RODRIGO H. NUÑEZ
The Florida State University
Suite 200 R. M. Johnson Building
East Paul Dirac Drive
Tallahassee, FL 32306 - 2840, U.S.A.
Since 1991 the Hydrographic and Oceanographic Service of the Chilean Navy (SHOA) has run numerous cruises in the WOCE PR-14 and SR-01 hydrographic regions ( Figure 1). These cruises include 6 to date in the PR-14 region and two in the SR-01 region. Another SR-01 cruise is planned for November 1995. The vessels used include the AGS Yelcho (decommissioned in 1992) and the AGOR 60 Vidal Gormaz (former AGOR 10 Thomas Washington - ONR/SCRIPPS; Figure 2).
Hydrographically these regions are of great interest. The PR-14 region covers the bifurcation point where the West Wind Drift of the Southeast Pacific divides upon impinging the coast of the South American continent. More details on the hydrography of the PR-14 region and some preliminary cruise results can be found in Rojas et al. (1995). The SR-01 region spans the Drake Passage to the west of the seamount topographic features within the Passage. The section extends into the Bransfield Strait, where evidence of deep convection has been found. This section will effectively complement and extend the time series measurements made during the ISOS program (1975-1981).
The focus of this article, however, is to introduce the meteorological data (often overlooked on some WOCE cruises) collected on the PR-14 and SR-01 cruises. The PR-14 and SR-01 study areas are located in regions of great meteorological variability. The PR-14 region lies in the heart of the 'Roaring Forties' where cyclones are frequent. Streten and Troup (1973) showed that the major cyclone tracks for the austral spring and early summer pass through or just south of the PR-14 region. The austral summer polar front, separating cold antarctic air masses from warmer northern air masses, is located on the southern end of the PR-14 region (Taljaard 1968). The majority of the PR-14 cruises conducted by Chile occurred during the austral spring and early summer, offering a unique opportunity to better understand the interaction of the sea surface gradients along the Antarctic Circumpolar Current (West Wind Drift) and these atmospheric features.
The SR-01 region covers the Drake Passage, known since the its first exploration by Sir Francis Drake as a region of severe storms. In 1993, Chile began WOCE cruises across this rough passage from Cape Horn to the Bransfield Strait. Four cruises are planned with the AGOR Vidal Gormaz. The first three in November 1993, 1994, and 1995 (relatively good weather conditions!) and one in July 1996 (Winter conditions). At the ocean-atmosphere interface, sea ice plays a key role in ocean fluxes and circulations. Carleton (1988) showed the existence of interannual signals in the sea ice concentration of the Weddell Sea to the east of the Drake Passage and related these variations to the Southern Oscillation of the tropical Pacific. Simmonds and Jacka (1995) also showed the sea ice extent in the Weddell Sea region to be highly variable on interannual time scales. With three SR-01 cruises collecting meteorological data in November of different years, some additional information regarding the interannual variations of sea ice and ocean fluxes in the Drake Passage may be realized.
The meteorological data collected on the Chilean PR-14 and SR-01 cruises was recorded every 6 hours as part of the Voluntary Observing Ship XBT Program. An incomplete set of these meteorological data are transmitted, within a three-hour window of the observation, via satellite to NOAA (note that the XBT data is forwarded by NOAA to the XBT DAC). The complete meteorological observations were manually recorded by the bridge crew and oceanographers, and contain information on wind direction and speed, air temperature, sea surface temperature, humidity, atmospheric pressure, weather occurring at the observation time, and cloud cover, height, and type. Also included are the ship's position, heading , and speed along with the characteristics of the sea (wave height, period, etc.). Table 1 lists the instruments used and the instrument heights for both ships. Once all the data for a particular PR-14 or SR-01 cruise are collected, the meteorological data are sent to the WOCE Data Assembly Center/Special Analysis Center (DAC/SAC) for Surface Meteorology associated with the Center for Ocean Atmosphere Prediction Studies (COAPS) at the Florida State University in Tallahassee, FL, USA.
The WOCE DAC is charged with collecting, quality controlling, archiving, and distributing all underway surface meteorological data from WOCE vessels worldwide. The overall goal of the DAC's quality control (QC) is to provide a well documented, reliable data set to the scientific community. The DAC combines the meteorological data with information provided about how the data were collected, i.e. metadata, to create one easily portable data set. Ideally the metadata contains information on the instrument configuration, calibrations, and formats for the meteorological data. When completed, the entire DAC archive of WOCE surface meteorological data will better define forcing fields for ocean models and climatologies over the oceans.
When the PR-14 and SR-01 data arrived, the DAC verified the accuracy of the measurements using a series of data quality evaluation procedures. The DAC QC procedure is a four step process fully detailed in a technical report by Smith et al. (1995). After the arrival of a WOCE meteorological data set and accompanying metadata, the data is converted to a standard format. Once converted, the data is run through an automated preprocessing program that checks for values outside a realistic range, statistical outliers, unrealistic ship movements, etc. During the third step of the QC process, the preprocessed meteorological data are visually inspected by a trained Data Quality Evaluator (DQE) using the Visual Data Assessment Tool (VIDAT). VIDAT is an interactive, graphically based, data evaluation and browser tool, developed at the FSU-DAC, that allows the DQE to add or remove QC flags from the meteorological data. When the meteorological data has passed all final inspections, the data are converted to a standard format suitable for public distribution and a QC report is written. The resulting quality controlled data and report are then made available for the WOCE community via the world wide web (coaps.fsu.edu/woce/) and anonymous ftp (wocemet.fsu.edu). Data are also available via disk, tape, and hard copy upon request.
Once meteorological data have completed quality control, the WOCE SAC at COAPS plans to utilize the data in scientific analyses. Currently, only a portion Chilean WOCE meteorological data are transmitted in real time to the Global Telecommunications System and the use of these transmitted data in the global operational weather analyses is unlikely. As a result, the SAC plans to compare the complete meteorological data from the Chilean, and other, WOCE cruises to the global surface analysis fields from numerical weather prediction centers, e.g. European Centre for Medium Range Weather Forecasts or National Center for Environmental Prediction. The differences between the analyses and the observed conditions will likely be large, especially over the southern oceans. Other SAC activities will include providing fields of surface meteorological variables and fluxes based on in- situ and remotely sensed data.
Carleton, A.M. 1988. Sea ice-atmosphere signal of the Southern Oscillation in the Weddell Sea, Antarctica. Journal of Climate, 1 , 379-388.
Rojas, R., N. Silva, W. Garcia, and Y. Guerrero. 1995. WHP repeated hydrography section PR14, offshore southern Chile. International WOCE Newsletter, 20, 31-33.
Simmonds, I. and T.H. Jacka. 1995. Relationships between the interannual variability of Antarctic sea ice and the Southern Oscillation. Journal of Climate, 8, 637-647.
Streten, N.A., and A.J. Troup. 1973. A synoptic climatology of satellite observed cloud vortices over the Southern Hemisphere. Quarterly Journal of the Royal Meteorological Society, 99 ,56-72.
Smith, S.R., C. Harvey, and D.M. Legler. 1995. Handbook of quality control procedures and methods. Technical report WOCEMET 95-5, COAPS, 020 Love Bldg., Florida State University, Tallahassee, FL 32306, USA, in press.
Taljaard, J.J. 1968. Climatic frontal zones of the Southern Hemisphere. NOTOS , 17, 23-34.