<p>Utilizing comprehensive multi-source sea surface temperature (SST) data, this study identifies and validates the existence of a cold water tongue (CWT) phenomenon in the northwestern top of the Beibu Gulf (BG) during winter. Further analysis reveals that the CWT emerges in October, with its intensity and spatial extent progressively increasing, peaking in January and February of the following year. Thereafter, it gradually weakens and dissipates entirely by May. An in-depth analysis of the CWT mechanism was conducted using the Price Weller Pinkel (PWP) one-dimensional mixed layer model. Results from numerical sensitivity experiments indicate that latent heat flux is the most critical factor governing the development and evolution of the CWT. The northwestern top of the BG experiences stronger winds during winter compared to adjacent regions, intensifying latent heat loss from the local ocean. This heightened heat loss accelerates convective cooling throughout the entire water column, ultimately leading to the formation of the CWT phenomenon. Additionally, the northwestern top of the BG is characterized by a series of river systems. Reanalysis meteorological product and observational data indicate that the surface air temperature over these river basins during winter is lower than that of non-CWT areas, contributing to the influx of cooler freshwater into the gulf, which supports the development of the CWT to some extent. Further research indicates that the strong La Niña event of 2010–2011 induced cyclonic wind anomalies over the BG, reinforcing the prevailing northeasterly monsoon in winter, thereby amplifying the latent heat loss in the CWT region. This process ultimately leads to a pronounced enhancement of the CWT. Conversely, during the strong El Niño event of 2014–2015, an anticyclonic wind anomaly emerged, weakening the northeasterly monsoon and reducing latent heat loss, which led to a remarkable attenuation of the CWT that year.</p>