Ocean feedback to tropical cyclones: climatology and processes

Abstract

International audienceThis study presents the first multidecadal and coupled regional simulation of cyclonic activity in the South Pacific. The long-term integration of state-of the art models provides reliable statistics, missing in usual event studies, of air-sea coupling processes controlling tropical cyclone (TC) intensity. The coupling effect is analyzed through comparison of the coupled model with a companion forced experiment. Cyclogenesis patterns in the coupled model are closer to observations with reduced cyclogenesis in the Coral Sea. This provides novel evidence of air-sea coupling impacting not only intensity but also spatial cyclogenesis distribution. Storm-induced cooling and consequent negative feedback is stronger for regions of shallow mixed layers and thin or absent barrier layers as in the Coral Sea. The statistical effect of oceanic mesoscale eddies on TC intensity (crossing over them 20 % of the time) is also evidenced. Anticyclonic eddies provide an insulating effect against storm-induced upwelling and mixing and appear to reduce sea surface temperature (SST) cooling. Cyclonic eddies on the contrary tend to promote strong cooling, particularly through storm-induced upwelling. Air-sea coupling is shown to have a significant role on the intensification process but the sensitivity of TCs to SST cooling is nonlinear and generally lower than predicted by thermodynamic theories: about 15 rather than over 30 hPa °C−1 and only for strong cooling. The reason is that the cooling effect is not instantaneous but accumulated over time within the TC inner-core. These results thus contradict the classical evaporation-wind feedback process as being essential to intensification and rather emphasize the role of macro-scale dynamics

Observed strong currents under global tropical cyclones

“…The local background flow (or eddies of various length scales) affects the observed current structure and velocity (Gill, 1984) as well as the ocean mixed-layer depth (e.g. Jullien et al, 2014). The drifters are drogued at the 15-m depth, which is still in a region of significant Stokes drift influence.…”

MetUM-GOML1: a near-globally coupled atmosphere–ocean-mixed-layer model

“…Ocean-atmosphere coupling in general circulation models (GCMs) has been shown to improve the spatial distribution of cyclogenesis (e.g. Jullien et al, 2014), as well as the representation of cyclone intensity (e.g. Sandery et al, 2010).…”

Role of barrier layer in the developing phase of “Category 6” super typhoon Haiyan

“…This process contributed to Haiyan's intensity. A different case shows in the Coral Sea that storm-induced cooling and consequent negative feedback is stronger in regions with thin or absent BL (Jullien et al, 2014). Though the influence of BL on TCs intensification is hard to be estimated from the observations, its role in the evolution in a super typhoon should not be overlooked.…”