A decrease in surface albedo over ice‐covered ocean leads to global warming and Arctic‐amplified warming. Numerical results indicate seasonal variation in Arctic amplification (AA) is a result of local forcing and feedbacks in the Arctic. A decrease in surface albedo leads to a positive feedback, which dominates the local forcing and feedback mechanism. Ocean heat storage in the subsurface acts as a heat forcing to delay the influence of surface albedo feedback. In summer (autumn), heat storage increases (discharges) and contributes to a negative (positive) heat forcing, which decreases (increases) the positive local forcing and feedback and triggers the occurrence of the minimum (maximum) AA. In addition, increased CO₂ forcing largely decreases the outgoing longwave radiation at the surface and increases surface temperatures, especially at low latitudes and in the Arctic winters, which decreases the AA magnitude and seasonal variation, although the AA remains nearly the same during winter.

Effects of wind and fresh water on the Atlantic meridional overturning circulation (AMOC) are investigated using a fully coupled climate model. The AMOC can change significantly when perturbed by either wind stress or freshwater flux in the North Atlantic. This study focuses on wind stress effect. Our model results show that the wind forcing is crucial in maintaining the AMOC. Reducing wind forcing over the ocean can cause immediately weakening of the vertical salinity diffusion and convection in the mid-high latitudes Atlantic, resulting in an enhancement of vertical salinity stratification that restrains the deep water formation there, triggering a slowdown of the thermohaline circulation. As the thermohaline circulation weakens, the sea ice expands southward and melts, providing the upper ocean with fresh water that weakens the thermohaline circulation further. The wind perturbation experiments suggest a positive feedback between sea-ice and thermohaline circulation strength, which can eventually result in a complete shutdown of the AMOC. This study also suggests that sea-ice variability may be also important to the natural AMOC variability on decadal and longer timescales.

The effect of the ocean surface winds on the meridional heat transports is studied in a coupled model. Shutting down the global surface winds causes significant reductions in both wind-driven and thermohaline ocean circulations, resulting in a remarkable decrease in the poleward oceanic heat transport (OHT). The sea surface temperature responds with significant warming in the equator and cooling off the equator, causing an enhancement and equatorward shift in the Hadley cell. This increases the poleward atmospheric heat transport (AHT), which in turn compensates the decrease in the OHT. This compensation implies a fundamental constraint in changes of ocean–atmosphere energy transports. Several other compensation changes are also identified. For the OHT components, the changes in the Eulerian mean and bolus OHT are compensated with each other in the Southern Ocean, since a stronger wind driven Ekman transport is associated with a stronger meridional density gradient (stronger bolus circulation) and vice versa. For the AHT components, the changes in the dry static energy (DSE) and latent energy transports are compensated within the tropics (30°N/S), because a stronger Hadley cell causes a stronger equatorward convergence of moisture. In the extratropics, the changes in the mean and eddy DSE transports show perfect compensation, as a result of the equatorward shift of the Ferrell Cell and enhancement of atmospheric baroclinicity in mid-high latitudes, particularly over the North Atlantic. This work also shows how the Earth’s climate is trying to maintain the balance between two hemispheres: the ocean in the Northern Hemisphere is colder than that in the Southern Hemisphere due to much reduced northward heat transports cross the Equator in the Atlantic, therefore, the atmosphere responds to the ocean with temperature colder in the Southern Hemisphere than in the Northern Hemisphere by transporting more heat northward cross the equator over the Pacific, in association with a southward shift of the intertropical convergence zone.

Observations of sea surface wind field are critical for typhoon prediction. The scatterometer observation is one of the most important sources of sea surface winds, which provides both wind speed and wind direction information. However, the spatial resolution of scatterometer wind is low. Synthetic Aperture Radar (SAR) can provide a more detailed wind structure of the tropical cyclone. In addition, the cross-polarization observation of SAR can provide more detailed information of high speed wind (>25 m·s ${}^{-1}$ ) than the scatterometer. Nevertheless, due to the narrow swath of SAR, the number of retrieved sea surface wind data used in the data assimilation is limited, and another limitation of SAR wind observation is that it does not provide true wind direction information. In this paper, the joint assimilation of the Advanced Scatterometer (ASCAT) wind and Sentinel-1 SAR wind was investigated. Another limitation in the current operational typhoon prediction is the inefficient quality control (QC) method used in the data assimilation since a large number of high speed wind observations was rejected by the traditional Gaussian distribution QC. We introduce the Huber norm distribution quality control (QC) into the data assimilation successfully. A numerical simulation experiment of typhoon by Lionrock (2016) is conducted to test the proposed method. The experimental results showed that the new quality control scheme not only greatly increases the availability of wind data in the area of the typhoon center, but also improves the typhoon track prediction, as well as the intensity prediction. The joint assimilation of scatterometer and SAR winds does have a positive impact on the typhoon prediction.

The roles of energy balance and climate feedback in Bjerknes compensation (BJC) are studied through wind-perturbation experiments in a coupled climate model. Shutting down surface winds over the ocean causes significant reductions in both wind-driven and thermohaline overturning circulations, leading to a remarkable decrease in poleward ocean heat transport (OHT). The sea surface temperature (SST) responds with an increasing meridional gradient, resulting in a stronger Hadley Cell, and thus an enhanced atmosphere heat transport (AHT), compensating the OHT decrease. This is the so-called BJC. Coupled model experiments confirm that the occurrence of BJC is an intrinsic requirement of local energy conservation, and local climate feedback determines the degree of BJC, consistent with our previous theoretical results. Negative (positive or zero) local feedback results in AHT change undercompensating (overcompensating or perfectly compensating) OHT change. Using the radiative kernel technique, the general local feedback between the radiative balance at the top of the atmosphere and surface temperature can be partitioned into individual feedbacks that are related to perturbations in temperature, water vapor, surface albedo, and clouds. We find that the overcompensation in the tropics (extratropics) is mainly caused by positive feedbacks related to water vapor and clouds (surface albedo). The longwave feedbacks related to SST and atmospheric temperature are always negative and strong outside the tropics, well offsetting positive feedbacks in most regions and resulting in undercompensation. Different dominant feedbacks give different BJC scenarios at different regions, acting together to maintain the local energy balance.