Seasonal variability of the ocean bottom pressure (OBP) in the world oceans is investigated using 15 years of GRACE observations and a Pressure Coordinate Ocean Model (PCOM). In boreal winter, negative OBP anomalies appear in the northern North Pacific, subtropical South Pacific and north of 40 °S in the Indian Ocean, while OBP anomaly in the Southern Ocean is positive. The summer pattern is opposite to that in winter. The centers of positive (negative) OBP signals have a good coherence with the mass convergence/divergence due to Ekman transport, indicating the importance of wind forcing. The PCOM model reproduces the observed OBP quite well. Sensitivity experiments indicate that wind forcing dominates the regional OBP seasonal variations, while the contributions due to heat flux and freshwater flux are unimportant. Experiments with daily sea level pressure (SLP) forcing suggest that at high frequencies the non-static effect of SLP is not negligible.

El Niño events can be classified into two categories according to the onset time: the spring (SP) El Niño with onset time from April to June and the summer (SU) El Niño with onset time from July to October. The SP El Niño is a basin-scale phenomenon and is closer to the conventional ENSO. It goes through the earlier and stronger heat build-up process, and the earlier occurrence of westerlies in the equatorial Pacific, which can partly explain its earlier onset time. For SU El Niño, in contrast, the anomalous signals, such as SSTAs, zonal wind anomalies, and subsurface variations, are much weaker, which can be attributed to the weaker accumulation of warm water and shorter duration of positive Bjerknes feedback. During its peak phase, anomalous southeasterlies over the eastern Pacific enhance the wind–evaporation–SST (WES) feedback and impede the development of positive SSTAs there, and then lead to a west shift of SSTA center. Recharge/discharge processes exist in both types of events but are weaker in the SU type, which may be caused by the lack of meridional Sverdrup transports as a result of weak zonal wind anomalies. A heat budget analysis demonstrates that the relative importance of thermocline (TH) and zonal advective (ZA) feedbacks in SP and SU El Niño is different. In SP El Niño, the TH feedback is dominant compared to ZA feedback in both the GODAS and SODA datasets. In SU El Niño, however, these two terms are equally important in GODAS, but not in the SODA dataset.

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The study of ocean bottom pressure (OBP) is useful for understanding the barotropic processes variability that contribute to sea level rise. Previous studies have reported the strong OBP anomalies in the Southern Ocean on different time scales. In this study, the characteristic and mechanisms of the energetic interannual OBP variability in the southeastern Paci c are examined using 14 years of GRACE data. It is found that the OBP anomalies are positive (negative) related to the convergence (divergence) of Ekman transport forced by local winds variability. The sea level pressure (SLP) anomalies shows a wavenumber-3 structure in the high latitude of the South Paci c, which bene ts a strong and persistent anticyclone over the southeastern Paci c, leading to the positive OBP anomalies there. Such SLP anomalies are similar to the second Paci c-South American (PSA2). Moreover, El Niño-Southern Oscillation (ENSO) plays an important role in the austral spring (August-November) OBP variability and leads the austral autumn (March-June) OBP variability by 1 season. These results highlight the in uence of atmospheric variability on OBP anomalies and are validated by a mass conservation (non-Boussinesq) ocean model, which is expected to not only better understanding of OBP mechanisms in a longer time, but also predict OBP variation in the global scale.

The change in ocean net surface heat flux plays an important role in the climate system. It is closely related to the ocean heat content change and ocean heat transport, particularly over the North Atlantic, where the ocean loses heat to the atmosphere, affecting the AMOC (Atlantic Meridional Overturning Circulation) variability and hence the global climate. However, the difference between simulated surface heat fluxes is still large due to poorly represented dynamical processes involving multiscale interactions in model simulations. In order to explain the discrepancy of the surface heat flux over the North Atlantic, data sets from nineteen AMIP6 and eight highresSST-present climate model simulations are analyzed and compared with the DEEPC (Diagnosing Earth's Energy Pathways in the Climate system) product. As an indirect check of the ocean surface heat flux, the oceanic heat transport inferred from the combination of the ocean surface heat flux, sea ice and ocean heat content tendency is compared with the RAPID (Rapid Climate Change-Meridional Overturning Circulation and Heat flux array) observations at 26°N in the Atlantic. The AMIP6 simulations show lower inferred heat transport due to less heat loss to the atmosphere. The heat loss from the AMIP6 ensemble mean north of 26°N in Atlantic is about 10 Wm -2 less than DEEPC, and the heat transport is about 0.30 PW lower than RAPID and DEEPC. The model horizontal resolution effect on the discrepancy is also investigated. Results show that by increasing the resolution, both surface heat flux north of 26°N and heat transport at 26°N of the Atlantic can be improved.