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We estimate ocean heat content (OHC) change in the upper 2000 m in the Gulf of Mexico (GOM) from 1950 to 2020 to improve understanding of regional warming. Our estimates are based on 192 890 temperature profiles from the World Ocean Database. Warming occurs at all depths and in most regions except for a small region at northeastern GOM between 200 and 600 m. GOM OHC in the upper 2000 m increases at a rate of 0.38 ± 0.13 ZJ decade-1 between 1970 and 2020, which is equivalent to 1.21 ± 0.41 terawatts (TW). The GOM sea surface temperature (SST) increased ~1.0° ± 0.25°C between 1970 and 2020, equivalent to a warming rate of 0.19° ± 0.05°C decade-1. Although SST in the GOM increases at a rate approximately twice that for the global ocean, the full-depth ocean heat storage rate in the GOM (0.86 ± 0.26 W m-2) applied to the entire GOM surface is comparable to that for the global ocean (0.82–1.11 W m-2). The upper-1000-m layer accounts for approximately 80%–90% of the total warming and variations in the upper 2000 m in the GOM. The Loop Current advective net heat flux is estimated to be 40.7 ± 6.3 TW through the GOM. A heat budget analysis shows the difference between the advective heat flux and the ocean heat storage rate (1.76 ± 1.36 TW, 1992–2017) can be roughly balanced with the annual net surface heat flux from ECCO (-37.9 TW).

A key challenge in lake ice modeling is quantifying the heat flux from water to ice. In shallow Central Asian lakes, where the seasonal ice cover mainly consists of columnar congelation ice, sunlight penetration enables strong interactions between ice and water. The evolution of ice cover in Lake Ulansu (Ulansuhai, Wuliangsuhai) in northern China was investigated via the High‐resolution Thermodynamic Snow and Ice (HIGHTSI) model. Atmospheric forcing was provided by calibrated ERA5 reanalysis data, and the initial freeze‐up dates were identified from remote sensing observations. A new parameterization of the water–ice heat flux (Fw), which is suitable for shallow lakes, was proposed as Fw = aQsw + b, where Qsw represents the solar heating of water and a and b are fitted coefficients. The model showed high correlations (>0.9) and low errors (<5 cm for ice thickness; <2°C for ice temperature) with respect to field observations. Throughout the ice season, long‐ and shortwave radiation promoted ice growth and melting, respectively. Surface melting and sublimation accounted for 9.5% and 9.8%, respectively, of the total ice decay, and the water–ice heat flux Fw = −17.5 ± 13.0 W m−2 was critical for simulation accuracy. Furthermore, despite the shallow depth, the lake released over 100 W m−2 of heat into the atmosphere for 2 days after break‐up. These findings highlight the climatic sensitivity and support sustainable water resource management of more than 10,000 shallow lakes in Central Asia.

An ensemble of initialized decadal prediction (DP) experiments using the Community Climate System Model, version 4 (CCSM4) shows considerable skill at forecasting changes in North Atlantic upper-ocean heat content and surface temperature up to a decade in advance. Coupled model ensembles were integrated forward from each of 10 different start dates spanning from 1961 to 2006 with ocean and sea ice initial conditions obtained from a forced historical experiment, a Coordinated Ocean-Ice Reference Experiment with Interannual forcing (CORE-IA), which exhibits good correspondence with late twentieth-century ocean observations from the North Atlantic subpolar gyre (SPG) region. North Atlantic heat content anomalies from the DP ensemble correlate highly with those from the CORE-IA simulation after correcting for a drift bias. In particular, the observed large, rapid rise in SPG heat content in the mid-1990s is successfully predicted in the ensemble initialized in January of 1991. A budget of SPG heat content from the CORE-IA experiment sheds light on the origins of the 1990s regime shift, and it demonstrates the extent to which low-frequency changes in ocean heat advection related to the Atlantic meridional overturning circulation dominate temperature tendencies in this region. Similar budgets from the DP ensembles reveal varying degrees of predictive skill in the individual heat budget terms, with large advective heat flux anomalies from the south exhibiting the highest correlation with CORE-IA. The skill of the DP in this region is thus tied to correct initialization of ocean circulation anomalies, while external forcing is found to contribute negligibly (and for incorrect reasons) to predictive skill in this region over this time period.

The Indian Ocean hosted a strong positive Indian Ocean Dipole (pIOD) event in 2019–2020, and a weak event in 2018–2019, such as the magnitude of the cold sea surface temperature anomaly (SSTA) during June-December in the former case is a factor of two higher (~ − 1.5 °C) than the latter (~ − 0.75 °C) at the western periphery of the eastern IOD zone at 5° S, 95° E. The plausible mechanisms responsible for this difference in the SSTA between these two events are examined using the mixed layer heat budget estimate using the moored buoy measurements. It is found that the enhanced cooling during June-December in 2019–2020 is determined primarily by the anomalous cooling due to the vertical processes associated with the combined effect of the anomalous thin barrier layer (BL), shallow thermocline, weak near-surface stratification, and strong wind speed induced vertical mixing, and secondarily by the enhancement in the latent heat flux ( LHF ) loss from the ocean. Conversely, the magnitude of cooling due to the vertical processes is much smaller in 2018–2019 due to the near-climatological states such as a thick BL, deep thermocline, and weak wind speed. During these events, the warming tendency by the horizontal advection dampens the cooling tendency associated with the vertical processes and LHF . These characteristics are distinct from the past study that suggested that the horizontal advection was responsible for the cool SSTA at the exact location during an extreme pIOD event in 2006–2007.

The oceanic intraseasonal variabilities (ISVs) are pronounced over the tropical Indian Ocean. Recently, a Central Indian Ocean (CIO) mode was proposed as an ocean–atmosphere coupled mode at intraseasonal timescales. It has a close relation with northward-propagating ISVs and intraseasonal precipitation during the Indian summer monsoon. In this study, the dynamics of tropical oceanic ISVs associated with the CIO mode are analyzed using reanalysis products and observations. A complete heat budget analysis shows that intraseasonal SST anomalies which propagate westward from the eastern to the central tropical Indian Ocean during the CIO mode are mainly attributable to zonal thermal advection. Surface heat flux is the second largest contributor. This is distinct from the traditional tropical oceanic ISVs as a response to the Madden–Julian Oscillation (MJO) in the atmosphere, in which surface heat flux is usually the dominant component. Current results along with the previously reported atmosphere dynamics during the CIO mode depict a framework for the ocean–atmosphere coupled mode over the tropical Indian Ocean. This represents a more comprehensive understanding of tropical ISVs and will ultimately contribute to the improvement in process understanding, simulations, and forecasts of the Indian summer monsoon.

Wintertime deepwater renewal, which is important for heat–oxygen–nutrient exchange in lakes, is traditionally considered to be mainly driven by 1D vertical convective cooling. However, differential cooling between shallow and deep waters can produce density currents that flow into deep layers. In order to determine the role that these two cooling processes play in deepwater renewal, field measurements and 3D numerical modeling were combined to investigate heat content dynamics in Lake Geneva's large basin, the Grand Lac (maximum depth 309 m), during an exceptionally cold air spell in early 2012 where complete overturning had been reported. In a novel approach, the heat budget of the lake was decomposed, which allowed the identification and quantification of the heat budget components. The heat budget decomposition revealed that vertical convective cooling only penetrated to 200 m and that lateral advection was not only caused by density currents being discharged from the shallow littoral zone of the Grand Lac, but also from the Lake's shallow Petit Lac basin (maximum depth 75 m); the latter was found to be the main driver of heat content decrease in the deep layers of the Grand Lac below ∼200‐m depth. These findings provide unique insight into heat exchange processes that cannot be obtained from field data or numerical simulations alone. Heat budget decomposition proved to be a powerful, universally applicable tool for quantifying the contribution of alternative deepwater renewal processes. This is important, since deepwater renewal by convective cooling is weakening due to persistent global warming. Plain Language Summary Winter cooling is an important process for heat–oxygen–nutrient exchange in lakes. Traditionally considered to be mainly driven by 1D vertical convective cooling, complete overturning occurs when temperatures become homogeneous over the full water depth. In most deep lakes, this only occurs occasionally. Complete overturning in 309‐m deep Lake Geneva was reported for winter 2012. Field measurements taken at the deepest part of the lake, however, showed that the lake's temperature profile during that winter was not uniform. Instead, it had a cooler, deep bottom layer. To understand this temperature profile, we used 3D numerical modeling to decompose the lake's heat budget. This novel approach made it possible to investigate and quantify the contribution of: (a) vertical top‐down convective cooling, and (b) lateral advection of cold water from shallower regions to the lake's deep layers. The results revealed that convective cooling did not reach below ∼200‐m depth. Instead, lateral cold‐water advection from the lake's shallow side basin and nearshore regions were responsible for the cooling and deepwater renewal below that depth. As climate change induced‐warming continues to weaken convective cooling, it will become increasingly important to understand and consider lateral advection as an alternative deepwater renewal process in large, deep lakes. Key Points Deepwater renewal in a large, 309‐m deep lake during a very cold air spell is investigated by field observations and 3D numerical modeling A novel heat budget decomposition methodology allows for quantification of contributions of different processes to deepwater renewal Lateral advection by density currents from shallow areas and a side basin was identified as the governing process in deepwater renewal

The decadal variability and long-term trend of upper-ocean temperature in the Southeast Indian Ocean since the 1950s are investigated based on an eddy-resolving ocean model and two observation-based products. All three datasets show increasing trends and significant decadal changes in both Southeast Indian Subantarctic Mode Water (SEISAMW) core layer temperature and mixed layer temperature (MLT). The temperature change in the SEISAMW core layer is induced by subducted heat change that is in turn due to a change in the MLT on both long-term trend and decadal time scale. A heat budget analysis finds that the long-term increase of MLT is mainly resulted from a combination of more heat flux from the atmosphere to the ocean and shallower mixed layer depth, while both the intensified zonal wind stress and the vertical entrainment have a negative contribution to the increase in the MLT. The decadal variability of MLT is dominated by changes of surface thermal forcing and horizontal advection, with the former contributing more than the latter. The change in the surface thermal forcing is a result of changes of both surface heat flux and mixed layer depth, and the change in the horizontal advection is due mainly to the change in the zonal wind stress. Further analysis suggests that the decadal changes of surface heat flux and zonal wind stress in the SEISAMW formation region are dominated by local zonal and meridional sea-level pressure gradients, respectively.

In the last decade, three persistent warm blob events (2013/14, 2015, and 2019/20) in the northeast Pacific (NEP) have been hotly debated given their substantial effects on climate, ecosystems, and the socioeconomy. This study investigates the changes of such long-lived NEP warm blobs in terms of their intensity, duration, structure, and occurrence frequency under Shared Socioeconomic Pathway (SSP) 119 and 126 low-warming scenarios of phase 6 of the Coupled Model Intercomparison Project. Results show that the peak timing of the warm blobs shifts from the cold season to boreal summer. For the summer-peak warm blobs, their maximum intensity increases by 6.7% (10.0%) under the SSP119 (SSP126) scenario, but their duration reduces by 31.0% (20.4%) under the SSP119 (SSP126) scenario. In terms of their vertical structure, the most pronounced temperature signal is located at the surface, and their vertical penetration is mostly confined to the mixed layer, which becomes shallower in warming climates. Based on a mixed layer heat budget analysis, we reveal that a shoaling mixed layer depth plays a dominant role in driving the stronger intensity of the warm blobs under low-warming scenarios, while the stronger magnitude of ocean heat loss after their peaks explains the faster decay and thus shorter duration. Regarding occurrence frequency, the total number of the warm blobs does not change robustly in the low-warming climates. Following the summer peak of the warm blobs, extreme El Ni˜no events may occur more frequently under the low-warming scenarios, possibly through stronger air–sea coupling induced by tropical Pacific southwesterly anomalies.

The interannual variability of sea surface temperature (SST) in the South China Sea (SCS) exhibits two warming peaks around January and August in the subsequent year of El Niño. Results show that the second basin-scale SST warming peak is only evident following strong El Niño events, not regular events, which can be well identified by the Empirical Orthogonal Function (EOF) method. The heat budget analysis reveals that the anomalous Ekman heat advection (Eka) and geostrophic heat advection (Goa) contribute to the second warming feature, associated with the local dynamic response in the SCS, such as wind-driven Ekman downwelling, weakened Vietnam coastal upwelling, and abnormal anticyclonic geostrophic currents. The abnormal Eka and Goa are further attributed to the advanced abnormal equatorial easterlies breakout in the western Pacific, coinciding with the development of the western North Pacific anomalous anticyclone (WNPAC) in response to strong El Niño. This suggests that the second warming feature of the SCS SST is the footprint of strong El Niño events via the establishment of the WNPAC, which is tightly related to Indo-Pacific remote processes.

In response to quadrupled CO 2 , the Southern Ocean primarily uptakes excess heat around 60°S, which is then redistributed by the northward ocean heat transport (OHT) and mostly stored in the ocean or released back to the atmosphere around 45°S. However, the relative roles of mean ocean circulation and ocean circulation change in the uptake and redistribution of heat in the Southern Ocean remain controversial. Here, a set of climate model experiments embedded with a novel partial coupling technique are used to separate the roles of mean ocean circulation (passive component) and ocean circulation change (active component). For the ocean heat uptake (OHU) response, the mean ocean circulation and ocean circulation change are of equal importance. The OHT response south of 50°S is mainly determined by mean ocean circulation, while the ocean circulation change generates an anomalous southward OHT north of 50°S. A heat budget analysis finds that the divergence of passive OHT acts to balance the passive surface heat gain to the south of ∼50°S, while the convergence of active OHT acts to balance the active surface heat loss to the north of ∼50°S. Intriguingly, all the increase in ocean heat storage (OHS) is attributable to the passive component, with the ocean circulation change playing almost no role. In the Southern Ocean, both the active and the passive ocean heat transports are overcompensated by the reverse atmospheric heat transport via the Bjerknes compensation.