The intricate currents of the Northwest Pacific Ocean, with strong manifestations along the westside rim, connect tropical and subtropical gyres and significantly influence East Asian and global climates. The El Niño/Southern Oscillation (ENSO) originates in the tropical Pacific Ocean and disrupts this ocean circulation system. However, the spatiotemporal dependence of the impact of ENSO events has yet to be elucidated because of the complexities of both ENSO events and circulation systems, as well as the increased availability of observational data. We thus combined altimeter and drifter observations to demonstrate the distinct tropical and subtropical influences of the circulation system on ENSO diversity. During El Niño years, the North Equatorial Current, North Equatorial Countercurrent, Mindanao Current, Indonesian Throughflow, and the subtropical Kuroshio Current and its Extension region exhibit strengthening, while the tropical Kuroshio Current weakens. The tropical impact is characterized by sea level changes in the warm pool, whereas the subtropical influence is driven by variations in the wind stress curl. The tropical and subtropical influences are amplified during the Centra Pacific El Niño years compared to the Eastern Pacific El Niño years. As the globe warms, these impacts are anticipated to intensify. Thus, strengthening observation systems and refining climate models are essential for understanding and projecting the enhancing influences of ENSO on the Northwest Pacific Oceanic circulation.
This study investigates the variability of the Southern Hemisphere super gyre (SHSG), using remotely sensed altimeter measurements, in situ Argo observations, and results from an ocean state estimate of the Consortium for Estimating the Circulation and Climate of the Ocean. Analyses of altimeter data show large trends of sea surface height, and their positive‐negative contrast suggests a strengthening of subtropical gyres in all the three Southern Hemisphere oceans since 1993. Analyses of Argo data and the Estimating the Circulation and Climate of the Ocean estimate indicate that these dynamic signals of southern subtropical gyres extend to at least 2,000 m. The three southern subtropical gyres are interconnected through the Tasman and Agulhas leakages and vary consistently during the period 1993–2016. The Tasman and Agulhas leakages also show an increasing trend of inter‐ocean water exchange with a typical increase of ~2 Sv (1 Sv = 106 m3/s) per decade, indicative of a two‐decade‐long spin‐up of the SHSG. The strengthening and poleward shift of westerly winds are associated with an increasing southern annular mode, which affect the midlatitude and high‐latitude Southern Hemisphere oceans and contribute to the spin‐up of the SHSG.
more » « less- NSF-PAR ID:
- 10375079
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 124
- Issue:
- 1
- ISSN:
- 2169-9275
- Page Range / eLocation ID:
- p. 154-170
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract The origins of the upper branch of the Atlantic meridional overturning circulation (AMOC) are traced with backward‐in‐time Lagrangian trajectories, quantifying the partition of volume transport between different routes of entry from the Indo‐Pacific into the Atlantic. Particles are advected by the velocity field from a recent release of “Estimating the Circulation and Climate of the Ocean” (ECCOv4). This global time‐variable velocity field is a dynamically consistent interpolation of over 1 billion oceanographic observations collected between 1992 and 2015. Of the 13.6 Sverdrups (1 Sv =
106 m3 /s) flowing northward across 6°S, 15% enters the Atlantic from Drake Passage, 35% enters from the straits between Asia and Australia (Indonesian Throughflow), and 49% comes from the region south of Australia (Tasman Leakage). Because of blending in the Agulhas region, water mass properties in the South Atlantic are not a good indicator of origin. -
more » « less
Abstract The origins of the upper limb of the Atlantic meridional overturning circulation and the partition among different routes has been quantified with models at eddy-permitting and one eddy-resolving model or with low-resolution models assimilating observations. Here, a step toward bridging this gap is taken by using the Southern Ocean State Estimate (SOSE) at the eddy-permitting 1/6° horizontal resolution to compute Lagrangian diagnostics from virtual particle trajectories advected between 6.7°S and two meridional sections: one at Drake Passage (cold route) and the other from South Africa to Antarctica (warm route). Our results agree with the prevailing concept attributing the largest transport contribution to the warm route with 12.3 Sv (88%) (1 Sv ≡ 106m3s−1) compared with 1.7 Sv (12%) for the cold route. These results are compared with a similar Lagrangian experiment performed with the lower-resolution state estimate from Estimating the Circulation and Climate of the Ocean. Eulerian and Lagrangian means highlight an overall increase in the transport of the major South Atlantic currents with finer resolution, resulting in a relatively larger contribution from the cold route. In particular, the Malvinas Current to Antarctic Circumpolar Current (MC/ACC) ratio plays a more important role on the routes partition than the increased Agulhas Leakage. The relative influence of the mean flow versus the eddy flow on the routes partition is investigated by computing the mean and eddy kinetic energies and the Lagrangian-based eddy diffusivity. Lagrangian diffusivity estimates are largest in the Agulhas and Malvinas regions but advection by the mean flow dominates everywhere.
-
more » « less
Abstract Both a quasi‐biennial variability and an overall linearly increasing trend are identified in the Sub‐Antarctic Mode Water (SAMW) subduction rate across the Southern Hemisphere ocean, using the Argo data during 2005–2019. The quasi‐biennial variability is mainly due to variability of the mixed layer depth. Variability of wind stress curl in the SAMW formation regions associated with the Southern Annular Mode plays a critical role in generating the quasi‐biennial variability of the mixed layer depth and consequently the SAMW subduction rates. The SAMW subduction rate across the Southern Hemisphere ocean, long‐term mean totaling 56 Sv, has increased at 0.73 ± 0.65 Sv year−1over the past 15 years. The increase has directly contributed to the observed increase in the total SAMW volume. Much of this increasing trend can be explained by the deepening mixed layers, which in turn are primarily forced by the strengthening westerly winds under an increasing Southern Annular Mode.
-
more » « less
Abstract The Gulf Stream is the only pathway in the subtropical North Atlantic by which warm water flows poleward. This transport of warm water and return of cold water at depth is called the Atlantic Meridional Overturning Circulation (AMOC). The dynamic method is applied to hydrocasts collected since the 1930s to estimate upper‐ocean transport (0–1,000 m) between the U.S. Continental Slope and Bermuda and separately to Africa with focus on the longest directly observable timescale. Calculating transport between the Slope and Bermuda eliminates the Gulf Stream's northern and southern recirculation gyres, while calculations between the Slope and Africa remove all other recirculating geostrophic flow. The net Slope‐Bermuda upper‐ocean transport is estimated to be 41.1 ± 0.4 Sv, decreasing by 2.0 ± 0.8 Sv between 1930 and 2020. The AMOC contribution is 18.4 ± 0.6 Sv, decreasing by 0.4 ± 0.6 Sv between 1930 and 2020.
