Making a contourfm plot over a map transparent

I need to make a contourfm plot over top of a map semi-transparent (aesthetic purpose) and am unsure how to do it. Below is an example code and I would need to make the "geoid" data appear transparent so the globe image was easier to see:

figure
axesm eckert4; framem; gridm; axis off; tightmap

load geoid
contourfm(geoid, geoidrefvec, -120:20:100, 'LineStyle', 'none');

coast = load('coast');
geoshow(coast.lat, coast.long, 'Color', 'black')

contourcbar