Advection by the North Equatorial Current of a Cold Wake due to Multiple Typhoons in the Western Pacific: Measurements From a Profiling Float Array

Abstract

Cold wakes of previous tropical cyclones (TCs) affect the development of subsequent TCs, but few subsurface data sets have sufficient persistence and spatial coverage to follow a cold wake as it is advected by currents. For >2 months in 2018, an array of eight floats obtained >20,000 temperature profiles from the surface to <200 m every <40 min before, during, and after Super Typhoons Mangkhut, Trami, Kong-Rey, and Yutu. Two floats were in/near Mangkhut's eye, experienced gale force winds during Trami and Kong-Rey, drifted over 1,000 km westward with the North Equatorial Current, and tracked the advection of the weakly stratified, cold wake produced by the sequence of TCs. Sea surface temperature shows the westward advection of the cumulative cold wake. While causation cannot be established, since atmospheric measurements were not made, Yutu weakened as it passed over the cold wake. The stratification and the energy needed to mix the water column in the cold wake decreased with each TC. One float directly in the path of Yutu showed that mixing to 125-150 m was likely, corresponding to a cooling of 0.5-1 • C under the eye. Sea surface temperature in the cold wake cooled by 1 • C within a 150 km radius of Yutu's eye, where the effect on air-sea heat fluxes is maximal. A cold wake can remain weakly stratified for many weeks during a sequence of TCs. These results also suggest that an advected cold wake from previous TCs may contribute many weeks later to the arrested development of a subsequent TC at a distant location. Plain Language SummaryCold wakes from previous tropical cyclones (TCs) are known to affect the development of subsequent TCs, but few subsurface data sets have sufficient persistence and spatial coverage to follow a cold wake as it is transported by currents. For >2 months in 2018, an array of eight profiling floats drifted >1,000 km westward and tracked the weakly stratified, cold wake produced by a sequence of three TCs. The subsequent Super Typhoon Yutu weakened into a typhoon, as it passed over the cold wake prior to landfall. Coincidence occurred, but causation cannot be established, since detailed atmospheric measurements were not made. The stratification and thus the energy needed to mix the water column in the cold wake decreased with each TC. One float directly in the path of Yutu showed that mixing to 125-150 m was likely, corresponding to a cooling of 0.5-1 • C under the eye, where the effect on air-sea heat fluxes that power TCs is maximal. These results show that a cold wake from previous TCs is transported by ocean currents to a distant location. The cold wake may possibly contribute many weeks later to the arrested development of a subsequent TC at a distant location.

Near‐Inertial Wave Propagation in the Wake of Super Typhoon Mangkhut: Measurements From a Profiling Float Array

“…With a sufficiently large array of rapidly‐profiling floats during TC passage, NIW forcing and related changes in oceanic heat content have been studied by intentional and fortuitous placement of assets (D'Asaro et al., 2007; Jayne & Bogue, 2017; Johnston et al., 2020; Mrvaljevic et al., 2013; Sanford et al., 2011). Our float array drifted in Super Typhoon Mangkhut's wake, which affected NIW propagation.…”

“…In the following section, we note the ongoing generation of NIWs in the transition layer, which may contribute to a longer τ e = 5.0 days at a later time for the closest float. The comparison for the longer decay time is complicated by the passage of Super Typhoon Trami on 23 September (Johnston et al., 2020), which although more distant may have generated further NIWs (Figures 7–11). The two northern floats (red and pink) were within 500 km of Trami and under tropical storm winds (18–32 m s −1 ).…”

“…E p was maximum on September 14, 1 day after the passage of Super Typhoon Mangkhut (Figure 16d). There is a quick decay almost to pre‐TC levels followed by a prolonged decay until the passage of Super Typhoon Trami on September 23 (Johnston et al., 2020). These two stages were noted in Section 6 with τ e = 2.7 days and 5.0 days.…”

“…Super Typhoon Mangkhut moved westward across the western North Pacific in September 2018. Ahead of this TC, we deployed an array of profiling floats, which measured NIW generation under the TC (Johnston et al., 2020). Since NIWs have a flat aspect ratio, it limits their vertical propagation and the extraction of near‐inertial (NI) energy from the mixed layer (Alford et al., 2016).…”

The Impact of Typhoon “Mangkhut” on Surface Water Nutrient and Chlorophyll Inventories of the South China Sea in September 2018

“…With about 10 typhoons per year cumulating end of summer, it can be expected that the effects of successive typhoons interact. So, if a series of typhoons affect the same area, the cold wake of a typhoon causes a weakening of a subsequent typhoon as the temperature decreases by 0.5–1 K (Johnston et al., 2020). Moreover, it is found from a series of typhoons that mixing is increasingly enabled by subsequent typhoons as stratification is reduced.…”