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William E. Johns
,
Thomas N. Lee
,
Dongxiao Zhang
,
Rainer Zantopp
,
Cho-Teng Liu
, and
Yih Yang

Abstract

Observations from the WOCE PCM-1 moored current meter array east of Taiwan for the period September 1994 to May 1996 are used to derive estimates of the Kuroshio transport at the entrance to the East China Sea. Three different methods of calculating the Kuroshio transport are employed and compared. These methods include 1) a “direct” method that uses conventional interpolation of the measured currents and extrapolation to the surface and bottom to estimate the current structure, 2) a “dynamic height” method in which moored temperature measurements from moorings on opposite sides of the channel are used to estimate dynamic height differences across the current and spatially averaged baroclinic transport profiles, and 3) an “adjusted geostrophic” method in which all moored temperature measurements within the array are used to estimate a relative geostrophic velocity field that is referenced and adjusted by the available direct current measurements. The first two methods are largely independent and are shown to produce very similar transport results. The latter two methods are particularly useful in situations where direct current measurements may have marginal resolution for accurate transport estimates. These methods should be generally applicable in other settings and illustrate the benefits of including a dynamic height measuring capability as a backup for conventional direct transport calculations. The mean transport of the Kuroshio over the 20-month duration of the experiment ranges from 20.7 to 22.1 Sv (1 Sv ≡ 106 m3 s−1) for the three methods, or within 1.3 Sv of each other. The overall mean transport for the Kuroshio is estimated to be 21.5 Sv with an uncertainty of 2.5 Sv. All methods show a similar range of variability of ±10 Sv with dominant timescales of several months. Fluctuations in the transport are shown to have a robust vertical structure, with over 90% of the transport variance explained by a single vertical mode. The moored transports are used to determine the relationship between Kuroshio transport and sea-level difference between Taiwan and the southern Ryukyu Islands, allowing for long-term monitoring of the Kuroshio inflow to the East China Sea.

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Dongxiao Zhang
,
Thomas N. Lee
,
William E. Johns
,
Cho-Teng Liu
, and
Rainer Zantopp

Abstract

Observations from the World Ocean Circulation Experiment PCM-1 moored current meter array in the East Taiwan Channel are analyzed and combined with TOPEX/Poseidon altimetry data and the Parallel Ocean Climate Model simulation to study Kuroshio variability and relationships to westward propagating sea surface height anomalies in the Philippine Sea.

Approximately 60% of the total subinertial velocity and temperature variance in the Kuroshio east of Taiwan is associated with so-called “transport” and “meandering” modes revealed from empirical orthogonal function analysis. The transport mode is dominated by a 100-day peak, while the most coherent energetic meandering signals are found in three limited frequency bands centered near periods of 100 days, 40 days, and 18 days. The detailed structure of the meanders is studied by frequency domain EOF analysis, which also reveals a higher frequency meander centered near 10 days confined to the western side of the channel.

On the 100-day timescale, the Kuroshio transport entering the East China Sea is strongly related to meandering of the Kuroshio, which in turn is caused by westward propagating anticyclonic eddies from the interior ocean. During low transport events, the Kuroshio meanders offshore and partly bypasses the East Taiwan Channel to flow northward along the eastern side of the Ryukyu islands. The interior eddy features that lead to the meandering can be identified as far east as 134°E, propagating westward to the coast of Taiwan at about 10 km day−1. The 100-day variability that is so dominant in the Kuroshio is virtually absent in the Florida Current but is strongly present east of Bahamas in the Antilles Current and deep western boundary current, presumably being blocked from entering the Straits of Florida by the Bahamas Island chain.

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Thomas N. Lee
,
William E. Johns
,
Rainer J. Zantopp
, and
Eve R. Fillenbaum

Abstract

A 5.8-year time series of moored current meter observations is used with hydrographic section data, CME model results, and gridded wind fields over the North Atlantic to describe the mean structure and variability of circulation and volume transports east of Abaco, Bahamas, at 26.5°N. A mean Antilles Current, with 5 Sv of northward transport, is confined against the Bahamas boundary in the upper 800 m and combines with approximately 19 Sv of Florida Current transport to balance the Sverdrup interior circulation, and does not contribute to interhemispheric exchange. The mean transport of the deep western boundary current (DWBC) off the Bahamas is approximately 40 Sv, of which 13 Sv compensates the upper branch of the thermohaline circulation, requiring a 27 Sv deep recirculation.

Robust annual and semiannual cycles of meridional are found in both moored observations and model results with remarkable agreement in amplitude (±13 Sv) and phase. Maximum northward transports occur in winter and summer, and minimums occur in fall and spring due to a predominantly barotropic response to remote and local seasonal wind forcing. Transport variability on timescales less than semiannual is dominated by mesoscale eddies that propagate westward into the Bahamas boundary in the thermocline at periods of 70–100 days, wave speeds of about 4 cm s−1, and wavelengths of about 335 km. These events are frequently correlated with offshore shifts of the DWBC core.

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Eve R. Fillenbaum
,
Thomas N. Lee
,
William E. Johns
, and
Rainer J. Zantopp

Abstract

Data from almost five years of current meter moorings located across the Bahamas Escarpment at 26.5°N are used to investigate meridional heat transport variability in the section and its impact on transatlantic heat flux. Estimates of heat transport derived from the moored arrays are compared to results from the Community Modeling Effort (CME) Atlantic basin model and to historical hydrographic section data. A large fraction of the entire transatlantic heat flux is observed in this western boundary region, due to the opposing warm and cold water flows associated with the Antilles Current in the thermocline and the deep western boundary current at depth. Local heat transport time series derived from the moored arrays exhibit large variability over a range of ± 2 PW relative to 0°C, on timescales of roughly 100 days. An annual cycle of local heat transport with a range of 1.4 PW is observed with a summer maximum and fall minimum, qualitatively similar to CME model results. Breakdown of the total heat transport into conventional “barotropic” (depth averaged) and “baroclinic” (transport independent) components indicates an approximately equal contribution from both components. The annual mean value of the baroclinic heat transport in the western boundary layer is 0.53 ± 0.08 PW northward, of opposite direction and more than half the magnitude of the total southward baroclinic heat transport between Africa and the Bahamas (about −0.8 PW) derived from transatlantic sections. Combination of the results from the moored arrays with Levitus climatology in the interior and historical Florida Current data yields an estimate of 1.44 ± 0.33 PW for the annual mean transatlantic heat flux at 26.5°N, approximately 0.2 PW greater than the previously accepted value of 1.2–1.3 PW at this latitude.

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Richard L. Bankert
,
Jeremy E. Solbrig
,
Thomas F. Lee
, and
Steven D. Miller

Abstract

The Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) nighttime visible channel was designed to detect earth–atmosphere features under conditions of low illumination (e.g., near the solar terminator or via moonlight reflection). However, this sensor also detects visible light emissions from various terrestrial sources (both natural and anthropogenic), including lightning-illuminated thunderstorm tops. This research presents an automated technique for objectively identifying and enhancing the bright steaks associated with lightning flashes, even in the presence of lunar illumination, derived from OLS imagery. A line-directional filter is applied to the data in order to identify lightning strike features and an associated false color imagery product enhances this information while minimizing false alarms. Comparisons of this satellite product to U.S. National Lightning Detection Network (NLDN) data in one case as well as to a lightning mapping array (LMA) in another case demonstrate general consistency to within the expected limits of detection. This algorithm is potentially useful in either finding or confirming electrically active storms anywhere on the globe, particularly those occurring in remote areas where surface-based observations are not available. Additionally, the OLS nighttime visible sensor provides heritage data for examining the potential usefulness of the Visible-Infrared Imager-Radiometer Suite (VIIRS) Day/Night Band (DNB) on future satellites including the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Preparatory Project (NPP). The VIIRS DNB will offer several improvements to the legacy OLS nighttime visible channel, including full calibration and collocation with 21 narrowband spectral channels.

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Jonathan B. Butcher
,
Mark Fernandez
,
Thomas E. Johnson
,
Afshin Shabani
, and
Sylvia S. Lee

Abstract

Cyanobacteria blooms are an increasing concern in U.S. freshwaters. Such blooms can produce nuisance conditions, deplete oxygen, and alter the food chain, and in some cases they may produce potent toxins, although many factors may modulate the relationships between biomass and toxin production. Cyanobacterial blooms are in turn associated with nutrient enrichment and warm water temperatures. Climate change is expected to increase water temperatures and, in many areas, surface runoff that can transport nutrient loads to lakes. While some progress has been made in short-term prediction of cyanobacterial bloom and toxin risk, the long-term projections of which lakes will become more vulnerable to such events as a result of climate change is less clear because of the complex interaction of multiple factors that affect bloom probability. We address this question by reviewing the literature to identify risk factors that increase lake vulnerability to cyanobacterial blooms and evaluating how climate change may alter these factors across the sample of conterminous U.S. lakes contained in the 2007 National Lakes Assessment. Results provide a national-scale assessment of where and in which types of lakes climate change will likely increase the overall risk of cyanobacterial blooms, rather than finer-scale prediction of expected cyanobacterial and toxin levels in individual lakes. This information can be used to guide climate change adaptation planning, including monitoring and management efforts to minimize the effects of increased cyanobacterial prevalence.

Significance Statement

Cyanobacteria blooms and associated algal toxins are an increasing problem in U.S. freshwater lakes and reservoirs. Climate change may further increase bloom frequency and severity. We survey the literature on relationships between bloom formation and climate. These relationships are combined with projections of future climate and lake response to develop indices of where and in what types of lakes such blooms are most likely to increase relative to current conditions. The results can help to focus monitoring and management measures to mitigate potential impacts on human health, wildlife, and aquatic biota.

Open access
Craig M. Lee
,
Thomas B. Sanford
,
Eric Kunze
,
Jonathan D. Nash
,
Mark A. Merrifield
, and
Peter E. Holloway

Abstract

Full-depth velocity and density profiles taken along the 3000-m isobath characterize the semidiurnal internal tide and bottom-intensified turbulence along the Hawaiian Ridge. Observations reveal baroclinic energy fluxes of 21 ± 5 kW m−1 radiating from French Frigate Shoals, 17 ± 2.5 kW m−1 from Kauai Channel west of Oahu, and 13 ± 3.5 kW m−1 from west of Nihoa Island. Weaker fluxes of 1–4 ± 2 kW m−1 radiate from the region near Necker Island and east of Nihoa Island. Observed off-ridge energy fluxes generally agree to within a factor of 2 with those produced by a tidally forced numerical model. Average turbulent diapycnal diffusivity K is (0.5–1) × 10−4 m2 s–1 above 2000 m, increasing exponentially to 20 × 10−4 m2 s–1 near the bottom. Microstructure values agree well with those inferred from a finescale internal wave-based parameterization. A linear relationship between the vertically integrated energy flux and vertically integrated turbulent dissipation rate implies that dissipative length scales for the radiating internal tide exceed 1000 km.

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Jeffrey D. Hawkins
,
Jeremy E. Solbrig
,
Steven D. Miller
,
Melinda Surratt
,
Thomas F. Lee
,
Richard L. Bankert
, and
Kim Richardson

Abstract

Global monitoring of tropical cyclones (TC) is enhanced by the unique capabilities provided by the day–night band (DNB), a sensor included on the Visible Infrared Imaging Radiometer Suite (VIIRS) flying on board the Suomi National Polar-Orbiting Partnership (SNPP) satellite. The DNB, a low-light visible–near-infrared-band passive radiometer, can leverage unconventional (i.e., nonsolar) sources of visible light illumination such as moonlight to infer storm structure at night. The DNB provides an unprecedented capability to resolve moonlit clouds at high resolution, offering numerous potential benefits to both operational TC analysts and researchers developing new methods of monitoring TCs occurring within the largely data-void tropical oceanic basins. DNB digital data provide significant enhancements over older nighttime visible data from the Defense Meteorological Satellite Program’s (DMSP) Operational Linescan System (OLS) by leveraging accurate calibration, high sensitivity, and sub-kilometer-scale imagery that covers 2–3 times the moon’s lunar cycle than the OLS. By leveraging these attributes, DNB data can enable the use of automated objective applications instead of subjective image interpretation. Here, the authors detail ways in which critical information about TC structure, location, intensity changes, shear environment, lightning, and other characteristics can be extracted when the DNB data are used in isolation or in a multichannel approach with coincident infrared (IR) channels.

Open access
Thomas E. Lee
,
Steven D. Miller
,
F. Joseph Turk
,
Carl Schueler
,
Richard Julian
,
Steve Deyo
,
Patrick Dills
, and
Sherwood Wang

The National Polar-orbiting Operational Environmental Satellite System (NPOESS) will feature the Visible-Infrared Imager-Radiometer Suite (VIIRS), a 22-channel imager that will contribute to nearly half of the NPOESS environmental data records. Included on VIIRS will be the Day/Night band (DNB), a visible channel designed to image the Earth and its atmosphere in all conditions ranging from bright solar illumination, to nocturnal lunar illumination, and negligible external illumination. Drawing heritage from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) instruments orbiting since the late 1960s, the DNB will be used to detect clouds at night, understand patterns of urban development based on the emissions of cities, monitor fires, and image scenes of snow and ice at the surface of the Earth. Thanks to significant engineering improvements, the DNB will produce superior capabilities to the OLS for a number of new applications.

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NPOESS

Next-Generation Operational Global Earth Observations

Thomas F. Lee
,
Craig S. Nelson
,
Patrick Dills
,
Lars Peter Riishojgaard
,
Andy Jones
,
Li Li
,
Steven Miller
,
Lawrence E. Flynn
,
Gary Jedlovec
,
William McCarty
,
Carl Hoffman
, and
Gary McWilliams

The United States is merging its two polar-orbiting operational environmental satellite programs operated by the Department of Commerce and the Department of Defense into a single system, which is called the National Polar-orbiting Operational Environmental Satellite System (NPOESS). During the next decade, NPOESS will provide global operational data to meet many of the needs of weather forecasters, climate researchers, and global decision makers for remotely sensed Earth science data and global environmental monitoring. The NPOESS Preparatory Project (NPP) will be launched in 2011 as a precursor to NPOESS to reduce final development risks for NPOESS and to provide continuity of global imaging and atmospheric sounding data from the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) missions. Beginning in 2014, NPOESS spacecraft will be launched into an afternoon orbit and in 2016 into an early-morning orbit to provide significantly improved operational capabilities and benefits to satisfy critical civil and national security requirements for space-based, remotely sensed environmental data. The European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Meteorological Operation (MetOp) spacecraft will complement NPOESS in a midmorning orbit. The joint constellation will provide global coverage with a data refresh rate of approximately four hours. NPOESS will observe more phenomena simultaneously from space and deliver a data volume significantly greater than its operational predecessors with substantially improved data delivery to users. Higher-resolution (spatial and spectral) and more accurate imaging and atmospheric sounding data will enable improvements in short- to medium-range weather forecasts. Multispectral and hyperspectral instruments on NPOESS will provide global imagery and sounding products useful to the forecaster that are complementary to those available from geostationary satellites. NPOESS will support the operational needs of meteorological, oceanographic, environmental, climatic, and space environmental remote sensing programs and provide continuity of data for climate researchers. This article that describes NPOESS was completed and accepted for publication prior to the White House decision in February 2010 ordering a major restructuring of the NPOESS program. The Department of Commerce will now assume primary responsibility for the afternoon polar-orbiting operational environmental satellite orbit and the Department of Defense will take primary responsibility for the early morning orbit. However, NPP, as described in this article, is still scheduled to be launched in 2011. Several of the instruments and program elements described in this article are also likely to be carried forward into future U.S. polar-orbiting operational environmental satellite missions.

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