Southern Caribbean upwelling system

The Southern Caribbean Upwelling system (SCUS) is a low latitude tropical upwelling system at about 10°N on the southern coast of the Caribbean sea basin, off Colombia, Venezuela, and Trinidad. The intensity of the Caribbean low-level jet and the coastal orientation are determining the timing and spatial variability of the system. The system is likely to be responsible for a major part in the primary production due to the additon of nutrients that are added to the system through the transportation.

There are two main upwelling zones in the system that vary in intensity throughout the year; The Western Upwelling Zone (WUZ); and the Eastern Upwelling Zone; (EUZ).

The western the WUZ is situated between 74-71°W and generates mainly seasonal upwelling and High offshore transport due to intense winds. The EUZ, situated between 71-60°W is less intense but is more favourable throughout the year.

The last thirty years the upwelling is intensifying producing a cooling of the sea surface temperature (SST) in the WUZ, This is in contrast to the general temperature in the Caribbean sea which has shown to increase. The "typical" Caribbean surface water is a mixture of North Atlantic Surface Water (NASW) and riverine waters from the Orinoco and Amazon rivers.

Under the Caribbean surface waters higher salinity water is found with values close to those typical for the Subtropical underwater (SUW) SA ~37, Θ ~22°C. this forms a subsurface maximum(SSM). after the rainy season the SSM is lower due to dilution of the surface waters

Characterization of the SCUS

From 1994 until 2009 SST data was used to characterize the SCUS. Variations in upwelling are studied using cycles of satellite SST, this is used as a proxy for upwelling in this tropical region; chlorophyll-a and sea-wind as well as hydrographic data from the world ocean atlas.

The Rossby radius $R={\sqrt {{g}{\frac {\Delta \rho }{\rho }}{\frac {h}{f}}}}$ characterizes the cross-shelff scale of upwelling. The rossby radius for this region is ~19km; estimated using a mean depth h= 35m, gravity g = 9,81 m s -1. The upwelling zones are found close to the coast roughly withing the 19km found within the Rossby radius. however, in exceptional cases upwelled water move offshore up to over 250km from the coast.

Upwelled waters in the SCUS are consistent in geochemical compositions of the SUW and are sourced from this water mass.

Sea surface temperature (SST)

To SCUS is studied through the SST at a high resolution (1km grid) radiometer (National Oceanic and Atmospheric Administration / NOAA) this data is used to identify differences in the SST and with that locate upwelling regions. There is a semi-annual cycle of SST within the upwelling areas. With cooling periodically occurring between December and April, showing 2-4 upwelling pulses peaking during February-March. Around May there is a typical increase in SST followed by cooling during June-August due to a midyear upwelling.

Wind

The Trade winds that blow over the southern Caribbean Sea generate northward Ekman transport The intensity of the trade winds vary per season and explain the variation in upwelling and the measured differences in SST that are mentioned above. The driving forces of the Ekman transport is wind-stress or "τ" The driving winds are divided in multiple areas. East of 68°W have relative stable wind speed (>6 m $s^{-}1$), and slightly lower during August-November (4 - 6 m $s^{-}1$).

The direction of the wind is generally parallel to the southern Caribbean Sea margin, however, between May and October the EUZ has more along shore winds that are ~1.7° varying from the along shore direction and During November until April the direction is more onshore with ~12°. In the WUZ the wind is more offshore in the majority of the year ~-14°. This changes during October-November when winds are aligned with the shoreline ~0.2°.

These wind directions respectively to the coast produce the northward and thus offshore Ekman transport that are favourable year round for the SCUS.

Chlorophyll-a

Chlorophyll a is used to see the productivity of zooplankton, amounts of Chlorophyll-a increase with higher nutrient concentrations that are found in upwelled water and can therefore be used as a proxy for upwelling systems . Withing the SCUS there are strong correlations between the SST and Chlorophyll-a. These show a Chlorophyll-a maximum in December and April and a shorter maximum between June and July.

Biological impact

Nutrient concentrations that come with the water from the SUW have a strong positive impact on the phytoplankton productivity, it is estimated that up to 95% of the small pelagic biomass is the southern Caribbean sea is sustained by the primary production that comes with these upwelled waters.

In the EUZ there a four time higher amount of small pelagic biomass compared to the WUZ. This difference is contributed to the prolonged duration of the upwelling. The water in the EUZ has a SST < 26C° for 8.5 months and the WUZ for 6.9. In addition to that, the EUZ has a wider continental shelf. Upwelling over wide and shallow continental shelves can generate resuspension and transport of essential microelements from the benthic boundary layer to the surface.