AMOCatlas Format AC1

This document defines the AC1 (Atlantic Circulation) standard data format which is an OceanSITES variant with small deviations. This format provides interoperability between moored estimates of overturning transport from the RAPID, OSNAP, MOVE and SAMBA arrays while ensuring compliance with international oceanographic data standards. Note the deviations below (2.1 Deviations from OceanSITES Standard).

Relationship to Other Format Documents:

Array Format (Native / Original) - Documents native data formats from each array
Converting to OceanSITES format - Describes conversion strategies from native to standardized formats
AMOCatlas Format AC1 - Current standardized output format implementation
This document (format_AC1) - AC1 format with full OceanSITES integration

1. Overview & Context 

The AC1 format incorporates OceanSITES v1.4 compliance for enhanced discoverability and interoperability, while adding additional metadata (vocabularies) and standardised naming conventions.

The AC1 format provides enhanced compliance with:

Full Standards Compliance: Complete implementation of CF Conventions 1.8, OceanSITES 1.4, and ACDD 1.3
Enhanced Discoverability: Rich metadata using controlled vocabularies for global data catalogs
Workflow Integration: Compatible with existing AMOCatlas workflows
International Interoperability: Compliance with OceanSITES and GDAC requirements
Provenance Tracking: Comprehensive attribution to original data providers
Extensibility: Supports future variables and array additions

1.1 Relationship to Standards 

The AC1 format represents the standardization level in the AMOCatlas hierarchy:

Native Formats → Internal standardised → AC1 Standard
(format_orig)       (format_Atlas)       (format_AC1)

Compliance Framework: OceanSITES AC1 datasets are designed to meet:

CF Conventions 1.8 compliance (validation tools to be implemented)
OceanSITES 1.4 compatibility (with documented deviations as specified in 2.1 Deviations from OceanSITES Standard)
AMOCatlas-specific validation rules (see amocatlas.compliance_checker)
ACDD-1.3 metadata structure

Standards Integration: The format integrates multiple international standards:

CF Conventions 1.8: Climate and Forecast metadata conventions
OceanSITES 1.4: Ocean observing time series data format
ACDD 1.3: Attribute Convention for Data Discovery
NERC Vocabularies: Controlled vocabularies for oceanographic parameters

2. Key Design Decisions 

The AC1 format incorporates several design decisions that enhance interoperability while maintaining scientific accuracy and usability.

2.1 Deviations from OceanSITES Standard 

AC1 implements OceanSITES 1.4 with the following deviations optimized for AMOC array data:

Deviations from OceanSITES Standard
Feature	OceanSITES Standard	AC1 Format
Date Format	`"YYYY-MM-DDThh:mm:ssZ"`	Compact ISO 8601: `"YYYYmmddTHHMMss"`
Contributor Metadata	`creator_`, `principal_investigator_`	`contributor_*` attributes (unified pattern)
Density Coordinates	Depth/pressure coordinates only	`SIGMA0` coordinate allowed (array-specific)
Component Dimension	Not specified	`N_COMPONENT` for transport decomposition
Coordinate Units	`degrees_north`, `degrees_east`	`degree_north`, `degree_east` (UDUNITS-2)
Transport Units	`Sv` for transport	`sverdrup` (avoids confusion with sievert)

Rationale for Deviations:

Contributor Pattern: Unified contributor_* approach simplifies metadata management while providing equivalent functionality
Sigma Coordinates: Essential for density-based transport calculations in some arrays
Component Dimension: Enables systematic representation of transport decompositions across arrays
Coordinate Units: UDUNITS-2 singular forms provide better tool compatibility than OceanSITES plural forms
Sverdrup Unit: Full spelling prevents confusion with Sv (sievert radiation unit)

These deviations maintain CF compliance and ISO 8601 compatibility while optimizing for AMOC-specific scientific requirements.

3. File Organisation & Naming 

3.1 File Naming Convention 

Files follow the OceanSITES naming pattern with AMOC-specific modifications:

Basic Pattern: OS_[PLATFORM]_[DEPLOYMENT]_[MODE]_[PARAMS].nc

Components:

OS = OceanSITES prefix (maintains compatibility)
[PLATFORM] = Platform identifier (e.g., “RAPID”, “OSNAP”)
[DEPLOYMENT] = Deployment code (e.g., “20040401-20230211” for date range)
[MODE] = Data mode: R (real-time), P (provisional), D (delayed-mode)
[PARAMS] = Parameter identifier (e.g., “transports_T12H”, “sections_T1M”)

Examples:

OS_RAPID_20040401-20230211_D_transports_T12H.nc - Delayed-mode transport data
OS_OSNAP_20140801-20200601_D_sections_T1M.nc - Delayed-mode section data

Reference: See OceanSITES file naming in “4.1.1 Deployment Data files Naming Convention” of the OceanSITES manual (https://ocean-uhh.github.io/oceanarray/oceanSITES_manual.html#data-files).

4. Global Attributes 

Following OceanSITES 1.4, ACDD 1.3, and CF 1.8 requirements for comprehensive metadata.

Note

Requirement Status: M = Mandatory, HD = Highly Desired, S = Suggested

4.1 Discovery and Identification 

Discovery and Identification Attributes
Attribute	Example	Description	Vocabulary	RS
`site_code`	“RAPID”	OceanSITES site identifier	OceanSITES Registry	M
`array`	“RAPID”	Array grouping identifier	Custom AMOCatlas	M
`data_mode`	“D”	Data mode: R=real-time, P=provisional, D=delayed	OceanSITES Standard	M
`title`	“RAPID-MOCHA Transport Time Series”	Human-readable dataset title	Free text	HD
`theme`	“Transport Moored Arrays”	OceanSITES theme classification	OceanSITES Themes	S
`naming_authority`	“AMOCatlas”	Authority providing the dataset ID	Reverse DNS recommended	S
`id`	“OS_RAPID_20040402-20240327_DPR_transports_T12H”	Unique dataset identifier (filename without .nc)	OceanSITES Pattern	M
`summary`	“Oceanographic mooring data from the RAPID array at 26°N…”	Extended description for discovery (≤100 words)	Free text	S
`source`	“subsurface mooring”	Platform type from controlled vocabulary	SeaVoX L06	HD
`keywords`	“EARTH SCIENCE > Oceans > Ocean Circulation”	Discovery keywords (comma-separated)	GCMD preferred	S
`keywords_vocabulary`	“GCMD Science Keywords”	Vocabulary source for keywords	Standards reference	S
`comment`	“Preliminary version; subject to revision”	Miscellaneous information	Free text	S

4.2 Provenance 

Consolidates OceanSITES creator_* and principal_investigator_* fields into unified contributor_* attributes supporting multiple contributors following OG1 patterns.

Contributor and Attribution Attributes
Attribute	Example	Description	Vocabulary	RS
`contributor_name`	“Dr. Jane Doe, Dr. John Smith”	Names of dataset contributors (comma-separated)	Free text	M
`contributor_email`	“jane.doe@example.org, john.smith@noc.ac.uk”	Email addresses (aligned with names)	Email format	M
`contributor_id`	“https://orcid.org/0000-0002-1825-0097, …”	Persistent IDs (ORCID preferred)	ORCID/ISNI URLs	HD
`contributor_role`	“principalInvestigator, creator”	Roles (aligned with names)	NERC W08	M
`contributor_role_vocabulary`	“https://vocab.nerc.ac.uk/collection/W08/current/”	Vocabulary for contributor roles	Standards reference	M
`contributing_institutions`	“University of Hamburg, National Oceanography Centre”	Institutional contributors	Free text	M
`contributing_institutions_vocabulary`	“https://ror.org/”	Institutional identifier vocabulary	ROR/EDMO preferred	HD
`contributing_institutions_role`	“operator, dataProvider”	Institutional roles	NERC W08	M
`contributing_institutions_role_vocabulary`	“https://vocab.nerc.ac.uk/collection/W08/current/”	Vocabulary for institutional roles	Standards reference	M

Standard Contributor Roles: Data scientist, Manufacturer, PI, Technical Coordinator, Operator, Owner

Provenance and Data History

Provenance and Source Attribution
Attribute	Example	Description	Format	RS
`source_acknowledgement`	“RAPID data collected and made freely available by the RAPID program…”	Attribution to original data providers (semicolon-separated)	Free text	M
`source_doi`	“https://doi.org/10.35090/gatech/70342; https://doi.org/10.1029/2018GL077408”	DOIs of source datasets (semicolon-separated)	DOI URLs	M
`amocatlas_version`	“0.3.0”	Version of amocatlas used for processing	Semantic version	M
`web_link`	“https://www.rapid.ac.uk/; https://www.o-snap.org/”	Links to project websites (semicolon-separated)	URLs	S
`start_date`	“2004-04-02T00:00:00Z”	Overall dataset start time	ISO 8601	M
`generated_doi`	“https://doi.org/10.xxxx/amocatlas-ac-proposed-2025”	DOI assigned to converted dataset (if available)	DOI URL	S

4.3 Geospatial and Temporal Coverage 

Geospatial and Temporal Attributes
Attribute	Example	Description	Format	RS
`geospatial_lat_min`	26.0	Southernmost latitude	Decimal degrees	M
`geospatial_lat_max`	26.5	Northernmost latitude	Decimal degrees	M
`geospatial_lat_units`	“degrees_north”	Latitude units	UDUNITS-2	S
`geospatial_lon_min`	-80.0	Westernmost longitude	Decimal degrees	M
`geospatial_lon_max`	-13.0	Easternmost longitude	Decimal degrees	M
`geospatial_lon_units`	“degrees_east”	Longitude units	UDUNITS-2	S
`geospatial_vertical_min`	0.0	Minimum depth/height	Meters	M
`geospatial_vertical_max`	5000.0	Maximum depth/height	Meters	M
`geospatial_vertical_positive`	“down”	Vertical direction convention	“up” or “down”	S
`geospatial_vertical_units`	“m”	Vertical coordinate units	UDUNITS-2	S
`time_coverage_start`	“2004-04-02T00:00:00Z”	Dataset start time	ISO 8601	M
`time_coverage_end`	“2024-03-27T23:59:59Z”	Dataset end time	ISO 8601	M
`time_coverage_duration`	“P19Y11M25D”	Dataset duration	ISO 8601 Duration	S
`time_coverage_resolution`	“PT12H”	Temporal resolution	ISO 8601 Duration	S
`sea_area`	“North Atlantic Ocean”	Geographical coverage	SeaVoX C19	S

Time Format Rationale: The compact YYYYmmddTHHMMss format reduces attribute string length while maintaining human readability and ISO 8601 compatibility.

File dates: The file dates, date_created and date_modified, are our interpretation of the file dates as defined by ACDD. Date_created is the time stamp on the file, date_modified may be used to represent the ‘version date’ of the geophysical data in the file. The date_created may change when e.g. metadata is added or the file format is updated, and the optional date_modified MAY be earlier.

Geospatial extents: (geospatial_lat_min, max, and lon_min, max) are preferred to be stored as strings for use in the GDAC software, however numeric fields are acceptable. This information is linked to the site information, and may not be specific to the platform deployment.

4.4 Publication and Licensing 

Publication and Licensing Attributes
Attribute	Example	Description	Format	RS
`publisher_name`	“AMOCatlas Development Team”	Data publisher name	Free text	S
`publisher_url`	“https://github.com/AMOCcommunity/amocatlas”	Publisher web address	URL	S
`references`	“http://www.oceansites.org, https://doi.org/10.1029/2018GL077408”	Relevant publications and resources (semicolon-separated)	URLs/DOIs	S
`license`	“CC-BY-4.0”	Data license	License identifier	S
`citation`	“These data were collected and made freely available by the OceanSITES program…”	Recommended citation text	Free text	S
`acknowledgement`	“Principal funding provided by Horizon Europe EPOC project…”	Funding and support acknowledgements	Free text	S

4.5 Technical and Processing Information 

Technical and Processing Attributes
Attribute	Example	Description	Format	RS
`featureType`	“timeSeries”	CF discrete sampling geometry type	CF Standard	M
`data_type`	“OceanSITES time-series data”	OceanSITES data type classification	OceanSITES Standard	M
`format_version`	“1.4”	OceanSITES format version	Version string	M
`Conventions`	“CF-1.8, OceanSITES-1.4, ACDD-1.3”	Metadata conventions followed	Standards list	S
`platform_code`	“RAPID26N”	Unique platform identifier	Free text	M
`QC_indicator`	“excellent”	Overall quality assessment	OceanSITES QC levels	S
`processing_level`	“Data verified against model or other contextual information”	Processing level description	OceanSITES levels	S
`date_created`	“2025-01-15T10:30:00Z”	File creation timestamp	ISO 8601	M
`date_modified`	“2025-01-15T10:30:00Z”	Last modification timestamp	ISO 8601	S
`history`	“2025-01-15T10:30:00Z: Converted to AC1 using amocatlas v0.3.0”	Processing history log	Timestamped entries	S

5. Dimensions & Coordinates 

Following CF conventions, dimensions are ordered as T, Z, Y, X with component dimensions leftmost:

Dimension Ordering
Category	Dimensions	Description
Component	`N_COMPONENT`	Transport components (optional)
Temporal	`TIME`	Time coordinate (unlimited)
Vertical	`DEPTH`, `PRESSURE`	Vertical coordinates (optional)
Horizontal	`LATITUDE`, `LONGITUDE`	Horizontal coordinates (optional)

Warning

All datasets must include the TIME dimension. Other dimensions are optional depending on data type (timeSeries vs timeSeriesProfile).

Coordinate Variables
Variable	Dimension	Attributes and Requirements	RS
`TIME`	`TIME`	Data Type: double (datetime64[ns]) Required Attributes: long_name = “Time” standard_name = “time” units = “seconds since 1970-01-01T00:00:00Z” calendar = “gregorian” axis = “T”	M
`LATITUDE`	scalar or `LATITUDE`	Data Type: float32 Required Attributes: long_name = “Latitude” standard_name = “latitude” units = “degree_north” valid_min = -90.0 valid_max = 90.0 axis = “Y”	HD
`LONGITUDE`	scalar or `LONGITUDE`	Data Type: float32 Required Attributes: long_name = “Longitude” standard_name = “longitude” units = “degree_east” valid_min = -180.0 valid_max = 180.0 axis = “X”	HD
`DEPTH`	`DEPTH`	Data Type: float32 Required Attributes: long_name = “Depth below sea surface” standard_name = “depth” units = “m” positive = “down” valid_min = 0.0 axis = “Z”	S
`PRESSURE`	`PRESSURE`	Data Type: float32 Required Attributes: long_name = “Sea water pressure” standard_name = “sea_water_pressure” units = “dbar” positive = “down” valid_min = 0.0 axis = “Z”	S
`SIGMA0`	`SIGMA0`	Data Type: float32 Required Attributes: long_name = “Sea water sigma-theta” standard_name = “sea_water_sigma_theta” units = “kg m-3” axis = “Z” positive = “down”	S

6. Data Variables & QC 

6.1 Transport Variables 

Transport Variables
Variable Name	Variable Attributes	RS
`MOC_TRANSPORT` Data Type: float32 Dimensions: `TIME`	long_name = “Maximum meridional overturning circulation transport” standard_name = “ocean_volume_transport_across_line” units = “sverdrup” vocabulary = “http://vocab.nerc.ac.uk/collection/P07/current/W946809H/” coordinates = “TIME, LONGITUDE, LATITUDE, DEPTH” (required if variable does not have 4 coordinates in its definition) _FillValue = NaNf valid_min = -50.0 (optional) valid_max = 50.0 (optional)	HD
`TRANSPORT` Data Type: float32 Dimensions: `N_COMPONENT`, `TIME`	long_name = “Ocean volume transport components across line” standard_name = “ocean_volume_transport_across_line” units = “sverdrup” vocabulary = “http://vocab.nerc.ac.uk/collection/P07/current/W946809H/” coordinates = “TIME, LONGITUDE, LATITUDE, DEPTH” (required if variable does not have 4 coordinates in its definition) _FillValue = NaNf	HD
`HEAT_TRANSPORT` Data Type: float32 Dimensions: `TIME`	long_name = “Northward ocean heat transport” standard_name = “northward_ocean_heat_transport” units = “PW” vocabulary = “http://vocab.nerc.ac.uk/collection/P07/current/CFSN0483/” coordinates = “TIME, LONGITUDE, LATITUDE, DEPTH” (required if variable does not have 4 coordinates in its definition) _FillValue = NaNf valid_min = -5.0 (optional) valid_max = 5.0 (optional)	S
`FRESHWATER_TRANSPORT` Data Type: float32 Dimensions: `TIME`	long_name = “Northward ocean freshwater transport” standard_name = “northward_ocean_freshwater_transport” units = “sverdrup” vocabulary = “http://vocab.nerc.ac.uk/collection/P07/current/CFSN0507/” coordinates = “TIME, LONGITUDE, LATITUDE, DEPTH” (required if variable does not have 4 coordinates in its definition) _FillValue = NaNf valid_min = -5.0 (optional) valid_max = 5.0 (optional)	S

6.2 Hydrographic Variables 

Hydrographic Variables
Variable Name	Variable Attributes	RS
`TEMP` data type: float32 dimensions: (`TIME`, `DEPTH`, `LATITUDE`, `LONGITUDE`)	long_name = “Sea water temperature” standard_name = “sea_water_temperature” units = “degree_Celsius” vocabulary = “https://vocab.nerc.ac.uk/collection/P07/current/CFSN0335/” valid_min = -2.0 valid_max = 40.0 _FillValue = NaNf ancillary_variables = “TEMPERATURE_QC” coordinates = “TIME, DEPTH, LATITUDE, LONGITUDE”	HD
`PSAL` data type: float32 dimensions: (`TIME`, `DEPTH`, `LATITUDE`, `LONGITUDE`)	long_name = “Sea water practical salinity” standard_name = “sea_water_practical_salinity” units = “1” vocabulary = “http://vocab.nerc.ac.uk/collection/P07/current/IADIHDIJ/” valid_min = 0.0 valid_max = 50.0 _FillValue = NaNf ancillary_variables = “SALINITY_QC” coordinates = “TIME, DEPTH, LATITUDE, LONGITUDE”	HD
`UCUR` data type: float32 dimensions: (`TIME`, `DEPTH`, `LATITUDE`, `LONGITUDE`)	long_name = “Eastward sea water velocity” standard_name = “eastward_sea_water_velocity” units = “m s-1” vocabulary = “http://vocab.nerc.ac.uk/collection/P07/current/CFSN0650/” valid_min = -2.0 valid_max = 2.0 _FillValue = NaNf ancillary_variables = “VELOCITY_EAST_QC” coordinates = “TIME, DEPTH, LATITUDE, LONGITUDE”	S
`VCUR` data type: float32 dimensions: (`TIME`, `DEPTH`, `LATITUDE`, `LONGITUDE`)	long_name = “Northward sea water velocity” standard_name = “northward_sea_water_velocity” units = “m s-1” vocabulary = “https://vocab.nerc.ac.uk/collection/P07/current/CFSN0494/” valid_min = -2.0 valid_max = 2.0 _FillValue = NaNf ancillary_variables = “VELOCITY_NORTH_QC” coordinates = “TIME, DEPTH, LATITUDE, LONGITUDE”	S

6.3 Descriptive Variables 

Descriptive Variables (Component Transport Arrays)
Variable Name	Variable Attributes	RS
`TRANSPORT_NAME` data type: \|S64 (string) dimensions: (`N_COMPONENT`)	long_name = “Transport component names” coordinates = “N_COMPONENT” content = “Short descriptive names (e.g., Ekman, UMO, AMOC, Florida_Current)”	HD
`TRANSPORT_DESCRIPTION` data type: \|S256 (string) dimensions: (`N_COMPONENT`)	long_name = “Transport component descriptions” coordinates = “N_COMPONENT” content = “Detailed descriptions of transport components”	S

Note

Requirement Status: M = Mandatory, HD = Highly Desired, S = Suggested

6.4 Variable-Level Quality Control 

For variables requiring quality control, implement OceanSITES QC conventions:

Quality Control Variables
QC Variable	Dimensions	Attributes and Values	RS
`<PARAM>_QC`	Same as parent variable	Data Type: byte Required Attributes: - long_name = “Quality flag for <parameter_name>” - flag_values = [0, 1, 2, 3, 4, 7, 8, 9] - flag_meanings = “unknown good_data probably_good_data potentially_correctable_bad_data bad_data nominal_value interpolated_value missing_value” - valid_min = 0 - valid_max = 9	S
`<PARAM>_UNCERTAINTY`	Same as parent variable	Data Type: float32 Required Attributes: - long_name = “Uncertainty estimate for <parameter_name>” - units = Same as parent variable - technique_title = “Description of uncertainty estimation method”	S

7. Conversion Tools and Implementation 

7.1 Enhanced Conversion Function 

To produce AC1 compliant datasets from standardized inputs:

from amocatlas.convert import to_AC1

# Convert with enhanced metadata
ds_ac1 = to_AC1(
    ds_standardized,
    array_metadata_yaml="metadata/rapid_array.yml",
    validate=True,
    gdac_compliant=True
)

7.2 Conversion Process 

The conversion function performs these operations:

Input Validation: Verify standardized dataset structure
Metadata Integration: Load and apply array-specific YAML metadata
Attribute Enhancement: Add comprehensive global attributes following OceanSITES/ACDD standards
Variable Standardization: Ensure proper standard names, units, and vocabularies
Quality Control: Apply QC flags and uncertainty estimates where available
File Naming: Generate OceanSITES-compliant filename
Compliance Validation: Run CF Checker and OceanSITES validation
Output Generation: Write NetCDF4 file with optimal compression and chunking

7.3 Validation Tools 

All AC datasets must pass comprehensive validation:

Validation Checks
Validation Category	Requirements
File Naming	Must match OceanSITES pattern: `OS_[PSPANCode]_[StartEndCode]_[ContentType]_[PARTX].nc`
Global Attributes	All mandatory (M) attributes must be present with valid values
Coordinate Variables	TIME dimension required; appropriate axis attributes; valid units
Data Variables	Valid standard_name attributes; UDUNITS-2 compliant units; appropriate _FillValue
CF Compliance	Must pass CF Checker with zero errors
OceanSITES Compliance	Must meet OceanSITES 1.4 requirements for GDAC submission
Vocabulary Compliance	All controlled vocabulary references must resolve to valid terms

from amocatlas.validation import validate_AC_proposed

# Comprehensive validation
validation_result = validate_AC_proposed(
    "OS_RAPID_20040402-20240327_DPR_transports_T12H.nc",
    checks=["cf", "oceansites", "acdd", "amocatlas"]
)

if validation_result.is_valid:
    print("Dataset is fully compliant with AC1 format")
else:
    print("Validation errors:", validation_result.errors)

8. Examples and Use Cases 

8.1 RAPID Transport Time Series Example 

File: OS_RAPID_20040402-20240327_DPR_transports_T12H.nc

netcdf OS_RAPID_20040402-20240327_DPR_transports_T12H {
dimensions:
    TIME = UNLIMITED ; // (14600 currently)
    N_COMPONENT = 8 ;
    LATITUDE = 1 ;

variables:
    double TIME(TIME) ;
        TIME:long_name = "Time" ;
        TIME:standard_name = "time" ;
        TIME:units = "seconds since 1970-01-01T00:00:00Z" ;
        TIME:calendar = "gregorian" ;
        TIME:axis = "T" ;

    float LATITUDE(LATITUDE) ;
        LATITUDE:long_name = "Latitude" ;
        LATITUDE:standard_name = "latitude" ;
        LATITUDE:units = "degrees_north" ;
        LATITUDE:valid_min = -90.0f ;
        LATITUDE:valid_max = 90.0f ;
        LATITUDE:axis = "Y" ;

    float MOC_TRANSPORT(TIME) ;
        MOC_TRANSPORT:long_name = "Maximum meridional overturning circulation transport" ;
        MOC_TRANSPORT:standard_name = "ocean_volume_transport_across_line" ;
        MOC_TRANSPORT:units = "sverdrup" ;
        MOC_TRANSPORT:coordinates = "TIME" ;
        MOC_TRANSPORT:_FillValue = NaNf ;
        MOC_TRANSPORT:vocabulary = "http://vocab.nerc.ac.uk/collection/P07/current/W946809H/" ;

    float TRANSPORT(N_COMPONENT, TIME) ;
        TRANSPORT:long_name = "Ocean volume transport components across line" ;
        TRANSPORT:standard_name = "ocean_volume_transport_across_line" ;
        TRANSPORT:units = "sverdrup" ;
        TRANSPORT:coordinates = "TIME" ;
        TRANSPORT:_FillValue = NaNf ;

    string TRANSPORT_NAME(N_COMPONENT) ;
        TRANSPORT_NAME:long_name = "Transport component names" ;
        TRANSPORT_NAME:coordinates = "N_COMPONENT" ;

// global attributes:
        :Conventions = "CF-1.8, OceanSITES-1.4, ACDD-1.3" ;
        :format_version = "1.4" ;
        :data_type = "OceanSITES time-series data" ;
        :featureType = "timeSeries" ;
        :data_mode = "D" ;
        :site_code = "RAPID" ;
        :array = "RAPID" ;
        :platform_code = "RAPID26N" ;
        :naming_authority = "AMOCatlas" ;
        :id = "OS_RAPID_20040402-20240327_DPR_transports_T12H" ;
        :title = "RAPID-MOCHA Transport Time Series at 26°N" ;
        :summary = "Meridional overturning circulation and component transports from the RAPID mooring array at 26°N in the Atlantic Ocean. Data processed to 12-hourly resolution with comprehensive quality control." ;
        :geospatial_lat_min = 26.0 ;
        :geospatial_lat_max = 26.5 ;
        :geospatial_lon_min = -80.0 ;
        :geospatial_lon_max = -13.0 ;
        :time_coverage_start = "2004-04-02T00:00:00Z" ;
        :time_coverage_end = "2024-03-27T23:59:59Z" ;
        :contributor_name = "Dr. David Smeed, Dr. Molly Baringer" ;
        :contributor_email = "david.smeed@noc.ac.uk, molly.baringer@noaa.gov" ;
        :contributor_role = "principalInvestigator, principalInvestigator" ;
        :source_acknowledgement = "RAPID data were collected and made freely available by the RAPID program and the national programs that contribute to it" ;
        :source_doi = "https://doi.org/10.5285/8cd7e7bb-9a20-05d8-e053-6c86abc012c2" ;
        :amocatlas_version = "0.3.0" ;
        :date_created = "2025-01-15T10:30:00Z" ;
        :history = "2025-01-15T10:30:00Z: Converted to AC1 using amocatlas v0.3.0" ;
}

9. Reference Tables 

9.1 UDUNITS-2 Compliance 

All units must follow the UDUNITS-2 standard for maximum compatibility and interoperability.

Unit Specifications for AC1 Format
Quantity	UDUNITS Format	Notes
Coordinates
Time	`seconds since 1970-01-01T00:00:00Z`	ISO 8601 epoch reference (Unix timestamp)
Latitude	`degree_north`	UDUNITS-2 standard (singular form)
Longitude	`degree_east`	UDUNITS-2 standard (singular form)
Depth	`m`	Standard SI unit, positive downward
Pressure	`dbar`	Standard oceanographic unit (decibars)
Density	`kg m-3`	SI derived unit for sigma coordinates
Physical Variables
Temperature	`degree_Celsius`	Preferred over `degC` (full spelling)
Salinity	`1`	Dimensionless (practical salinity scale)
Velocity	`m s-1`	SI derived unit (not `m/s`)
Transport Variables
Ocean Volume Transport	`sverdrup`	1 sverdrup = 10^6 m³/s (avoid `Sv` confusion)
Heat Transport	`petawatt`	1 PW = 10^15 W (preferred over `W` with scale factors)
Freshwater Transport	`sverdrup`	Same as volume transport

Warning

Use lowercase sverdrup (not Sv) to avoid confusion with the sievert radiation unit. UDUNITS-2 recognizes sverdrup as the standard oceanographic transport unit.

9.2 OceanSITES Reference table 1: data_type 

The data_type global attribute should have one of the valid values listed here.

Data Type Values
Data type
OceanSITES profile data
OceanSITES time-series data
OceanSITES trajectory data

9.3 OceanSITES Reference table 2: QC_indicator 

The quality control flags indicate the data quality of the data values in a file. The byte codes in column 1 are used only in the <PARAM>_QC variables to describe the quality of each measurement, the strings in column 2 (‘meaning’) are used in the attribute <PARAM>:QC_indicator to describe the overall quality of the parameter.

When the numeric codes are used, the flag_values and flag_meanings attributes are required and should contain lists of the codes (comma-separated) and their meanings (space separated, replacing spaces within each meaning by ‘_’).

QC Flag Values
Code	Meaning	Comment
0	unknown	No QC was performed
1	good data	All QC tests passed.
2	probably good data
3	potentially correctable bad data	These data are not to be used without scientific correction or re-calibration.
4	bad data	Data have failed one or more tests.
5		Not used
6		Not used.
7	nominal value	Data were not observed but reported. (e.g. instrument target depth.)
8	interpolated value	Missing data may be interpolated from neighboring data in space or time.
9	missing value	This is a fill value

9.4 OceanSITES Reference table 3: Processing level 

This table describes the quality control and other processing procedures applied to all the measurements of a variable. The string values are used as an overall indicator (i.e. one summarizing all measurements) in the attributes of each variable in the processing_level attribute.

Processing Level Values
Processing Level
Raw instrument data
Instrument data that has been converted to geophysical values
Post-recovery calibrations have been applied
Data has been scaled using contextual information
Known bad data has been replaced with null values
Known bad data has been replaced with values based on surrounding data
Ranges applied, bad data flagged
Data interpolated
Data manually reviewed
Data verified against model or other contextual information
Other QC process applied

9.5 OceanSITES Reference table 4: Data mode 

The values for the variables “<PARAM>_DM”, the global attribute “data_mode”, and variable attributes “<PARAM>:DM_indicator” are defined as follows:

Data Mode Values
Value	Meaning	Description
R	Real-time data	Data coming from the (typically remote) platform through a communication channel without physical access to the instruments, disassembly or recovery of the platform. Example: for a mooring with a radio communication, this would be data obtained through the radio.
P	Provisional data	Data obtained after instruments have been recovered or serviced; some calibrations or editing may have been done, but the data is not thought to be fully processed. Refer to the history attribute for more detailed information.
D	Delayed-mode data	Data published after all calibrations and quality control procedures have been applied on the internally recorded or best available original data. This is the best possible version of processed data.
M	Mixed	This value is only allowed in the global attribute “data_mode” or in attributes to variables in the form “<PARAM>:DM_indicator”. It indicates that the file contains data in more than one of the above states. In this case, the variable(s) <PARAM>_DM specify which data is in which data mode.

9.6 OceanSITES Reference Table 6: Identifying data variables (subset)

OceanSITES Variable Names (subset)
Parameter	CF Standard name or suggested Long name
CDIR	direction_of_sea_water_velocity
CNDC	sea_water_electrical_conductivity
CSPD	sea_water_speed
DEPTH	depth
DOX2	moles_of_oxygen_per_unit_mass_in_sea_water was dissolved_oxygen
DOXY	mass_concentration_of_oxygen_in_sea_water was dissolved_oxygen
DOXY_TEMP	temperature_of_sensor_for_oxygen_in_sea_water
DYNHT	dynamic_height
FLU2	fluorescence
HCSP	sea_water_speed
HEAT	heat_content
ISO17	isotherm_depth
PCO2	surface_partial_pressure_of_carbon_dioxide_in_air
PRES	sea_water_pressure
PSAL	sea_water_practical_salinity
TEMP	sea_water_temperature
UCUR	eastward_sea_water_velocity
VCUR	northward_sea_water_velocity

10. Metadata Requirements and YAML Integration 

10.1 Array-Specific Metadata Files 

Metadata are provided as enhanced YAML files for each array, defining variable mappings, unit conversions, attributes, and contributor information.

Enhanced YAML Structure (osnap_array.yml):

# Array identification
array:
  name: "OSNAP"
  site_code: "OSNAP"
  platform_code: "OSNAP60N"
  sea_area: "North Atlantic Ocean"

# Spatial coverage
geospatial:
  lat_min: 59.0
  lat_max: 61.0
  lon_min: -45.0
  lon_max: -10.0
  vertical_min: 0.0
  vertical_max: 3000.0

# Contributors
contributors:
  - name: "Susan Lozier"
    email: "susan.lozier@duke.edu"
    orcid: "https://orcid.org/0000-0002-1234-5678"
    role: "PI"
    institution: "Duke University"
    institution_ror: "https://ror.org/00py81415"
    institution_role: "operator"

# Variable definitions
variables:
  temp:
    name: TEMPERATURE
    long_name: "Sea water temperature"
    standard_name: "sea_water_temperature"
    units: "degree_Celsius"
    vocabulary: "https://vocab.nerc.ac.uk/collection/P07/current/CFSN0335/"
    valid_min: -2.0
    valid_max: 40.0

  sal:
    name: SALINITY
    long_name: "Sea water practical salinity"
    standard_name: "sea_water_practical_salinity"
    units: "1"
    vocabulary: "http://vocab.nerc.ac.uk/collection/P07/current/IADIHDIJ/"
    valid_min: 0.0
    valid_max: 50.0

  moc_transport:
    name: MOC_TRANSPORT
    long_name: "Atlantic meridional overturning circulation transport"
    standard_name: "ocean_volume_transport_across_line"
    units: "sverdrup"
    vocabulary: "http://vocab.nerc.ac.uk/collection/P07/current/W946809H/"

# Provenance
provenance:
  source_acknowledgement: "OSNAP data were collected and made freely available by the OSNAP project and all the national programs that contribute to it (www.o-snap.org)"
  source_doi: "https://doi.org/10.35090/gatech/70342"
  web_link: "https://www.o-snap.org/"

# Processing
processing:
  qc_indicator: "excellent"
  processing_level: "Data verified against model or other contextual information"

11. Future Development and Extensions 

11.1 Planned Enhancements 

Multi-Array Integration: Support for datasets combining multiple arrays
Real-Time Data Streams: Extensions for operational oceanography
Machine-Readable Provenance: Integration with Research Data Alliance metadata standards
Cloud-Optimized Formats: Zarr and COG variants for cloud computing

11.2 Community Integration 

AC1 format is designed for:

OceanSITES GDAC Submission: Full compliance for global data archive
CMIP Integration: Compatible with climate model evaluation workflows
ARGO Coordination: Harmonized with autonomous profiling float data standards
Regional Programs: Adaptable for other ocean observing arrays globally

12. Summary and Recommendations 

The AC1 format represents the next evolution of AMOCatlas data standardization, combining the proven AC1 implementation with comprehensive international standards compliance. Key benefits include:

For Data Providers: - Simplified workflow for OceanSITES GDAC submission - Enhanced discoverability through rich metadata - Maintained compatibility with existing tools

For Data Users: - Consistent interface across all AMOC arrays - Full metadata for proper data citation and attribution - Guaranteed interoperability with international tools and standards

For the Community: - Foundation for global AMOC data integration - Template for other observing array programs - Future-ready architecture for emerging requirements

We recommend adopting AC1 format for all new AMOCatlas releases while maintaining AC1 support for existing workflows. The enhanced metadata and standards compliance provide immediate value for data discovery and long-term preservation while ensuring continued scientific productivity.

—

Project Funding: AC1 format development is supported by the Horizon Europe project EPOC - Explaining and Predicting the Ocean Conveyor (Grant Agreement No. 101081012).

Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.