A 200-hPa zonal momentum budget is performed to examine the role that western North Pacific tropical cyclones (TCs) play in helping to organize intraseasonal extratropical circulation anomalies that occur with the Madden–Julian oscillation (MJO). Zonal wind is linearly decomposed into components that occur on MJO time scales (i.e., 20–100-day periods), as well as those that occur with lower and higher frequency. Dates during Northern Hemisphere fall that feature nonrecurving TCs within a search radius centered on a South China Sea grid point when the MJO is convectively active over the Maritime Continent and west Pacific warm pool are used to generate composites of relevant budget terms. These composites are then compared to others that are based on the full list of dates that feature a convectively active MJO in the same location during NH fall without regard for TC presence. Composite results highlight the primary momentum sources that guide the evolution of the NH extratropical zonal wind and associated mass field in each event set. TCs help to accelerate the East Asian subtropical jet that evolves with the MJO by modulating the high-frequency subtropical circulation over Southeast Asia. The phasing of this circulation with its underlying MJO time-scale component enables it to transfer momentum to the emerging subtropical jet. This momentum is integrated into the more slowly evolving flow and carried forward by other processes, which leads to the development of a westerly momentum surge along the subtropical jet that spans the length of the North Pacific Ocean.
Atmospheric angular momentum (AAM) is used to study the variability of Earth’s atmospheric circulation during the past 45 years, a time of considerable climate change. Using global AAM, two interdecadal states are defined covering the periods 1977–98 (hereinafter P1) and 1999–2022 (P2). Global AAM decreased from P1 to P2 and was accompanied by weakened subtropical jet streams in both hemispheres, strong convection around the northern Maritime Continent, and a strengthened sea surface temperature (SST) gradient across the tropical Pacific Ocean. The period differences project onto 1) internal interdecadal Pacific variability (IPV), 2) a postulated transient ocean thermostat response to greenhouse gas and aerosol emissions, and 3) circulation anomalies related to the ozone hole. During 1977–2023, the first two processes are forcing the climate toward larger Pacific Ocean SST gradients and a poleward expansion of the Indo-Pacific warm pool (IPWP), especially into the Northern Hemisphere. The ozone hole produces its own distinct pattern of anomalies in the Southern Hemisphere that tend to become persistent in the early 1990s. The zonal and vertical mean AAM variations during P1 have frequent westerly wind anomalies between 40°N and 40°S with poleward propagation on interannual time scales. During P2, the circulation is dominated by subtropical easterly wind anomalies, poleward-shifted jets, and weaker propagation. Locally, the zonal mean anomalies manifest as midlatitude ridges that lead to continental droughts. Case studies illustrate the weakened subtropical jet streams of P2 and examine the factors behind a transition to La Niña in early 2020 that maintains the P2 pattern.
more » « less- Award ID(s):
- 2048770
- PAR ID:
- 10452894
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 36
- Issue:
- 19
- ISSN:
- 0894-8755
- Format(s):
- Medium: X Size: p. 6597-6611
- Size(s):
- p. 6597-6611
- Sponsoring Org:
- National Science Foundation
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