Restore tob Kelvin conversion

src/access_moppy/derivations/__init__.py

@@ -71,7 +71,7 @@
     "power": lambda a, b: a**b,
     "sum": lambda x, **kwargs: x.sum(**kwargs),
     "mean": lambda *args: sum(args) / len(args),
-    "kelvin_to_celsius": lambda x: x - 273.15,
+    "kelvin_to_celsius": lambda x: _kelvin_to_celsius(x),
     "celsius_to_kelvin": lambda x: x + 273.15,
     "cli_level_to_height": cli_level_to_height,
     "clw_level_to_height": clw_level_to_height,
@@ -128,6 +128,26 @@
 }
 
 
+def _kelvin_to_celsius(value):
+    """Convert Kelvin to Celsius with a fallback for mislabeled Celsius-scale data."""
+    attrs = getattr(value, "attrs", {}) or {}
+    units = str(attrs.get("units", "")).strip().lower()
+
+    if "celsius" in units or "degc" in units or units in {"c", "deg c"}:
+        return value
+
+    if units in {"k", "kelvin"}:
+        valid_range = attrs.get("valid_range")
+        try:
+            if valid_range is not None and len(valid_range) >= 1:
+                if float(valid_range[0]) < 100.0:
+                    return value
+        except Exception:
+            pass
+
+    return value - 273.15
+
+
 def evaluate_expression(expr, context):
     if isinstance(expr, dict):
         if "literal" in expr:

src/access_moppy/mappings/ACCESS-ESM1-6_mappings.json

@@ -5382,8 +5382,11 @@
                 "pot_temp"
             ],
             "calculation": {
-                "type": "direct",
-                "formula": "pot_temp"
+                "type": "formula",
+                "operation": "kelvin_to_celsius",
+                "operands": [
+                    "pot_temp"
+                ]
             },
             "CF standard Name": "",
             "model_variable_units": {

tests/unit/test_derivations.py

@@ -62,6 +62,107 @@ def test_optional_used_as_kwarg_in_formula(self):
         result = evaluate_expression(expr, ctx)
         np.testing.assert_allclose(result.values, 2.0)
 
+    def test_kelvin_to_celsius_converts_kelvin_values(self):
+        """kelvin_to_celsius should convert canonical Kelvin metadata."""
+        da = xr.DataArray(
+            np.array([300.0], dtype=np.float32),
+            attrs={"units": "K", "valid_range": [250.0, 350.0]},
+        )
+        result = evaluate_expression(
+            {"operation": "kelvin_to_celsius", "operands": ["temp"]},
+            {"temp": da},
+        )
+        assert float(result.values[0]) == pytest.approx(26.85, abs=1e-3)
+
+    def test_kelvin_to_celsius_skips_celsius_scale_metadata(self):
+        """kelvin_to_celsius should not offset known Celsius-scale mislabeled data."""
+        da = xr.DataArray(
+            np.array([10.0], dtype=np.float32),
+            attrs={"units": "K", "valid_range": [-10.0, 500.0]},
+        )
+        result = evaluate_expression(
+            {"operation": "kelvin_to_celsius", "operands": ["temp"]},
+            {"temp": da},
+        )
+        assert float(result.values[0]) == pytest.approx(10.0)
+
+    def test_kelvin_to_celsius_passes_through_celsius_units(self):
+        """kelvin_to_celsius should leave Celsius-labelled values unchanged."""
+        da = xr.DataArray(np.array([12.5], dtype=np.float32), attrs={"units": "degC"})
+        result = evaluate_expression(
+            {"operation": "kelvin_to_celsius", "operands": ["temp"]},
+            {"temp": da},
+        )
+        assert float(result.values[0]) == pytest.approx(12.5)
+
+    def test_kelvin_to_celsius_converts_non_kelvin_units(self):
+        """Non-Celsius, non-Kelvin units should still apply the Kelvin offset."""
+        da = xr.DataArray(np.array([273.15], dtype=np.float32), attrs={"units": "m"})
+        result = evaluate_expression(
+            {"operation": "kelvin_to_celsius", "operands": ["temp"]},
+            {"temp": da},
+        )
+        assert float(result.values[0]) == pytest.approx(0.0, abs=1e-3)
+
+    def test_kelvin_to_celsius_ignores_malformed_valid_range(self):
+        """Malformed valid_range metadata should fall back to the Kelvin offset."""
+
+        class BadRange:
+            def __len__(self):
+                raise RuntimeError("bad valid_range")
+
+        da = xr.DataArray(
+            np.array([280.0], dtype=np.float32),
+            attrs={"units": "K", "valid_range": BadRange()},
+        )
+        result = evaluate_expression(
+            {"operation": "kelvin_to_celsius", "operands": ["temp"]},
+            {"temp": da},
+        )
+        assert float(result.values[0]) == pytest.approx(6.85, abs=1e-3)
+
+    def test_kelvin_to_celsius_without_valid_range_uses_offset(self):
+        """Kelvin units without valid_range metadata should still convert."""
+        da = xr.DataArray(np.array([280.0], dtype=np.float32), attrs={"units": "K"})
+        result = evaluate_expression(
+            {"operation": "kelvin_to_celsius", "operands": ["temp"]},
+            {"temp": da},
+        )
+        assert float(result.values[0]) == pytest.approx(6.85, abs=1e-3)
+
+    def test_ocean_floor_then_kelvin_to_celsius(self):
+        """The tob-style chain should convert the floor value from Kelvin to Celsius."""
+        data = xr.DataArray(
+            np.array(
+                [[300.0, 280.0], [295.0, np.nan], [290.0, np.nan]], dtype=np.float32
+            ),
+            dims=["st_ocean", "xt_ocean"],
+            attrs={"units": "K", "valid_range": [250.0, 350.0]},
+        )
+        result = evaluate_expression(
+            {
+                "operation": "kelvin_to_celsius",
+                "operands": [
+                    {"operation": "ocean_floor", "operands": ["temp"]},
+                ],
+            },
+            {"temp": data},
+        )
+        assert float(result.isel(xt_ocean=0).values) == pytest.approx(16.85, abs=1e-3)
+        assert float(result.isel(xt_ocean=1).values) == pytest.approx(6.85, abs=1e-3)
+
+    def test_list_expression_is_evaluated_recursively(self):
+        """evaluate_expression should recurse through list expressions."""
+        ctx = self._make_context()
+        result = evaluate_expression(["var1", {"literal": 3}], ctx)
+        assert result[0] is ctx["var1"]
+        assert result[1] == 3
+
+    def test_unsupported_expression_raises(self):
+        """Unsupported expressions should raise a ValueError."""
+        with pytest.raises(ValueError, match="Unsupported expression"):
+            evaluate_expression(object(), {})
+
 
 class TestCalculateMonthlyMinimum:
     """Tests for calculate_monthly_minimum() — covers decode_cf and ME frequency fix."""