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The PIRATA Program: An Extenstion of the TAO Array in the Atlantic
J. Servain, ORSTOM/Brest
It is well recognized that atmosphere-ocean interactions throughout the global tropics are potentially important to the earth's climate system on time scales of years to decades. Among the regions of particular interest is the tropical Atlantic where two main modes of interannual and longer-term variability are observed: (i) an "equatorial" mode, operating preferentially at seasonal and interannual time scales, has many similarities to the ENSO phenomenon in the Pacific, and involves trade wind variations and the excitation of equatorial Kelvin and Rossby waves; (ii) the so-called "dipole" mode, which operates primarily at decadal and longer time scales, involving north-south interhemispheric variations in sea surface temperature.

Unfortunately, the in-situ observational system that exists in the tropical Atlantic is relatively poorly developed. It relies mainly on volunteer observing ships and occasional research vessels that pass through the area. The generally infrequent sampling provided shipboard data limit our ability to better describe and understand climatically relevant ocean-atmosphere interactions in this region.

The Pilot Research Moored Array in the Tropical Atlantic (PIRATA) is an initiative put forward by a group of scientists from Brazil, France, and the USA involved in tropical climatic studies. The PIRATA program, which will be implemented as a collaborative multinational effort, proposes to install and maintain in the tropical Atlantic an array of 14 moored ATLAS buoys in the region 15°N 10°S, 0° 35°W during the years 1997 to 2000. In addition to the ATLAS mooring array, wind measurements and tide-gauge data will be available in real-time from St. Peter and St. Paul Rocks, Atol das Rocas, and Sao Tom Island. Brazil will also instrument 0°,44°W with a coastal meteorological buoy.
There are several specific scientific and technical goals of PIRATA:

• To provide an improved description of the seasonal-to-interannual variability in the upper ocean and at the air-sea interface in the tropical Atlantic.
• To improve our understanding of the relative contributions of the different components of the surface heat flux and ocean dynamics to the seasonal and interannual variability of SST within the tropical Atlantic basin.
• To provide a data set that can be used to develop and improve predictive models of the coupled Atlantic climate system.
• To design, deploy and operate a pilot array of moored buoys, similar to the ones used during the TOGA program (the TOGA-TAO array) in the tropical Pacific.
• To collect and transmit via satellite in real-time a set of oceanic and atmospheric data to monitor and study the upper ocean and atmosphere of the tropical Atlantic. These measurements will be available to all interested users in the research and operational communities.

The PIRATA document is available via World-Wide-Web (http://www.ifremer.fr/ird/pirata/pirataus.html) or directly via anonymous ftp (ftp.ifremer.fr/ifremer/orstom/pirata30.rtf.Z).

In addition to the observations by research vessels, JMA is making an effort to promote subsurface observations by Voluntary Observing Ships along the routes of Japan - Persian Gulf and Hong Kong - New Zealand - Japan and in the TRANSPAC region.

JMA has been providing oceanographic information services for more than fifty years. Based on in situ data from ships and buoys including TAO array and satellite data, in addition to the observations made by JMA itself, the Agency has been making a variety of oceanographic products. These include analyses and forecasts of 10-day and/or monthly mean sea surface temperature, subsurface temperature and sea surface current. Some of these products are broadcast on an operational basis through meteorological radio facsimile to the ships and JMA also publishes the Monthly Ocean Report which is distributed to interested institutes and agencies. JMA also disseminates information on the latest states of El Niño events to the domestic public.

For the purpose of monitoring the climate, especially ocean variability related to ENSO, JMA has developed an Ocean Data Assimilation System (ODAS) and started to use it operationally in February 1995. The system consists of an ocean general circulation model (OGCM) and a subsurface temperature analysis scheme using optimum interpolation. The analysed temperatures are continuously assimilated into the wind-driven OGCM. Only data that arrive via the GTS are used in the ocean temperature analysis, some TAO array data are available this way. Other TAO data, from the current meter moorings along the equator, do not come via the GTS. Because these data are not incorporated in the temperature analysis, they provide a valuable source of verification of the fields produced by the ODAS. We find that the subsurface thermal field produced by the ODAS is more closely correlated with the subsurface temperature data from TAO than with the same field derived from the wind-driven OGCM, thus the TAO data are clearly having a major impact on the JMA ocean temperature product. We find also that the intra-seasonal variation of the currents produced by the ODAS compare well with the observed sea surface current. At present, JMA is developing a new objective analysis method for the current field, using current observations obtained through traces of drifting buoys and geostrophic current data obtained through dynamic height observed by TOPEX/Poseidon altimeter. The verification of this new method also needs current data of TAO array. Aiming at operational forecasting of ENSO, JMA is developing a coupled ocean-atmosphere general circulation model. The oceanic field obtained by the ODAS is supposed to serve as the initial condition for the prediction. Impacts of TAO data on the prediction will be examined.

References:

Climate and Marine Department : Monthly Ocean Report, Japan Meteorological Agency, Tokyo 36 pp. Kimoto, M., I. Yoshikawa and M. Ishii, 1996: An Ocean Data Assimilation System for Climate Monitoring , J. Meteor. Soc. Japan, special issue ( in press).

The scientific objectives of PACS are to understand and more realistically model (1) the seasonally varying mean climate of the Americas and adjacent ocean regions; (2) the role of boundary processes in forcing seasonal-to-interannual climate variability over the Americas; (3) the coupling between the oceanic mixed layer in the tropical Atlantic and eastern Pacific; and (4) the processes that determine the structure and evolution of the tropical sea-surface temperature field. As a result of the first two proposal cycles for PACS, three studies requiring observations beyond those provided by the TAO array will be deploying into the field in 1997 and 1998:

• A study of the relative importance between ocean dynamics and surface heat fluxes in determining the evolution of cold tongue SST will deploy on the equator, near an existing ATLAS mooring, a downward looking 600 kHz ADCP to obtain, with high vertical resolution, upper ocean currents over a 6 month to 1 year period (Figure 5). Together with data from the TAO Array, this will allow detailed investigations of the nature of the near surface shear and its relationship to the local wind stress.
• The temporal and spatial variability of the air-sea fluxes and the upper ocean will be examined by collecting one-year long time series of fluxes of momentum, heat, fresh water, upper ocean temperatures, velocities, and salinities using IMET moorings deployed along 125°W (Figure 5). One mooring will be at 10°N, which places it in the ITCZ for much of the year, a region characterized by high precipitation rates and warm sea surface temperatures. The second mooring will be in the equatorial cold tongue (0° 2°N), a region of clear skies and very little precipitation except during strong ENSO episodes.
• The establishment of an enhanced atmospheric sounding and monitoring network over the eastern tropical Pacific Ocean and bordering regions. This program will begin with the establishment of a radiosonde station at Cocos Island and pilot balloon observations at selected sites and is intended to operate for a period of up to 18 months. At the end of this period, it is believed that enough data (both meteorological and operational experience) will have been collected to determine the desirability and feasibility of sustaining the network.

The PACS Implementation Plan is, and will be, maintained as an electronic version at http://www.jisao.washington.edu/ because it is meant to be a dynamic document to be updated as programs as detailed plans for studies are developed, studies implemented, and new foci developed. The period immediately after the above programs (e.g., 1998 - 2000) will include a major observational program focussed on the ITCZ/Cold Tongue complex in the extreme eastern Pacific and will utilize the in situ observations provided by the TAO Array along 95°W, supplemented by shipboard observations and aircraft (manned and, possibly, unmanned) based out of the Galapagos Islands, southern Mexico and/or Costa Rica. Included within this study is the region of extreme deep convection centered over the Panama Basin, southern Central America, and northwest South America. Plans under consideration include augmenting the existing radiosonde network in the region and establishing two sounding stations on islands in the region. This program may be further augmented with periodic dropsonde missions flown by the NOAA Gulfstream IV aircraft out of Tampa, Florida.

Plans are also under consideration to extend the proposed Atlantic Circulation and Climate Experiment with PACS support for some observations from 5°N to 5°S. Longer range proposals include more comprehensive atmospheric observations in the extreme western tropical Atlantic.

CLIVAR Upper Ocean Panel Report (W. Kessler, NOAA/PMEL)

Perhaps the most important conclusion of the UOP meeting was that despite the many advances that have been made in modeling Pacific dynamics and thermodynamics, and the increased quantity and quality of wind observations, there remains a strong need for in situ sub-surface observations to correct wind-forced GCMs by assimilation. Several talks focused on the NCEP GCM runs and reanalysis product, comparing the output to observations of various types, and evaluating this model's ability to hindcast features of the Pacific climate such as ENSO. There was discussion of how to best use the in situ observations for assimilation. At present, an observed temperature profile (from TAO or XBT), is assimilated directly. In regions where the model has significant mean errors (for example the wrong depth of the mean thermocline), most of the information content of an observed profile serves to correct the mean error, and observed anomalies may not have much influence. On the other hand, one might try to assimilate observed anomalies. However, evidence was shown of significant decadal changes which make definition of anomalies problematic, particularly when the time range of data available to compute climatologies is not consistent from region to region. Another unsolved problem is how to make use of altimetric sea level from the TOPEX/Poseidon satellite. It is not obvious how one would use sea level as a constraint in the GFDL model formulation. One group at GFDL has developed a regression technique to make "synthetic XBTs" from the altimeter values. Unfortunately this produces some quite odd profiles on occasion. Another method attempts to use sea level as an integral constraint on the model; this work is still in progress. Overall, it is clear that to date, even in the relatively well-studied tropical Pacific, large amounts of in situ data will continue to be necessary for the foreseeable future.

Another subject of discussion at the meeting was the Pacific wind products (NCEP, ECMWF, FNOC, and FSU). Although TAO winds flow to all these centers, large differences remain among the wind fields and between TAO and the wind products, indicating that the observed winds may not be constraining the fields appropriately. Reasons for this might be problems in the model planetary boundary layer physics or surface pressure fields that result in observed winds appearing too anomalous and being rejected.

Salinity was discussed, and evidence was shown for the need for in situ salinity profiles for the computation of dynamic height and geostrophic currents. Salinity will be investigated for next years' meeting, including questions of where the need for in situ salinity is most important, to what extent surface salinity alone is useful, and how densely salinity must be sampled to be useful.

Other regions of the world ocean were briefly discussed with an eye towards choosing a focus for next year's meeting. Possible topics include the subtropical-tropical connection, which is seen to be important in regards to decadal changes; the Southern Ocean, where observations of barometric pressure that can be collected by drifting buoys might have a large influence on the AGCMs; the Atlantic, which is of interest in connection with climate over the Americas; and the Indian Ocean and monsoon variability.

Ocean Rain Measurements for TRMM Validation (O. Thiele, NASA/GSFC)

Another line of investigation centers around a low cost disdrometer that can be used on ships and buoys as well as on land. The current development approach is based on an early Johns Hopkins' Applied Physics Laboratory design. The disdrometer instrument has the advantage of measuring rain drop size distribution (in addition to rain-rate and amount) which is highly important for quantifying latent heat release in convective/cumulus cloud models, and for developing reliable radar reflectivity to rain (i.e., Z-R) relationships. Although this type disdrometer is an order of magnitude less expensive than the one commercially available Swiss disdrometer, it, like the ORG of similar cost, is not suitable for large scale deployment. However, a number of these on buoys located in a few representative regions in the tropics could provide invaluable information on variable rain systems. Approximately 50 of these units will be completed by the Summer of 1997 and deployed at the several TRMM ground validation sites and at various experimental locations.

While both of these systems, especially the disdrometer, can contribute significantly to important and specialized ocean measurements of rainfall, a simpler and less expensive rain measuring system is needed for wide spread ocean buoy deployment such as the TAO Array. The most promising solution at this time for extending Pacific Ocean rain measurements at this time would appear to be the R.M. Young syphon gauge. A significantly large deployment of these gauges on ATLAS moorings in the Pacific would be of great benefit to TRMM.

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