Enhanced North Pacific subtropical gyre circulation during the late Holocene

The North Pacific Subtropical Gyre circulation redistributes heat from the Western Pacific Warm Pool towards the mid- to high-latitude North Pacific. However, the driving mechanisms of this circulation and how it changed over the Holocene remain poorly understood. Here, we present alkenone-based sea surface temperature reconstructions along the Kuroshio, California and Alaska currents that cover the past ~7,000 years. These and other paleorecords collectively demonstrate a coherent intensification of the boundary currents, and thereby the basin-scale subtropical gyre circulation, since ~3,000–4,000 years ago. Such enhanced circulation during the late Holocene appears to have resulted from a long-term southward migration of the Intertropical Convergence Zone, associated with Holocene ocean cooling. Our results imply that the North Pacific Subtropical Gyre circulation could be weakened under future global warming.

On the Role of Bottom Pressure Torques in Wind-Driven Gyres

Previous studies have concluded that the wind-input vorticity in ocean gyres is balanced by bottom pressure torques (BPT), when integrated over latitude bands. However, the BPT must vanish when integrated over any area enclosed by an isobath. This constraint raises ambiguities regarding the regions over which BPT should close the vorticity budget, and implies that BPT generated to balance a local wind stress curl necessitates the generation of a compensating, nonlocal BPT and thus nonlocal circulation. This study aims to clarify the role of BPT in wind-driven gyres using an idealized isopycnal model. Experiments performed with a single-signed wind stress curl in an enclosed, sloped basin reveal that BPT balances the winds only when integrated over latitude bands. Integrating over other, dynamically motivated definitions of the gyre, such as barotropic streamlines, yields a balance between wind stress curl and bottom frictional torques. This implies that bottom friction plays a nonnegligible role in structuring the gyre circulation. Nonlocal bottom pressure torques manifest in the form of along-slope pressure gradients associated with a weak basin-scale circulation, and are associated with a transition to a balance between wind stress and bottom friction around the coasts. Finally, a suite of perturbation experiments is used to investigate the dynamics of BPT. To predict the BPT, the authors extend a previous theory that describes propagation of surface pressure signals from the gyre interior toward the coast along planetary potential vorticity contours. This theory is shown to agree closely with the diagnosed contributions to the vorticity budget across the suite of model experiments.

On the use of the data- and physics-driven approaches for quasi-geostrophic double-gyre problem: application of Genetic Programming

<p>In this study, we investigate Genetic Programming as a data-driven approach to reconstruct eddy-resolved simulations of the double-gyre problem. Stemming from Genetic Algorithms, Genetic Programming is a method of symbolic regression which can be used to extract temporal or spatial functionalities from simulation snapshots.  The double-gyre circulation is simulated by a stratified quasi-geostrophic model which is solved using high-resolution CABARET scheme. The simulation results are compressed using proper orthogonal decomposition and the time variant coefficients of the reduced-order model are fed into a Genetic Programming code. Due to the multi-scale nature of double-gyre problem, we decompose the time signal into a meandering and a fluctuating component. We next explore the parameter space of objective functions in Genetic Programming to capture the two components separately. The data-driven predictions are cross-compared with original double-gyre signal in terms of statistical moments such as variance and auto-correlation function.</p><p> </p>

Southwest Pacific Paleoceanography During the Late Holocene: 3000 years of ocean circulation and marine biogeochemistry reconstructed from New Zealand deep-sea black corals

<p>The global climate results from interactions between the ocean and atmosphere. Ocean gyres are perhaps one of the most significant interactions; they regulate temperature, salinity and nutrient flow across the ocean basins. Gyres transport warm, tropical waters to higher latitudes and cold waters to lower latitudes and act as the dominant heat-transport mechanism in the Earth’s climate system. They also influence spatial patterns in marine primary production by distributing nutrients between the equator and poles. However, gyre circulation in the subtropics has been strengthening, leading to marine heat waves, changing biogeochemistry and reducing primary production since the early 1900s. These changes are often interpreted as a consequence of anthropogenic climate change. However, ocean circulation and primary production can exhibit natural variations on a variety of timescales. Could these recent changes be a part of a long-term natural cycle or a product of anthropogenic change? </p> <b>This research aims to reconstruct South Pacific Gyre (SPG) circulation and biogeochemistry using a suite of New Zealand black corals. The primary research goal is determining if there is a precedent for the ocean changes observed over the instrumental period. Black corals are an ideal paleoceanographic archive for this work; they provide high-resolution, multi-millennial records of biogeochemistry and ocean circulation within their skeletons, derived using radiocarbon (14C) and stable isotopes (d13C and d15N). In this thesis, I show that late Holocene SPG strength has been highly variable and the relationship between circulation and biogeochemistry is timescale dependent. </b><p>The black coral radiocarbon records suggest late Holocene SPG circulation has been controlled by westerly wind strength. Our records show the SPG exhibits natural variability on multi-centennial and millennial timescales that corresponds to the variability within the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation (ENSO). The black coral circulation record shows that the modern gyre circulation is not without precedent over the last 3000 years. </p> <p>The black coral d13C and d15N records show significant variability on multi-decadal to multi-centennial timescales. Multi-centennial variability in black coral d13C and d15N appears to be driven by sea surface temperature (SST), nitrogen fixation rates and wind-driven upwelling and is possibly forced by the mean state of the Southern Oscillation Index and ocean circulation strength. A trend in black coral d13C over the last 1500 years also suggests a shift in phytoplankton community structure towards larger and faster growing phytoplankton. These records also reveal a shift in mean coral d13C and d15N between the 0-2000BP and 2000-3000BP period, the latter corresponding to a period of stronger gyre circulation inferred from the radiocarbon records. </p> <p>This work shows that: 1) New Zealand’s black corals are a promising archive for studying paleoceanography; they can extend instrumental ocean records and fill the gap between traditional southwest Pacific paleoceanographic proxy records (tropical corals, sediment cores); 2) SPG circulation has been highly variable over the last 3000 years; circulation is controlled by atmospheric patterns (e.g. SAM) on multi-centennial to millennial timescales; 3) Gyre circulation is only one of many forcing factors on southwest Pacific primary production and marine biogeochemistry; comparisons between the ∆R, d13C and d15N proxies show that variations in SPG biogeochemical patterns and productivity are likely driven by local dynamics such as phytoplankton community structure, SST, upwelling and gyre circulation. Finally, this research demonstrates the key role that a distributed set of deep-sea coral paleoceanographic reconstructions could play in characterizing the dynamical variability in southwest Pacific Ocean circulation, biogeochemistry and primary production. This information is critical for detecting and attributing past and future anthropogenic impacts on the southwest Pacific Ocean. </p>

Stochastic modeling of the oceanic mesoscale eddies

&lt;p&gt;In this work, a stochastic representation [Bauer2020a, Bauer2020b] based on a physical transport principle is proposed to account for mesoscale eddy effects on the the large-scale oceanic circulation. This stochastic framework [M&amp;#233;min2014] arises from a decomposition of the Lagrangian velocity into a time-smooth component and a highly oscillating noise term. One important characteristic of this random model is that it conserves the energy of any transported tracer. Such an energy-preserving representation has been successfully implemented in a well established multi-layered quasi-geostrophic dynamical core (http://www.q-gcm.org). The empirical spatial correlation of the small-scale noise is estimated from the eddy-resolving simulation data. In particular, a sub-grid correction drift has been introduced in the noise due to the bias ensuing from the coarse-grained procedure. This non intuitive term seems quite important in reproducing on a coarse mesh the meandering jet of the wind-driven double-gyre circulation. In addition, a new projection method has been proposed to constrain the noise living along the iso-surfaces of the vertical stratification. The resulting noise enables us to improve the intrinsic low-frequency variability of the large-scale current. From some statistical studies and energy transfers analysis, this improvement is well demonstrated.&lt;/p&gt;&lt;ul&gt;&lt;li&gt;&lt;span&gt;[&lt;/span&gt;Bauer2020a] W. Bauer, P. Chandramouli, B. Chapron, L. Li, and E. M&amp;#233;min. Deciphering the role&amp;#160;of small-scale inhomogeneity on geophysical flow structuration: a stochastic approach. Journal of Physical Oceanography, 50(4):983-1003, 2020a.&lt;span&gt;&amp;#160; &amp;#160; &amp;#160; &amp;#160;&lt;/span&gt;&lt;/li&gt; &lt;li&gt;&lt;span&gt;[&lt;/span&gt;Bauer2020b] W. Bauer, P. Chandramouli, L. Li, and E. M&amp;#233;min. Stochastic representation of mesoscale&amp;#160;eddy effects in coarse-resolution barotropic models. Ocean Modelling, 151:101646 (2020b). &lt;span&gt;&amp;#160; &amp;#160;&lt;/span&gt;&lt;/li&gt; &lt;li&gt;[M&amp;#233;min2014] E. M&amp;#233;min. Fluid flow dynamics under location uncertainty. Geophysical &amp; Astrophysical Fluid Dynamics, 108(2):119-146, 2014.&lt;span&gt;&amp;#160; &amp;#160; &amp;#160;&lt;/span&gt;&lt;/li&gt; &lt;/ul&gt;

Southwest Pacific Paleoceanography During the Late Holocene: 3000 years of ocean circulation and marine biogeochemistry reconstructed from New Zealand deep-sea black corals

<p>The global climate results from interactions between the ocean and atmosphere. Ocean gyres are perhaps one of the most significant interactions; they regulate temperature, salinity and nutrient flow across the ocean basins. Gyres transport warm, tropical waters to higher latitudes and cold waters to lower latitudes and act as the dominant heat-transport mechanism in the Earth’s climate system. They also influence spatial patterns in marine primary production by distributing nutrients between the equator and poles. However, gyre circulation in the subtropics has been strengthening, leading to marine heat waves, changing biogeochemistry and reducing primary production since the early 1900s. These changes are often interpreted as a consequence of anthropogenic climate change. However, ocean circulation and primary production can exhibit natural variations on a variety of timescales. Could these recent changes be a part of a long-term natural cycle or a product of anthropogenic change? </p> <b>This research aims to reconstruct South Pacific Gyre (SPG) circulation and biogeochemistry using a suite of New Zealand black corals. The primary research goal is determining if there is a precedent for the ocean changes observed over the instrumental period. Black corals are an ideal paleoceanographic archive for this work; they provide high-resolution, multi-millennial records of biogeochemistry and ocean circulation within their skeletons, derived using radiocarbon (14C) and stable isotopes (d13C and d15N). In this thesis, I show that late Holocene SPG strength has been highly variable and the relationship between circulation and biogeochemistry is timescale dependent. </b><p>The black coral radiocarbon records suggest late Holocene SPG circulation has been controlled by westerly wind strength. Our records show the SPG exhibits natural variability on multi-centennial and millennial timescales that corresponds to the variability within the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation (ENSO). The black coral circulation record shows that the modern gyre circulation is not without precedent over the last 3000 years. </p> <p>The black coral d13C and d15N records show significant variability on multi-decadal to multi-centennial timescales. Multi-centennial variability in black coral d13C and d15N appears to be driven by sea surface temperature (SST), nitrogen fixation rates and wind-driven upwelling and is possibly forced by the mean state of the Southern Oscillation Index and ocean circulation strength. A trend in black coral d13C over the last 1500 years also suggests a shift in phytoplankton community structure towards larger and faster growing phytoplankton. These records also reveal a shift in mean coral d13C and d15N between the 0-2000BP and 2000-3000BP period, the latter corresponding to a period of stronger gyre circulation inferred from the radiocarbon records. </p> <p>This work shows that: 1) New Zealand’s black corals are a promising archive for studying paleoceanography; they can extend instrumental ocean records and fill the gap between traditional southwest Pacific paleoceanographic proxy records (tropical corals, sediment cores); 2) SPG circulation has been highly variable over the last 3000 years; circulation is controlled by atmospheric patterns (e.g. SAM) on multi-centennial to millennial timescales; 3) Gyre circulation is only one of many forcing factors on southwest Pacific primary production and marine biogeochemistry; comparisons between the ∆R, d13C and d15N proxies show that variations in SPG biogeochemical patterns and productivity are likely driven by local dynamics such as phytoplankton community structure, SST, upwelling and gyre circulation. Finally, this research demonstrates the key role that a distributed set of deep-sea coral paleoceanographic reconstructions could play in characterizing the dynamical variability in southwest Pacific Ocean circulation, biogeochemistry and primary production. This information is critical for detecting and attributing past and future anthropogenic impacts on the southwest Pacific Ocean. </p>

The Antilles Current and wind-driven gyre circulation at 26oN

&lt;p&gt;The Antilles Current is a narrow, northward flowing boundary current in the western Atlantic just east of the Bahamas. Its role in the larger scale circulation has been debated: alternately thought to be part of the western boundary closure of the gyre circulation or the northward flowing limb of the meridional overturning circulation (MOC). From 19 years of moored current meter observations (1987--1991, 2004--2018), we define the strength of the Antilles Current by the net transport between the Bahamas and 76.5&amp;#176;W (spanning about 45 km zonally) and in the thermocline (0&amp;#8211;1000 m). We find a mean northward transport of 3.5 Sv, substantial interannual variability, and no discernable trend since 1987. The interannual variability of the AC transport is independent of the variability of the Florida Current (the Gulf Stream through the Florida Straits). Instead, the Antilles Current contributes to the interannual variability of the MOC at 26&amp;#176;N, while the trend in the strength of the gyre circulation (defined as the transbasin thermocline transport minus the AC) is responsible for the trend in the MOC. In particular, the 2009/10 slowdown of the MOC resulted from a weaker northward AC transport, rather than an intensified gyre transport. Using the recent 14 years of in situ transport records, we compare the interannual variability of the gyre circulation to that of wind stress curl forcing via a Sverdrup transport calculation, identifying a potential role for wind stress curl (WSC) forcing at 26&amp;#176;N with a ~2 year lag until 2016.&lt;span&gt;&amp;#160; &lt;/span&gt;From 2016, the predicted gyre circulation using WSC diverges from the measured gyre strength.&lt;/p&gt;

An Idealized Model of Ocean Gyres near Pine Island Ice Shelf and Thwaites Ice Shelf

&lt;p&gt;Floating ice shelves buttress the Antarctic Ice Sheet, which is losing mass rapidly mainly due to oceanic melting and the associated disruption to glacial dynamics. The local oceanic circulation near ice shelves is therefore important for the prediction of future ice mass loss and related sea-level rise as it determines the water mass exchange, heat transport under the ice shelf, and the resultant melting. However, the dynamics controlling the near-coastal circulation are not fully understood, particularly relating to seasonal and interannual changes in wind stress curl and ice cover. A gyre circulation (27 km radius, cyclonic) in front of the Pine Island Ice Shelf has been identified in both numerical models and velocity observations. In 2019 in the west of Thwaites Ice Shelf, for the first time in this habitually ice-covered region, another gyre circulation rotating in a different direction (13 km, anticyclonic) was detected by velocity observations. Here we use an idealised configuration of MITgcm, with idealised forcing based on ERA-5 climatological wind fields and simplified sea ice conditions from MODIS satellite images, to reproduce key features of the observed gyres near Pine Island Ice Shelf and Thwaites Ice Shelf. A barotropic version of the model is able to reproduce the gyres driven solely by the wind. We show that the modelled gyre direction depends upon the angle between the wind direction and the sea ice front. Gyres generated by wind in sea-ice-free conditions have directions controlled by the wind stress curl. When sea ice is present, the wind stress exerted on the sea surface is reduced, leading to a modified wind stress curl and a resultant change in gyre direction.&lt;/p&gt;

Added and redistributed heat and carbon in climate model projections for the Southern Ocean

&lt;p&gt;Projected changes in ocean heat and carbon storage are assessed in terms of the added and &amp;#8232;redistributed tracer using a transport-based framework for 6 CMIP5 Earth system models following an annual 1% rise in atmospheric &amp;#8232;CO2. Heat and carbon budgets for the added and redistributed tracer are used to compare the reasons for the relatively-reduced storage of heat and carbon within the Southern Ocean. Here the added tracer takes &amp;#8232; account of the net tracer source and the advection of the added tracer, while the redistributed tracer takes account of the time-varying advection of the pre-industrial tracer &amp;#160;distribution. The added heat and carbon are nearly always positive over the Southern Ocean with the net source acting to supply tracer. However, there is a relatively-reduced local storage of heat and carbon in the Southern Ocean due to the passive northward transport of heat and carbon by the overturning, which is augmented by a passive northward carbon transport for the gyre circulation. In contrast, the redistributed heat is usually negative and the redistributed carbon is positive over the Southern Ocean due to the transport effects of a strengthening residual circulation and the opposing gradients in the pre-industrial temperature and &amp;#8232;carbon. Hence, climate projections for the Southern Ocean are expected to have heat anomalies of a variable sign and carbon anomalies of a consistently positive &amp;#160;sign, since the effects of added and redistribution heat are opposing in sign, while the effects of added and redistributed &amp;#8232;carbon reinforce each other.&amp;#8232;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Arctic Cyclone Activity and the Beaufort High

The Beaufort High (BH) and its accompanying anticyclonic winds drive the Arctic Ocean’s Beaufort Gyre, the major freshwater reservoir of the Arctic Ocean. The Beaufort Gyre circulation and its capacity to accumulate or release freshwater relies on the BH intensity. The migration of Nordic Seas cyclones into the Arctic has been hypothesized to moderate the strength of the BH. We explore this hypothesis by analyzing reanalysis sea-level pressure (SLP) fields to characterize the BH and identify and track cyclones north of 60°N during 1948-2019. A cluster analysis of Nordic Seas cyclone trajectories reveals a western pathway (through the Arctic interior) associated with a relatively weak BH and an eastern pathway (along the Arctic periphery) associated with a relatively strong BH. Furthermore, we construct cyclone activity indices (CAIs) in the Arctic and Nordic Seas which take into account multiple cyclone parameters (number, strength, duration). There are significant correlations between the BH and the CAIs in the Arctic and Nordic Seas during 1948-2019, with anomalously strong cyclone activity related to an anomalously weak BH, and vice versa. We show how the Arctic and Nordic Seas CAIs experienced a regime shift towards increased cyclone activity between the first four decades analyzed (1948-1988) and the most recent three decades (1989-2019). Over the same two time periods, the BH exhibits a weakening. Increased cyclone activity and an accompanying weakening of the BH may be consistent with expectations in a warming Arctic, and has implications for Beaufort Gyre dynamics and freshwater.