The　chlorophyll-a　concentration　(CHL)　is　an　essential　climate　variable.　Extremes　of　CHL　events　directly　reflect　the　condition　of　marine　ecosystems.　Here,　we　applied　the　statistical　framework　for　defining　marine　heatwaves　to　study　the　extremes　of　winter　CHL　blooms　off　the　Luzon　Strait　(termed　as　LZB),　northeastern　South　China　Sea　(SCS),　from　a　set　of　remote　sensing　data.　The　application　was　enabled　by　a　recent　gap-free　CHL　dataset,　the　SCSDCT　data.　We　present　the　basic　properties　and　the　long-term　trends　of　these　LZB　events,　which　had　become　fewer　but　stronger　in　recent　years.　We　further　statistically　analyze　the　LZB　events＇　controlling　factors,　including　the　submesoscale　activity　quantified　by　a　heterogeneous　index　or　surface　temperature　gradients.　It　was　revealed　that　the　submesoscale　activity　was　also　a　vital　modulating　factor　of　the　bloom　events　in　addition　to　the　well-understood　wind　and　upwelling　controls.　This　modulation　can　be　explained　by　the　stratification　introduced　by　submesoscale　mixed-layer　instabilities.　In　the　winter,　the　intensified　winter　monsoon　provides　a　background　front　and　well-mixed　upper　layer　with　replenished　nutrients.　During　the　wind　relaxation,　submesoscale　baroclinic　instabilities　developed,　leading　to　rapid　stratification　and　scattered　submesoscale　fronts.　Such　a　scenario　is　favorable　for　the　winter　blooms.　For　the　first　time,　this　study　identifies　the　bloom　events　in　a　typical　marginal　sea　and　highlights　the　linkage　between　these　events　and　submesoscale　activity.　Furthermore,　the　method　used　to　identify　extreme　blooms　opens　up　the　possibility　for　understanding　trends　of　multiple　marine　extreme　events　under　climate　change.