Observed strong subsurface marine heatwaves in the tropical western Pacific Ocean

Subsurface-intensified warming is one of the key features of MHWs occurred in the tropical western Pacific Ocean. As shown in figure 2, discrete warming events are detected at every buoy site and from surface to deeper than 700 m depth. The maximum intensity of almost every MHW event is found in the subsurface layer, and many of MHWs occurred even when no significant warming anomalies in the surface layer. The vertical structure of these MHWs suggests that they are very different from surface intensified MHWs and is why we call them subsurface MHWs.

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The intensity and duration of subsurface MHWs differ from buoy sites, but the occurrence of these MHWs seems to be coherent changes within the tropical western Pacific Ocean. For example, the MHWs in 2005, 2007, 2008, 2010, 2011, and 2012 are detected at basically all stations simultaneously (figure 2). A comparison between MHWs and isotherms indicates that a subsurface MHW event corresponds to a deepening of thermoclines.

Figures 3(a)–(c) present statistical features, including mean intensity, the yearly number of events and duration of the subsurface MHWs at each buoy site. As suggested in previous studies, the intensity of MHWs in the surface layer is quite small. However, the intensity of subsurface MHWs in the tropical western Pacific Ocean is clearly intensified in the 50–300 m layer, which is also identified by figure 2, with a significant peak at 150–200 m depth (figure 3(a)). A comparison between figures 3(a) and (b) indicates that the subsurface MHWs occur in the thermocline layer. The vertical structure of MHWs' intensity at each buoy site is very similar (figure 3(a)). The maximum of the ensemble mean intensity (average of all the 19 sites) is about 5.2 °C, which is much greater than surface MHWs (∼1.3 °C) and is twice as big as the STD of temperature at the same depth. The subsurface MHWs occur about 1.3 ± 0.3 times a year on average for the whole period (vertical mean over 50–300 m), and the mean duration of the MHWs in this region is about 13–22 days at depths with a mean of about 17 days (figures 3(b) and 3(c)).

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Given that the 150–200 m layer is a typical layer of subsurface MHWs, we present the intensity of subsurface MHWs averaged over 150–200 m in figure 3(d). Vertical bands shown in figure 3(d) suggest that the MHWs' intensity at different positions have coherent features in terms of dates and duration, implying that the MHWs occurred at these buoy sites might be a result of a common large-scale process. It is shown that subsurface MHWs have occurred almost every year during 1985–2020 in the tropical western Pacific Ocean, but in some particular years they were distributed on a broader area than in other years and showed stronger intensity.

To investigate the relationship between ENSO and the interannual changes of subsurface MHWs, we present the monthly ONI during 1984–2020 in figure 3(e). The subsurface MHWs in 1988, 1995, 1996, 1999, 2000, 2003, 2004, 2007, 2008, 2009, 2010, 2011, 2013, 2016, and 2017 have greater intensities than in other years (figure 3(d)), but it seems that the subsurface MHWs are not unequivocally related to ENSO. The four strongest subsurface MHWs occurred during: (a) a phase transition from an El Niño in 1987 to a La Niña in 1988; (b) a La Niña event in 2007; (c) a phase transition from an El Niño in 2009 to a La Niña in 2010; and (d) a neutral phase in the first half of 2017 (figure 3(e)). As shown in figure 3(e), during the observation period, about 22% of subsurface MHWs have occurred during warming phases (ONI ⩾ 0.5), 36% during cold phases (ONI ⩽ −0.5), and 42% under normal conditions (−0.5 < ONI < 0.5), suggesting that the vast majority (∼78%) of subsurface MHWs have occurred in durations without La Niña events. The intensity of subsurface MHWs is 5.0 ± 1.1 °C during warm phases (ONI ⩾ 0.5), 5.2 ± 0.9 °C during cold phases (ONI ⩽ −0.5), and 5.2 ± 1.0 °C during normal conditions (−0.5 < ONI < 0.5). Hence, the difference in MHWs' intensity between El Niño and La Niña phases is much smaller than the uncertainty, indicating that the intensifying of subsurface MHWs during La Niña phases is very weak and not significant. Therefore, we suggest that, although the ENSO cycle is probably able to affect subsurface MHWs, the occurrence of subsurface MHWs have no statistically significant correlation with the ENSO index.

The subsurface MHWs in the tropical western Pacific Ocean show interesting seasonality. We present the climatological monthly intensity and percentage number of the subsurface (150–200 m) MHWs in figures 3(f) and (g). The intensity ranges from about 3.7 °C in September to 5.4 °C in April–June with a mean of 4.9 °C and an STD of about 0.5 °C, indicating that the seasonal change of the subsurface MHWs' intensity is weak (figure 3(f)). In contrast, the percentage number of subsurface MHWs shows a well-defined seasonal cycle: about 55% of subsurface MHWs occur in March–June, but only about 13% of subsurface MHWs occur in August–November and merely 4% in September and October (figure 3(g)). Hence, it seems that the subsurface MHWs in September and October are relatively weaker and less frequent than in other months.

Temperature anomalies associated with subsurface MHWs occurred mainly in the layer between 50 and 300 m centered at 150–200 m, in the depths of the surface Ekman layer and upper thermocline layer. For example, the surface Ekman layer depth at 5° N, assuming a steady, homogeneous and horizontal flow with friction on a rotating earth, is 102 m when the sea surface wind speed is 5 ${\text{m}}\,{{\text{s}}^{ - 1}}$ and 257 m when the sea surface wind speed is 10 ${\text{m}}\,{{\text{s}}^{ - 1}}$ (e.g. Stewart 2008). The tropical western Pacific Ocean is a complex place of strong equatorial currents and western boundary currents: the North Equatorial Current and South Equatorial Current flows westward and feed the Mindanao Current and New Guinea Coastal Current and New Guinea Coastal Undercurrent, which converge in the equatorial western Pacific Ocean and provide sources of outflowing North Equatorial Counter Current, Equatorial Undercurrent and Indonesian Throughflow (Hu et al 2015, 2016, 2018). Hence, temperature variability in the subsurface tropical western Pacific Ocean could be caused by vertical heat transport (isotherm heaving) related to upwelling/downwelling and horizontal heat advection. Hu et al (2017) found that enhanced upwelling leads to significant cooling in the upper layer and the western Pacific warm pool split. If that is the case, a downwelling anomaly may give rise to significant warming in the tropical western Pacific Ocean. To examine the role of related ocean dynamics in generating subsurface MHWs, we investigate changes in sea surface wind forcing, Ekman pumping velocity, sea level, and ocean currents during subsurface MHWs. The Ekman pumping velocity ${w_{\text{E}}}$ can be estimated with sea surface wind stress when its curl is non-zero (Cushman-Roisin and Beckers 2011):

$$\begin{equation}{w_{\text{E}}} = \frac{1}{\rho }\left[ {\frac{\partial }{{\partial x}}\left( {\frac{{{\tau _y}}}{f}} \right) - \frac{\partial }{{\partial x}}\left( {\frac{{{\tau _x}}}{f}} \right)} \right]\end{equation} \tag{ 1 }$$

where $\rho$ is sea water density, $f\,$ is Coriolis coefficient, and ${\tau _x}$ and ${\tau _y}$ are zonal and meridional components of sea surface wind stress.

Figure 4(a) presents composited upwelling velocity anomaly and surface wind stress curl anomaly during subsurface MHWs. Daily wind stress data from NOAA/ERDDA, available during 2000–2009, are used here. Negative upwelling velocity anomaly and wind stress curl anomaly can be found in the tropical western Pacific Ocean, indicating anomalous down-welling during the MHWs. The spatial distribution of the intensity of subsurface MHWs is in agreement with the Ekman down-welling velocity anomaly (figure 4(a)), except the one at 5° N, 137° E where the meandering of North Equatorial Counter Current (NECC) is also important (e.g. Messié and Radenac 2006).

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The anomalous down-welling in the tropical western Pacific Ocean is accompanied by ocean convergence in the sea surface. We composite the sea level anomaly and surface meridional velocity anomaly during the subsurface MHWs, using monthly AVISO sea level anomaly and OSCAR surface current velocity products. As shown in figure 4(b), positive sea level anomalies relative to a multi-year mean are found in the western Pacific Ocean, indicating a strengthening of convergence in the surface layer. The composite surface meridional velocity anomalies relative to the multi-year mean suggest that southward current anomalies occurred in the northern hemisphere but northward current anomalies occurred in the southern hemisphere, indicating a clear meridional convergence of surface water in the equatorial western Pacific Ocean. It should be noted that changes in the zonal currents, i.e. North Equatorial Current, South Equatorial Current, and NECC, are also important components of the processes of surface convergence (e.g. Hu et al 2015).

The spatial pattern of Ekman down-welling and surface convergence is in agreement with the spatial structure of isothermals during subsurface MHWs. Given that no daily subsurface temperature observations are available on synoptic scales and in such a big domain, we examined the temperature anomaly at 150–200 m in a specific MHW event using a monthly temperature product from RG Argo. We choose the month of February 2011 during a strong La Niña event, when a MHW event occurred at several buoy sites, as shown in figure 3(d). The spatial patterns of the MHW related temperature anomalies and MHW's intensity (figure 4(c)) are very similar to that of down-welling velocity anomaly and surface wind stress curl anomaly during MHWs (figure 4(a)). Given that the temperature gradient in the mixed layer is much smaller than that in the thermocline layer, the temperature anomalies induced by down-welling are expected to be larger in the thermocline layer than in the surface, and this explains why the subsurface MHWs have a peak intensity in the thermocline layer and are independent from surface MHWs that usually related to heat flux.

In addition to the local wind forcing, equatorial Kelvin waves and intra-seasonal Rossby waves might also be important mechanisms of the subsurface. But as shown in figure 2, subsurface warming events were detected at 5 and 8° N/S, where is away from the equator and beyond the equatorial Kelvin waves' sphere of influence. Given the existence of intra-seasonal Rossby waves in the tropical Pacific Ocean (e.g. Hu et al 2018), the westward propagation of intra-seasonal Rossby waves may also play an important role.

The subsurface layer between 50 and 300 m depth is the biota of several fishes like tuna and thermal changes in these depths may affect the fishery catches (e.g. Cai et al 2020, Kim et al 2020). Kim et al (2020) suggested that the annual catch amount of skipjack tuna in the Federated States of Micronesia (FSM) was much less than normal during 1997–2001 and 2008–2012, but much greater than normal during 1991–1995 and 2003–2007.

The intensities of subsurface MHWs in the FSM region (here we use observations collected at 165° E, and 5° N and 8° N) were 4.1 °C during 1997–2001 and 4.9 °C during 2008–2012. In contrast, when the catch amount of skipjack tuna was greater than normal years, the average intensities of subsurface MHWs were 2.5 °C during 1991–1995 and 2.8 °C during 2003–2007. The differences of subsurface MHWs' intensities are about 1.6 °C between 1991–1995 and 1997–2001, and 2.1 °C between 2003–2007 and 2008–2012, which are greater than the STD of subsurface MHWs' intensities (about 1.6 °C), indicating the difference is significant.

Thus, strong subsurface MHWs are likely able to affect the fishery production of the tropical western Pacific. But it should be noted that, it is not clear whether the correlation between subsurface MHWs and catch amount of skipjack tuna is significant or not, and it is expected that their relationship would be complex. Further and more detailed studies are needed to uncover the influence of subsurface MHWs on fishery production in the tropical western Pacific Ocean.