Ship Observations Research Articles

A Comparison of Sensible and Latent Heat Flux Estimates for the North Atlantic Ocean

Abstract Previously published estimates of the surface turbulent fluxes over the North Atlantic Ocean have been compared by applying the calculation methods used by each author to a common dataset of ship observations. The major differences between the various flux estimations were due to whether or not the data were corrected for observation height, the method of calculating averaged fluxes and the choice of transfer coefficient. By correcting the ship observations obtained from the Voluntary observing ship Special Observing Programme–North Atlantic for known observational biases an estimate of the correct values for the fluxes has been made. The previous studies were found to have overestimated the North Atlantic sea to air heat transfer by up to 30%. All of the schemes overestimated the annual cycle, giving fluxes approximately equal to the best estimate values in the summer but much higher fluxes in the winter. The use of transfer coefficient values much larger than the values determined by air-sea in...

Wind Forcing over the Southwest Atlantic: Comparison between Observations and ECMWF Analyses

Abstract Oceanic variability in the southwest Atlantic is dominated by temporal scales at the annual, semiannual, and few-month periods. It is now examined if the timescales present in the local wind forcing (wind vector and wind stress curl) match the timescales in oceanic variability. Wind observations from land stations in South America and from ship observations in the Argentine Basin are examined to assess the validity and limitations of the European Centre for Medium-Range Weather Forecasts (ECMWF) wind analyses in the South Atlantic. The comparison is carried out over the five-year period 1986–90. The amount of observations, their statistics, and the temporal scales are examined. The major discrepancy the authors find is due to land–sea effects that are not resolved in the ECMWF model. Furthermore, the agreement in amplitude and phase of the two dominant signals, the annual and semiannual signals, is remarkable. The temporal scales present in the wind stress curl derived from ECMWF analyses are the...

Water Vapor Corrections for Radar Altimeter and Its Variability over the Indian Ocean

Abstract A new regional relationship is derived for wet tropospheric range correction h for radar altimeter and precipitable water (W) over the Indian Ocean using ship observations for the period 1982–91. The W varied over the range of 20–80 mm, thus providing total variability expected over tropical oceans. A fifth-order polynomial between h and W gave an rms error of 2.3 mm when compared with h computed using direct relation. Model results have also been compared with an earlier relation over the Indian Ocean and with the global relation, and it has been found that the present model yields the lowest rms error in h values over the Indian Ocean. Comparisons with earlier models show a factor of 2 improvement in the accuracy of the correction over this region. The W values have also been derived using the NOAA High-Resolution Infrared Sounder data for the years 1980, 1981, and 1984. These monthly mean W (3-yr averaged) values have been used to study the variabilities in W. The Indian Ocean depicts large va...

Comparison between Global Latent Heat Flux Computed from Multisensor (SSM/I and AVHRR) and from In Situ Data

The accurate estimate of the latent heat flux (LHF) is important to understand better the coupling between the atmosphere and the ocean and their respective circulation. In the near future, the availability of satellite-derived datasets over long periods will allow us to perform studies that, so far, have only been possible with historic in situ datasets. Therefore, a natural issue to explore is how the computation derived from both data types agree on LHF estimates, Comprehensive Ocean-Atmosphere Data Set (COADS) on one hand and satellite-derived parameters on the other hand are input to a similarity theory-based model and treated in completely equivalent ways to compute global latent heat flux. In order to compute latent heat flux exclusively from satellite measurements, an empirical relationship (Q−W relationship) is used to compute the air mixing ratio from Special Sensor Microwave/Imager precipitable water W and a new one is derived to compute the air temperature also from retrieved W (T−W relationship). First analyses indicate that in situ and satellite LHF computations compare within 40%, but systematic errors increase the differences up to 100% in some regions. By investigating more closely the origin of the discrepancies, the spatial sampling of ship reports has been found to be an important source of error in the observed differences. When the number of in situ data records increases (more than 20 per month), the agreement is about 50 W m−2 rms (40 W m−2 rms for multiyear averages). Limitations of both empirical relationships and W retrieval errors strongly affect the LHF computation. Systematic LHF overestimation occurs in strong subsidence regions and LHF underestimation occurs within surface convergence zones and over oceanic upwelling areas. The analysis of time series of the different parameters in these regions confirms that systematic LHF discrepancies are negatively correlated with the differences between COADS and satellite-derived values of the air mixing ratio and air temperature. To reduce the systematic differences in satellite-derived LHF, a preliminary ship-satellite blending procedure has been developed for the air mixing ratio and air temperature. The T−W relationship is not used any more and the air temperature is computed by adding the 3-yr-averaged COADS air-sea temperature difference to the satellite SST maps. The method to get the air mixing ratio is based on a weighted combination of COADS and satellite values according to the number of COADS observations available. After the blending proem is applied, large improvements are observed in the Northern Hemisphere where both datasets are complementary. At midlatitudes, the blending procedure does not modify LHF values since satellite and COADS air mixing ratio do not differ by more than the expected satellite uncertainty (1 g kg−1). In the eastern and northern part of the basin, where the air mixing ratio difference is large and ship observations are numerous, blended LHF values are efficiently corrected toward in situ estimates compensating for the limitations of Q−W relationship. In the Southern Hemisphere, the number of in situ observations rarely exceeds four values per month, and without “ground truth,” more confidence is given to the satellite-derived values. Statistically, the rms difference drops to 28 W M−2 when 20 ship observations are available, which is likely a good approximation of the lowest bound of the blended LHF uncertainty compared with optimal in situ estimates. However, along the eastern boundaries in the southern oceans, local differences are expected to be much larger.

Warm Pool Heat Budget and Shortwave Cloud Forcing: A Missing Physics?

Ship observations and ocean models indicate that heat export from the mixed layer of the western Pacific warm pool is small (<20 watts per square meter). This value was used to deduce the effect of clouds on the net solar radiation at the sea surface. The inferred magnitude of this shortwave cloud forcing was large ( approximately - 100 watts per square meter) and exceeded its observed value at the top of the atmosphere by a factor of about 1.5. This result implies that clouds (at least over the warm pool) reduce net solar radiation at the sea surface not only by reflecting a significant amount back to space, but also by trapping a large amount in the cloudy atmosphere, an inference that is at variance with most model results. The excess cloud absorption, if confirmed, has many climatic implications, including a significant reduction in the required tropics to extrattropics heat transport in the oceans.

Sea Surface Winds—A Critical Input to Oceanic Models, but Are They Accurately Measured?

Wind, driving oceans, and the links between them to the atmosphere compose a critical parameter for the world circulation model as well as for the evaluation of climate changes. Traditionally, wind velocities have been reported by ships of oppurtunity and recorded on a network of buoys; they have also recently been generated by numerical weather prediction models and mapped with spaceborne remote sensors. Wind speeds from buoy measurements, ship observations, and model computations are compared, using the globally available altimeter returns that they have in common. Large, systematic deviations are found among the results obtained with these techniques, cautioning against use of these wind speeds.

The Volunteer Observing Ship and Future Ocean Monitoring

Widespread and sustained in situ ocean measurements are essential to an improved understanding of the state of the ocean and its role in global change. Merchant marine vessels can play a major role in ocean monitoring, yet apart from routine weather observations and upper-ocean temperature measurements, they constitute a vastly underutilized resource due to lack of suitable instrumentation. Examples of ways in which vessels can assist include profiling techniques of physical properties, chemical sampling via automated water samplers, optical techniques to measure various biological parameters, and ground truth measurements for remote sensing from orbiting and geostationary satellites. Further, ships can act as relays between subsurface instrumentation and satellite communication services. To take advantage of the opportunities that the maritime industry can provide, two steps must be taken. The first is to initiate an instrumentation development program with emphasis on techniques optimized for highly automated use onboard ships at 15-20-kt speeds. The second is to forge partnerships or links between academic and government laboratories and the maritime industry for the institution and maintenance of such monitoring programs. No doubt significant resources will be required, but in the long run the improved ability to monitor the state of ocean in situ will make the effort more than worthwhile.

Noaa tovs‐derived moisture fields over the arabian sea and the bay of bengal and their association with the south‐west monsoon

AbstractMoisture fields derived from the Tiros‐N Operational Vertical Sounder (TOVS) data onboard previous National Oceanic and Atmospheric Administration (NOAA) series of satellites have been used to study monthly moisture fields over the Arabian Sea and the Bay of Bengal for the years 1979–1981. It is observed that monthly averaged moisture fields over the Bay of Bengal are always more than those over the Arabian Sea at a given latitudinal belt. Further, the monthly mean moisture flux divergence has been calculated using monthly mean winds from ship observations over the same areas of the Arabian Sea and the Bay of Bengal. It is found that net moisture flux divergence over the Arabian Sea is consistently higher than that over the Bay of Bengal for the years, 1979, 1980, and 1981, which are deficient, and excess monsoon rainfall years in terms of departure from long‐term climatological rainfall values. Moisture variability over the Arabian Sea and the Bay of Bengal on a monthly scale has also been studied for the months of June, July, and August for the years 1979–1981.A plot of the zonally averaged (at every 2° latitude) differential moisture signal (DMS) by latitude indicates the approximate position of the low‐level westerly jet. The overall differential signal between zonally averaged moisture fields over the Arabian Sea and the Bay of Bengal has also been found to be an indicator of total Indian rainfall excluding orographically influenced regions, although more data is needed to firmly establish the relationship quantitatively.

Features of the hydrological structure and circulation in the north-western Black Sea: shipboard and satellite observations in 1989–1990

The results of shipboard and satellite observations of the synoptic variability in the region of the continental slope south of the Tarkhankut Cape are considered. Ship observations revealed a two-layer circulation system of currents. An anticyclonic meander is localized within the upper mixed layer. A cyclonic disturbance, related to the main Black Sea current, occupies the water column below the seasonal thermocline. The interaction between the thermohaline fields is presumably induced by the anticyclonic atmospheric circulation. The satellite-derived images revealed a complex pattern of the large-scale dynamics of seawater over the entire north-western Black Sea.

Correction of Marine Air Temperature Observations for Solar Radiation Effects

The effect of incoming solar radiation on merchant ships' observations of air temperature was assessed as part of the Voluntary Observing Ships' Special Observing Project for the North Atlantic (VSOP-NA), The ships' reports were compared with interpolated output from a numerical weather model. Differences between the ship values and the model values for air temperature (ΔTa) were found, in the mean, to be independent of instrument type, ship size, and, except for very badly exposed sensors, exposure. The differences were related to the relative wind speed over the ship (V) and the incoming shortwave radiation (R). The formula derived for the radiative heating error δT was δT = 2.7 × 10−3 R − 3.2 × 10−5 RV, where δt has units of degrees Celsius, R is in watts per square meter, and V is in knots. After correcting the ΔTa values, an approximately constant bias remained with the ship reports on average 0.4°C lower than the model air temperatures. This offset probably represents a mean bias in the model estimates; however, a residual bias in the ship observations is also a possibility. There was also evidence that heat generated by the ship caused a temperature overestimate of about 0.4°C at zero relative wind, decreasing to a negligible level at a relative wind speed of 20 kt. For the North Atlantic dataset used, the correction reduced daytime marine air temperature reports by 0.63°C on average. Applying the correction to the VSOP-NA air temperature data was found to significantly change estimates of sensible and latent heat fluxes.

Atmospheric‐hydrospheric mechanisms of climate anomalies in the western equatorial Indian Ocean

Atmosphere‐ocean mechanisms of rainfall anomalies at the coast of eastern Africa are studied using long‐term ship observations in the Indian Ocean, surface current measurements, subsurface casts, upper air analyses by the European Centre for Medium Range Weather Forecasts, rain gauge series in eastern Africa and India, and an index of the Southern Oscillation (SO), the high‐SO phase being defined by anomalously high/low pressure at Tahiti/Darwin. The causalities of precipitation anomalies at the coast of eastern Africa differ for the two rainy seasons centered on April‐May and October‐November, and only the latter is strongly related to the SO. In the high‐SO phase, April‐May pressure is low over the entire Indian Ocean domain, whereas in October‐November, pressure is high in the west and low in the east. Concomitantly, surface waters are anomalously cold in the west, and strong westerlies sweep the equatorial zone of the Indian Ocean. Eastern African rainfall anomalies are related to the SO through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern Africa. (1) Equatorial westerly winds are conducive to lower tropospheric divergence over equatorial East Africa, and in the high‐SO phase these are accelerated, especially in October‐November, owing to the anomalous eastward pressure gradient. (2) The equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere, which entails cold‐water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds. (3) In addition, cold‐water anomalies in the western Indian Ocean, most pronounced in October‐November during the high‐SO phase, also suppress convection. (4) In the high‐SO phase, the Indian summer monsoon tends to be strong, leaving behind an anomalously cold western Indian Ocean, which in turn feeds into mechanisms 1 to 3. The eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern Africa.

Amphetamine and drugged driving

The quantitative assessment of the influence of rough weather avoidance on the wave climate ships experience in their oceanic routes is presented. The only meteorological source that naturally includes this effect is the database derived from the Voluntary Observing Ships’ programme, which is here compared with the state-of-the-art ERA-interim numerical re-analysis. The results of this work allow quantifying the reduction of the mean significant wave height encountered by ships in their operations for a more aware planning of the missions and better assessment of the environmental conditions.

Circulation anomaly mechanisms in the tropical Atlantic sector during the northeast Brazil rainy season: Results from the GISS general circulation model

The role of the sea surface temperature (SST) pattern in the tropical Atlantic on the overlying atmosphere is studied from a 7‐year run of the general circulation model (GCM) of the Goddard Institute for Space Studies (GISS) (period 1980–1986) and in comparison to upper air analyses of the European Centre for Medium‐Range Weather Forecasts (ECMWF), surface ship observations of the SST and wind fields (COADS), and rainfall measurements in Northeast Brazil (Nordeste). Observations for March to April show that reduced northward SST decrease in the tropical Atlantic is accompanied by reduced northward decrease of lower‐tropospheric thickness, which accounts for an inverse change in the meridional pressure gradient and reduced northerly wind component; this in turn entails a northward displaced Intertropical Convergence Zone and drought in the Nordeste. The GCM simulation reproduces the thermal and hydrostatic forcing of SST on the meridional gradients of lower‐tropospheric thickness and pressure in the proper sense, albeit more weakly than observed. The modeled meridional wind component follows the meridional pressure gradient in the proper sense, but mean and range are smaller than observed, and the modeled interannual variability is not altogether consistent with observational evidence. The modeled March to April precipitation in the Nordeste shows little relation to the observed rainfall, although it is weakly related with the observed interhemispheric SST contrast and with the modeled near‐surface meridional wind component. Both modeled and observed March to April Nordeste rainfall are negatively related with SST anomalies in the equatorial Pacific in March to April as well as in the immediately preceding January. Both in January and in March to April the modeled soil moisture exhibits a strong dependence on the modeled precipitation, but neither resembles in interannual variability nor in absolute amounts of the observed rainfall. Overall the GISS GCM seems to reproduce reasonably well the response of lower‐tropospheric thickness and near‐surface pressure to SST, while the simulation of wind and rainfall is somewhat deficient.

The Accuracy of Voluntary Observing Ships' Meteorological Observations-Results of the VSOP-NA

For the Voluntary Observing Ships Special Observing Project for the North Atlantic (VSOP-NA), the layout, meteorological instrumentation, and observing practices of 45 voluntary observing ships (VOS) operating in the North Atlantic were cataloged. Over a two-year period these ships provided extra information with each observation, and the effect of different observing practices has been quantified by using analysis fields from an atmospheric forecast model as a comparison standard. Biases of order several tenths of a degree Celsius were detected in sea surface temperature data from engine intake thermometers, in dewpoint temperatures from screens (and to a lesser extent, psychrometers), and in air temperatures due to solar heating. Wind speeds from anemometers were high compared to visual winds by about 2 kt for winds up to about 25 kt. The VSOP-NA data do not, however, indicate which is the more accurate. Correction for anemometer height and use of the WMO Commission for Marine Meteorology version of the Beaufort scale reduced this difference significantly. The result of these corrections on mean heat flux estimates was only a few watts per square meter but much greater changes resulted for particular areas and seasons. The project identified observing methods that are to be preferred for future use on the VOS, and demonstrated that the combined use of VOS data and a forecasting model allowed the detection of biases both in the observations and in the model analyses.

Estimates of wave height data for New Zealand waters by numerical modelling

Abstract A procedure has been developed for estimating the wave climate in New Zealand waters. The key elements are the specification of surface wind fields and the diagnosis of wave conditions using a numerical wave model. Wave data have been generated for a period in 1989 and the deduced significant wave heights have been analysed. The distributions at a number of selected sites indicate wave features consistent with existing knowledge of the longer‐term wave climate—despite differences between wind characteristics for this season and the longer‐term norm as derived from ship observations. The wave‐fields were partitioned into wind‐sea and swell components and the probability distributions for each feature were also found to be consistent with longer‐term studies.

The monsoonal heat budget of the hydrosphere‐atmosphere system in the Indian Ocean sector

The heat budget of the combined hydrosphere‐atmosphere system in the Indian Ocean sector (30°N to 30°S, 30°–120°E) is studied on the basis of surface ship observations and subsurface temperature casts to 400 dbar, analyses of the atmospheric heat and moisture transport in the layer surface to 100 mbar, published estimates of precipitation, and satellite measurements of net all‐wave radiation at the top of the atmosphere. During boreal winter (November‐April) the hydrosphere of the southern tropical Indian Ocean stores (+2 × 1014 W) and exports (7 × 1014 W) heat, of which about half (3 × 1014 W) is carried northward across the equator; of similar magnitude (9 × 1014 W) is the heat export in the atmosphere accomplished primarily in the upper tropospheric westerlies. In the northern portion of the basin the hydrosphere stores heat (+8 × 1014 W), in part supplied by the hydrospheric heat import from the southern hemisphere; the atmospheric column exports heat (4 × 1014 W), primarily owing to a southward moisture transport. In boreal summer (May‐October) the water body of the northern Indian Ocean cools drastically (−8 × 1014 W), with the net heat gain through the ocean surface being overcompensated by the larger (12 × 1014 W) southward heat transport across the equator, which in turn results in a large heat import (8 × 1014 W) to the hydrosphere of the southern Indian Ocean. This and the depletion of the oceanic heat content (−2 × 1014 W) supply about a third of the energy required for the vigorous evaporation from the southern tropical Indian Ocean (34 × 1014 W), which is further favored by the strong southeast trade winds peaking at this season of the year. The evaporation sustains the large latent heat export from the overlying atmospheric column (+14 × 1014 W). The latent heat surplus from the southern portion of the basin is primarily carried northward across the equator by the lower atmospheric boreal summer monsoon airflow, and in the June‐August core of the season this latent heat flux becomes as large as the southward cross‐equatorial heat transport in the ocean (13 × 1014 W). Associated with the summer monsoon airflow, latent heat import takes place in the northern portion of the basin (−2 × 1014 W) and southern Asia (−6 × 1014 W). In context, hydrosphere and atmosphere cooperate in the energetics of the Indian summer monsoon. The water body to the north of the equator cools (despite the summer season) and exports heat to the southern Indian Ocean, where this energy helps sustain the large evaporation associated with the strong winds, which further carry the moisture northward to supply the abundant monsoon rainfall and latent heat release that fuels the atmospheric portion of the monsoon machine. It is during boreal winter that the northern Indian Ocean water body replenishes its heat content, in preparation for the next summer monsoon.

Circulation mechanisms related to northeast Brazil rainfall anomalies

This study explores the role of the upper ocean for circulation anomalies in the overlying atmosphere, with focus on the late boreal winter and extreme climatic events in northeast Brazil. Data sources comprise rainfall records in the Nordeste, surface ship observations of the sea surface temperature (SST), pressure, and wind fields (COADS, period 1948–1990), satellite observations of tropical convection (HRC, 1971–1988), and upper air analyses of the European Centre for Medium Range Weather Forecasts (ECMWF, 1980–1991), all for the tropical Atlantic sector. Interhemispheric SST gradients are most strongly associated with north‐south contrasts in pressure and the meridional wind component, whereas the inverse SST‐pressure relations in situ are less close. Enhanced northward temperature increase in the tropical Atlantic is accompanied by steeper meridional pressure gradient and accelerated southerly wind component, which is representative of a northward displaced Intertropical Convergence Zone (ITCZ), with the latter in turn leading to drought in northeast Brazil. Interhemispheric contrasts in the 850/1000 mbar layer mean temperature are similar to those of SST and account for most of the meridional surface pressure gradient. During wet as compared to dry years in northeast Brazil, the tropical North Atlantic is cooler and the South Atlantic warmer, and accordingly the lower tropospheric thickness is reduced to the north but inflated to the south of the equator, resulting in increased/reduced surface pressure over the North/South Atlantic, as well as reduced southerly surface wind, stronger subsidence over the outer tropics of the northern hemisphere and intensified ascending motion and convective activity over the Nordeste, related to a southward displaced near‐equatorial convection belt. The surface and upper air evidence thus indicates the ways in which the interhemispheric SST gradients exert a hydrostatic control on the lower tropospheric thickness pattern, and thus force the south‐north surface pressure gradients and the surface meridional wind component, and hence modulate the latitude position of the ITCZ and Nordeste rainfall.

Observed high temperatures in the sunglint area over the North Indian Ocean

Abstract Sea surface temperatures (SSTs) have been derived operationally at the National Remote Sensing Agency using NOAA AVHRR data since early 1991. The channel 4 temperature variations in the sunglint area and its surroundings are presented. The SSTs derived using satellite data with those of ship observations are compared in the sunglint area. The errors in sea surface temperature estimations using satellite data in an area of sunglint are described.

Interannual variability of surface climate in the Amazon basin

AbstractThis empirical study examines the circulation mechanisms of anomalously wet and dry years in the Amazon basin. Data sources include surface ship observations in the tropical Atlantic, eastern Pacific, and Indian Oceans, as well as various hydrometeorological indices in Amazonia during 1951–1983. Rainfall and river water level anomalies in northern Amazonia are associated with distinct circulation patterns in the tropical Atlantic, particularly at the March–April peak of the rainy season. Thus, abundant rainfall in northern Amazonia and high Rio Negro water levels are accompanied by a strong North Atlantic high, steep meridional pressure gradient on its equatorward side, accelerated North‐east trades, and cool surface waters in the tropical North Atlantic. The intensified North‐east trades may entail enhanced boundary‐layer moisture transport from the tropical North Atlantic into the Amazon basin. This ensemble of atmospheric and oceanic anomalies is characteristic of, but not limited to, the high phase of the Southern Oscillation (defined by anomalously high/low pressure at Tahiti/Darwin).

An Evaluation of the WOCE Volunteer Observing Ship–XBT Network in the Atlantic

Abstract A volunteer observing ship (VOS)-expendable bathythermograph (XBT) network has been proposed for the Atlantic Ocean to satisfy World Ocean Circulation Experiment (WOCE) objectives in the upper water column. These objectives include measuring changes in upper-layer temperature. An evaluation of the proposed WOCE XBT network to resolve variability in sea surface temperature (SST), temperature distribution at 150 m (T150), and average temperature of the upper 400 m (T400L) between 25°S and 35°N is performed. A sampling design study based on an optimum interpolation (OI) of the historical XBT dataset is used to construct uncertainty distributions for various XBT networks. The OI technique requires statistical representations of the variability (in the form of structure functions) of the three variables that are derived from the historical database. The structure functions and various sampling grids are used to construct uncertainty maps. Two seasons are used in the analysis of SST. In both seasons, u...