5.1. Impact of Climatic and Non-Climatic Factors on Cirques
The Last Glacial period (MIS 4–2) is assumed to have had a major effect on cirque development [
2]. The climate of the TP during MIS 4 and 2 was characterised by strong westerlies and a weak ISM [
34]. From west to east, the reconstructed temperature and precipitation during the Last Glacial Maximum (LGM, a period of MIS 2) of the study area decreased [
21,
35], which had a similar spatial pattern to the modern one (
Figure 2). MIS 3 had a strong ISM and weak westerlies [
34], which implies that the climatic pattern during MIS 3 was similar to the modern situation. Therefore, the spatial patterns of temperature and precipitation during the Last Glacial period were similar to the present, and the modern climatic pattern was used to analyse the relation of the cirque and the climate in the study area. The western part of the study area was characterised by mountain ranges and thus low temperatures. The study area was dominated by the ISM, which entered this region from the southwestern direction. Most cirques were located in the western part of the study area, where a cold and wet climate dominated (
Figure 2 and
Figure 3). This reveals the role of climate on the cirque locations. In contrast, few cirques were found in the eastern part of the study area due to the warm and arid climate. However, the cirque size (L, W, H, and area) increased with temperature and decreased with precipitation (
Figure 2 and
Figure 3A–D,
Table 2), implying that large cirques tended to develop under a warm and dry climate. This was inconsistent with the findings of the Gangdise Mountains and the central TP, where the cirque sizes showed consistent spatial distributions with those of precipitation [
6,
7]. One explanation for this is that cirque enlargement is mainly limited to the duration when a cirque is occupied by a cirque-type glacier [
2]; when low temperature and high precipitation levels surpass certain ranges, the glacier extends beyond the cirque boundary and limits cirque enlargement [
7]. The positive correlation between CFA and MAAT implies that high temperature limits cirque development. However, this correlation is weak, and it implies that non-climatic factors affect CFA. The cirques tended to be located on leeward slopes because snow drifting from the windward slope promoted cirque development, and the enhanced heat exchange on the windward slope limited cirque development [
28,
36]. The cirques in this area tended to face NE and E (
Table 5), and this may reveal this effect, because the ISM originated from the southwestern direction. The tendency of facing NE may also reveal the ‘morning–afternoon effect’, which documents that the polar tendency of the cirque aspects in the northern hemisphere often displace to the NE due to the coincidence of low solar radiation and low air temperatures in the morning [
28,
37].
The cirques tended to develop in the mountainous regions of the study area, where high altitudes dominated. This implies that local topography controlled the locations of the cirques. With altitude, the cirque aspects shifted from N to SE, implying that high altitude promoted cirque development on an unfavourable aspect, which is in line with the finding of the central TP [
6]. The cirque size (L, W, H, and area) decreased with altitude, which is contrary to the results of the Gangdise Mountains and the central TP, where the cirque size increased with the altitude [
6,
7]. Two possible reasons were put forward for this result in this study area: (i) with altitude, temperature decreases and precipitation increases, which results in the development of valley-type glaciers and thus limits cirque enlargement; (ii) with altitude, the accommodation space for cirque enlargement is limited (cf. [
25]). The negative correlation between the plan closure and Z
mean indicates that the cirques in high altitudes had a low development degree, which was in line with their small sizes. The CFA had a negative correlation with Z
mean, which was contrary to the consensus that CFA increased with altitude. However, this correlation was weak (
r = −0.10;
p < 0.05). Furthermore, no statistical differences were found between altitudinal groups at the 0.05 level (
Table 3), indicating that altitude did not play a major role in CFA.
The dominant cirque aspects of NE and E were partly caused by the NW–SE axis of the mountain orientation, which was in line with the results of the Gangdise Mountains [
7] and the central TP [
6]. The cirque size and shape did not show statistical differences between the aspects (
Table 4), indicating that the aspect did not affect the cirque size and shape. This implies that the ISM had a limited effect on the cirque size and shape. If NW, N, and NE are taken as the ‘northern slope’ and SW, S, and SE as ‘southern slope’, the profile closures of the cirques on the ‘northern slope’ were larger than those on the ‘southern slope’, indicating that cirques on the poleward slope were better developed in the vertical direction. If NE, E, and SE are taken as the ‘eastern slope’ and NW, W, and SW as the ‘western slope’, then the larger profile closure for the cirques on the eastern slope may reflect that the cirques on the leeward slope were better developed in the vertical direction. The cirques located on the ‘southern slope’ had a higher altitude than those on the ‘northern slope’, implying that high solar radiation levels led to cirque development at a high altitude.
Cirques on soft bedrocks usually tend to be larger and have lower CFAs than those on hard bedrocks [
6,
7,
16]. Indeed, the cirques on sand–slate, marine clastic/limestone, and clastic with coal (i.e., soft bedrocks) had a larger H than those on monzonite, granodiorite, and syengranite (i.e., hard bedrocks) (
Table 5), which is in line with the findings mentioned above. However, the cirques on the soft bedrocks had a smaller L, W, and area than those on the hard bedrocks (
Table 5). This implies that soft bedrocks promote subglacial erosion other than rock wall erosion.
5.2. Monsoon’s Role on Cirques
The MAAT and MAP values of the central and southeastern TP and the Gangdise Mountains showed that the strength of the ISM was greatest in the southeastern TP and then decreased westward to the central–eastern Gangdise Mountains, whilst the western Gangdise Mountains and the central TP were dominated by a continental climate with coldness and aridity (
Table 6) [
6,
7,
16]. The cirque density of the study area was 0.043 n km
–2, which was larger than those of the central TP and the western sector of the Gangdise Mountains but smaller than those of the central and eastern sectors of the Gangdise Mountains (
Table 6). This does not support the suggestion that with the strengthening of the ISM (i.e., temperature and precipitation increase), cirque density increases [
6]. One reason is that the southeastern TP was the most strongly ISM-dominated region on the TP; this result may imply that when the monsoon strength (temperature and precipitation levels) surpasses a certain range, cirque development is limited. Another reason is that cirque density is affected by non-climatic factors (e.g., topography and accommodation space), and there is no clear relation between monsoon strength and cirque density.
The mean values of the cirque L and H of this study were smaller than those of the central TP and the western, central, and eastern sectors of the Gangdise Mountains. The mean value of the cirque W of this study was smaller than those of the central TP, western, and central sectors of the Gangdise Mountains, but it was larger than that of the eastern sector of the Gangdise Mountains. The mean cirque area was smaller than those of the central TP, the western, and central sectors of the Gangdise Mountains, but it was slightly larger than that of the eastern sector of the Gangdise Mountains (
Table 6) [
6,
7]. It was found that the cirques in the ISM-dominated regions had smaller sizes than those in the continental climate, indicating that a strengthened ISM limits cirque size in a regional scale. This finding is in line with that of Zhang et al. [
6]. The CFA was highest in the central sector of the Gangdise Mountains, followed by the western and eastern sectors of the Gangdise and the central TP, while the mean CFA in the southeastern TP was the lowest. Among the five regions, the western sector of the Gangdise Mountains and the central TP receives the least monsoonal moisture, but they do not have the highest CFAs. The spatial trend of the CFA was consistent with that of the Z
mean in these regions, implying that the spatial trend of the CFA is greatly affected by cirque altitude at a regional scale. This indicates that the CFA does not necessarily reflect climate, which supports the finding of the Gangdise Mountains [
16].