What are these triangles and ribbons on this MD83's wing, and what do they do?
These are Ice Detection Tufts that check for ice. If they are stuck, it indicates there's a layer of ice on the wing, disturbing airflow and presenting a danger for the engines when it detaches. They are focused around the rear root side since this is most likely to be the spot with the most buildup, as remaining cold fuel below flows to this corner.
SAS had an incident with ice a few years ago in the Gottröra Accident (SAS Flight 751) that thankfully ended relatively well.
Some more information about this particular item.
AD 2002-21-06 addresses the issues with detecting wing ice on the DC-9 series of aircraft (which includes the MD-83).
Some aircraft of this type have a wing surface ice detection system installed.
There is also a modification available that automatically moves fuel between the wing and center fuel tanks to prevent the supercooled fuel from accumulating in wing tanks.