Externals in E3SM

Some of the subdirectories in the E3SM code contain code that was brought in from another repository.


  • external any subdirectory in E3SM whose entire content is from another repository
  • upstream master  the repository everyone is sending their changes to.  Only one so that everyone pulling from the same source is working with all the same changes.   The source of an external.

Methods for including externals

  • submodule  a link that points to another git repository.  See Sharing External Code using Git Submodules .
  • recursive submodule:   a submodule that is inside another submodule.  
  • subtree a set of files and commits from another git repo, brought in to your repo so that it looks like original content.  See Sharing External Code using Git Subtree
  • monorepo the entire repo is added once (usally with subtree) and developed within E3SM.  Might maintain a separate build/test system.
  • buildtime  the repo is checked out to the $CIME_OUTPUT_ROOT/$CASE/bld when compiling a case for the first time.  Code does not exist in E3SM repo.

Rules for adding externals

  1. Group leads must approve adding a new external.  The group needing the external can decide on the method of inclusion.  A POC for the upstream master must be identified and the IG group leads notified.
  2. The upstream master for the external must be build-able and testable on its own.
  3. For submodules, the linked repo must be readable by all E3SM developers.  If the upstream master can not be made readable, a fork must be maintained in E3SM-Project github and E3SM then links to that fork.

Development in externals

In general, there are several ways to develop with external code

  • strict 1-way:  all development is merged to the upstream master first and flows down to E3SM.  No exceptions.
  • mostly 1-way:  prefer strict 1-way but some exceptions can be made for emergency fixes
  • 2-way:  developers can make changes in E3SM or the upstream master.  POC keeps them in sync.

More specific instructions for externals are below.

Table of Externals currently in ACME

NameSubdirectory in ACMEexternal methodupstream masterdevelopment methodPOCNotes



mostly 1-way

usually brought in with CIME

mostly 1-way

usually brought in with CIME



submodulehttps://github.com/E3SM-Project/scorpiostrict 1-wayJayesh KrishnaE3SM uses this and ignores cime/externals/pio
Kokkosexternals/kokkossubmodulegit@github.com:E3SM-Project/kokkos.git branch e3sm/kokkosstrict 1-wayJames Foucar




strict 1-wayMPAS-Ocean, Seaice and land ice all use code from here.
RRTMGPcomponents/cam/src/physics/rrtmgp/externalsubmodulegit@github.com:RobertPincus/rte-rrtmgp.git branch developstrict 1-wayBen Hillman

git@github.com:CFMIP/COSPv2.0.git branch CESM_v2.1.4

strict 1-way



submodulehttps://github.com/MPP-LSM/MPP branch alm/developstrict 1-wayGautam Bisht




strict 1-way
SBETrcomponents/clm/src/external_models/sbetrsubmodulegit@github.com:BeTR-biogeochemistry-modeling/sbetr.gitstrict 1-wayGautam Bisht
CVMixcomponents/mpas-source/src/core_ocean/cvmixrescursive submodulehttps://github.com/CVMix/CVMix-srcstrict 1-wayJon Wolfeused to be not in-source and fetched at build time.
ocean BGCcomponents/mpas-source/src/core_ocean/BGCrecursive submodulehttps://github.com/E3SM-Project/Ocean-BGCstrict 1-wayJon Wolfeused to be not in-source and fetched at build time.

 Externals planned for E3SM

NameSubdirectory in ACMEexternal methodupstream masterdevelopment methodPOCNotes
CICE column physicsTBDTBD?



GCAMTBDsubmodule?https://github.com/JGCRI/gcam-corestrict 1-wayKatherine Calvin (Unlicensed)

Specific external development instructions


Code location:  E3SM/cime

Any code development in E3SM/cime/config/acme can be committed directly to E3SM through the normal PR process.

Code development in other parts of CIME should be committed first with a PR in https://github.com/ESMCI/cime.  It will be then brought to E3SM by a subtree merge as in Maintaining the CIME subtree in ACME.

Report issues about the Case Control System, driver or coupler directly to  https://github.com/ESMCI/cime.   config file issues can be reported to E3SM gitub.