By Norberto Pereira

The sea state is defined by the characteristics of the waves at a particular location in a specific moment. To better define it, we have to identify the wave height (meters), being the same assigned by the average value of the trough of the wave crest height and the wave period (seconds), which is the average time interval between the passage of successive wave crests.

In this description we can still distinguish ripple from wave. A ripple is generated by the wind on its own place, it presents an irregular appearance with irregular ridges and waves from several directions in the course of the wind. A swell is generated by the spread of the ripple caused by the wind on a distant place, it presents a more regular appearance with rounded crests and the wave has a well-defined direction.

So we can characterize the sea as a large system of waves, each with its height, period and direction.

Sea characteristics in the Portuguese Coast

Continental Portugal maritime coast has different characteristics between the West Coast (Rio Minho to Cape St. Vincent) and the South Coast (Cape St. Vincent to the Guadiana River).

West Coast

The west coast is exposed to the waves formed in the North Atlantic, and this phenomenon influences the sea waves more than the simple action of the local wind.
The height of the waves is higher than 1 m for 95% of the year and more than 4m in the remaining 5% of the year.

Sea characteristics in the Portuguese Coast

Northwest Sea

This is the most frequently maritime agitation condition on the West Coast, occurring in 80% of the year. This results from the NW waving formed in the North Atlantic, and the wave associated with the dominant local winds of N and NW.

In the winter the sea waves are about 2.5 m high and 9s period on the North Cape Raso, falling to 2m 8s on south of Cape Espichel. That is related with the generation of areas in the NE edge of the Azores Anticyclone (Map 1), or situations with post-front NW circulation or depression (map 2).

Map 1:

Sea characteristics in the Portuguese Coast

Map 2:

Sea characteristics in the Portuguese Coast

In the summer it occurs a Sea NW, with sea characteristics with 1 to 1.5 m high and 8 period, associated with the meteorological situation of this season, and North wind (Nortada) scheme (Map 3) .The regions between Cascais and the Tagus bar, and from Cape Espichel to Sado bar has a sea state conditions much calmer because they are more sheltered

Map 3:

Sea characteristics in the Portuguese Coast

South West Sea

It occurs Sea SW when we have sea of 3 to 4m and with possibility to reach up to 7m and periods of 10s; with maritime agitation conditions associated with depression centered the SW of the Iberian Peninsula (Map 5), or approaching cold front surfaces (Map 4). This results in SW training areas along the coast, occurring mainly in winter and on transition periods. For including NW components, they formed a cross sea.
In the Summer, sea of SW is rare and, when it occurs, usually does not exceed 3m.

Map 4:

Sea characteristics in the Portuguese Coast

Map 5:

Sea characteristics in the Portuguese Coast

Outside Sea

It occurs when the wind is weak, or its course is the land quadrants.
In the winter, these conditions are influenced by the Blocking Anticyclone (Map 6).
In the summer this happens when there is no North wind (Nortada) and maritime agitation has characteristics up to 2m in height and period of about 14s

Mapa 6:

Sea characteristics in the Portuguese Coast

Sea Smooth (Banzeiro)

It occurs in situations where the wind is weak, or its course is the land quadrants.

The maritime agitation is predominant from NW or WNW with 0.5m and it happens about 4% of the year the North, and 10% in the South West coast. (Map 7)

Map 7:

Sea characteristics in the Portuguese Coast

South Coast

More sheltered than the West Coast, the South Coast has much milder maritime conditions, with wave height less than 1 m for 60% of the year, and the maximum height is on average about 4m.

South West Sea

We have Sea SW with sea characteristics of 2 to 3m and period of 8s, associated with weather events that originate in the western coast sea of SW or temporal W (Map 5). These combined conditions cause storms on the south coast, especially on the region between Cape Sagres until Cape Santa Maria.

South East Sea (Levante)

It occurs Sea SE when there's a sea characteristic with about 2m and periods of 6s for 10% of the year. This sea type is associated with the occurrence of wind E in the Atlantic region on SE of Algarve, especially in the Strait of Gibraltar where sometimes the wind is very strong (map 8); so it's usual to rise in a few hours SE waving that could reach up to 4m.

Map 8:

Sea characteristics in the Portuguese Coast

Sea North (Nortada)

It is generated by the occurrence of wind quadrant N, sometimes fresh and very fresh wind, and in particular for W of Lagos. The sea state is characterized with waves of 0.5m high, decreasing as we move away from the coast.

Breeze Sea

It is the most frequent in the southern Algarve coast, occurring during 70% of the year. It is formed by the local breeze, maintaining throughout the day a SW swell very weak.
It begins to rise SE crisply morning with 0.5m, and rotates with the wind, making it small SW wave 1m, falling to the end of the day to less than 0.5m.

Regarding these resumed explanations I hope that you have a better understanding of weather and waves systems in general, and especially on Portuguese coast. Understanding how waves are made, how they normally behave, and how to predict their response to weather can make the difference between a smooth, comforting time on the water, or a bumpy and frightening experience.
For your safety, observe the weather stations and stay always alert to notices on VHF radio.

For navigators

If you are planning to travel in west coast;

  • From South to North, it is better to do it in the first hours of the day until 2 Pm. After that hour for sure you will catch Sea and wind from North.
  • From North to South, you can do it at any hour without major problems.
  • If you are planning to travel in southern Algarve coast;
  • From leeward to windward, you will catch throughout the day a SW swell very weak (0.5m) until Lagos. From Lagos to Cape S.Vicente is better to rib on the first hours of the day until 2 Pm. After that hour for sure you will catch Sea and wind from SW and, in general, it only decreases after sunset.
  • From windward to the leeward, you can do it at any hour without major problems.

Remember, the waves can be dangerous and destructive to any vessel at open sea!

References

  • Charts and Maps
  • IM-Instituto de Meteorologia, atual Instituto Português do Mar e da Atmosfera

 

...keep Ribbing!