Monthly BLT

In NL, the monthly BLT became obviously lower (about −0.4 ◦C/decade) from July to September with a 0.01 significance level (Figure 5c). Its warming surface intensified the stability in the stratification period of the deep lake and resulted in less heat being transferred to the bottom. Changes in NL BLT in other months were not insignificant.

Owing to the shallow depth of D1F and HSP, there were no big differences between monthly LSWT, MLT, and BLT in the two runs during ice-free periods (Figure 5). The shallow lake depth and high salinity made the winter BLT increase faster in HSP than in other runs.

### *3.3. Effects of Local Climate Drivers on the Lake Warming* 3.3.1.Correlationbetween LSWTandMeteorologicalForcing

The study region was experiencing the same rapid climate change as most of the TP during the study period [52]. All trends of ITPCAS meteorological variables (Figure 6) over the study region passed the 0.01 significance level except for solar radiation, with a 0.1 significance level. Ta increased by 0.49 ◦C/decade, downward longwave radiation LWD increased at a rate of 3.40 W m<sup>−</sup>2/decade, and the specific humidity Q grew at 0.16 g kg−1/decade.

**Figure 6.** The ITPCAS annual mean meteorological variables and their trends: air temperature Ta and wind speed WS (**a**) and downward shortwave radiation SWD, downward longwave radiation LWD and specific humidity Q (**b**).

The increasing Ta, LWD, and Q had positive effects on lake warming, with around 0.8 correlation with LSWT in the three experiments (Figure 7a). Wind speed decreased by −0.23 m s<sup>−</sup>1/decade and was negatively correlated with the LSWT (r = −0.6). The downward shortwave radiation SWD decreased at −1.51 W m<sup>−</sup>2/decade, acting in the opposite direction, with a correlation coefficient of −0.15.

**Figure 7.** Correlations between the meteorological variables and LSWT (**a**), the percentage difference of LSWT warming rate between the control simulation S-(lake) and the sensitivity simulations S- (Lake)-d (variable) in NL, D1F, and HSP (**b**), the difference of LSWT and the warming rate between the S-HSP control simulation and S-HSP-(parameter of salinity effect) sensitivity simulations (**c**).

### 3.3.2. Quantified Contribution of Individual Meteorological Forcing to Lake Warming

The above correlation analysis showed the effects of meteorological variables on lake warming, but their quantified contributions were still unknown. Therefore, detrended sensitivity experiments, referred to as S-(Lake)-d (variable) in Table 2, were performed to attempt to answer this question, although the sensitivity experiments were a little artificial. The increase of atmospheric longwave radiation contributed the most to the warming of TP lakes, causing 30–40% of the annual LSWT change (Figure 7b). Increasing Ta induced a 30% increase of LSWT. The combined increase of atmospheric longwave radiation and Ta could explain almost the all of the observed LSWT warming. The decreasing WS caused a 20–30% increase in LSWT, especially in HSP. Air humidity increase accelerated the lake surface warming by 20% and, consequently, should not be ignored, as it was in a study of the Nam Co Lake in TP [15]. The decrease of SWD decelerated warming by about 10%.

### *3.4. Effects of Salinity Parameters on the Lake Warming*

The cumulative effect of salinity on the lake water properties caused the simulated lake surface to be 2.6 ◦C warmer and experience 0.02 ◦C/decade faster warming in S-HSP than in its freshwater counterpart (Figure 4).

The salinity effect on Tf and Rsvp were simulated, making the saline lake surface 2.34 ◦C and 0.9 ◦C warmer (Figure 7c) in S-HSP than in the two sensitivity experiments of salinity effects (S-HSP-Tf and S-HSP-Rsvp), inducing about 90% and 31% annual differences between the saline lake and the freshwater one. The lower freezing point also significantly accelerated the increase of simulated LSWT in saline lake warming to 0.07 ◦C/decade (3.5 times the warming rate difference between the saline lake and the hypothetical freshwater lake with the same 1 m depth), while the salinity effect on the saturation water vapor pressure had no impact on the long-term temperature trend.

The differences between the S-HSP and S-HSP-η0 simulations showed that the lower transparency of salt water with higher η0 increased the annual LSWT by only 0.02 ◦C and slowed the warming by 0.01 ◦C/decade.

The difference between the S-HSP and S-HSPα simulations showed that the higher α of salt water cooled the lake surface by 1.24 ◦C annually (about −47% of the annual LSWT difference between the saline lake and the freshwater one) and slowed the warming rate by 0.02 ◦C/decade (the same magnitude as the warming rate caused by all the salinity effects).

Changes in the temperature of the maximum density (Tmaxd) of saline water and the absorption of solar radiation (β) by the lake surface caused by salinity did not affect the annual mean temperature or the changing rate.

### **4. Discussion**

### *4.1. Salinity Effects and Parameterizations*

Most TP lakes are saline, but existing numerical studies have focused exclusively on several large lakes with small amounts of salt because of the lack of observations and salinity parameterizations in lake models. Based on previous salinity parameterizations coupled in CLM applied to the Great Salt Lake in USA and the significant impacts of Tmaxd on density convection and thermal stratification, the effects of Tmaxd were further parameterized in our lake model. The improved model was first applied to the TP saline lake and significantly reduced errors in the simulation of LSWT and LT in the saline lake, especially in winter. The salinity-extended lake model will be an efficient tool for studying saline lakes in the TP.

Salinity parameterization of Tmaxd had no obvious impacts on the warming of a small saline lake, mainly because of the shallow lake depth, absence of ice cover and the strong effects of wind in terms of turbulence mixing in HSP. However, Tmaxd could alter the thermal structure of a lake with a certain depth [2,24]. Therefore, considering the salinity effect on Tmaxd makes the developed saline lake model more accurate.

Salinity will play a major role in terms of the impact of climate change on TP lakes in future. Salinity decreases due to increased precipitation and inflow of glacial meltwater, but

it could also increase due to increased evaporation. Since many lakes are fairly small, even small changes in the water balance can be important. Also, salinity evolution influences the vertical stratification of lakes and, consequently, the water temperature structure. Variations in salinity will be addressed in future according to the mass balance.

### *4.2. Simulated Warming Rates of LSWT in Different Studies*

LSWTs globally have increased rapidly, with a mean trend of 0.34 ◦C/decade in summer between 1985 and 2009 [12]. Although the TP is warming at twice or even three times the global warming rate [8,11], TP lakes have overall been warming with a trend of 0.37 ◦C/decade, based on data from 374 inland lakes. They are warming slightly more rapidly than the global mean because the warming of TP lakes is highly heterogeneous [14]. Most TP lakes are warming with the higher 0.76 ◦C/decade rate, while some lakes are cooling due to glacier meltwater inflow or reduced salinity [3,13–17].

The simulated LSWT warming rate of NL was 0.68 ◦C/decade, as shown by remote sensing data that the lake was warming [14,23], and its warming rate was between the simulated rates of 0.52 ± 0.25 ◦C/decade in Nam Co by the GLM and 0.74 ◦C/decade in Qinghai Lake by Flake. The trend was not insignificant as in the previous NL simulation, in which the model was forced by the NCEP and ERA data where solar radiation was larger and decreased quickly compared to the observations [22]. Thus, insignificant warming in NL was concluded in the previous study. Although field work is hard, enough observations should be performed and accurate forcing datasets should be built for more accurate simulation studies.

Our results make it clear that the largest freshwater lake in the TP and a nearby small saline lake have indeed warmed over the last several decades and are warming faster owing to the amplification effect of their high altitude. The warming rate of LSWT in NL (0.68 ◦C/decade) significantly exceeded that of the regional air temperature (0.49 ◦C/decade) and was similar to Qinghai Lake and Lake Superior due to reduced ice cover [16,53]. While lakes in some temperate climate regions are warming in line with increased air temperatures [50,54], even the warming rates of tropical lakes are smaller than those of air [55,56]. Moreover, the bottom layers of NL were simulated to isolate from direct atmospheric influence, and tended to show long-term cooling at a rate −0.03 ◦C/decade on account of strengthening stratification. This result is similar to those reported from other stratified dimictic lakes, such as Qinghai Lake, Heiligensee Lake, and so on [16,50].
