Increase in Cape Verde hurricanes during Atlantic Niño.

At seasonal-to-interannual timescales, Atlantic hurricane activity is greatly modulated by El Niño-Southern Oscillation and the Atlantic Meridional Mode. However, those climate modes develop predominantly in boreal winter or spring and are weaker during the Atlantic hurricane season (June-November). The leading mode of tropical Atlantic sea surface temperature (SST) variability during the Atlantic hurricane season is Atlantic Niño/Niña, which is characterized by warm/cold SST anomalies in the eastern equatorial Atlantic. However, the linkage between Atlantic Niño/Niña and hurricane activity has not been examined. Here, we use observations to show that Atlantic Niño, by strengthening the Atlantic inter-tropical convergence zone rainband, enhances African easterly wave activity and low-level cyclonic vorticity across the deep tropical eastern North Atlantic. We show that such conditions increase the likelihood of powerful hurricanes developing in the deep tropics near the Cape Verde islands, elevating the risk of major hurricanes impacting the Caribbean islands and the U.S.

Impacts of model resolution and ocean coupling on mean and eddy momentum transfer during the rapid intensification of super-typhoon Muifa (2011).

This study uses a coupled atmosphere-ocean model with different numerical settings to investigate the mean and eddy momentum transfer processes responsible for Typhoon Muifa's (2011) early rapid intensification (RI). Three experiments are conducted. Two use the coupled model with a horizontal resolution of either 1 km (HRL) or 3 km (LRL). The third (NoTCFB) is the same as LRL but excludes tropical cyclone (TC)-induced sea-surface temperature (SST) cooling. HRL reasonably reproduces Muifa's intensity during its rapid intensification and weakening periods. The azimuthal mean tangential and radial momentum budgets are analysed before the RI rates diverge between HRL and LRL. Results show that the dominant processes responsible for Muifa's intensification are different in HRL and LRL. For HRL, the net eddy effect intensifies the storm's circulation and contracts the eyewall during early RI, and it dominates the net mean-flow effect inside the radius of maximum wind (RMW), except near the surface and between 2 and 5 km close to the RMW. In contrast, the mean and eddy effects in LRL almost cancel inside the RMW, while the mean-flow effects dominate and intensify tangential winds outside. Without TC-induced SST cooling, Muifa in NoTCFB reaches a similar storm intensity as in HRL but its rapid weakening rate is substantially underestimated. The dominant mechanisms for tangential wind intensification in NoTCFB are similar to those in LRL, but their magnitudes are larger, implying a misrepresentation of the dominant momentum transfer processes in NoTCFB during RI. For the radial momentum budget analysis, the dominant processes are similar among the three experiments except for some differences in their locations and strengths.

Surface cooling caused by rare but intense near-inertial wave induced mixing in the tropical Atlantic.

The direct response of the tropical mixed layer to near-inertial waves (NIWs) has only rarely been observed. Here, we present upper-ocean turbulence data that provide evidence for a strongly elevated vertical diffusive heat flux across the base of the mixed layer in the presence of a NIW, thereby cooling the mixed layer at a rate of 244 W m-2 over the 20 h of continuous measurements. We investigate the seasonal cycle of strong NIW events and find that despite their local intermittent nature, they occur preferentially during boreal summer, presumably associated with the passage of atmospheric African Easterly Waves. We illustrate the impact of these rare but intense NIW induced mixing events on the mixed layer heat balance, highlight their contribution to the seasonal evolution of sea surface temperature, and discuss their potential impact on biological productivity in the tropical North Atlantic.

Hydrological cycle changes under global warming and their effects on multiscale climate variability.

Despite a globally uniform increase in the concentrations of emitted greenhouse gases, radiatively forced surface warming can have significant spatial variations. These define warming patterns that depend on preexisting climate states and through atmospheric and oceanic dynamics can drive changes of the hydrological cycle with global-scale feedbacks. Our study reviews research progress on the hydrological cycle changes and their effects on multiscale climate variability. Overall, interannual variability is expected to become stronger in the Pacific and Indian Oceans and weaker in the Atlantic. Global monsoon rainfall is projected to increase and the wet season to lengthen despite a slowdown of atmospheric circulation. Strong variations among monsoon regions are likely to emerge, depending on surface conditions such as orography and land-sea contrast. Interdecadal climate variability is expected to modulate the globally averaged surface temperature change with pronounced anomalies in the polar and equatorial regions, leading to prolonged periods of enhanced or reduced warming. It is emphasized that advanced global observations, regional simulations, and process-level investigations are essential for improvements in understanding, predicting, and projecting the modes of climate variability, monsoon sensitivity, and energetic fluctuations in a warming climate.

Global warming-induced upper-ocean freshening and the intensification of super typhoons.

Super typhoons (STYs), intense tropical cyclones of the western North Pacific, rank among the most destructive natural hazards globally. The violent winds of these storms induce deep mixing of the upper ocean, resulting in strong sea surface cooling and making STYs highly sensitive to ocean density stratification. Although a few studies examined the potential impacts of changes in ocean thermal structure on future tropical cyclones, they did not take into account changes in near-surface salinity. Here, using a combination of observations and coupled climate model simulations, we show that freshening of the upper ocean, caused by greater rainfall in places where typhoons form, tends to intensify STYs by reducing their ability to cool the upper ocean. We further demonstrate that the strengthening effect of this freshening over the period 1961-2008 is ∼53% stronger than the suppressive effect of temperature, whereas under twenty-first century projections, the positive effect of salinity is about half of the negative effect of ocean temperature changes.