diff --git a/components/omega/configs/Default.yml b/components/omega/configs/Default.yml
index 620f790cdc65..24524c17613d 100644
--- a/components/omega/configs/Default.yml
+++ b/components/omega/configs/Default.yml
@@ -19,7 +19,7 @@ Omega:
     IOStride: 1
     IOBaseTask: 0
     IORearranger: box
-    IODefaultFormat: NetCDF4
+    IODefaultFormat: pnetcdf
   State:
     NTimeLevels: 2
   Advection:
@@ -114,7 +114,7 @@ Omega:
         - Restart
     History:
       UsePointerFile: false
-      Filename: ocn.hist.$SimTime
+      Filename: ocn.hist.$Y-$M-$D_$h.$m.$s
       Mode: write
       IfExists: replace
       Precision: double
diff --git a/components/omega/src/base/IO.cpp b/components/omega/src/base/IO.cpp
index ac523904d914..c7d5dd3a2914 100644
--- a/components/omega/src/base/IO.cpp
+++ b/components/omega/src/base/IO.cpp
@@ -189,11 +189,11 @@ void init(const MPI_Comm &InComm // [in] MPI communicator to use
    CHECK_ERROR_ABORT(Err, "IO: IO group not found in input Config");
 
    // Read default file format
-   std::string InFileFmt = "netcdf4c"; // set default value
+   std::string InFileFmt = "pnetcdf"; // set default value
    Err                   = IOConfig.get("IODefaultFormat", InFileFmt);
    CHECK_ERROR_WARN(Err, "IO: DefaultFileFmt not found in Config - using {}",
                     InFileFmt);
-   FileFmt DefaultFileFmt = FileFmtFromString(InFileFmt);
+   DefaultFileFmt = FileFmtFromString(InFileFmt);
 
    // Read parallel IO settings - default to single-task if config
    // values do not exist
@@ -249,6 +249,7 @@ void openFile(
 
    int PIOErr = 0;        // internal SCORPIO/PIO return call
    int Format = InFormat; // coerce to integer for PIO calls
+   int IsCDF5 = (InFormat == PIO_IOTYPE_PNETCDF) ? PIO_64BIT_DATA : 0;
 
    switch (InMode) {
 
@@ -269,7 +270,7 @@ void openFile(
       // file exists, we use create and fail with an error
       case IfExists::Fail:
          PIOErr = PIOc_createfile(SysID, &FileID, &Format, Filename.c_str(),
-                                  NC_NOCLOBBER | InMode);
+                                  NC_NOCLOBBER | IsCDF5 | InMode);
          if (PIOErr != PIO_NOERR)
             ABORT_ERROR("IO::openFile: PIO error opening file {} for writing",
                         Filename);
@@ -279,7 +280,7 @@ void openFile(
       // we use create with the CLOBBER option
       case IfExists::Replace:
          PIOErr = PIOc_createfile(SysID, &FileID, &Format, Filename.c_str(),
-                                  NC_CLOBBER | InMode);
+                                  NC_CLOBBER | IsCDF5 | InMode);
          if (PIOErr != PIO_NOERR)
             ABORT_ERROR("IO::openFile: PIO error opening file {} for writing",
                         Filename);
@@ -291,10 +292,10 @@ void openFile(
       case IfExists::Append:
          if (std::filesystem::exists(Filename)) {
             PIOErr = PIOc_openfile(SysID, &FileID, &Format, Filename.c_str(),
-                                   InMode);
+                                   IsCDF5 | InMode);
          } else {
             PIOErr = PIOc_createfile(SysID, &FileID, &Format, Filename.c_str(),
-                                     InMode);
+                                     IsCDF5 | InMode);
          }
 
          if (PIOErr != PIO_NOERR)
diff --git a/components/omega/src/base/IO.h b/components/omega/src/base/IO.h
index e06c9629dbf8..044228676b50 100644
--- a/components/omega/src/base/IO.h
+++ b/components/omega/src/base/IO.h
@@ -74,7 +74,7 @@ enum FileFmt {
    FmtHDF5     = PIO_IOTYPE_HDF5,     ///< native HDF5 format
    FmtADIOS    = PIO_IOTYPE_ADIOS,    ///< ADIOS format
    FmtUnknown  = -1,                  ///< Unknown or undefined
-   FmtDefault  = PIO_IOTYPE_NETCDF4C, ///< NetCDF4 is default
+   FmtDefault  = PIO_IOTYPE_PNETCDF,  ///< PNETCDF is default
 };
 
 /// File operations
diff --git a/components/omega/src/infra/IOStream.cpp b/components/omega/src/infra/IOStream.cpp
index 114853d878a1..2eaf41181d16 100644
--- a/components/omega/src/infra/IOStream.cpp
+++ b/components/omega/src/infra/IOStream.cpp
@@ -2285,7 +2285,7 @@ Error IOStream::readStream(
 
    // Open input file
    int InFileID;
-   IO::openFile(InFileID, InFileName, Mode, IO::FmtDefault, ExistAction);
+   IO::openFile(InFileID, InFileName, Mode, IO::DefaultFileFmt, ExistAction);
 
    // Read any requested global metadata
    for (auto Iter = ReqMetadata.begin(); Iter != ReqMetadata.end(); ++Iter) {
@@ -2438,7 +2438,8 @@ void IOStream::writeStream(
 
    // Open output file
    int OutFileID;
-   IO::openFile(OutFileID, OutFileName, Mode, IO::FmtDefault, ExistAction);
+   IO::openFile(OutFileID, OutFileName, Mode, IO::DefaultFileFmt, ExistAction);
+   bool NeedEndDef = false; // check if we need to call end-define-mode
 
    // For files with multiple frames or time slices, we need to determine the
    // default Frame number for time-dependent fields. If the frame/time already
@@ -2491,6 +2492,7 @@ void IOStream::writeStream(
    if (Frame < 1) {
       writeFieldMeta(CodeMeta, OutFileID, IO::GlobalID);
       writeFieldMeta(SimMeta, OutFileID, IO::GlobalID);
+      NeedEndDef = true;
    }
 
    // Create and write a field for any global data that is file or time
@@ -2506,6 +2508,10 @@ void IOStream::writeStream(
       FileField->addMetadata(SimTimeName, SimTimeStr);
    }
    // Write and then destroy temporary field
+   if (!NeedEndDef) { // in data-mode, need to redef
+      PIOc_redef(OutFileID);
+      NeedEndDef = true;
+   }
    writeFieldMeta("FileField", OutFileID, IO::GlobalID);
    Field::destroy("FileField");
 
@@ -2557,8 +2563,12 @@ void IOStream::writeStream(
       // Now we can write the field metadata
       if (Frame < 1) { // only write if it's the first time
          writeFieldMeta(FieldName, OutFileID, FieldID);
+         NeedEndDef = true;
       }
    }
+   if (NeedEndDef) {
+      IO::endDefinePhase(OutFileID);
+   }
 
    // Now write data arrays for all fields in contents
    for (auto IFld = Contents.begin(); IFld != Contents.end(); ++IFld) {
diff --git a/externals/scorpio b/externals/scorpio
index 8b8450444c0e..9df851035a89 160000
--- a/externals/scorpio
+++ b/externals/scorpio
@@ -1 +1 @@
-Subproject commit 8b8450444c0e6f900042f57d93813065443bb5b5
+Subproject commit 9df851035a89ba1ca0581ccdbb88fcfc0ec38992