Coastal upwelling regions Coast of North Africa; Phytoplankton bloom through upwelling. UJCC now combined with HiGEM. John Donners UK-Japan Climate Collaboration / HiGEM NCAS Climate University of Reading

Introduction Coastal upwelling regions are a strongly coupled system of clouds, winds and SST. Important for fisheries, carbon cycle and coastal communities. Observations suggest intensification in the last century. Modelling studies used either regional, uncoupled models or coarse, global models. The SSTs in HiGEM vs. HadGEM decrease in these regions, improving the simulation.

Changes in summer SSTs at increased atmosphere res. Annual meanCoastal SSTs

Changes in summer sw. rad. at increased atmosphere res.

More low clouds at increased atmosphere resolution More low clouds near the coast, but less further away. Unfortunately no further explanation of the mechanism.

RH@1000mbar increases in the NH HadGEM HiAtmos

Mechanism unclear in SH Liquid cloud water@925mbar HiGEM-HadGEM RH@925mbar HiGEM-HadGEM

Seasonal cycle of SST So the atmosphere clearly cools SSTs through shielding shortwave radiation. What does the ocean do?

Upwelling summer winter Net upwelling is in balance with ekman transport, but upwelling is much more concentrated along the coast in the hi-resolution ocean models. Also note that Canary islands are now represented.

Ocean temperature

Deep upwelling Warning: different color scales! ~ 5mm/s ==> 13 m/month ~ 25mm/s ==> 65 m/month Observations: tens to hundreds m/month

Cloud feedback to upwelled SST

Future upwelling? Transient climate change simulation: 2% CO2 incr./year Averaged over 20 years, ~ 2x CO2 Suggests that upwelling regions intensify and warm up less than their surroundings.

Summary Global coupled models lacked resolution to represent upwelling adequately; regional studies used atmosphere- or ocean-only models. Both higher atmosphere and ocean resolution improve the seasonal cycle of SSTs: either due to cloud or upwelling effects. Upwelling at higher ocean resolution is concentrated near the coastline, drawing colder water from deeper down the water column. Cloud feedback enhances SST cooling due to upwelling.

The end

Unsolved mysteries

Seasonal cycle of SSTs in SH No decrease in the amplitude of the seasonal cycle. Cloud feedback to cold, upwelled SSTs only important in the NH? Influence of Agulhas rings? Orographic forcing of the Andes mountain range?

SST variability SST variability decreases with resolution in the NH; in closer agreement with obs. However, the SH shows again a different behaviour, with hardly any change in SST variability. Suggests stronger coupling of cloud and SST in the NH than in the SH?