Potential Strengthening of Madden-Julian Oscillation Circulations with Enhanced Modulation of Tropical Cyclone Genesis

Several recent observational studies have reported a gradual strengthening of Madden Julian Oscillation (MJO) circulations in the late 20th and/or early 21st centuries. While some climate models predict such strengthening to continue as the oceans warm, others models predict insignificant changes or even a gradual weakening of MJO circulations over the remainder of the 21st century. Whether or not MJO circulations strengthen or weaken in the future is not just of academic interest, as the MJO has global impacts on weather and climate, modulating tropical cyclone (TC) genesis, monsoon onset, and atmospheric rivers, among other things.

In this study we examine predicted changes to MJO circulations resulting from global warming, and we relate them to changes in tropical cyclone genesis. The primary model we use is the Lagrangian Atmospheric Model (LAM), which has been carefully tuned to reproduce observed vertical and horizontal structures of the MJO for each of the developing, mature, and dissipating stages of its convective envelope. To further understand the nature of MJO circulation changes, as well as their impacts on TC genesis, we partition MJO circulations into components associated with Kelvin and Rossby waves. TCs in the LAM are identified when cyclonic vorticity exceeds 4 × 10−5 s−1, a warm core exists for at least 48 hours, and maximum wind speed exceeds 12.5m/s for at least 36 consecutive hours. This methodology yields a TC genesis distribution that is broadly consistent with that observed in the Southern Indian and Western Pacific Oceans.

The LAM simulations suggest that a continued enhancement of greenhouse gases, consistent with the Shared Socioeconomic Pathway 585 scenario, will cause both the Kelvin and Rossby wave components of the MJO's circulation to intensify, leading to enhanced modulation of TC genesis. The model predicts the percentage enhancement of meridionally-integrated TC genesis just west of the MJO's convective center to increase from 73 percent to 170 percent of the background value. The predicted enhancement of the MJO's Kelvin wave circulation is consistent with that reported in a recent observational study by the lead author, which also showed some evidence of increased MJO modulation of TC genesis in recent decades. In addition to documenting predicted changes to MJO circulations and TC genesis simulated by the LAM, we plan to relate TC genesis perturbations to those expected from vertical motion, moisture, and vorticity perturbations associated with Kelvin and Rossby wave components of the MJO's circulation.