What is surge? What is the typical phenomenon when the turbine compressor has surge?

Surge is a kind of vibration under abnormal working condition when the flow of turbine compressor (also called vane compressor) decreases to a certain extent. Centrifugal compressor is a form of turbine compressor, and surge is very harmful to centrifugal compressor.
When centrifugal compressor has surge, typical phenomena are:
1) the outlet pressure of the compressor firstly rises, then falls sharply, and fluctuates periodically;
2) the flow rate of the compressor drops sharply and fluctuates greatly. In severe cases, air is even poured backward to the suction pipe;
3) the electric current and power meter of the motor dragging the compressor indicate instability and large fluctuation;
4) the machine generates strong vibration and generates abnormal airflow noise at the same time.