The monsoon effect on energy and carbon exchange processes over a highland lake in the southwest of China

Abstract

Abstract. Erhai Lake is a subtropical highland shallow lake on the southeast margin of the Tibetan Plateau, which is influenced by both South Asian and East Asian summer monsoons. Based on 4 years of continuous eddy covariance (EC) data over Erhai Lake, the monsoon effect on water–atmosphere exchange processes is investigated by comparing the energy and CO2 flux patterns and their main drivers during pre-monsoon (March–April), monsoon (May–October) and post-monsoon (November–December) periods. The results show that the atmospheric properties display a large difference during the three different periods. There is a negative difference between water surface and air temperature (T) during the pre-monsoon period, while a positive T during the post-monsoon period. The diurnal sensible heat flux (Hs) is larger during the post-monsoon period, while the latent heat flux (LE) is larger during the monsoon period. The monthly average Hs and heat storage (Q) in the lake remain negative during the pre-monsoon period and the early monsoon period, and they become positive in the middle monsoon period, which indicates that the lake absorbs heat at first and releases it subsequently. LE plays a dominating role in the energy partitioning of the lake. The Bowen ratio is higher during the post-monsoon period. The uptake of CO2 flux is observed in the middle of the day during monsoon and post-monsoon periods. The ΔT is the main driver for Hs and the effect of ΔT is increased as timescales are extended from half-hourly to monthly. The wind speed has a weak effect on Hs but a strong effect on LE and CO2 fluxes. Similar main drivers for Hs are found during the pre-monsoon and post-monsoon periods, which is also found for CO2 flux, indicating a large impact of the monsoon on the heat and carbon exchange processes over Erhai Lake.