How to Predict and Spot an Upwelling Event off the Florida Keys
Upwelling is what brings cold, green, food-rich water up onto the reef. The Gulf Stream wall is the on/off switch. This guide shows you how to know days ahead if the green water will be there — and exactly what to look for when you arrive on the water.
What is upwelling? (the short version)
Imagine the ocean in two layers: warm water on top, cold water below. The boundary between them is called the thermocline. The cold water down deep is full of nutrients — the stuff plants need to grow.
Upwelling is when that cold, nutrient-rich water gets pushed UP from the deep onto the reef. When it arrives, tiny plants (phytoplankton) bloom and turn the water green. Small fish eat the plants. Big fish eat the small fish. So green water = food = fish.
The thing that pushes the cold water up is the Gulf Stream pressing against the underwater wall at the edge of the reef. When the Stream is close, the upwelling engine is ON. When it pulls away, the engine is OFF.
The on/off switch: where the Gulf Stream wall sits
Upwelling on the reef needs the Gulf Stream wall pressed close to the underwater shelf edge (within 2–5 miles). When the wall is close, the Stream squeezes water against the wall, makes internal waves, and the cold pulses fire. When the wall pulls away, the engine shuts off. So predicting the upwelling is the same as predicting whether the wall will be close or far when you get there.
Wall is close (2–5 miles)
The Gulf Stream is pressed right up against the edge of the reef. Its fast flow squeezes water against the underwater wall and makes internal waves. Those waves push cold green water up onto the reef. The engine is running.
Wall moves away (15+ miles)
The Gulf Stream pulls away from the reef. With no current pressing in, there are no internal waves and no cold green water. The reef turns blue and the food chain shuts down until the wall comes back.
Wall coming back (getting closer)
As the wall moves back in, the engine starts up again. The first cold pulses show up within one tide cycle of the wall returning to 2–5 miles. The green water rebuilds over 1–3 days. This is when you get back on the spots.
Step 1 · Forecast (days ahead): will the upwelling be ON?
Check these before you commit to the trip. They tell you whether the engine will be running when you get there.
Step 1: Check how far the wall is
If the Gulf Stream wall is within 1–6 miles of the reef, the upwelling engine is ON. If it is 15+ miles away, the engine is OFF. This is your best single clue. You pull it from your Gulf Stream readings.
Step 2: Check the big picture in the Gulf
Is the Loop Current stretched out or pulled back? Is there a spinning eddy growing in the Gulf? A pulled-back Loop plus an eddy means the slow period (and no upwelling) lasts 3–7 days, not just hours. The Gulf tells you how long the off-period will last.
Step 3: Read the trend
Is the wall moving away (engine shutting down) or moving closer (engine restarting)? The trend matters more than the exact number. A wall moving away is your early warning that the green water is about to leave.
Step 4: Check the season
Upwelling needs a layered ocean — warm water on top, cold water below, with a clear boundary between them. That only happens in summer (June–September, peak July–September). Outside summer, the water mixes up and no upwelling happens. No layering, no upwelling.
Step 5: Look for green water on satellite
On a satellite chlorophyll map (SatFish, ROFFS, or Hilton), a green plume pushing onto the reef at Conch, Alligator, or Molasses means cold pulses have been firing for days. Green on the reef = the upwelling has been running.
Step 2 · Timing (day of): when do the cold pulses hit?
The cold pulses run on a clock — but not the tide clock. Here is how to time it on the day.
About 2 pulses a day (~12 hours apart)
Expect roughly 2 cold pulses every day, about 12 hours apart. Once you see the first temperature drop, write down the time — the next pulse comes about 12.4 hours later. The rhythm is reliable; which tide it lands on is NOT.
Do NOT trust your tide app
The underwater wave (internal tide) does NOT match the surface tide you see on your app. The cold pulse can hit on a rising tide, a falling tide, or slack water. There is no fixed rule you can look up. The timing shifts with how deep the cold layer is and where the Stream sits.
The bottom temp probe is the proof
Drop a temperature probe and watch for a sudden 5–12°F drop in just a few minutes. When the number falls fast, the cold pulse just arrived on your spot. Everything else is a guess; the probe is the proof.
Step 3 · Spotting (on the water): how to see it
These tell you the cold pulse is arriving right now. No guessing — it is happening in front of you.
Water changes color (blue to green)
This is the easiest sign to see. Blue Stream water turns green on the surface or on your sonar. The green is tiny plants (phytoplankton) growing because the cold water brought nutrients up from the deep. Mark that color-change line — it is your highway.
A sharp color-change LINE
You want a clear edge where blue meets green — not a blurry green wash everywhere. Bait fish stack right on that line. The bait holds the green side; the big fish hold the blue side looking in. A sharp edge always beats a blurry one.
Bait stacking on the edge
Watch your sonar. When bait suddenly shows up stacked right on the color-change line, the cold pulse just delivered lunch to the reef. That is your real-time proof the upwelling food chain is lit up — and the muttons will be on it.
The sonar wedge (the cold pulse itself)
A dark, slanted band lifting off the bottom and climbing the wall, with bait stacked along its top. The cold dense water sends a stronger signal back to your sonar than the warm water above. That wedge rising on your screen is the cold green water arriving right now.
A fast temperature drop
A sudden 5–12°F drop in just a few minutes on your bottom temp probe or fishfinder temp sensor. Scientists measured drops up to about 10°F in 1–20 minutes at Conch Reef. When the number falls, the cold pulse is on you right now.
Algae and hermit crabs on your hook
If you pull up green algae (like Halimeda or sea lettuce) or hermit crabs, you are catching proof that the upwelling food chain is running. The algae is the cover on the bottom; the hermit crabs are the food. Lots of algae = the upwelling engine is running. Stay on that spot.
The sonar wedge: how to tell a cold pulse from lookalikes
A normal sonar shows flat water between the surface and the bottom. A cold pulse shows a dark, slanted wedge lifting off the bottom and climbing the wall, with bait stacked along its top edge. Here is how to tell it apart from other things that look similar:
Thermocline (no cold pulse)
A thin, flat, horizontal line across the screen — the temperature boundary sitting still.
Cold pulse: A cold pulse is that same layer TILTED and moving, getting thicker as it climbs.
Bait ball
A round, dense blob, usually in the middle of the water column and moving around.
Cold pulse: Cold-pulse bait stacks ALONG the slanted layer, not in a free-floating ball.
False bottom / hard return
A solid line that does not move over time.
Cold pulse: The cold-pulse layer MOVES — it rises and falls over a few minutes.
Why the algae is here today, gone tomorrow
You mark a spot thick with algae one day, run back to the same numbers the next day, and the sonar looks bare. The reef did not move — the algae did, or your view of it did. Here are the four reasons, and how to tell which one you are seeing:
It lays flat vs. stands up (current)
Algae has no stiff stalk — it flops with the current. When the current is ripping, the whole mat lays flat against the bottom and your sonar shows bare hard bottom. When the current slows or reverses, the same algae stands up and looks like a fuzzy carpet. Same spot, same algae, two different sonar pictures.
The tide moved the loose algae
A lot of what you see is not attached algae — it is loose drift rolling along the bottom with the current. An outgoing tide can sweep a whole patch off a ledge and drop it 200 yards away. The next day the tide brings it back. The reef did not change; the algae just moved.
Upwelling pushed it up (or pulled it down)
When a cold pulse hits, nutrient-rich water rushes onto the reef and the algae grows fast — it gets taller, thicker, and easier to see. When the upwelling stops, the algae settles back down and thins out. A spot that looked alive yesterday can look bare today if the cold pulse shut off overnight.
Your sonar settings or boat speed changed
Run the boat faster and the algae layer blurs into the bottom and disappears. Turn up the color gain and it pops out; turn it down and it vanishes. CHIRP and DownScan catch the fuzzy layer where a basic 200 kHz sonar misses it. Same water, different picture.
How to tell which one: Note the current speed and tide direction when you mark it vs. when you do not. If it only shows at slack or on one tide, it is laying flat or drifting. If the bottom temp dropped a few degrees before it appeared, it is upwelling-driven growth. If the spot, tide, and current are all the same and it still vanishes, check your gain setting.
The simple workflow
You cannot predict the exact minute a cold pulse arrives from shore. The timing between the surface tide and the underwater wave shifts with how deep the cold layer is and where the Stream sits. The reliable workflow is:
- Pick the day (Step 1): Summer + wall close + green plume on satellite = go.
- Set the timing (Step 2): Expect about 2 pulses, 12 hours apart. Write down the first one you see.
- Catch the pulse (Step 3): Watch the bottom temp probe and the sonar wedge. When the temp drops and the wedge climbs, the cold pulse is on you — set up on the green/blue edge and fish the 4-hour window the green water sits there.
The bottom temp probe is the tie-breaker. Everything else is a guess; the probe is the proof.
The one-page checklist
| Check | What to look for | Tool |
|---|---|---|
| Season | June–September (layered ocean present) | Calendar |
| Wall distance | Within 1–6 miles of reef = engine ON | Gulf Stream readings |
| Wall trend | Getting closer = restarting; moving away = shutting down | Readings over time |
| Gulf view | Loop pulled back + eddy = off for 3–7 days | Gulf current map |
| Green plume | Green water plume on the reef | SatFish / ROFFS |
| Water color | Blue turns green on the spot | Eyeball / sonar |
| Sonar wedge | Dark slanted band climbing the wall + bait on top | Fishfinder |
| Bottom temp | Fast 5–12°F drop in minutes | Temp probe |
| Bioindicator | Algae / hermit crabs on the hook | Check the hook |
When all 9 line up — wall close, summer layering, green plume on satellite, blue-to-green color change on the spot, a sonar wedge climbing the wall, a fast temp drop, and algae/hermit crabs on the hook — you have found an active upwelling pulse on a mutton spot. Fish the green/blue edge, not the middle of the green.
Go deeper
Upwelling Prediction Guide (8th Grade Edition) · Florida Keys reef tract
Simplified from Leichter et al. (1996), NOAA/RTOFS, and SatFish chlorophyll imagery.