*3.3. Spatial Variability of Cold Point Tropopause Temperature during December 2015*

Changes in the CPT-T are crucial for the understanding the lower stratosphere WV changes [27,52]. It has been reported warm CPT-T (~3.5 K) over the SEA and WP regions in December 2015 from the reanalysis data sets [7]. In this study, we have utilized COSMIC GPS RO data for estimating the CPT temperatures. Figure 8 shows the spatial variability in the CPT-T in December 2015 along with the background climatology of CPT-T. The coldest CPT-T (~185 K) is situated over the central pacific (CP) (Figure 8b) in 2015 while background climatological minimum CPT-T is located over

the WP region (Figure 8a). It clearly indicates the shifting of the minimum CPT-T region towards CP with extension towards EP in December 2015. The spatial distribution of CPT-T anomalies shows two strong warm anomaly areas over SEA/WP and Atlantic Ocean regions in December (Figure 8c). However, the strongest warm anomalies of ~5 K are observed over the SEA and WP regions. The height-longitudinal cross section of COSMIC temperature also shows the minimum temperature over eastern pacific region (Figure 8d). The thickness of the minimum temperature layer was much lower over the Indian Ocean and WP regions than the EP region.

**Figure 8.** Spatial distribution of COSMIC GPS RO observed cold point tropopause temperature (**a**) December climatological (2006–2014) CPT, (**b**) December 2015 CPT and (**c**) December 2015 CPT anomaly. (**d**) Height-longitudinal cross section of COSMIC temperature over 10◦N–10◦S in December 2015.

The observed results matched well with the previous report by using reanalysis data sets [7] but with higher CPT-T anomalies during the same period. However, this warm CPT-T is formed well before December (figure not shown). The formation of warm CPT-T anomalies initiated during October gradually increased from November and reached the maximum warm CPT-T anomaly (~5 K) in December 2015 over the SEA and WP regions.
