are consistent with observations, the model shows biases when the results are examined in detail. These biases appear to be consequences in part of the lack of parameterizations of ice dynamics and the ice phase in atmospheric moist processes in winter, but appear to have other causes in summer.

The inclusion of sea ice dynamics has substantial impacts on the model results for winter. Locally, the fluxes of sensible and latent heat increase by over 100 W m−2 in regions where offshore winds evacuate sea ice. Averaged over the entire domain, these effects result in root-mean-square differences of sensible heat flux and temperatures of 15 W m−2 and 2°C. Other monthly simulations have addressed the model sensitivity to the subgrid-scale moisture treatment, to ice-phase physics in the explicit moisture parameterization, and to changes in the relative humidity threshold for the autoconversion of cloud water to rainwater. The results suggest that the winter simulation is most sensitive to the inclusion of ice phase physics, which results in an increase of precipitation of approximately 50% and in a cooling of several degrees over large portions of the domain. The summer simulation shows little sensitivity to the ice phase and much stronger sensitivity to the convective parameterization, as expected.

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