研究テーマ

Ocean mixed layer (ML) is typically a layer from surface to several tens of meters, which is well-mixed and has vertically uniform density, temperature and salinity. Heat budget within the ML is often used to diagnose the mechanisms of sea surface temperature variability. There are two main mechanisms; 1) atmospheric surface forcing (e.g. evaporative cooling due to the strong wind speed, heating by solar radiation) and 2) ocean dynamics (e.g. horizontal water transport, vertical mixing, entrainment of cold water across the bottom of the ML). Generally, in the extra-tropics, the role of ocean dynamics have received less attention than that of the atmospheric forcing. However, recent studies using numerical simulations have shown their importance. So, using observational datasets, one of our work tried to quantify "the relative importance of atmospheric and oceanic processes" for the high temperature anomalies in the ML around Hawaii in 2010s (figure a). We found that the Ekman advection processes driven by trade wind played a significant role to dampen the high temperature anomalies by atmospheric forcing. In other words, there is a compensated relationship with the two processes in the trade wind regions. In addition, shallower ML depth can be also found to enhance the extremely warming at the sea surface due to the decrease in effective heat capacity within the ML. Our work published from GRL in 2023 revealed the importance of ML depth for amplifying or suppressing the local SST variability compared with the role of anomalous heat fluxes.  

Oceanic low-level clouds (e.g. stratus, stratocumulus, fog) play a key role in modulating the Earth’s radiation budget, which generally have strong cooling effect. Variability of the low-level cloud interacts strongly with the sea surface temperature (SST) via two-way physical processes between them. Through the stratification process of the atmospheric boundary layer, decreasing SST promotes the low-level cloud formation, which promotes cooling of the sea surface itself by more reflection of the solar radiation by the low-level cloud. As a result, a positive feedback exists between the radiative impact of the low-level cloud and SST, especially in summertime North Pacific. Main issue about the relationship is “a time scale dependence for a trigger of the feedback loop, particularly for the role of SST (i.e. active role of ocean)”. We focused on the western boundary current regions, and investigated the causal relationship between the low-level cloud properties and SST on various timescales using the observational data and atmospheric regional model.