scholarly journals Do Winds Control the Confluence of Subtropical and Subantarctic Surface Waters East of New Zealand?

2021 ◽  
Author(s):  
◽  
Denise Fernandez
Keyword(s):  
Time Series ◽  
New Zealand ◽  
Kinetic Energy ◽  
Wind Stress ◽  
South Pacific ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Island Rule ◽  
Eddy Activity ◽  
Confluence Region

<p>The confluence region east of New Zealand is one of only a few places in the world where the Antarctic Circumpolar Current meets the strong southwardflowing boundary current of a subtropical gyre. The convergence of subtropical and subantarctic water creates strong fronts. The fronts have clear signatures in height and temperature that make them appropriate places to investigate ocean/climate variability. The location and extent of the New Zealand confluence should respond to changes in large-scale wind patterns, as changes in South Pacific currents have been linked to wind shifts. However, recent studies have shown that highly energetic eddies, local winds, and the bathymetry may be significant controls of currents and associated fronts. This thesis investigates the temporal and spatial variability of the confluence and evaluates its response to variability in South Pacific winds. Analysis of the 18-year time series, from January 1993 to December 2010, of sea surface height mapped from satellite altimetry was used to investigate the location and extent of fronts and the eddy activity and relate these to the wind forcing. Wind stress data were used with the Island Rule to estimate the winddriven transport of the western boundary currents that feed the confluence. In addition, the climate modes Southern Annular Mode (SAM) and Southern Oscillation Index (SOI) were used to examine the influence of the principal modes of atmospheric variability. Time series of the local wind stress curl and local climate indices were calculated and compared to the intensity of the confluence to test any influence of local forcing. In addition, bathymetric effects were investigated by evaluating evidence for preferred front locations near topographic features. Sea level anomalies in the confluence region are increasing at 3.4 cm decade⁻¹. The sea surface height gradients and the eddy kinetic energy are also increasing at a rate of 0.01 cm km⁻¹ and 23 cm² s⁻² per decade respectively, indicating an intensification of the fronts and eddy activity in the confluence. There is a high and significant correlation (r = 0.84) between the front and eddy signals reflecting baroclinic instabilities inherent in the fronts. Difference in transport anomalies across the confluence derived from the Island Rule are also increasing at 8.8 Sv decade⁻¹. SAM and SOI indices showed little or no correspondence with variability in the confluence intensity and eddy kinetic energy, and the same lack of correspondence was observed in local winds and local indices. While these results suggest a connection between the variability in the confluence and South Pacific winds, there is a preferential location of the strongest fronts and eddy activity northeast of Bounty Plateau and Bollons Seamount, indicating some bathymetric control. The correspondence between basin-scale winds and sea surface height gradients in the confluence region indicates that if wind stress continues to increase, as current trends predict, front intensity and eddy activity will also increase, enhancing the transfer of heat and nutrients that, respectively, influence energy transfer and biological productivity.</p>

scholarly journals Do Winds Control the Confluence of Subtropical and Subantarctic Surface Waters East of New Zealand?

2021 ◽  
Author(s):  
◽  
Denise Fernandez
Keyword(s):  
Time Series ◽  
New Zealand ◽  
Kinetic Energy ◽  
Wind Stress ◽  
South Pacific ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Island Rule ◽  
Eddy Activity ◽  
Confluence Region

<p>The confluence region east of New Zealand is one of only a few places in the world where the Antarctic Circumpolar Current meets the strong southwardflowing boundary current of a subtropical gyre. The convergence of subtropical and subantarctic water creates strong fronts. The fronts have clear signatures in height and temperature that make them appropriate places to investigate ocean/climate variability. The location and extent of the New Zealand confluence should respond to changes in large-scale wind patterns, as changes in South Pacific currents have been linked to wind shifts. However, recent studies have shown that highly energetic eddies, local winds, and the bathymetry may be significant controls of currents and associated fronts. This thesis investigates the temporal and spatial variability of the confluence and evaluates its response to variability in South Pacific winds. Analysis of the 18-year time series, from January 1993 to December 2010, of sea surface height mapped from satellite altimetry was used to investigate the location and extent of fronts and the eddy activity and relate these to the wind forcing. Wind stress data were used with the Island Rule to estimate the winddriven transport of the western boundary currents that feed the confluence. In addition, the climate modes Southern Annular Mode (SAM) and Southern Oscillation Index (SOI) were used to examine the influence of the principal modes of atmospheric variability. Time series of the local wind stress curl and local climate indices were calculated and compared to the intensity of the confluence to test any influence of local forcing. In addition, bathymetric effects were investigated by evaluating evidence for preferred front locations near topographic features. Sea level anomalies in the confluence region are increasing at 3.4 cm decade⁻¹. The sea surface height gradients and the eddy kinetic energy are also increasing at a rate of 0.01 cm km⁻¹ and 23 cm² s⁻² per decade respectively, indicating an intensification of the fronts and eddy activity in the confluence. There is a high and significant correlation (r = 0.84) between the front and eddy signals reflecting baroclinic instabilities inherent in the fronts. Difference in transport anomalies across the confluence derived from the Island Rule are also increasing at 8.8 Sv decade⁻¹. SAM and SOI indices showed little or no correspondence with variability in the confluence intensity and eddy kinetic energy, and the same lack of correspondence was observed in local winds and local indices. While these results suggest a connection between the variability in the confluence and South Pacific winds, there is a preferential location of the strongest fronts and eddy activity northeast of Bounty Plateau and Bollons Seamount, indicating some bathymetric control. The correspondence between basin-scale winds and sea surface height gradients in the confluence region indicates that if wind stress continues to increase, as current trends predict, front intensity and eddy activity will also increase, enhancing the transfer of heat and nutrients that, respectively, influence energy transfer and biological productivity.</p>

Contributions of Wind Forcing and Surface Heating to Interannual Sea Level Variations in the Atlantic Ocean

2006 ◽  
Vol 36 (9) ◽  
pp. 1739-1750 ◽  
Author(s):  
Cécile Cabanes ◽  
Thierry Huck ◽  
Alain Colin de Verdière
Keyword(s):  
Atlantic Ocean ◽  
Sea Level ◽  
Wind Stress ◽  
Large Scale ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Altimeter Data ◽  
Local Response ◽  
Sea Level Variability ◽  
Sea Level Variations

Abstract Interannual sea surface height variations in the Atlantic Ocean are examined from 10 years of high-precision altimeter data in light of simple mechanisms that describe the ocean response to atmospheric forcing: 1) local steric changes due to surface buoyancy forcing and a local response to wind stress via Ekman pumping and 2) baroclinic and barotropic oceanic adjustment via propagating Rossby waves and quasi-steady Sverdrup balance, respectively. The relevance of these simple mechanisms in explaining interannual sea level variability in the whole Atlantic Ocean is investigated. It is shown that, in various regions, a large part of the interannual sea level variability is related to local response to heat flux changes (more than 50% in the eastern North Atlantic). Except in a few places, a local response to wind stress forcing is less successful in explaining sea surface height observations. In this case, it is necessary to consider large-scale oceanic adjustments: the first baroclinic mode forced by wind stress explains about 70% of interannual sea level variations in the latitude band 18°–20°N. A quasi-steady barotropic Sverdrup response is observed between 40° and 50°N.

scholarly journals Decadal Variability in the Large-Scale Sea Surface Height Field of the South Pacific Ocean: Observations and Causes

2006 ◽  
Vol 36 (9) ◽  
pp. 1751-1762 ◽  
Author(s):  
Bo Qiu ◽  
Shuiming Chen
Keyword(s):  
Pacific Ocean ◽  
Rossby Wave ◽  
Large Scale ◽  
South Pacific ◽  
Sea Surface Height ◽  
Sea Surface ◽  
South American ◽  
The South ◽  
South Pacific Ocean ◽  
Spatially Varying

Abstract Large-scale sea surface height (SSH) changes in the extraequatorial South Pacific Ocean are investigated using satellite altimetry data of the past 12 yr. The decadal SSH signals in the 1990s were dominated by an increasing trend in the 30°–50°S band and a decreasing trend in the central South Pacific Ocean poleward of 50°S. In recent years since 2002 there has been a reversal in both of these trends. Spatially varying low-frequency SSH signals are also found in the tropical region of 10°–25°S where the decadal SSH trend is negative in the eastern basin, but positive in the western basin. To clarify the causes for these observed spatially varying SSH signals, a 1½-layer reduced-gravity model that includes the wind-driven baroclinic Rossby wave dynamics and the responses forced by SSH changes along the South American coast was adopted. The model hindcasts the spatially varying decadal trends in the midlatitude and the eastern tropical regions well. Accumulation of the wind-forced SSH anomalies along Rossby wave characteristics is found to be important for both previously reported long-term trends and their reversals in recent years. The boundary forcing associated with the time-varying SSH signals along the South American coast is crucial for understanding the observed SSH signals of all time scales in the eastern tropical South Pacific basin, but it has little impact upon the midlatitude interior SSH signals.

scholarly journals Seasonal Variability of the Transport through the Yucatan Channel from Observations

2020 ◽  
Vol 50 (2) ◽  
pp. 343-360
Author(s):  
Gabriela Athié ◽  
Julio Sheinbaum ◽  
Julio Candela ◽  
José Ochoa ◽  
Paula Pérez-Brunius ◽  
...  
Keyword(s):  
Time Series ◽  
Kinetic Energy ◽  
Wind Stress ◽  
Seasonal Cycle ◽  
Loop Current ◽  
Wind Stress Curl ◽  
Channel Transport ◽  
Statistical Sense ◽  
Yucatan Channel ◽  
Western Side

AbstractThe seasonal cycle of transport through the Yucatan Channel is estimated from 59 months of direct mooring measurements and 23 years of a transport proxy from AVISO sea level across the channel. Both exhibit a seasonal cycle with a maximum in summer (July–August) but have a minimum in March for the mooring and in November for AVISO data. The annual and semiannual harmonics explain respectively 19% (~32%) and 6% (~4%) of the subinertial variance of the moored (proxy) transports. Seasonal variations of zonal wind stress and anticyclonic wind stress curl over the Cayman Sea appear to be positively correlated with transport in Yucatan Channel and the northward extension of the Loop Current during the summer, agreeing to some extent with modeling results previously reported. Transport increments during summer coincide with enhanced regional easterly winds and anticyclonic wind stress curl in 60% of the cases (of 23 years). However, this connection is not as tight as model results suggest during winter. The summer correlation only appears to be valid in a broad statistical sense since it is modulated by large interannual and higher-frequency variability. Moored time series confirm previous results that the transport signal on the western side of the channel is quite different from the total Yucatan Channel transport and that eddy kinetic energy at higher frequencies (50–100 days) dominates the variability and is characterized by a relatively low net transport signal, with flow of opposite signs on each side of the channel.

Assessment of the Jason-2 Extension to the TOPEX/Poseidon, Jason-1 Sea-Surface Height Time Series for Global Mean Sea Level Monitoring

2010 ◽  
Vol 33 (sup1) ◽  
pp. 447-471 ◽  
Author(s):  
B. D. Beckley ◽  
N. P. Zelensky ◽  
S. A. Holmes ◽  
F. G. Lemoine ◽  
R. D. Ray ◽  
...  
Keyword(s):  
Time Series ◽  
Sea Level ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Mean Sea Level ◽  
Global Mean Sea Level ◽  
Global Mean ◽  
Level Monitoring

Decadal Spinup of the South Pacific Subtropical Gyre

2007 ◽  
Vol 37 (2) ◽  
pp. 162-173 ◽  
Author(s):  
D. Roemmich ◽  
J. Gilson ◽  
R. Davis ◽  
P. Sutton ◽  
S. Wijffels ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
World Ocean ◽  
South Pacific ◽  
Sea Surface Height ◽  
Subtropical Gyre ◽  
Sea Surface ◽  
The South ◽  
Maximum Increase ◽  
Large Spatial Scale ◽  
Dynamic Height

Abstract An increase in the circulation of the South Pacific Ocean subtropical gyre, extending from the sea surface to middepth, is observed over 12 years. Datasets used to quantify the decadal gyre spinup include satellite altimetric height, the World Ocean Circulation Experiment (WOCE) hydrographic and float survey of the South Pacific, a repeated hydrographic transect along 170°W, and profiling float data from the global Argo array. The signal in sea surface height is a 12-cm increase between 1993 and 2004, on large spatial scale centered at about 40°S, 170°W. The subsurface datasets show that this signal is predominantly due to density variations in the water column, that is, to deepening of isopycnal surfaces, extending to depths of at least 1800 m. The maximum increase in dynamic height is collocated with the deep center of the subtropical gyre, and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre, by at least 20% at 1000 m. A comparison of WOCE and Argo float trajectories at 1000 m confirms the gyre spinup during the 1990s. The signals in sea surface height, dynamic height, and velocity all peaked around 2003 and subsequently began to decline. The 1990s increase in wind-driven circulation resulted from decadal intensification of wind stress curl east of New Zealand—variability associated with an increase in the atmosphere’s Southern Hemisphere annular mode. It is suggested (based on altimetric height) that midlatitude gyres in all of the oceans have been affected by variability in the atmospheric annular modes on decadal time scales.

scholarly journals Statistical Prediction of the South China Sea Surface Height Anomaly

2015 ◽  
Vol 2015 ◽  
pp. 1-9
Author(s):  
Caixia Shao ◽  
Weimin Zhang ◽  
Chunjian Sun ◽  
Xinmin Chai ◽  
Zhimin Wang
Keyword(s):  
Time Series ◽  
South China Sea ◽  
South China ◽  
Decadal Variability ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Statistical Prediction ◽  
Height Anomaly ◽  
China Sea ◽  
Sea Surface Height Anomaly

Based on the simple ocean data assimilation (SODA) data, this study analyzes and forecasts the monthly sea surface height anomaly (SSHA) averaged over South China Sea (SCS). The approach to perform the analysis is a time series decomposition method, which decomposes monthly SSHAs in SCS to the following three parts: interannual, seasonal, and residual terms. Analysis results demonstrate that the SODA SSHA time series are significantly correlated to the AVISO SSHA time series in SCS. To investigate the predictability of SCS SSHA, an exponential smoothing approach and an autoregressive integrated moving average approach are first used to fit the interannual and residual terms of SCS SSHA while keeping the seasonal part invariant. Then, an array of forecast experiments with the start time spanning from June 1977 to June 2007 is performed based on the prediction model which integrates the above two models and the time-independent seasonal term. Results indicate that the valid forecast time of SCS SSHA of the statistical model is about 7 months, and the predictability of SCS SSHA in Spring and Autumn is stronger than that in Summer and Winter. In addition, the prediction skill of SCS SSHA has remarkable decadal variability, with better phase forecast in 1997–2007.

scholarly journals Interannual Variability of Sea Surface Temperature in the Southwest Pacific and the Role of Ocean Dynamics

2017 ◽  
Vol 30 (18) ◽  
pp. 7481-7492 ◽  
Author(s):  
Melissa Bowen ◽  
Jordan Markham ◽  
Philip Sutton ◽  
Xuebin Zhang ◽  
Quran Wu ◽  
...  
Keyword(s):  
New Zealand ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Interannual Variability ◽  
Heat Transport ◽  
South Pacific ◽  
Ocean Dynamics ◽  
Sea Surface ◽  
Upper Ocean ◽  
Ocean Heat Transport

Abstract This paper investigates the mechanisms causing interannual variability of upper ocean heat content and sea surface temperature (SST) in the southwest Pacific. Using the ECCOv4 ocean reanalysis it is shown that air–sea heat flux and ocean heat transport convergence due to ocean dynamics both contribute to the variability of upper ocean temperatures around New Zealand. The ocean dynamics responsible for the ocean heat transport convergence are investigated. It is shown that SSTs are significantly correlated with the arrival of barotropic Rossby waves estimated from the South Pacific wind stress over the latitudes of New Zealand. Both Argo observations and the ECCOv4 reanalysis show deep isotherms fluctuate coherently around the country. The authors suggest that the depth of the thermocline around New Zealand adjusts to changes in the South Pacific winds, modifies the vertical advection of heat into the upper ocean, and contributes to the interannual variability of SST in the region.

Sign in / Sign up

Export Citation Format

Share Document