Shawn Nafziger
Travel · Mexico and the Caribbean

Where the seaweed lands

Every summer, seaweed drifts in from the Atlantic. It hits Mexico's Caribbean coast and Punta Cana most weeks, while Aruba, Nassau and Turks & Caicos almost never see it. And along any one coast, the beach you pick matters: some get seaweed close to shore all season, while others a few kilometers away almost never do.

Beach detail: Mexico's Caribbean coast only

Every summer, it rides in from the open Atlantic

Sargassum grows in a huge belt across the tropical Atlantic, out past the right edge of this map. Ocean currents carry it west into the Caribbean.

Each dot shows seaweed a satellite spotted floating that month. Press play to watch a year.

Seaweed off Mexico's Caribbean coast, by month

In 2024 most of it stayed out east. In 2025 and 2026 it piled up off Mexico.

Mexico or the Caribbean?

How to read: each bar is how many summer weeks (May–September, 2018–2026) satellites saw seaweed in the open water off that coast, about 12–40 km out.

This compares whole coasts, not beaches. Along any coast, which way a beach faces matters a lot, as the Mexico maps below show.

The coastline tells you where it lands

The shoreline is colored by how often satellites spotted seaweed floating within 3 km of that beach, May–September. Darker brown means more often.

Satellites can see seaweed in the water, not on the sand. Seaweed this close to the beach is usually on its way in.

What pushes it ashore

Teal arrows: the ocean current runs north along the coast, about , carrying seaweed up from the south.

Dashed arrows: the wind blows in from the east, about on average, pushing floating seaweed toward the shore.

That helps explain the pattern: beaches facing east into the wind collect it, while beaches tucked behind a point or facing north or west see much less.

May–September averages. Current: 2018–2025. Wind: 2018–2021.

The open coast south of Playa del Carmen gets hit often

Around Akumal and Tulum, seaweed floated near the beach on about and of . Farther south, Mahahual: .

Same island, two coasts

Cozumel's east side, open to the sea: seaweed near the beach on of .

The west side, facing the mainland, where the town and cruise piers are: .

Cancún splits the same way

The hotel zone's north-facing beaches, on the Bay of Mujeres: seaweed near the beach on of .

The Caribbean side, a few kilometers south: , as often.

Pick your destination

Month by month, 2016–2026
Each square is one month, same colors as the map.
Outlined: your travel month, 2018–2026 (the years counted above).
Gray: too cloudy to tell.
Every beach, every summer (May–Sep)

Across all 13 places, 2025 and 2026 were the two worst summers since 2018. Calm summers still happen: 2024 was the quietest.

How this was measured

Mexico's national Earth-observation lab (LANOT, at UNAM) maps every patch of seaweed it can see in Sentinel-2 satellite images, about every five days. We counted how often a patch showed up within 3 km of each stretch of coast, and divided by the number of clear-sky days, when a satellite could actually see the water there, so cloudy months don't look falsely clean.

What it does and doesn't tell you

  • The Atlantic map shows where seaweed was floating and how it moved, not how many tons there were.
  • The coastline map shows seaweed approaching the coast. It doesn't measure what washes up on the sand on a given day.
  • Resorts often clean their own beaches every morning, so a stretch can look worse here than it does in front of one hotel.
  • Each colored piece of coastline is about 1 km long, but it counts seaweed within 3 km of its middle, so neighboring beaches blend a little.
  • "A seaweed month" means satellites spotted seaweed within 3 km of the beach on at least 2 separate days that month. One sighting can be a passing patch; two or more means it kept coming back. A month gets only about 7 clear-sky days, so this is a yes-or-no count, not a precise rate.
  • Months with fewer than two clear-sky days are shown gray, not clean.
Measure: satellite days with seaweed within 3 km of shore ÷ clear-sky days (LANOT's cloud-adjusted count of satellite passes; a half-cloudy pass counts as half a day), May–September 2018–2026. Places and coastline pieces: a 3 km circle around each point.
Seaweed month: at least 2 satellite days with seaweed within 3 km of the beach in that month; months with fewer than 2 clear-sky days are left out.
Source: LANOT, Instituto de Geografía, UNAM, Sentinel-2 sargassum detections, 2016 to Sep 10, 2026. Arellano-Verdejo, J., et al. (2025). Towards enhanced Sargassum monitoring in the Caribbean Sea. Scientific Reports 15, 8965. https://sargazo.lanot.unam.mx/home/sargazo/
Atlantic map: University of South Florida Floating Algae Index (AFAI), 7-day composites from NASA's MODIS satellites, served by NOAA AOML (dataset noaa_aoml_atlantic_oceanwatch_AFAI_7D). Two composites per month (the 8th and the 22nd).
Atlantic dots: 0.3° squares (about 33 km); color = share of clear water in that square where the index stood out from its surroundings (more than 0.0015 above the ~60 km ). Levels: a few patches 1.2–3% of the water, some 3–7%, a lot 7–15%, thick 15% or more; below 1.2% is not shown (mostly noise). Water within ~12 km of land, water shallower than 15 m (NOAA ETOPO1 depths; mostly the Bahamas banks), and water next to cloud gaps is left out, because murky shallow water, a bright sea floor and cloud edges light up the index too. Dots off Florida and the US East Coast are mostly the older Sargasso Sea seaweed carried by the Gulf Stream.
Atlantic coverage: the product covers the Gulf, the Caribbean and the tropical Atlantic to 38° W; the gray area has no coverage. Near Mexico it agrees with the LANOT data month by month ( 0.84 over 28 months, 2024–2026).
Mexico or the Caribbean: same USF index, four 7-day composites a month (the 4th, 11th, 18th and 25th), May–September 2018–2026. For each destination, open water within 40 km, leaving out the 12 km nearest land. A "seaweed week" is a composite with seaweed in at least 3% of that clear water; weeks with less than a quarter of the water clear are left out. The order of destinations barely changes with the cutoff (1% to 10%).
Mexico or the Caribbean, limits: this is offshore water, not the beach. Near Mexico, its year-to-year ups and downs only partly match the near-shore LANOT measure (rank agreement 0.62 over 27 destination-summers), so use it to compare coasts, not to predict a beach.
Currents: NOAA AOML Near Real Time Geostrophic Currents (0.2°), May–Sep 2018–2025. These show the broad flow offshore, not the surf zone.
Wind: NOAA AOML, METOP-A ASCAT 10 m winds (0.25°), May–Sep 2018–2021.
Shoreline: GSHHG (Wessel and Smith), full resolution.
Map: MapLibre GL JS.