What causes the climate in Tibet to be "cold in the south and warm in the north"?

Your question involves the unique atmospheric circulation in the Qinghai-Tibet Plateau. It is the knowledge in university meteorology.

Simply put.

Due to the high altitude of the Qinghai-Tibet Plateau, the air density of the free atmosphere on the plateau is lower than that outside the plateau in winter, which leads to the existence of cold high pressure on the plateau, that is, the cold source, whose center is west of the central plateau, but the north and south sides of the cold source are different. Due to the southerly airflow prevailing in the north, the temperature is higher, and the south is also warmer due to the influence of Indian Ocean monsoon currents.

In summer, the plateau surface is a heat source, which is hot in the middle, cold in the north and south, and northerly airflow in the north, so it is cold in essence.

Your question is one-sided and should be understood according to my statement.