bnlawrence · Oct 10, 2026 · Oct 10, 2026 · Oct 15, 2026 · Oct 23, 2026 · Oct 23, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -14,12 +14,17 @@ of being developed elsewhere.
 -----
 
 
-### Version `3.5.X`, first released 2026-10-09
+### Version `3.5.X`, first released XXXXXX
 
 #### Version `3.5.1`, released 2026-??-??
 
 * Update code to prevent deprecation-related warnings
   (https://github.com/NCAS-CMS/cf-plot/issues/72)
+* Prevent striped artefacts on curvilinear longitude/latitude grids
+  such as NEMO (https://github.com/NCAS-CMS/cf-plot/issues/126)
+* Add new example 'Example 44: NEMO data on a curvilinear T grid'
+  to the documentation and integration testing
+  (https://github.com/NCAS-CMS/cf-plot/issues/142)
 
 #### Version `3.5.0`, released 2026-10-09
 

diff --git a/cfplot/blockfill.py b/cfplot/blockfill.py
@@ -108,9 +108,11 @@ def _bfill(
                 colarr[pts] = np.size(levels) - 2
 
     if isinstance(field, np.ma.MaskedArray):
-        pts = np.ma.where(field.mask)
-        if np.size(pts) > 0:
-            colarr[pts] = -1
+        # np.ma.where on a scalar False mask raises on newer NumPy.
+        # Use a normalized boolean mask array instead.
+        mask = np.ma.getmaskarray(field)
+        if np.any(mask):
+            colarr[mask] = -1
 
     norm = matplotlib.colors.BoundaryNorm(levels, cmap.N)
 

diff --git a/cfplot/contour.py b/cfplot/contour.py
@@ -682,7 +682,7 @@ def render_filled(
         runtime.image = runtime.mymap.contourf(
             lons,
             lats,
-            self.data.field * self.data.fmult,
+            _mask_curvilinear_contour_seams(self.data) * self.data.fmult,
             self.data.levels,
             extend=scale.levels_extend,
             cmap=cmap,
@@ -792,10 +792,11 @@ def render_lines(
 
         runtime = plotvars.runtime
         dec = plotvars.decoration
+        field = _mask_curvilinear_contour_seams(self.data) * self.data.fmult
         cs = runtime.mymap.contour(
             self.data.x,
             self.data.y,
-            self.data.field * self.data.fmult,
+            field,
             self.data.levels,
             colors=colors,
             linewidths=linewidths,
@@ -825,7 +826,7 @@ def render_lines(
             cs0 = runtime.mymap.contour(
                 self.data.x,
                 self.data.y,
-                self.data.field * self.data.fmult,
+                field,
                 [-1e-32, 0],
                 colors=colors,
                 linewidths=zero_thick,
@@ -1697,6 +1698,70 @@ def _window_irregular_map_data(
     return field_irregular, lons_irregular, lats_irregular
 
 
+def _is_curvilinear_map_grid(data: ContourData) -> bool:
+    """Return True for map plots using 2D lon/lat coordinates.
+
+    Curvilinear grids (for example NEMO/ORCA ocean meshes) are prone to
+    severe contourf/contour artefacts. These are better rendered through
+    blockfill/pcolormesh.
+    """
+    if data.ptype != 1 or data.x is None or data.y is None:
+        return False
+    if np.ndim(data.field) != 2:
+        return False
+    return np.ndim(data.x) == 2 and np.ndim(data.y) == 2
+
+
+def _mask_curvilinear_contour_seams(data: ContourData) -> np.ndarray:
+    """Exclude vertices adjoining longitude jumps from structured contours."""
+    if not _is_curvilinear_map_grid(data):
+        return data.field
+
+    mask = np.ma.getmaskarray(data.field).copy()
+    for axis in (0, 1):
+        jumps = np.abs(np.diff(data.x, axis=axis)) > 180.0
+        before = [slice(None), slice(None)]
+        after = before.copy()
+        before[axis] = slice(None, -1)
+        after[axis] = slice(1, None)
+        # Mask both ends so corner masking cannot retain a seam-crossing
+        # triangle and interpolate a contour across the whole map.
+        mask[tuple(before)] |= jumps
+        mask[tuple(after)] |= jumps
+
+    return np.ma.array(data.field, mask=mask)
+
+
+def _resolve_curvilinear_render_defaults(
+    *,
+    data: ContourData,
+    kwargs: dict[str, Any],
+    fill: bool,
+    lines: bool,
+    blockfill: bool,
+    blockfill_fast: bool | None,
+) -> tuple[bool, bool, bool, bool | None]:
+    """Adjust renderer defaults for curvilinear lon/lat map grids.
+
+    Returns the possibly updated (fill, lines, blockfill, blockfill_fast)
+    tuple.
+    """
+    if not _is_curvilinear_map_grid(data):
+        return fill, lines, blockfill, blockfill_fast
+
+    # Use robust defaults for 2D map grids unless the caller explicitly
+    # requested alternative behavior.
+    if "blockfill" not in kwargs:
+        blockfill = True
+        fill = False
+    if blockfill and blockfill_fast is None:
+        blockfill_fast = True
+    if "lines" not in kwargs:
+        lines = True
+
+    return fill, lines, blockfill, blockfill_fast
+
+
 def _render_with_new_xy(
     f: Any, x: Any, y: Any, kwargs: dict[str, Any]
 ) -> bool:
@@ -1776,6 +1841,7 @@ def _render_with_new_xy(
     fill = kwargs.get("fill", global_fill)
     lines = kwargs.get("lines", global_lines)
     blockfill = kwargs.get("blockfill", global_blockfill)
+    blockfill_fast = kwargs.get("blockfill_fast", None)
     line_labels = kwargs.get("line_labels", True)
     zero_thick = kwargs.get("zero_thick", False)
     colors = kwargs.get("colors", "k")
@@ -1787,6 +1853,17 @@ def _render_with_new_xy(
     if blockfill:
         fill = False
 
+    fill, lines, blockfill, blockfill_fast = (
+        _resolve_curvilinear_render_defaults(
+            data=data,
+            kwargs=kwargs,
+            fill=fill,
+            lines=lines,
+            blockfill=blockfill,
+            blockfill_fast=blockfill_fast,
+        )
+    )
+
     colorbar = kwargs.get("colorbar", True)
     if not fill and not blockfill:
         colorbar = False
@@ -2171,7 +2248,7 @@ def _render_with_new_xy(
         )
     if blockfill:
         renderer.render_blockfill(
-            fast=kwargs.get("blockfill_fast", None), alpha=alpha, zorder=zorder
+            fast=blockfill_fast, alpha=alpha, zorder=zorder
         )
     if lines:
         renderer.render_lines(
@@ -2324,7 +2401,7 @@ def _render_with_new_xy(
 
 
 def con(f=None, x=None, y=None, **kwargs):
-    """The interface to contouring in cf-plot.
+    """Create a contour or blockfill (or combined contour-blockfill) plot.
 
     The minimum use is con(f)
     where f is a 2 dimensional array. If a cf field is passed then an
@@ -2350,7 +2427,14 @@ def con(f=None, x=None, y=None, **kwargs):
     |                       6 = rotated pole
     | negative_linestyle='solid' - set to one of 'solid', 'dashed'
     | zero_thick=False - add a thick zero contour line. Set to 3 for example.
-    | blockfill=False - set to True for a blockfill plot
+    | blockfill=False - set to True for a blockfill plot. For 2D lon/lat
+    |                   grids, defaults to fast blockfill so blockfill=True.
+    |                   For 2D lon/lat grids, defaults to fast blockfill,
+    |                   i.e. blockfill=True, fill=False, and
+    |                   blockfill_fast=True.
+    |                   Explicit False selects contours; vertices adjoining
+    |                   longitude wrap jumps are excluded to avoid stripes,
+    |                   leaving a narrow gap at the seam.
     | colorbar_title=colbar_title - title for the colour bar
     | colorbar=True - add a colour bar if a filled contour plot
     | colorbar_label_skip=None - skip colour bar labels. Set to 2 to skip
@@ -2428,7 +2512,10 @@ def con(f=None, x=None, y=None, **kwargs):
     |                        then it is set to True.
     | blockfill_fast=None - Use pcolormesh blockfill. This is possibly less
     |                       reliable that the usual code but is
-    |                       faster for higher resolution datasets
+    |                       faster for higher resolution datasets.
+    |                       For 2D lon/lat grids, defaults to fast
+    |                       blockfill, i.e. blockfill=True, fill=False and
+    |                       blockfill_fast=True.
     | nlevs=False - Let Matplotlib work out the levels for the contour plot
     | orca=None - User specifies this is an orca tripolar grid. Internally
     |             cf-plot tries to detect this by looking for a single

diff --git a/docs/_images/ref_fig_1.png b/docs/_images/ref_fig_1.png
diff --git a/docs/_images/ref_fig_10.png b/docs/_images/ref_fig_10.png
diff --git a/docs/_images/ref_fig_11.png b/docs/_images/ref_fig_11.png
diff --git a/docs/_images/ref_fig_12.png b/docs/_images/ref_fig_12.png
diff --git a/docs/_images/ref_fig_13.png b/docs/_images/ref_fig_13.png
diff --git a/docs/_images/ref_fig_14.png b/docs/_images/ref_fig_14.png
diff --git a/docs/_images/ref_fig_15.png b/docs/_images/ref_fig_15.png
diff --git a/docs/_images/ref_fig_16a.png b/docs/_images/ref_fig_16a.png
diff --git a/docs/_images/ref_fig_16b.png b/docs/_images/ref_fig_16b.png
diff --git a/docs/_images/ref_fig_16c.png b/docs/_images/ref_fig_16c.png
diff --git a/docs/_images/ref_fig_17.png b/docs/_images/ref_fig_17.png
diff --git a/docs/_images/ref_fig_18.png b/docs/_images/ref_fig_18.png
diff --git a/docs/_images/ref_fig_19a.png b/docs/_images/ref_fig_19a.png
diff --git a/docs/_images/ref_fig_2.png b/docs/_images/ref_fig_2.png
diff --git a/docs/_images/ref_fig_20.png b/docs/_images/ref_fig_20.png
diff --git a/docs/_images/ref_fig_22.png b/docs/_images/ref_fig_22.png
diff --git a/docs/_images/ref_fig_27.png b/docs/_images/ref_fig_27.png
diff --git a/docs/_images/ref_fig_28.png b/docs/_images/ref_fig_28.png
diff --git a/docs/_images/ref_fig_29.png b/docs/_images/ref_fig_29.png
diff --git a/docs/_images/ref_fig_3.png b/docs/_images/ref_fig_3.png
diff --git a/docs/_images/ref_fig_30.png b/docs/_images/ref_fig_30.png
diff --git a/docs/_images/ref_fig_31.png b/docs/_images/ref_fig_31.png
diff --git a/docs/_images/ref_fig_33.png b/docs/_images/ref_fig_33.png
diff --git a/docs/_images/ref_fig_34.png b/docs/_images/ref_fig_34.png
diff --git a/docs/_images/ref_fig_35.png b/docs/_images/ref_fig_35.png
diff --git a/docs/_images/ref_fig_36.png b/docs/_images/ref_fig_36.png
diff --git a/docs/_images/ref_fig_37.png b/docs/_images/ref_fig_37.png
diff --git a/docs/_images/ref_fig_38.png b/docs/_images/ref_fig_38.png
diff --git a/docs/_images/ref_fig_39.png b/docs/_images/ref_fig_39.png
diff --git a/docs/_images/ref_fig_39b.png b/docs/_images/ref_fig_39b.png
diff --git a/docs/_images/ref_fig_4.png b/docs/_images/ref_fig_4.png
diff --git a/docs/_images/ref_fig_40.png b/docs/_images/ref_fig_40.png
diff --git a/docs/_images/ref_fig_41.png b/docs/_images/ref_fig_41.png
diff --git a/docs/_images/ref_fig_42a.png b/docs/_images/ref_fig_42a.png
diff --git a/docs/_images/ref_fig_42b.png b/docs/_images/ref_fig_42b.png
diff --git a/docs/_images/ref_fig_5.png b/docs/_images/ref_fig_5.png
diff --git a/docs/_images/ref_fig_6.png b/docs/_images/ref_fig_6.png
diff --git a/docs/_images/ref_fig_7.png b/docs/_images/ref_fig_7.png
diff --git a/docs/_images/ref_fig_8.png b/docs/_images/ref_fig_8.png
diff --git a/docs/_images/ref_fig_9.png b/docs/_images/ref_fig_9.png
diff --git a/docs/_sources/gallery_of_examples.rst.txt b/docs/_sources/gallery_of_examples.rst.txt
@@ -40,77 +40,77 @@ by clicking through to the category pages.
    :gutter: 1
 
    .. grid-item-card:: Contour plots
-      :img-top: /../../tests/reference-example-images/ref_fig_1.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_1.png
       :link: cylindrical.html#example-1-basic-contour-plot-in-default-projection
       :text-align: center
 
    .. grid-item-card:: Blockfill plots
-      :img-top: /../../tests/reference-example-images/ref_fig_2.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_2.png
       :link: cylindrical.html#example-2-basic-blockfill-plot-in-default-projection
       :text-align: center
 
    .. grid-item-card:: Vector plots
-      :img-top: /../../tests/reference-example-images/ref_fig_13.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_13.png
       :link: vectors.html#example-13-basic-vector-plot
       :text-align: center
 
    .. grid-item-card:: Stream plots
-      :img-top: /../../tests/reference-example-images/ref_fig_16c.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_16c.png
       :link: vectors.html#example-16c-stream-plot-in-a-colour-scale
       :text-align: center
 
    .. grid-item-card:: Stipple plots
-      :img-top: /../../tests/reference-example-images/ref_fig_17.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_17.png
       :link: stipple_plots.html#example-17-basic-stipple-plot
       :text-align: center
 
    .. grid-item-card:: Trajectories
-      :img-top: /../../tests/reference-example-images/ref_fig_42b.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_42b.png
       :link: trajectories.html#example-42b-tracks-displayed-in-a-colour-scale
       :text-align: center
 
    .. grid-item-card:: Hovmöller plots
-      :img-top: /../../tests/reference-example-images/ref_fig_11.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_11.png
       :link: hovmuller.html#example-11-latitude-time-subset-view-hovmoller-plot
       :text-align: center
 
    .. grid-item-card:: Vertical (pressure or height) plots
-      :img-top: /../../tests/reference-example-images/ref_fig_8.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_8.png
       :link: pressure.html#example-8-latitude-against-log-of-pressure-over-longitude-zonal-mean
       :text-align: center
 
    .. grid-item-card:: Lineplots
-      :img-top: /../../tests/reference-example-images/ref_fig_27.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_27.png
       :link: graphs.html#example-27-basic-line-plot
       :text-align: center
 
    .. grid-item-card:: Support for multiple projections
-      :img-top: /../../tests/reference-example-images/ref_fig_34.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_34.png
       :link: projections.html#example-34-cropping-the-lambert-conformal-conic-lcc-projection
       :text-align: center
 
    .. grid-item-card:: Polar projection views
-      :img-top: /../../tests/reference-example-images/ref_fig_40.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_40.png
       :link: trajectories.html#example-40-tracks-in-the-polar-stereographic-projection
       :text-align: center
 
    .. grid-item-card:: Unstructured grids (UGRID) support
-      :img-top: /../../tests/reference-example-images/ref_fig_24a.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_24a.png
       :link: unstructured.html#example-24a-ugrid-blockfill-plot-with-lfric-cubed-sphere-mesh-output
       :text-align: center
 
    .. grid-item-card:: Rotated pole support
-      :img-top: /../../tests/reference-example-images/ref_fig_22.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_22.png
       :link: rotated_pole.html#example-22-plot-of-rotated-pole-data-on-its-native-grid
       :text-align: center
 
    .. grid-item-card:: Multiple plots on one figure
-      :img-top: /../../tests/reference-example-images/ref_fig_19b.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_19b.png
       :link: multiple_plots.html#example-19b-multiple-plots-with-user-specified-plot-positions
       :text-align: center
 
    .. grid-item-card:: Flexible customisation e.g. user-defined axes
-      :img-top: /../../tests/reference-example-images/ref_fig_21a.png
+      :img-top: /../../tests/v350-controlled-env-ref-images/ref_fig_21a.png
       :link: user_defined.html#example-21a-user-defined-axes
       :text-align: center
 
@@ -236,6 +236,7 @@ Listing of all examples
    examples/example_42a.rst
    examples/example_42b.rst
    examples/example_43.rst
+   examples/example_44.rst
 
 
 .. include:: datasets.rst
diff --git a/docs/_sources/plot_types.rst.txt b/docs/_sources/plot_types.rst.txt
@@ -20,4 +20,4 @@ Plots by type/theme
    graphs
    projections
    trajectories
-   wrf
+   specific_models
diff --git a/docs/_sources/projections.rst.txt b/docs/_sources/projections.rst.txt
@@ -1,7 +1,7 @@
 .. _projections:
 
-Projections in cf-plot
-**********************
+Projections
+***********
 
 The cylindrical and polar stereographic projections are detailed
 separately in :ref:`cylindrical plots <cylindrical>` and

diff --git a/docs/api/con.html b/docs/api/con.html
@@ -492,7 +492,13 @@
 </div>
 <div class="line">negative_linestyle='solid' - set to one of 'solid', 'dashed'</div>
 <div class="line">zero_thick=False - add a thick zero contour line. Set to 3 for example.</div>
-<div class="line">blockfill=False - set to True for a blockfill plot</div>
+<div class="line">blockfill=False - set to True for a blockfill plot. For 2D lon/lat</div>
+<div class="line-block">
+<div class="line">grids, defaults to fast blockfill without lines.</div>
+<div class="line">Explicit False selects contours; vertices adjoining</div>
+<div class="line">longitude wrap jumps are excluded to avoid stripes,</div>
+<div class="line">leaving a narrow gap at the seam.</div>
+</div>
 <div class="line">colorbar_title=colbar_title - title for the colour bar</div>
 <div class="line">colorbar=True - add a colour bar if a filled contour plot</div>
 <div class="line">colorbar_label_skip=None - skip colour bar labels. Set to 2 to skip</div>

diff --git a/docs/colour_scales.html b/docs/colour_scales.html
@@ -428,9 +428,9 @@
 <li class="toctree-l1"><a class="reference internal" href="rotated_pole.html">Rotated pole plots</a></li>
 <li class="toctree-l1"><a class="reference internal" href="unstructured.html">Unstructured grids and UGRID</a></li>
 <li class="toctree-l1"><a class="reference internal" href="graphs.html">Line plots i.e. graphs</a></li>
-<li class="toctree-l1"><a class="reference internal" href="projections.html">Projections in cf-plot</a></li>
+<li class="toctree-l1"><a class="reference internal" href="projections.html">Projections</a></li>
 <li class="toctree-l1"><a class="reference internal" href="trajectories.html">Trajectories</a></li>
-<li class="toctree-l1"><a class="reference internal" href="wrf.html">WRF data</a></li>
+<li class="toctree-l1"><a class="reference internal" href="specific_models.html">Specific Models</a></li>
 </ul>
 </div>
 </nav></div>

diff --git a/docs/cylindrical.html b/docs/cylindrical.html
@@ -428,9 +428,9 @@
 <li class="toctree-l1"><a class="reference internal" href="rotated_pole.html">Rotated pole plots</a></li>
 <li class="toctree-l1"><a class="reference internal" href="unstructured.html">Unstructured grids and UGRID</a></li>
 <li class="toctree-l1"><a class="reference internal" href="graphs.html">Line plots i.e. graphs</a></li>
-<li class="toctree-l1"><a class="reference internal" href="projections.html">Projections in cf-plot</a></li>
+<li class="toctree-l1"><a class="reference internal" href="projections.html">Projections</a></li>
 <li class="toctree-l1"><a class="reference internal" href="trajectories.html">Trajectories</a></li>
-<li class="toctree-l1"><a class="reference internal" href="wrf.html">WRF data</a></li>
+<li class="toctree-l1"><a class="reference internal" href="specific_models.html">Specific Models</a></li>
 </ul>
 </div>
 </nav></div>

diff --git a/docs/examples/example_1.html b/docs/examples/example_1.html
@@ -468,6 +468,7 @@
 <li class="toctree-l1"><a class="reference internal" href="example_42a.html">Example 42a: Tracks with labelled data points</a></li>
 <li class="toctree-l1"><a class="reference internal" href="example_42b.html">Example 42b: Tracks displayed in a colour scale</a></li>
 <li class="toctree-l1"><a class="reference internal" href="example_43.html">Example 43: CF-compliant WRF model output data</a></li>
+<li class="toctree-l1"><a class="reference internal" href="example_44.html">Example 44: NEMO data on a curvilinear T grid</a></li>
 </ul>
 </div>
 </nav></div>

diff --git a/docs/examples/example_10.html b/docs/examples/example_10.html
@@ -468,6 +468,7 @@
 <li class="toctree-l1"><a class="reference internal" href="example_42a.html">Example 42a: Tracks with labelled data points</a></li>
 <li class="toctree-l1"><a class="reference internal" href="example_42b.html">Example 42b: Tracks displayed in a colour scale</a></li>
 <li class="toctree-l1"><a class="reference internal" href="example_43.html">Example 43: CF-compliant WRF model output data</a></li>
+<li class="toctree-l1"><a class="reference internal" href="example_44.html">Example 44: NEMO data on a curvilinear T grid</a></li>
 </ul>
 </div>
 </nav></div>

diff --git a/docs/examples/example_11.html b/docs/examples/example_11.html
@@ -468,6 +468,7 @@
 <li class="toctree-l1"><a class="reference internal" href="example_42a.html">Example 42a: Tracks with labelled data points</a></li>
 <li class="toctree-l1"><a class="reference internal" href="example_42b.html">Example 42b: Tracks displayed in a colour scale</a></li>
 <li class="toctree-l1"><a class="reference internal" href="example_43.html">Example 43: CF-compliant WRF model output data</a></li>
+<li class="toctree-l1"><a class="reference internal" href="example_44.html">Example 44: NEMO data on a curvilinear T grid</a></li>
 </ul>
 </div>
 </nav></div>

diff --git a/docs/examples/example_12.html b/docs/examples/example_12.html
@@ -468,6 +468,7 @@
 <li class="toctree-l1"><a class="reference internal" href="example_42a.html">Example 42a: Tracks with labelled data points</a></li>
 <li class="toctree-l1"><a class="reference internal" href="example_42b.html">Example 42b: Tracks displayed in a colour scale</a></li>
 <li class="toctree-l1"><a class="reference internal" href="example_43.html">Example 43: CF-compliant WRF model output data</a></li>
+<li class="toctree-l1"><a class="reference internal" href="example_44.html">Example 44: NEMO data on a curvilinear T grid</a></li>
 </ul>
 </div>
 </nav></div>