A marine heatwave is a stretch of at least five consecutive days when the local sea-surface temperature sits above the 90th-percentile threshold for that calendar day-of-year. The threshold is calibrated against a 30-year baseline — 1982–2011 here — using the standard published in Hobday et al. 2016. Events are then categorized by mean intensity above climatology — Moderate, Strong, Severe, Extreme — following Hobday et al. 2018.
This revision upgrades from the prior ERA5 2m air-temperature proxy to real NOAA OISST v2.1 sea-surface temperature. The prior proxy compressed all intensity anomalies into a narrow range, producing only Moderate events — a known artifact of using air temperature as SST substitute. Real SST anomalies should produce a broader intensity distribution including Severe and Extreme events in El Niño years (2015, 2023). At La Saladita, the analysis uses an offshore OISST grid cell at lat=17.375°N, lon=−101.625°W, roughly 25 km from the beach, because the immediate near-coast cell is land-masked on the 1/4° grid.
Coverage status: NOAA ERDDAP servers (CoastWatch PFEG and PSL THREDDS) were unavailable during the analysis run. Three years of real OISST data are confirmed from a prior fetch cache: 1988, 1989, 1990. Results for those 3 years are genuine. The remaining 41 years are marked no_data. The full 44-year analysis will be complete once ERDDAP access is restored — run python3 scripts/analyze_marine_heatwaves.py to refresh.
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Loading event totals and trend confidence from the artifact…
Four measures, where the data lands.
Annual trend by ordinary-least-squares regression; change per ten years across the 44-year record. Significant slopes (p < 0.05) highlighted.
Moderate, Strong, Severe, Extreme.
Hobday 2018 categories use the local p90 threshold as a 1× unit. Moderate < 2×, Strong 2–3×, Severe 3–4×, Extreme ≥ 4×.
Below 2×
2–3×
3–4×
≥ 4×
Heat-stress days by decade.
Mean MHW-days per year and mean events per year, grouped by decade.
The years that broke through.
Each year's worst event by category, total MHW-days within the calendar year, and the maximum daily intensity above the climatological threshold.
Only 3 years of OISST confirmed (1988–1990). NOAA ERDDAP servers were unavailable during this analysis run. The 8 events and 52 MHW-days reported are from real OISST for 1988, 1989, and 1990 only. The 41 remaining years are absent from this result. Do not interpret the category distribution or trend statistics as representative of the full 1982–2025 record.
The 3-year climatology is unreliable. Hobday's standard requires a 30-year baseline for the 90th-percentile threshold. A 3-year baseline overfits to those years. The thresholds are not the same as a 1982–2011 baseline would produce. With full data, the intensity distribution and event counts will differ — likely more events at higher categories, especially in El Niño years.
All Moderate — expected for 1988–1990. These years span the tail of the 1987-88 moderate El Niño, normal conditions in 1988-89, and the onset of 1990's moderate warmth. They are not El Niño extreme years. Even with real SST, 1988–1990 would be expected to produce mostly Moderate events. The prior "all 149 Moderate is a proxy artifact" claim remains untested — it requires full OISST data to confirm or refute.
The grid cell is offshore, not in the surf zone. OISST is a 1/4° (~25 km) product; the immediate near-coast cell at La Saladita is land-masked. Coastal SST can diverge from offshore SST by ~0.3–0.8°C during strong upwelling or onshore-wind episodes.
OISST blends satellite + buoy data. The product is well-validated over the open ocean (RMSE ≈ 0.3°C against drifters and moored buoys). The decadal trend in the full record is dominated by basin-scale warming, not measurement noise.
This describes what happened, not why. Drivers — basin-scale ocean warming, the 2014–16 "Warm Blob," the 2023–24 El Niño — are active research topics. This page is a count, not a mechanism.
Real OISST SST for 1988–1990 produces 8 events, 52 MHW-days, ~17 days/year — all Moderate, as expected for non–El Niño years. The "all Moderate" finding of the prior proxy analysis cannot be confirmed or refuted until ERDDAP delivers the full 1982–2025 record. The analysis infrastructure is ready; the data pipeline is blocked by NOAA server availability.Status update · June 2026 · partial_data: true
Sources & method
NOAA OISST v2.1 daily sea-surface temperature. Cell: lat=17.375°N, lon=−101.625°W — the nearest valid offshore cell to La Saladita (17.5897°N, −101.4317°W); the target cell is land-masked at 1/4° resolution. Climatology baseline: 1982–2011, ±11-day smoothing window per day-of-year. MHW detection: ≥ 5 consecutive days above the day-of-year 90th-percentile threshold, with gaps of up to 2 days bridged within an event (Hobday 2016 §2.1). Categories assigned by mean intensity / (p90 − climatological mean), per Hobday 2018.
Data acquisition: OISST accessed via CoastWatch PFEG ERDDAP (dataset ncdcOisst21Agg_LonPM180), year-chunked requests. Fallback: PSL THREDDS OPeNDAP (sst.day.mean.{year}.nc, lat-index 429, lon-index 1034). Both endpoints were unavailable at time of analysis. 3-year cache (1988–1990) from scripts/build_sst_climatology.py used. partial_data: true flag set in artifact.
Trend test: ordinary least-squares regression of annual aggregates against year. Two-tailed p-value from slope's standard error. With n=3 years of data, trends are not statistically meaningful.
Prior analysis used ERA5 2m air temperature as SST proxy. Proxy analysis produced 149 events, all Moderate — a known artifact of using air temperature (lower variance, systematically compressed anomalies) in place of SST. Real OISST data is expected to produce Severe and Extreme events in strong El Niño years (2015, 2023, 2024) once full coverage is available.
References: Hobday, A.J. et al. (2016) A hierarchical approach to defining marine heatwaves. Progress in Oceanography 141: 227–238. Hobday, A.J. et al. (2018) Categorizing and naming marine heatwaves. Oceanography 31(2): 162–173.
Script: scripts/analyze_marine_heatwaves.py (OISST_PRIMARY mode) → functions/api/_findings_marine_heatwaves.js. Re-run when ERDDAP access is restored.