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umsonst.bsky.social

@umsonst.bsky.social

You could call it another critical system change:

Upper ocean currents speed up in the tropical Pacific

"Strengthening of the Equatorial Pacific Upper-Ocean Circulation Over the Past Three Decades"; agupubs.onlinelibrary.wiley.com/doi/10.1029/...

#climate #uöäü1currents #Earth #Pacific #oceans

Abstract

Thirty years (1993–2022) of concurrent satellite and in-situ observations show a long-term strengthening of the equatorial Pacific upper-ocean circulation. Enhanced southeasterly and cross-equatorial winds have caused an annual mean, basin-wide acceleration of the equatorial westward near-surface currents by ∼20% and an acceleration of poleward flow north (south) of the equator by ∼60% (∼20%). Additional moored velocity data reveal a deepening of the Equatorial Undercurrent (EUC) core at 170°W and significant shoaling at 140°W and 110°W, but no significant changes in EUC core velocity. The strongest subsurface zonal velocity trends are observed above the EUC core in the central to eastern Pacific and occur before and after the seasonal maximum of EUC core velocity, causing enhanced upper-ocean vertical current shear. Consistent with trends of the 20°C isotherm depth along the equatorial Pacific, a significant basin-wide steepening of the equatorial thermocline is observed. Both the accelerating equatorial current system and the enhanced thermocline slope are consistent with an observed steepening of the zonal sea surface height gradient due to increased wind-driven westward mass transport at the surface. During February-March, both surface and subsurface currents along the equator show eastward velocity trends, in contrast to westward near-surface current trends during the remainder of the year. The trend reversal is attributed to both a long-term shift in equatorial Kelvin wave activity and to the impact of strong interannual variability due to El Niño Southern Oscillation and other modes of natural variability on decadal to multidecadal time scales.

(a) Decadal trends of zonal wind stress (τx, color shading) and horizontal wind stress (τ, arrows) and precipitation (color contours) in the tropics between 1993 and 2022. The ±0.25 mm day−1 decade−1 contour lines are indicated with negative trends in red and positive trends in green. Speckled regions indicate significant τx trends at 95% confidence. (b) Decadal trend of τx (color shading) and climatological τx (color contours) as a function of month and zonally averaged for the central tropical Pacific Ocean region (170°E−100°W) indicated in (a). Blue contour lines indicate negative climatological τx and red contour lines indicate positive climatological τx (in N m−2). (c) Zonally averaged (170°E−100°W) annual mean τx (black line; N m−2) and annual mean decadal trend (red and blue bars; N m−2 decade−1). (d) Same as (b) but for meridional wind stress, τy. (e) Same as (c) but for meridional wind stress, τy. Gray dots in (b) and (d) indicate significant trends at 95% confidence. The same color bar is used for the panels (a), (b), and (d).

(a) Decadal trend of horizontal wind stress divergence (∇h · τ) in the tropics. Three regions are indicated, in the tropical North Pacific (170°E−100°W, 5°N−10°N; black box), equatorial Pacific (170°E−100°W, 2°S−5°N; red box), and in the tropical South Pacific (170°E−120°W, 15°S−2°S; green box). Speckled regions indicate significant ∇h · τ trends at 95% confidence. Monthly time series of ∇h · τ averaged in the (b) tropical North Pacific, (c) equatorial Pacific, and (d) tropical South Pacific. (e) Decadal trend of precipitation in the tropics. Note the color map is flipped for this panel. The black contour in (e) represents the climatological 5 mm day−1 isoline. Speckled regions indicate significant precipitation trends at 95% confidence. The same three regions as in (a) are depicted. Monthly time series of precipitation averaged in the (f) tropical North Pacific, (g) equatorial Pacific, and (h) tropical South Pacific. In (b)–(d) and (f)–(h), the linear trends (thick lines), 95% confidence interval (thin solid lines), and the 95% prediction interval (dashed lines) are indicated.

(a) Decadal trend of SSH in the tropics. The black isoline indicates the global tropical (30°S−30°N) annual mean SSH trend (0.03 m decade−1). At 99.3% of all grid points, SSH trends are significant at 95% confidence. (b) Same as (a) but with the global tropical annual mean SSH trend removed. (c) Monthly SSH difference between the western (W; 150°E−170°W, 3°S−3°N) and eastern (E; 170°W−85°W, 3°S−3°N) equatorial Pacific regions indicated in (a). The thin curve represents the monthly time series, while the thick curve shows a 25 month running mean. The linear trend (thick solid line), 95% confidence interval (thin solid lines and gray shading), and the 95% prediction interval (dashed lines) are indicated.

December 7, 2024 at 1:05 PM

umsonst.bsky.social

@umsonst.bsky.social

On the other hand model results and reanalysis datasets showed that subsurface currents could speed up also below 200m.

"Deep-reaching acceleration of global mean ocean circulation over the past two decades"; www.science.org/doi/10.1126/...

#climate #uöäü1currents

November 24, 2024 at 9:07 PM

umsonst.bsky.social

@umsonst.bsky.social

One of the more fascinating Earth system changes:

Stratification is discussed to cause the speed up of surface currents (1), while it suppresses water moments below 200m (2).

#climate #uöäü1OHU #uöäü1currents

Our results show that the surface warming effect, a robust feature of anthropogenic climate change, dominates and accelerates the upper ocean currents in 77% of the global ocean. Specifically, the increased
vertical stratification intensifies the upper subtropical gyres and equatorial currents by shoaling these systems, while the differential warming between the Southern Ocean upwelling zone and the region to the north accelerates surface zonal currents in the Southern Ocean.

Globally integrated kinetic energy (KE) of large-scale ocean circulations in the upper layer,

Eddy KE (EKE)

KE of large-scale ocean circulations (mean KE and MKE for short)

Satellite observations26 reveal an enhanced surface eddy activity, measured by eddy KE (EKE), over the past three decades, especially in eddy-rich regions such as the western boundary currents and their extensions (WBCEs) and the Antarctic Circumpolar Current. This finding is in broad agreement with the projected future EKE change based on a global climate simulation with regionally refined grids to resolve mesoscale eddies in the eddy-rich regions27. However, little is known about the EKE trend below the sea surface. It remains unclear whether the surface intensification of mesoscale eddies is a useful fingerprint of total EKE change in the global ocean under global warming. Neither do we know its contribution to the total KE change.

This is consistent with the existing literature14,18 suggesting accelerated large-scale ocean circulations in the upper ocean under global warming. In addition, the surface horizontally integrated EKE (EKE) shows a significant increase (Supplementary Fig. 2), which is also in line with the previous estimates26,27. However, the trend of EKE attenuates rapidly with increasing depth and reverses its sign below 100 m (Supplementary Fig. 2), so that the integrated EKE over the upper 200 m is almost unchanged (Fig. 3f). It thus suggests that the surface imprint of the trend is not representative of its vertical integral.

Below 200 m, KE shows a negative trend (−21.79 ± 5.75 × 1016 J per century or equivalently −6.5 ± 1.7% per century) mainly contributed by that (−15.22 ± 3.69 × 1016 J per century) of EKE (Fig. 3).

November 24, 2024 at 8:38 PM

umsonst.bsky.social

@umsonst.bsky.social

Variability and Trends of the South Atlantic Subtropical Gyre

We find a significant southward migration of the southern gyre boundary but no trends in the gyre size or gyre strength from 1993to2018

Fgyre strength follows the seasonal heating and cooling of the upper ocean

#climate #uöäü1currents