Seasonal outlook — 500 hPa pressure
Three-month forecast of the 500 hPa geopotential (Z500) from ECMWF SEAS5: it tells us whether the central Mediterranean is expected to be dominated by a persistent subtropical ridge (the precondition for marine heatwaves) or by a more ventilated Atlantic flow.
Overall picture over the central Mediterranean
Mean anomaly of 500 hPa geopotential height against the 1993-2016 reference. Positive values (red) = stronger than normal high pressure; negative values (blue) = more Atlantic circulation.
Z500 anomaly map — next 3 months
The three panels show the predicted monthly evolution. Black rectangles are the 6 TYRN SEA zone boxes: the pressure above Calabria is the most relevant indicator for the risk of marine heatwaves along our coast.
Detail by coastal zone
Z500 anomaly point value above each TYRN SEA zone. The seasonal grid is about 100 km, so adjacent zones often show very similar values: that is the spatial scale at which pressure travels.
| Zone | Settembre 2026 | Ottobre 2026 | Novembre 2026 |
|---|---|---|---|
| Tirreno Nord | +47,5 m Ridge subtropicale forte | +3,0 m Neutra | +20,2 m Ridge subtropicale forte |
| Tirreno Sud | +44,2 m Ridge subtropicale forte | +3,4 m Neutra | +21,5 m Ridge subtropicale forte |
| Estremo Sud | +44,2 m Ridge subtropicale forte | +3,4 m Neutra | +21,5 m Ridge subtropicale forte |
| Ionio Sud | +44,6 m Ridge subtropicale forte | +4,5 m Neutra | +21,0 m Ridge subtropicale forte |
| Ionio Nord | +48,0 m Ridge subtropicale forte | +4,1 m Neutra | +19,7 m Ridge lieve |
| Amantea-Lamezia | +48,0 m Ridge subtropicale forte | +4,1 m Neutra | +19,7 m Ridge lieve |
How to read it
The 500 hPa geopotential height is the altitude (in metres) at which the 500 hPa pressure surface is found. When this surface is higher than normal (positive anomaly), there is a robust high pressure below: warm air, clear sky, weak wind. This is the pattern that recent literature links to summer Mediterranean marine heatwaves [7].
Classification thresholds
| Anomaly | Class | Meaning for the Calabrian sea |
|---|---|---|
| > +20 m | Strong ridge | Robust persistent high pressure: strong likelihood of marine heatwaves and of preconditions for algal blooms and mucilage. |
| +5 ÷ +20 m | Mild ridge | Tendency to high pressure, MHW episodes possible but not guaranteed. |
| −5 ÷ +5 m | Neutral | Circulation near the seasonal norm, evolution driven by short-term fluctuations. |
| < −5 m | Atlantic flow | Low pressure, westerly winds, well-mixed sea: scenario unfavourable to MHWs and stagnation. |
Important caveat: summer skill is low
Seasonal forecasts for the summer Mediterranean have limited skill beyond the first month: in the warm season the circulation is dominated by small-scale convective phenomena hard to capture three months ahead [7]. The outlook should therefore be read as a qualitative indication of the trend, not a precise forecast. For this reason we do not use it in the 5-day bloom and mucilage precondition indices, which rely only on observed data and short-range weather forecasts.
Data and updates
- Model: ECMWF SEAS5 (system 51, 51 membri ensemble)
- CDS dataset:
seasonal-postprocessed-pressure-levels - Variable:
geopotential_anomaly@ 500 hPa, media mensile - Reference climatology: 1993–2016 (hindcast ECMWF)
- Update: once a month, around the 14th (after the ECMWF release on the 13th).