**Review of the PIRATA report**

***Accomplishments of PIRATA: 1997-2005, Status and Perspectives***

By a subgroup of the Ocean Observations Panel for Climate (OOPC): D. E.
Harrison, F. Schott, R. Weller

*October 2006*

**Summary:** The PIRATA array and related observations taken during
mooring exchange/refit and ancillary cruises have allowed a multitude of
studies leading to a much better understanding of Tropical Atlantic
Variability (TAV), and have made possible the initiation of ocean
forecasting for the region. Prior to the multi-national establishment of
the PIRATA network and related regular cruises, observations of the
Tropical Atlantic were much sparser than for the Tropical Pacific. OOPC
fully endorses the concept of sustained observations of TAV, including
continuation of the network and of the southwestern and planned eastern
extensions. However sustained observing of the region should be
undertaken with an integrated observing system in which surface moorings
are used where they are the most effective technology, and other
technologies are used where they offer advantages. OOPC thus recommends
that annual meetings of the TAV community be established which include
PIRATA, and that development of a plan for of an integrated observing
system for the tropical Atlantic be a priority for the TAV community.

**1. PIRATA status 2006**

OOPC compliments PIRATA for its accomplishments under challenging
conditions. The existence of PIRATA and of its associated exchange
cruises has resulted in a seasonal and interannual coverage of the
tropical Atlantic that was impossible before, except for brief periods
during international experiments like the GARP programs GATE and FGGE.
An important result of PIRATA is that it has spawned significant
enthusiasm and progress among adjacent nations, and in particular
Brazil. PIRATA observations are important for operational oceanography
and climate forecasting in the tropical Atlantic. Much has been learned
in the analysis of PIRATA data. Doubts on the mean zonal circulation on
the equator were resolved using PIRATA moored data, and revealed the
existence of two westward intermediate currents (EICs) underneath the
Equatorial Undercurrent (EUC), separated by an eastward flow in between.
As was accomplished in the Pacific earlier using TAO exchange cruises
(e.g. Johnson et al., 2002; Sloyan et al., 2003), PIRATA ship sections
have recently allowed us to better quantify the transports of the
eastward equatorial and off-equatorial undercurrents (Schott et al.,
2003; Brandt et al., 2006; Fischer et al., 2006). Furthermore, the
existence of PIRATA has allowed process studies (for example, a study on
mixing in the eastern cold-tongue lead by M. Dengler of IFM-GEOMAR) that
are essential in improving models, and has certainly helped triggering
planning activities for internationally-coordinated equatorial (TACE) or
near-equatorial (AMI, AMMA) studies. A multi-year study of the eastern
equatorial regime has also been proposed to the German Deutsche
Forschungsgemeinschaft (DFG), which also benefits from the momentum and
perseverance of PIRATA.

**2. Recommendations**

***Importance of moored or profiling subsurface observations at
intraseasonal timescales***

Recent observations in the central tropical Atlantic (Brandt et al.,
2006) have emphasized the need for subsurface coverage of intraseasonal
variability at time scales shorter than a month. Their spatial
structures and dynamics are not yet sufficiently well understood, and
their surface signatures are not satisfactorily resolved by satellite
observations. This requires subsurface moored ADCP or rotor current
measurements with a best data quality standard. Mooring survivability
would be greatly increased if subsurface floatation is used, for example
with a top float near the 100m level carrying an upward
(high-resolution) and a downward (longranger) ADCP. Whether this task
can also be carried out by gliders in the future needs to be
investigated.

***The need for eastern tropical Atlantic and cold tongue studies***

Coupled models have significant difficulty in reproducing tropical
Atlantic stratification and SST (e.g., IPCC-AR3 and CLIVAR/TACE
reports), with the cold spot essentially appearing in the central basin
and too-warm eastern tropics. Improved parameterization of the shallow
eastern mixed layers is therefore important. Since advection is
important in the tropical belt and eastern upwelling regimes (Foltz et
al., 2003), moored current observations are essential in the region,
including documentation of the poleward propagation of coastal-trapped
waves (CTWs). The proposed southeastern extension (SEE) focuses on the
Benguela Niños with moorings at 6-8° S; and the proposed northeastern
extension (NEE) on the ITCZ migration, i.e. on issues of interest for
AMMA/AMI, with planned moorings along 23° W in a region important for
climate anomalies and hurricane generation. Therefore the OOPC strongly
endorses both extensions and the associated research foci.

***Southwest Extension***

After international review, Brazil has started a program with three
moored stations off its northeastern coast, between 8-19° S . In this
region, surface buoys are much less prone to vandalism and
meteorological observations are useful for forecasts in Brazil. Recent
observations have shown that there is large intraseasonal variability in
the region, and from altimetry and high-resolution model analyses (Ph.D.
study K. v. Schuckmann, IFM-GEOMAR), a relation to the interior basin
has been suggested. Therefore, moored observations in the zonal inflow
regime will be helpful for understanding the ocean response and boundary
interaction.

***The*** ***PIRATA mooring concept***

At the start of PIRATA, there was a general interest in and
preoccupation with the "SST Dipole Mode", leading to the two NW and SE
off-equatorial mooring lines that run through the presumed dipole
maxima. Good coverage of the equatorial belt and boundary interaction
regions (PIRATA extensions) by moored stations is of continued highest
importance, as observations and model studies have shown a large
contribution of advection and diffusion to the heat and salt balance on
the equator, and for the propagating boundary signals. At the NW and SE
off-equatorial stations, advection plays a less important role. Given
the availability of other types of observations of stratification and
water mass variability (Argo float profiles in particular) a
reassessment of how much of an off-equatorial moored station coverage
will be needed in the future should take place. Furthermore there were
high hopes in obtaining much more realistic model simulations through
data assimilation. However, it is fair to say that at the present stage
of evolution of ocean data assimilation systems, it is unclear how well
sparse data points from a dozen or so widely distributed moored stations
actually influence model simulations.

***Use of surface moorings***

The biggest problem PIRATA has faced is vandalism of surface moorings,
which has resulted in significant losses, data gaps and data quality
deterioration. There should be a discussion of why the use of surface
moorings is essential at each of the present locations, in particular in
the east, where vandalism is higher, given the technical advances in
autonomous observations since the beginnings of PIRATA. Many of the use
examples cited in the review document could be addressed by other
sampling means: for example (Figures 7&9) -- satellite SST can also
produce the dipole index. More generally, the possibility of gliders
deployed as virtual moorings should be explored, as they may be able to
produce the same benefit with a far reduced risk of vandalism. Certainly
the surface met and flux data have high value. However, some questions
should be addressed by the TAV community: Does daily-averaged data of
coarse vertical resolution from the ocean justify the quality challenges
posed by vandalism to surface moorings? What is the density of moorings
needed to get the surface meteorology and fluxes? Are these independent
observations or are they too coarse to have any coherence?

***Development of an Integrated Observing System for TAV***

It is suggested that the TAV community work with the PIRATA community
and the tropical Atlantic operational oceanography community to design
an observing system that makes best use of all available technologies.
It is suggested that OSSEs be carried out in which the importance of
individual stations and sampling strategies for recovering the
energetically important parts of the variability are investigated, by
subsampling and reconstructing output fields of high-resolution models,
such as those available from the 1/12 degree FLAME or HYCOM models. This
strategy was successfully applied for investigating sampling strategies
in the tropical-subtropical Indian Ocean with its strong intraseasonal
variability (Vecchi et al., 2006). It is also recommended that model
simulations with ocean data assimilation be tested by using different
subsets of PIRATA location data.

**3. Vessel support**

Ship time is one of the limiting factors for successfully continuing
PIRATA into the future. Through OSSEs and interaction with the
meteorological/assimilation community it should be determined which
surface stations with met observations are considered essential.
Subsurface moorings could be deployed for one year at a time with little
risk of vandalism, but there would be technical challenges in obtaining
the data in real time. These systems should be tested before any
transition. The impact of delayed mode access to some observations
should also be weighed against the probability of ultimately more
complete data sets made possible by less vandalism-prone observational
techniques.

**4. Technical mooring issues**

This section has a number of specific questions from OOPC based on the
PIRATA review document.

1\) There needs to be more information provided about calibration,
quality control, quality assurance, and accuracies of the sensors,
instrument systems, and resulting data. There is no discussion of
calibration procedures (factory or lab calibrations how often?), of how
sensors degrade with time in the field (certainly the salinity and
meteorological sensors do or can), of problems with ADCP data due to
fish (in the TAO array they have gone to separate subsurface moorings to
get ADCP data as the fish return could not be beat so how does PIRATA do
QC on the ADCP data?). On page 28 the report mentions the "bad quality
of the buoy data" -- what does this mean, certainly the buoys cannot
measure SST and make a temperature measurement at some depth below the
hull which is never mentioned -- so how much bias arises due to this?
Page 45 talks about dust deposits-how do these affect the met sensors,
especially the radiometers? Several discussions about the use of the
buoy data talk about using the buoy data as a truth to identify biases
and then points to differences; yet, none of this discussion has much
impact as one does not know how accurate the buoy data are and thus
cannot judge if any of the differences are valid and/or significant. For
an operational continuation of PIRATA in the future, there must be

-   a thorough and careful statement of practices and procedures for
    QC/QA and calibration,

-   justification based on field comparison with other data of the in
    situ accuracies,

-   a discussion of the observational challenges (bioufouling of
    salinity, degradation of met sensors, fish swimming in the ADCP
    sampling field).

There is a reference in the text to a TAO website, but it does not carry
sufficient information. It is the procedures and practices used in
PIRATA that must be documented, and the accuracies and uncertainties are
too important not to be discussed in some depth in this report. Page 26
uses the phrase "high quality buoy observations" but there is no
substantiation of what this means. On page 40 there is a discussion of
differences between the buoys and models of 0.4 °C in SST, 0.6 °C for
air temp, 1.3 m/s for wind speed---are these significant? Which is
correct? Certainly in low winds buoy air and sea temperatures can have
at least this much error.

2\) Following the above, what is the procedure for updating the
real-time data with corrections based on post-deployment calibrations?
How are users alerted to these corrections? A table of sampling rates is
needed.

3\) The document needs more detailed description of the sampling and
cruises. How often are the cruises ("...at least once a year ...", page
7)? What observations are made on the land-based meteorological stations
(page 15)? A further explanation of the statement on page 51 is needed,
that work is being done to get higher sampling rate data sent via
satellite with the result that PIRATA data sets would be "more useful
and even indispensable for more skillful predictions."

4\) The utility of the telemetered data to meet some of the goals is
unclear. The telemetered data is daily-averaged (top of page 15), so how
does this get used to check or initialize models? The vertical spacing
of the temperature sensors is coarse; is the resulting accuracy in the
mixed layer depth determination good enough? What are the error bars on
Figure 25 -- the mixed layer salinity balance? How was the vertical
sampling strategy chosen? -- Argo floats give much more vertical
resolution -- to get a mixed layer salinity balance, would not that be a
better approach?