The Impact of the El Niño-Southern Oscillation and Atlantic Meridional Mode on Seasonal Atlantic Tropical Cyclone Activity

The influence of the El Niño-Southern Oscillation (ENSO) on Atlantic tropical cyclone (TC) activity on the interannual timescale is well understood, with El Niño (La Niña) inhibiting (supporting) TCs through enhanced (reduced) vertical wind shear in the Atlantic main development region (MDR). The switch from relatively inactive to active Atlantic TC seasons in the mid-1990's has highlighted the influence of interannual to decadal modes of Atlantic climate variability, including the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Mode (AMM).

An important issue is the relative strengths of ENSO and AMM in modulating Atlantic TC activity on interannual timescales, and possible interactions between these two influences. We have carried out mechanistic simulations using a 27-km resolution regional climate model (the NCAR WRF model) to address this issue. The model simulates the observed 1980-2000 interannual variability of Atlantic accumulated cyclone energy (ACE; 10⁴ kt²) with a correlation of 0.58 when observed Atlantic and eastern tropical Pacific sea surface temperature (SST) and lateral boundary conditions (LBCs) are prescribed.

Mechanistic experiments are configured by forcing the model with a strong El Niño or La Niña by prescribing LBCs and eastern tropical Pacific SST of 1987 and 1999 respectively, in combination with strongly positive, neutral/weakly positive, and strongly negative phases of the AMM through prescribed Atlantic SST of 2005, 1987 or 1999, and 1984, respectively. Two-member ensembles of each experiment are run.

During the strong El Niño, simulated Atlantic ACE is weaker than the simulated 1980-2000 mean regardless of AMM phase. The ACE changes relatively little between the negative and neutral AMM, but increases moderately between the neutral and positive AMM during El Niño. This, together with the response in MDR vertical wind shear suggests that the threshold MDR shear for TC suppression occurs between the neutral and positive AMM phase during El Niño.

During the strong La Niña and strongly negative AMM, Atlantic ACE is close to the mean, and anomalies in MDR wind shear and temperature and moisture profiles are relatively weak, suggesting compensation between the TC-supporting La Niña and TC-inhibiting negative AMM. Atlantic ACE is greater than the 1980-2000 mean by 61% during a strong La Niña and weakly positive AMM, and is supported by reduced shear and increased MDR low-level water vapor. The La Niña and strongly positive AMM result in a similar reduction of shear but a further increase in low-level moisture, and ACE that is greater than the mean by 179%. This suggests that the upper bound of seasonal Atlantic TC activity is not limited by La Niña or positive AMM alone, rather the two together produce prime conditions for TCs. Whereas ENSO impacts TC development mainly through altering MDR vertical wind shear, AMM affects both shear and low-level water vapor. Changes in frequency and intensity of ENSO and AMM should be considered together in studies of natural variability and anthropogenic changes in seasonal Atlantic TC activity.