Search for: All records

The variability of the Hadley circulation strength (HCS), crucial to tropical climate variability, is attributed to both oceanic and atmospheric forcings. El Niño–Southern Oscillation (ENSO) and variations in the extratropical upper-tropospheric eddies are the known drivers of the interannual HCS variability. However, the relative contributions of these oceanic and atmospheric forcings to the hemispheric HCS variability are not well understood. In particular, how much anomalous wind stress–driven ocean dynamics, including ENSO, impact HCS variability remains an open question. To address these gaps, we investigate the drivers of the interannual HCS variability using global coupled model experiments that include or exclude anomalous wind stress–driven ocean circulation variability. We find that the anomalous wind stress–driven ocean circulation variability significantly amplifies HCS variability in the Southern Hemisphere (SH). ENSO is the leading modulator of the SH HCS variability, which offers the potential to improve the predictability of Hadley circulation (HC)–related hydrological consequences. On the other hand, the Northern Hemisphere (NH) HCS variability is predominantly influenced by the eddy-driven internal atmospheric variability with little role in ocean dynamics. We hypothesize that the large eddy variability in the NH and concentrated ENSO-associated heating and precipitation in the SH lead to the hemisphere-dependent differences in the interannual HCS variability.

During certain years, a synoptic scale vortex called the monsoon onset vortex (MOV) forms within the northward advancing zone of precipitating convection over the Arabian Sea. The MOV does not form each year and the reason is unclear. Since the Madden‐Julian Oscillation (MJO) is known to modulate convection and tropical cyclones in the tropics, we examined its role in the formation of the MOV. While the convective and transition phases of the MJO do not always lead to MOV formation, the suppressed phase of the MJO hinders the formation of the MOV more consistently. This asymmetric relationship between the MJO and MOV can be partially explained by the modulation of the large‐scale environment, measured by a tropical cyclone genesis index. It also suggests that the Arabian Sea is generally near a critical state that is favorable for MOV formation during the monsoon onset period.

The dynamics of African easterly waves (AEWs) are investigated from the perspective of potential vorticity (PV) using data from global reanalysis projects. To a leading order, AEW evolution is governed by four processes: advection of the wave-scale PV by background flow, advection of background PV by the AEW, diabatic forcing due to wave-scale moist convection, and coupling between the wave and background diabatic forcing. Moist convection contributes significantly to the growth of AEWs in the midtroposphere, and to both growth and propagation of AEWs near the surface. The former is associated with stratiform clouds while the latter with deep convection. Moist convection helps maintain a more upright AEW PV column against the background shear, which makes the wave structure conducive for tropical cyclogenesis. It is also argued that—contrary to the hypothesis in some prior studies—the canonical diabatic Rossby wave model is likely not applicable to AEWs.

Studies have quantified the contribution of tropical cyclones (TCs) toward seasonal precipitation, but limited analysis is available on TC contribution toward seasonal streamflow over the southeastern and southcentral (SESC) United States (U.S.). Using an extensive network of hydroclimatic data that consists of 233 TC tracks and daily precipitation and streamflow, we estimate TC contribution toward precipitation and streamflow during the hurricane season over the SESC U.S. We found that TCs account for 12% of seasonal streamflow and 6% of seasonal precipitation over the region. Florida, North Carolina, and Louisiana have the highest fractional occurrence of TC‐generated annual maximum precipitation (∼20%–32%) and streamflow (∼15%–27%). We also found the fractional occurrence of TCs associated with peak‐over threshold precipitation (streamflow) events ranges from 5% to 8% in coastal regions (10%–20% over FL and 5%–10% over coastal NC). Increased antecedent moisture results in increased TCs contribution to streamflow leading to different land‐surface responses for similar hurricane events.

Perturbation kinetic and available energy budgets are used to explore how convectively coupled equatorial Kelvin waves (KWs) impact African easterly wave (AEW) activity. The convective phase of the Kelvin wave increases the African easterly jet’s meridional shear, thus enhancing the barotropic energy conversions, leading to intensification of southern track AEWs perturbation kinetic energy. In contrast, the barotropic energy conversion is reduced in the suppressed phase of KW. Baroclinic energy conversion of the southern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. AEWs in the convective phase of a Kelvin wave have stronger perturbation available potential energy generation by diabatic heating and stronger baroclinic overturning circulations than in the suppressed phase of a Kelvin wave. These differences suggest that southern track AEWs within the convective phase of Kelvin waves have more vigorous convection than in the suppressed phase of Kelvin waves. Barotropic energy conversion of the northern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. The convective phase of the Kelvin wave increases the lower-tropospheric meridional temperature gradient north of the African easterly jet, thus enhancing the baroclinic energy conversion, leading to intensification of northern track AEWs perturbation kinetic energy. In contrast, the baroclinic energy conversion is reduced in the suppressed phase of KW. These results provide a physical basis for the modulation of AEWs by Kelvin waves arriving from upstream.

We examine the upscale effect of moist convection on African easterly waves (AEWs) by limiting condensational heating and initial ambient moisture in convection‐permitting simulations. Moist convection is fundamental in maintaining and destabilizing AEWs. The contribution from barotropic‐baroclinic instability, albeit important, is relatively smaller. Mesoscale convective systems (MCSs) are initiated downstream of the AEW troughs and are associated with extensive trailing stratiform cloud regions. Using a potential vorticity (PV) budget, we show that the attendant diabatic heating profile reinforces the AEW. A model for destabilization is proposed that relies on the phasing of stratiform heating and the PV anomaly of the AEW. It qualitatively resemblesstratiform instabilityandstretched building blockshypotheses introduced in previous studies. The generation of PV by deep moist convection in the vicinity of the trough counters the shearing effect of the background flow. This helps maintain an upright PV column, which is conducive for formation of tropical cyclones. AEW propagation is dominated by advective processes and intermittently modified by moist convection when large MCSs move ahead of the AEW.