diff --git a/config/cesm/machines/config_compilers.xml b/config/cesm/machines/config_compilers.xml
index 3c48f915dd3..8ef5b9fe3bb 100644
--- a/config/cesm/machines/config_compilers.xml
+++ b/config/cesm/machines/config_compilers.xml
@@ -109,7 +109,7 @@ using a fortran linker.
-mcmodel=medium -std=gnu99
-fopenmp
- -g -Wall -ffpe-trap=invalid,zero,overflow -fcheck=bounds
+ -g -Wall -Og -fbacktrace -ffpe-trap=invalid,zero,overflow -fcheck=bounds
-O
@@ -126,11 +126,11 @@ using a fortran linker.
-mcmodel=medium -fconvert=big-endian -ffree-line-length-none -ffixed-line-length-none
-fopenmp
- -g -Wall -ffpe-trap=zero,overflow -fcheck=bounds
+ least with some versions of gfortran (confirmed with 5.4.0, 6.3.0 and
+ 7.1.0), gfortran's isnan (which is called in cime via the
+ CPRGNU-specific shr_infnan_isnan) causes a floating point exception
+ when called on a signaling NaN. -->
+ -g -Wall -Og -fbacktrace -ffpe-trap=zero,overflow -fcheck=bounds
-O