Surface layer temperature inversion in the Bay of Bengal: Main characteristics and related mechanisms

Abstract

Surface layer temperature inversion (SLTI), a warm layer sandwiched between surface and subsurface colder waters, has been reported to frequently occur in conjunction with barrier layers in the Bay of Bengal (BoB), with potentially commensurable impacts on climate and postmonsoon tropical cyclones. Lack of systematic measurements from the BoB in the past prevented a thorough investigation of the SLTI spatiotemporal variability, their formation mechanisms, and their contribution to the surface temperature variations. The present study benefits from the recent Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction (RAMA) buoys located in BoB along 90°E at 4°N, 8°N, 12°N, and 15°N over the 2006–2014 period. Analysis of data from these RAMA buoys indicates that SLTI forms after the summer monsoon and becomes fully developed during winter (December–February). SLTI exhibits a strong geographical dependency, with more frequent (80% times during winter) and intense inversions (amplitude, ΔT ∼ 0.7°C) occurring only in the northern BoB compared to central and southern Bay. SLTI also exhibits large interannual and intraseasonal variations, with intraseasonal amplitude significantly larger (ΔT ∼ 0.44°C) than the interannual amplitude (∼0.26°C). Heat budget analysis of the mixed layer reveals that the net surface heat loss is the most dominant process controlling the formation and maintenance of SLTI. However, there are instances of episodic advection of cold, low‐saline waters over warm‐saline waters leading to the formation of SLTI as in 2012–2013. Vertical processes contribute significantly to the mixed layer heat budget during winter, by warming the surface layer through entrainment and vertical diffusion.

Observed Seasonal Variations of the Upper Ocean Structure and Air‐Sea Interactions in the Andaman Sea

“…When strong surface heat loss occurs in areas with strong salinity stratification, vertical temperature inversions tend to form (Drushka et al, 2014;Girishkumar et al, 2013;Thadathil et al, 2016). The analysis of historical data (Thadathil et al, 2002) and measurements from Argo floats (Thadathil et al, 2007;Thompson et al, 2006) have indicated that organized surface layer temperature inversions occur along the coastal regions of the western and northeastern parts of the BOB during the winter.…”

Observed Upper Ocean Seasonal and Intraseasonal Variability in the Andaman Sea

“…From Figure c, it is further evident that magnitude of temperature inversion is very weak during summer compared to winter. It is worth to note that the magnitude of temperature inversions in the AS are very small compared to the values reported from the northern BoB, where the amplitude of temperature inversions go beyond 1 °C during the peak phase of winter (Thadathil et al, ; Thangaprakash et al, ). As reported in earlier studies in the BoB, the presence of thick BL is the necessary but not the sufficient condition to the formation of temperature inversion (Girishkumar, Ravichandran, & McPhaden, ; Thadathil et al, , ).…”

“…As suggested by earlier studies, cooling of ML by net surface heat loss and/or horizontal advection and warming of BL through penetrative shortwave radiation below the ML are the important processes that leads to the formation of temperature inversions in the presence of BL (Girishkumar, Ravichandran, & McPhaden, ; Kurian & Vinayachandran, ; Thadathil et al, , ). In order to understand the relative importance of these two processes on the formation of temperature inversion in the study area, heat flux into the ML by the sum of horizontal advection and net surface heat flux ( Q net − Q pen ) and penetrative shortwave radiation absorbed at depths below the ML (Q pen ) are presented along with magnitude of temperature inversion in Figure .…”

“…Near‐surface salinity shows annual signal (Figure ) with low values (~30.5 psu) during September–February and high values (~33 psu) during March–August (Figure ). Vertical structure of salinity shows relatively low saline water (31–32 psu) sitting on the top of higher saline water (34.5 psu) in the subsurface layer, and it leads to the persistent occurrence of halocline in the near‐surface layer similar to the BoB (Girishkumar, Ravichandran, & McPhaden, ; Rao & Sivakumar, ; Sprintall & Tomczak, ; Thadathil et al, , , ; Thangaprakash et al, ; Vinayachandran et al, ). Moreover, strong vertical gradient of temperature (0.20 °C) is observed around the depth of the 23 °C isotherm (D23), and it is considered as representative depth for thermocline (Girishkumar, Ravichandran, & Han, ).…”

“…It is worth to note that the magnitude of temperature inversions in the AS are very small compared to the values reported from the northern BoB, where the amplitude of temperature inversions go beyond 1 °C during the peak phase of winter (Thadathil et al, ; Thangaprakash et al, ). As reported in earlier studies in the BoB, the presence of thick BL is the necessary but not the sufficient condition to the formation of temperature inversion (Girishkumar, Ravichandran, & McPhaden, ; Thadathil et al, , ). In the presence of thick BL, relative cooling of ML, as compared to the BL, can lead to formation of temperature inversion, provided that the overall density profile is statically stable (Kurian & Vinayachandran, ).…”