diff --git a/components/elm/bld/namelist_files/namelist_definition.xml b/components/elm/bld/namelist_files/namelist_definition.xml
index 0f8b03dfec38..55206b5de6b4 100644
--- a/components/elm/bld/namelist_files/namelist_definition.xml
+++ b/components/elm/bld/namelist_files/namelist_definition.xml
@@ -1423,7 +1423,7 @@ Representative concentration pathway for future scenarios [radiative forcing at
 
 <entry id="mask" type="char*20" category="default_settings"
        group="default_settings"
-       valid_values="USGS,gx3v7,gx1v6,navy,test,tx0.1v2,tx1v1,T62,TL319,cruncep,oEC60to30v3,oEC60to30v3wLI,ECwISC30to60E1r2,EC30to60E2r2,WC14to60E2r3,WCAtl12to45E2r4,SOwISC12to60E2r4,ECwISC30to60E2r1,oRRS18to6,oRRS18to6v3,oRRS15to5,oARRM60to10,oARRM60to6,ARRM10to60E2r1,oQU480,oQU240,oQU240wLI,oQU120,oRRS30to10v3,oRRS30to10v3wLI,360x720cru,NLDASww3a,NLDAS,tx0.1v2,ICOS10,IcoswISC30E3r5,IcosXISC30E3r7">
+       valid_values="USGS,gx3v7,gx1v6,navy,test,tx0.1v2,tx1v1,T62,TL319,cruncep,oEC60to30v3,oEC60to30v3wLI,ECwISC30to60E1r2,EC30to60E2r2,WC14to60E2r3,WCAtl12to45E2r4,SOwISC12to60E2r4,ECwISC30to60E2r1,oRRS18to6,oRRS18to6v3,oRRS15to5,oARRM60to10,oARRM60to6,ARRM10to60E2r1,oQU480,oQU240,oQU240wLI,oQU120,oRRS30to10v3,oRRS30to10v3wLI,360x720cru,NLDASww3a,NLDAS,tx0.1v2,ICOS10,IcoswISC30E3r5,IcosXISC30E3r7,RRSwISC6to18E3r5">
 Land mask description
 </entry>
 
diff --git a/components/mpas-ocean/bld/namelist_files/namelist_defaults_mpaso.xml b/components/mpas-ocean/bld/namelist_files/namelist_defaults_mpaso.xml
index 2880bf2ce5cb..ffd45353943e 100644
--- a/components/mpas-ocean/bld/namelist_files/namelist_defaults_mpaso.xml
+++ b/components/mpas-ocean/bld/namelist_files/namelist_defaults_mpaso.xml
@@ -54,6 +54,7 @@
 <config_dt ocn_grid="FRISwISC04to60E3r1">'00:04:00'</config_dt>
 <config_dt ocn_grid="FRISwISC02to60E3r1">'00:02:00'</config_dt>
 <config_dt ocn_grid="FRISwISC01to60E3r1">'00:01:00'</config_dt>
+<config_dt ocn_grid="RRSwISC6to18E3r5">'00:05:00'</config_dt>
 <config_time_integrator>'split_explicit_ab2'</config_time_integrator>
 <config_number_of_time_levels>2</config_number_of_time_levels>
 
@@ -83,6 +84,7 @@
 <config_hmix_scaleWithMesh ocn_grid="FRISwISC04to60E3r1">.true.</config_hmix_scaleWithMesh>
 <config_hmix_scaleWithMesh ocn_grid="FRISwISC02to60E3r1">.true.</config_hmix_scaleWithMesh>
 <config_hmix_scaleWithMesh ocn_grid="FRISwISC01to60E3r1">.true.</config_hmix_scaleWithMesh>
+<config_hmix_scaleWithMesh ocn_grid="RRSwISC6to18E3r5">.true.</config_hmix_scaleWithMesh>
 <config_maxMeshDensity>-1.0</config_maxMeshDensity>
 <config_hmix_use_ref_cell_width>.false.</config_hmix_use_ref_cell_width>
 <config_hmix_ref_cell_width>30.0e3</config_hmix_ref_cell_width>
@@ -149,6 +151,7 @@
 <config_mom_del4 ocn_grid="FRISwISC04to60E3r1">4.37e08</config_mom_del4>
 <config_mom_del4 ocn_grid="FRISwISC02to60E3r1">5.46e07</config_mom_del4>
 <config_mom_del4 ocn_grid="FRISwISC01to60E3r1">6.83e06</config_mom_del4>
+<config_mom_del4 ocn_grid="RRSwISC6to18E3r5">3.2e09</config_mom_del4>
 <config_mom_del4_div_factor>1.0</config_mom_del4_div_factor>
 <config_use_tracer_del4>.false.</config_use_tracer_del4>
 <config_tracer_del4>0.0</config_tracer_del4>
@@ -165,6 +168,7 @@
 <config_use_Redi ocn_grid="oRRS30to10v3wLI">.false.</config_use_Redi>
 <config_use_Redi ocn_grid="oRRS18to6v3">.false.</config_use_Redi>
 <config_use_Redi ocn_grid="oRRS15to5">.false.</config_use_Redi>
+<config_use_Redi ocn_grid="RRSwISC6to18E3r5">.false.</config_use_Redi>
 <config_Redi_closure>'constant'</config_Redi_closure>
 <config_Redi_constant_kappa>400.0</config_Redi_constant_kappa>
 <config_Redi_constant_kappa ocn_forcing="datm_forced_restoring">400.0</config_Redi_constant_kappa>
@@ -203,6 +207,7 @@
 <config_use_GM ocn_grid="oRRS30to10v3wLI">.false.</config_use_GM>
 <config_use_GM ocn_grid="oRRS18to6v3">.false.</config_use_GM>
 <config_use_GM ocn_grid="oRRS15to5">.false.</config_use_GM>
+<config_use_GM ocn_grid="RRSwISC6to18E3r5">.false.</config_use_GM>
 <config_GM_closure>'EdenGreatbatch'</config_GM_closure>
 <config_GM_closure ocn_grid="EC30to60E2r2">'constant'</config_GM_closure>
 <config_GM_closure ocn_grid="WC14to60E2r3">'constant'</config_GM_closure>
@@ -399,6 +404,7 @@
 <config_land_ice_flux_mode ocn_grid="FRISwISC04to60E3r1">'pressure_only'</config_land_ice_flux_mode>
 <config_land_ice_flux_mode ocn_grid="FRISwISC02to60E3r1">'pressure_only'</config_land_ice_flux_mode>
 <config_land_ice_flux_mode ocn_grid="FRISwISC01to60E3r1">'pressure_only'</config_land_ice_flux_mode>
+<config_land_ice_flux_mode ocn_grid="RRSwISC6to18E3r5">'pressure_only'</config_land_ice_flux_mode>
 <config_land_ice_flux_formulation>'Jenkins'</config_land_ice_flux_formulation>
 <config_land_ice_flux_useHollandJenkinsAdvDiff>.false.</config_land_ice_flux_useHollandJenkinsAdvDiff>
 <config_land_ice_flux_attenuation_coefficient>10.0</config_land_ice_flux_attenuation_coefficient>
@@ -417,6 +423,7 @@
 <config_land_ice_flux_explicit_topDragCoeff ocn_grid="FRISwISC04to60E3r1">4.48e-3</config_land_ice_flux_explicit_topDragCoeff>
 <config_land_ice_flux_explicit_topDragCoeff ocn_grid="FRISwISC02to60E3r1">4.48e-3</config_land_ice_flux_explicit_topDragCoeff>
 <config_land_ice_flux_explicit_topDragCoeff ocn_grid="FRISwISC01to60E3r1">4.48e-3</config_land_ice_flux_explicit_topDragCoeff>
+<config_land_ice_flux_explicit_topDragCoeff ocn_grid="RRSwISC6to18E3r5">4.48e-3</config_land_ice_flux_explicit_topDragCoeff>
 <config_land_ice_flux_ISOMIP_gammaT>1e-4</config_land_ice_flux_ISOMIP_gammaT>
 <config_land_ice_flux_rms_tidal_velocity>5e-2</config_land_ice_flux_rms_tidal_velocity>
 <config_land_ice_flux_jenkins_heat_transfer_coefficient>0.011</config_land_ice_flux_jenkins_heat_transfer_coefficient>
@@ -430,6 +437,7 @@
 <config_land_ice_flux_jenkins_heat_transfer_coefficient ocn_grid="FRISwISC04to60E3r1">0.00295</config_land_ice_flux_jenkins_heat_transfer_coefficient>
 <config_land_ice_flux_jenkins_heat_transfer_coefficient ocn_grid="FRISwISC02to60E3r1">0.00295</config_land_ice_flux_jenkins_heat_transfer_coefficient>
 <config_land_ice_flux_jenkins_heat_transfer_coefficient ocn_grid="FRISwISC01to60E3r1">0.00295</config_land_ice_flux_jenkins_heat_transfer_coefficient>
+<config_land_ice_flux_jenkins_heat_transfer_coefficient ocn_grid="RRSwISC6to18E3r5">0.00295</config_land_ice_flux_jenkins_heat_transfer_coefficient>
 <config_land_ice_flux_jenkins_salt_transfer_coefficient>3.1e-4</config_land_ice_flux_jenkins_salt_transfer_coefficient>
 <config_land_ice_flux_jenkins_salt_transfer_coefficient ocn_grid="oEC60to30v3wLI">8.42e-5</config_land_ice_flux_jenkins_salt_transfer_coefficient>
 <config_land_ice_flux_jenkins_salt_transfer_coefficient ocn_grid="ECwISC30to60E1r2">8.42e-5</config_land_ice_flux_jenkins_salt_transfer_coefficient>
@@ -441,6 +449,7 @@
 <config_land_ice_flux_jenkins_salt_transfer_coefficient ocn_grid="FRISwISC04to60E3r1">8.42e-5</config_land_ice_flux_jenkins_salt_transfer_coefficient>
 <config_land_ice_flux_jenkins_salt_transfer_coefficient ocn_grid="FRISwISC02to60E3r1">8.42e-5</config_land_ice_flux_jenkins_salt_transfer_coefficient>
 <config_land_ice_flux_jenkins_salt_transfer_coefficient ocn_grid="FRISwISC01to60E3r1">8.42e-5</config_land_ice_flux_jenkins_salt_transfer_coefficient>
+<config_land_ice_flux_jenkins_salt_transfer_coefficient ocn_grid="RRSwISC6to18E3r5">8.42e-5</config_land_ice_flux_jenkins_salt_transfer_coefficient>
 
 <!-- advection -->
 <config_vert_advection_method>'flux-form'</config_vert_advection_method>
@@ -469,6 +478,7 @@
 <config_implicit_top_drag_coeff ocn_grid="FRISwISC04to60E3r1">4.48e-3</config_implicit_top_drag_coeff>
 <config_implicit_top_drag_coeff ocn_grid="FRISwISC02to60E3r1">4.48e-3</config_implicit_top_drag_coeff>
 <config_implicit_top_drag_coeff ocn_grid="FRISwISC01to60E3r1">4.48e-3</config_implicit_top_drag_coeff>
+<config_implicit_top_drag_coeff ocn_grid="RRSwISC6to18E3r5">4.48e-3</config_implicit_top_drag_coeff>
 <config_loglaw_bottom_roughness>1.0e-3</config_loglaw_bottom_roughness>
 <config_loglaw_layer_depth_max>10.0</config_loglaw_layer_depth_max>
 <config_loglaw_bottom_drag_min>2.5e-3</config_loglaw_bottom_drag_min>
@@ -555,6 +565,7 @@
 <config_btr_dt ocn_grid="FRISwISC04to60E3r1">'0000_00:00:05'</config_btr_dt>
 <config_btr_dt ocn_grid="FRISwISC02to60E3r1">'0000_00:00:02.5'</config_btr_dt>
 <config_btr_dt ocn_grid="FRISwISC01to60E3r1">'0000_00:00:01.25'</config_btr_dt>
+<config_btr_dt ocn_grid="RRSwISC6to18E3r5">'0000_00:00:10'</config_btr_dt>
 <config_n_btr_cor_iter>2</config_n_btr_cor_iter>
 <config_vel_correction>.true.</config_vel_correction>
 <config_btr_subcycle_loop_factor>2</config_btr_subcycle_loop_factor>
@@ -601,6 +612,7 @@
 <config_check_ssh_consistency ocn_grid="FRISwISC04to60E3r1">.false.</config_check_ssh_consistency>
 <config_check_ssh_consistency ocn_grid="FRISwISC02to60E3r1">.false.</config_check_ssh_consistency>
 <config_check_ssh_consistency ocn_grid="FRISwISC01to60E3r1">.false.</config_check_ssh_consistency>
+<config_check_ssh_consistency ocn_grid="RRSwISC6to18E3r5">.false.</config_check_ssh_consistency>
 <config_filter_btr_mode>.false.</config_filter_btr_mode>
 <config_prescribe_velocity>.false.</config_prescribe_velocity>
 <config_prescribe_thickness>.false.</config_prescribe_thickness>
@@ -1126,6 +1138,7 @@
 <config_AM_mocStreamfunction_enable ocn_grid="FRISwISC04to60E3r1">.true.</config_AM_mocStreamfunction_enable>
 <config_AM_mocStreamfunction_enable ocn_grid="FRISwISC02to60E3r1">.true.</config_AM_mocStreamfunction_enable>
 <config_AM_mocStreamfunction_enable ocn_grid="FRISwISC01to60E3r1">.true.</config_AM_mocStreamfunction_enable>
+<config_AM_mocStreamfunction_enable ocn_grid="RRSwISC6to18E3r5">.true.</config_AM_mocStreamfunction_enable>
 <config_AM_mocStreamfunction_compute_interval>'0000-00-00_01:00:00'</config_AM_mocStreamfunction_compute_interval>
 <config_AM_mocStreamfunction_output_stream>'mocStreamfunctionOutput'</config_AM_mocStreamfunction_output_stream>
 <config_AM_mocStreamfunction_compute_on_startup>.true.</config_AM_mocStreamfunction_compute_on_startup>
@@ -1213,6 +1226,7 @@
 <config_AM_conservationCheck_enable ocn_grid="FRISwISC04to60E3r1">.true.</config_AM_conservationCheck_enable>
 <config_AM_conservationCheck_enable ocn_grid="FRISwISC02to60E3r1">.true.</config_AM_conservationCheck_enable>
 <config_AM_conservationCheck_enable ocn_grid="FRISwISC01to60E3r1">.true.</config_AM_conservationCheck_enable>
+<config_AM_conservationCheck_enable ocn_grid="RRSwISC6to18E3r5">.true.</config_AM_conservationCheck_enable>
 <config_AM_conservationCheck_compute_interval>'dt'</config_AM_conservationCheck_compute_interval>
 <config_AM_conservationCheck_output_stream>'conservationCheckOutput'</config_AM_conservationCheck_output_stream>
 <config_AM_conservationCheck_compute_on_startup>.false.</config_AM_conservationCheck_compute_on_startup>
@@ -1224,6 +1238,7 @@
 <config_AM_conservationCheck_compute_on_startup ocn_grid="FRISwISC04to60E3r1">.true.</config_AM_conservationCheck_compute_on_startup>
 <config_AM_conservationCheck_compute_on_startup ocn_grid="FRISwISC02to60E3r1">.true.</config_AM_conservationCheck_compute_on_startup>
 <config_AM_conservationCheck_compute_on_startup ocn_grid="FRISwISC01to60E3r1">.true.</config_AM_conservationCheck_compute_on_startup>
+<config_AM_conservationCheck_compute_on_startup ocn_grid="RRSwISC6to18E3r5">.true.</config_AM_conservationCheck_compute_on_startup>
 <config_AM_conservationCheck_write_on_startup>.false.</config_AM_conservationCheck_write_on_startup>
 <config_AM_conservationCheck_write_on_startup ocn_grid="SOwISC12to60E2r4">.true.</config_AM_conservationCheck_write_on_startup>
 <config_AM_conservationCheck_write_on_startup ocn_grid="ECwISC30to60E2r1">.true.</config_AM_conservationCheck_write_on_startup>
@@ -1233,6 +1248,7 @@
 <config_AM_conservationCheck_write_on_startup ocn_grid="FRISwISC04to60E3r1">.true.</config_AM_conservationCheck_write_on_startup>
 <config_AM_conservationCheck_write_on_startup ocn_grid="FRISwISC02to60E3r1">.true.</config_AM_conservationCheck_write_on_startup>
 <config_AM_conservationCheck_write_on_startup ocn_grid="FRISwISC01to60E3r1">.true.</config_AM_conservationCheck_write_on_startup>
+<config_AM_conservationCheck_write_on_startup ocn_grid="RRSwISC6to18E3r5">.true.</config_AM_conservationCheck_write_on_startup>
 <config_AM_conservationCheck_write_to_logfile>.true.</config_AM_conservationCheck_write_to_logfile>
 <config_AM_conservationCheck_restart_stream>'conservationCheckRestart'</config_AM_conservationCheck_restart_stream>
 
diff --git a/components/mpas-ocean/cime_config/buildnml b/components/mpas-ocean/cime_config/buildnml
index 47076dee726a..99c028a8808b 100755
--- a/components/mpas-ocean/cime_config/buildnml
+++ b/components/mpas-ocean/cime_config/buildnml
@@ -365,6 +365,20 @@ def buildnml(case, caseroot, compname):
             ic_date = '20240314'
             ic_prefix = 'mpaso.IcosXISC30E3r7.rstFromPiControlSpinup-chrysalis'
 
+    elif ocn_grid == 'RRSwISC6to18E3r5':
+        decomp_date = '20240404'
+        decomp_prefix = 'partitions/mpas-o.graph.info.'
+        restoring_file = 'sss.PHC2_monthlyClimatology.RRSwISC6to18E3r5.20240327.nc'
+        analysis_mask_file = 'RRSwISC6to18E3r5_mocBasinsAndTransects20210623.nc'
+        ic_date = '20240327'
+        ic_prefix = 'mpaso.RRSwISC6to18E3r5'
+        if ocn_ic_mode == 'spunup':
+            logger.warning("WARNING: The specified compset is requesting ocean ICs spunup from a G-case")
+            logger.warning("         But no file available for this grid.")
+        if ocn_ismf == 'data':
+            data_ismf_file = 'prescribed_ismf_paolo2023.RRSwISC6to18E3r5.20240327.nc'
+
+
     #--------------------------------------------------------------------
     # Set OCN_FORCING = datm_forced_restoring if restoring file is available
     #--------------------------------------------------------------------
@@ -491,7 +505,8 @@ def buildnml(case, caseroot, compname):
             lines.append('')
             lines.append('<immutable_stream name="mesh"')
             lines.append('                  type="none"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('                  io_type="pnetcdf,cdf5"')
             else:
                 lines.append('                  io_type="{}"'.format(ocn_pio_typename))
@@ -500,7 +515,8 @@ def buildnml(case, caseroot, compname):
             lines.append('/>')
             lines.append('<immutable_stream name="input"')
             lines.append('                  type="input"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('                  io_type="pnetcdf,cdf5"')
             else:
                 lines.append('                  io_type="{}"'.format(ocn_pio_typename))
@@ -521,7 +537,8 @@ def buildnml(case, caseroot, compname):
             lines.append('-->')
             lines.append('<immutable_stream name="restart"')
             lines.append('                  type="input;output"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('                  io_type="pnetcdf,cdf5"')
             else:
                 lines.append('                  io_type="{}"'.format(ocn_pio_typename))
@@ -540,7 +557,8 @@ def buildnml(case, caseroot, compname):
             lines.append('')
             lines.append('<stream name="output"')
             lines.append('        type="output"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('        io_type="pnetcdf,cdf5"')
             else:
                 lines.append('        io_type="{}"'.format(ocn_pio_typename))
@@ -826,7 +844,8 @@ def buildnml(case, caseroot, compname):
             lines.append('<stream name="eddyProductVariablesOutput"')
             lines.append('        type="output"')
             lines.append('        precision="single"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('        io_type="pnetcdf,cdf5"')
             else:
                 lines.append('        io_type="{}"'.format(ocn_pio_typename))
@@ -1206,7 +1225,8 @@ def buildnml(case, caseroot, compname):
             lines.append('<stream name="timeSeriesStatsMonthlyOutput"')
             lines.append('        type="output"')
             lines.append('        precision="single"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('        io_type="pnetcdf,cdf5"')
             else:
                 lines.append('        io_type="{}"'.format(ocn_pio_typename))
@@ -1252,7 +1272,7 @@ def buildnml(case, caseroot, compname):
             lines.append('    <var name="atmosphericPressure"/>')
             lines.append('    <var name="normalMLEvelocity"/>')
             lines.append('    <var name="vertMLEBolusVelocityTop"/>')
-            if not ocn_grid.startswith("oRRS1"):
+            if not (ocn_grid.startswith("oRRS1") or ocn_grid.startswith("RRSwISC6")):
                 lines.append('    <var name="normalGMBolusVelocity"/>')
                 lines.append('    <var name="vertGMBolusVelocityTop"/>')
                 lines.append('    <var name="cGMphaseSpeed"/>')
@@ -1438,7 +1458,8 @@ def buildnml(case, caseroot, compname):
             lines.append('<stream name="timeSeriesStatsMonthlyMaxOutput"')
             lines.append('        type="output"')
             lines.append('        precision="single"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('        io_type="pnetcdf,cdf5"')
             else:
                 lines.append('        io_type="{}"'.format(ocn_pio_typename))
@@ -1502,7 +1523,8 @@ def buildnml(case, caseroot, compname):
             lines.append('<stream name="timeSeriesStatsMonthlyMinOutput"')
             lines.append('        type="output"')
             lines.append('        precision="single"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('        io_type="pnetcdf,cdf5"')
             else:
                 lines.append('        io_type="{}"'.format(ocn_pio_typename))
@@ -1607,7 +1629,8 @@ def buildnml(case, caseroot, compname):
             lines.append('')
             lines.append('<stream name="timeSeriesStatsMonthlyRestart"')
             lines.append('        type="input;output"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('        io_type="pnetcdf,cdf5"')
             else:
                 lines.append('        io_type="{}"'.format(ocn_pio_typename))
@@ -1623,7 +1646,8 @@ def buildnml(case, caseroot, compname):
             lines.append('')
             lines.append('<stream name="timeSeriesStatsMonthlyMaxRestart"')
             lines.append('        type="input;output"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('        io_type="pnetcdf,cdf5"')
             else:
                 lines.append('        io_type="{}"'.format(ocn_pio_typename))
@@ -1639,7 +1663,8 @@ def buildnml(case, caseroot, compname):
             lines.append('')
             lines.append('<stream name="timeSeriesStatsMonthlyMinRestart"')
             lines.append('        type="input;output"')
-            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS"):
+            if ocn_grid.startswith("oRRS1") or ocn_grid.startswith("FRIS") \
+                    or ocn_grid.startswith("RRSwISC6"):
                 lines.append('        io_type="pnetcdf,cdf5"')
             else:
                 lines.append('        io_type="{}"'.format(ocn_pio_typename))
diff --git a/components/mpas-seaice/bld/namelist_files/namelist_defaults_mpassi.xml b/components/mpas-seaice/bld/namelist_files/namelist_defaults_mpassi.xml
index 1d8330ac86ef..dee59f726e25 100644
--- a/components/mpas-seaice/bld/namelist_files/namelist_defaults_mpassi.xml
+++ b/components/mpas-seaice/bld/namelist_files/namelist_defaults_mpassi.xml
@@ -30,6 +30,7 @@
 <config_dt ice_grid="FRISwISC04to60E3r1">240.0</config_dt>
 <config_dt ice_grid="FRISwISC02to60E3r1">120.0</config_dt>
 <config_dt ice_grid="FRISwISC01to60E3r1">60.0</config_dt>
+<config_dt ice_grid="RRSwISC6to18E3r5">900.0</config_dt>
 <config_calendar_type>'noleap'</config_calendar_type>
 <config_start_time>'2000-01-01_00:00:00'</config_start_time>
 <config_stop_time>'none'</config_stop_time>
@@ -89,6 +90,7 @@
 <config_initial_latitude_north ice_grid="ARRM10to60E2r1">75.0</config_initial_latitude_north>
 <config_initial_latitude_north ice_grid="oRRS30to10v3wLI">85.0</config_initial_latitude_north>
 <config_initial_latitude_north ice_grid="oRRS18to6v3">85.0</config_initial_latitude_north>
+<config_initial_latitude_north ice_grid="RRSwISC6to18E3r5">85.0</config_initial_latitude_north>
 <config_initial_latitude_south>-60.0</config_initial_latitude_south>
 <config_initial_latitude_south ice_grid="oEC60to30v3wLI">-75.0</config_initial_latitude_south>
 <config_initial_latitude_south ice_grid="ECwISC30to60E1r2">-75.0</config_initial_latitude_south>
@@ -103,6 +105,7 @@
 <config_initial_latitude_south ice_grid="ARRM10to60E2r1">-85.0</config_initial_latitude_south>
 <config_initial_latitude_south ice_grid="oRRS30to10v3wLI">-85.0</config_initial_latitude_south>
 <config_initial_latitude_south ice_grid="oRRS18to6v3">-85.0</config_initial_latitude_south>
+<config_initial_latitude_south ice_grid="RRSwISC6to18E3r5">-85.0</config_initial_latitude_south>
 <config_initial_velocity_type>'uniform'</config_initial_velocity_type>
 <config_initial_uvelocity>0.0</config_initial_uvelocity>
 <config_initial_vvelocity>0.0</config_initial_vvelocity>
@@ -166,6 +169,7 @@
 <config_dynamics_subcycle_number ice_grid="FRISwISC04to60E3r1">1</config_dynamics_subcycle_number>
 <config_dynamics_subcycle_number ice_grid="FRISwISC02to60E3r1">1</config_dynamics_subcycle_number>
 <config_dynamics_subcycle_number ice_grid="FRISwISC01to60E3r1">1</config_dynamics_subcycle_number>
+<config_dynamics_subcycle_number ice_grid="RRSwISC6to18E3r5">2</config_dynamics_subcycle_number>
 <config_rotate_cartesian_grid>true</config_rotate_cartesian_grid>
 <config_include_metric_terms>true</config_include_metric_terms>
 <config_elastic_subcycle_number>120</config_elastic_subcycle_number>
diff --git a/components/mpas-seaice/cime_config/buildnml b/components/mpas-seaice/cime_config/buildnml
index 8337fcfc7061..5522d8ac6511 100755
--- a/components/mpas-seaice/cime_config/buildnml
+++ b/components/mpas-seaice/cime_config/buildnml
@@ -318,6 +318,16 @@ def buildnml(case, caseroot, compname):
             grid_date = '20240314'
             grid_prefix = 'mpassi.IcosXISC30E3r7.rstFromPiControlSpinup-chrysalis'
 
+    elif ice_grid == 'RRSwISC6to18E3r5':
+        decomp_date = '20240404'
+        decomp_prefix = 'partitions/mpas-seaice.graph.info.'
+        grid_date = '20240327'
+        grid_prefix = 'mpassi.RRSwISC6to18E3r5'
+        data_iceberg_file = 'Iceberg_Climatology_Merino.RRSwISC6to18E3r5.20240327.nc'
+        if ice_ic_mode == 'spunup':
+            logger.warning("WARNING: The specified compset is requesting seaice ICs spunup from a G-case")
+            logger.warning("         But no file available for this grid.")
+
     elif ice_grid == 'ICOS10':
         grid_date = '211015'
         grid_prefix = 'seaice.ICOS10'
@@ -451,7 +461,8 @@ def buildnml(case, caseroot, compname):
             lines.append('<streams>')
             lines.append('<immutable_stream name="mesh"')
             lines.append('                  type="none"')
-            if ice_grid.startswith("oRRS1") or ice_grid.startswith("FRIS"):
+            if ice_grid.startswith("oRRS1") or ice_grid.startswith("FRIS") \
+                    or ice_grid.startswith("RRSwISC6"):
                 lines.append('                  io_type="pnetcdf,cdf5"')
             else:
                 lines.append('                  io_type="{}"'.format(ice_pio_typename))
@@ -460,7 +471,8 @@ def buildnml(case, caseroot, compname):
             lines.append('')
             lines.append('<immutable_stream name="input"')
             lines.append('                  type="input"')
-            if ice_grid.startswith("oRRS1") or ice_grid.startswith("FRIS"):
+            if ice_grid.startswith("oRRS1") or ice_grid.startswith("FRIS") \
+                    or ice_grid.startswith("RRSwISC6"):
                 lines.append('                  io_type="pnetcdf,cdf5"')
             else:
                 lines.append('                  io_type="{}"'.format(ice_pio_typename))
@@ -482,7 +494,8 @@ def buildnml(case, caseroot, compname):
             lines.append('')
             lines.append('<immutable_stream name="restart"')
             lines.append('                  type="input;output"')
-            if ice_grid.startswith("oRRS1") or ice_grid.startswith("FRIS"):
+            if ice_grid.startswith("oRRS1") or ice_grid.startswith("FRIS") \
+                    or ice_grid.startswith("RRSwISC6"):
                 lines.append('                  io_type="pnetcdf,cdf5"')
             else:
                 lines.append('                  io_type="{}"'.format(ice_pio_typename))
@@ -498,8 +511,9 @@ def buildnml(case, caseroot, compname):
             if ice_ic_mode == 'spunup':
                 lines.append('<immutable_stream name="restart_ic"')
                 lines.append('                  type="input"')
-                if ice_grid.startswith("oRRS1") or ice_grid.startswith("FRIS"):
-                    lines.append('                  io_type="pnetcdf,cdf5"')
+                if ice_grid.startswith("oRRS1") or ice_grid.startswith("FRIS") \
+                        or ice_grid.startswith("RRSwISC6"):
+                        lines.append('                  io_type="pnetcdf,cdf5"')
                 else:
                     lines.append('                  io_type="{}"'.format(ice_pio_typename))