Marine heatwaves (MHWs) have been recognized as a serious threat to marine life, yet, most studies so far have focused on the surface only. Here, we investigate the vertical dimension and propagation of surface MHWs in the Eastern Pacific using results from a high-resolution hindcast simulation (1979 to 2019), performed with the Regional Ocean Modeling System. We detect MHWs using a seasonally varying percentile threshold on a fixed baseline and track their vertical propagation across the upper 500 m. We find that nearly a third (∼ 29 %) of the MHWs extend beyond the surface mixed layer depth (MLD). On average, these deep-reaching MHWs (dMHWs) extend to 110 m below the MLD and last five times longer than MHWs that are confined to the mixed layer (184 vs. 36 days). The dMHWs can cause stronger temperature anomalies at depth than at the surface (maximum intensity of 5.0°C vs. 1.9°C). This general subsurface MHW intensification even holds when scaling the temperatures with the respective local variability. A clustering of dMHWs reveals that 41 % of them are block-like, i.e., continually remain in contact with the sea surface, 24 % propagate downward, 20 % propagate upward, while 15 % appear at the surface multiple times. Although the water column MHW duration, intensity and severity are only moderately correlated with their corresponding surface-based MHW characteristics, dMHWs have the potential to be detected from the surface. Our study can help to augment the remote sensing-based monitoring of upper ocean exposure to MHWs.

Shelled pteropods and planktic foraminifers are calcifying zooplankton that contribute to the biological carbon pump, but their importance for regional and global plankton biomass and carbon fluxes is not well understood. Here, we modelled global annual patterns of pteropod and foraminifer total carbon (TC) biomass and total inorganic carbon (TIC) export fluxes over the top 200m using an ensemble of five species distribution models (SDMs). An exhaustive newly assembled dataset of zooplankton abundance observations was used to estimate the biomass of both plankton groups. With the SDM ensemble we modeled global TC biomass depending on multiple environmental parameters. We found hotspots of mean annual pteropod biomass in the high Northern latitudes and the global upwelling systems, and in the high latitudes of both hemispheres and the tropics for foraminifers. This largely agrees with previously observed distributions. For the biomass of both groups, surface temperature is the strongest environmental correlate, followed by chlorophyll-a. We found mean annual standing stocks of 52 (48-57) Tg TC and 0.9 (0.6-1.1) Tg TC for pteropods and foraminifers, respectively. This translates to mean annual TIC fluxes of 14 (9-22) Tg TIC yr-1 for pteropod shells and 11 (3-27) Tg TIC yr-1 for foraminifer tests. These results are similar to previous estimates for foraminifers standing stocks and fluxes but approximately a factor of ten lower for pteropods. The two zooplankton calcifiers contribute approximately 1.5% each to global surface carbonate fluxes, leaving 40%-60% of the global carbonate fluxes unaccounted for. We make suggestions how to close this gap.

Filter-feeding gelatinous macrozooplankton (FFGM), namely salps, pyrosomes and doliolids, are increasingly recognized as an essential component of the marine ecosystem. Unlike crustacean zooplankton (e.g., copepods) that feed on preys that are an order of magnitude smaller, filter-feeding allows FFGM to have access to a wider range of organisms, with predator over prey ratios as high as 10$^5$:1. In addition, most FFGM produce carcasses and/or fecal pellets that sink 10 times faster than those of copepods. This implies a rapid and efficient export of organic matter to depth. Even if these organisms represent $<$5\% of the overall planktonic biomass, the induced organic matter flux could be substantial. Here we present a first estimate of the influence of FFGM organisms on the export of particulate organic matter to the deep ocean based on the marine biogeochemical model NEMO-PISCES. In this new version of PISCES, two processes characterize FFGM: the preference for small organisms due to filter feeding, and the rapid sinking of carcasses and fecal pellets. To evaluate our modeled FFGM distribution, we compiled FFGM abundance observations into a monthly biomass climatology using a taxon-specific conversion. A model-observation comparison supports the model ability to quantify the global and large-scale patterns of FFGM biomass distribution, but reveals an urgent need to better understand the factors triggering the boom-and-bust FFGM dynamics before we can reproduce the observed spatio-temporal variability of FFGM. FFGM contribute strongly to carbon export at depth (0.4 Pg C yr$^{-1}$ at 1000 m), particularly in low-productivity region (up to 40\% of organic carbon export at 1000 m) where they dominate macrozooplankton by a factor of 2. The FFGM-induced export increases in importance with depth, with a simulated transfer efficiency close to one.