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

<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>

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

<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>

Interdecadal changes in synoptic transient eddy activity over the Northeast Pacific and their role in tropospheric Arctic amplification

Arctic amplification refers to the greater surface warming of the Arctic than of other regions during recent decades. A similar phenomenon occurs in the troposphere and is termed “tropospheric Arctic amplification” (TAA). The poleward eddy heat flux and eddy moisture flux are critical to Arctic warming. In this study, we investigate the synoptic transient eddy activity over the North Pacific associated with TAA and its relationship with the subtropical jet stream, and propose the following mechanism. A poleward shift of the subtropical jet axis results in anomalies of the meridional gradient of zonal wind over the North Pacific, which drive a meridional dipole pattern of synoptic transient wave intensity over the North Pacific, referred to as the North Pacific Synoptic Transient wave intensity Dipole (NPSTD). The NPSTD index underwent an interdecadal shift in the late 1990s accompanying that of the subtropical jet stream. During the positive phase of the NPSTD index, synoptic eddy heat flux transports more heat to the Arctic Circle, and the eddy heat flux diverges, increasing Arctic temperature. This mechanism highlights the need to consider synoptic transient eddy activity over the North Pacific as the link between the mean state of the North Pacific subtropical upper jet and TAA.

Drivers of Mars' northern winter polar vortex

&lt;p&gt;Mars&amp;#8217; polar vortices play a mjaor role in determining the global-scale transport of trace gases and the composition of the polar caps. Potential vorticity (PV) is a key quantity in determining their dynamical and transport properties. Mars' winter polar vortices are annular in PV, a direct contrast to Earth&amp;#8217;s stratospheric polar vortices, whose PV values increase monotonically towards the poles. Given that a ring of high PV is known to be barotropically unstable, the persistence of this phenomenon in observations, simulations and reanalyses is somewhat surprising. Condensation of atmospheric carbon dioxide at the winter pole has been shown to be necessary to maintain the annulus in Martian Global Circulation Models (MGCM). Dust is also known to be a cause of internal and interannual variability in the polar vortices, but given the relatively few years of observations available, it is not yet fully understood. Here we present results of an attribution study of the driving mechanisms of the northern hemisphere Martian polar vortex. Using a reanalysis dataset and an idealized MGCM, we investigate the combined effects of dust, latent heat release, and topography on the polar vortex.&lt;/p&gt;&lt;p&gt;We show that the vertical PV structure of the polar vortex in the reanalysis is dependent on the observations assimilated, and that high atmospheric dust loading (such as that seen during a global dust storm) can disrupt the vortex and cause the destruction of PV in the low-mid altitudes. We also demonstrate that high dust loading can significantly reduce eddy activity within the core of the vortex over the course of a Martian winter. Latent heat release from carbon dioxide condensation is an important driver of variability within the polar vortex, but it is dust in the model that primarily drives the eddy activity throughout the Martian year.&lt;/p&gt;

The Role of Mesoscale Dynamics over Northwestern Cuba in the Loop Current Evolution in 2010, during the Deepwater Horizon Incident

The Loop Current (LC) system controls the connectivity between the northern Gulf of Mexico (GoM) region and the Straits of Florida. The evolution of the LC and the shedding sequence of the LC anticyclonic ring (Eddy Franklin) were crucial for the fate of the hydrocarbons released during the Deepwater Horizon (DwH) oil spill in 2010. In a previous study, we identified LC-related anticyclonic eddies in the southern GoM, named “Cuba anticyclones” (“CubANs”). Here, we investigate the relation between these eddies and LC evolution in 2010, focusing on the DwH period. We use high-resolution model results in tandem with observational data to describe the connection between the LC system evolution within the GoM (LC extensions, Eddy Franklin and LC Frontal Eddies—LCFEs) and the mesoscale dynamics within the Straits of Florida where CubANs propagate. Five periods of CubAN eddy activity were identified during the oil spill period, featuring different formation processes under a combination of local and regional conditions. Most of these cases are related to the retracted LC phases, when the major LC anticyclone (Eddy Franklin in 2010) is detached from the main body and CubAN eddy activity is most likely. However, two cases of CubAN eddy presence during elongated LC were detected, which led to the attenuation of the eastward flows of warm waters through the Straits (Florida Current; outflow), allowing the stronger supply of Caribbean waters through the Yucatan Channel into the Gulf (inflow), which contributed to short-term LC northward extensions. Oceanographic (LCFEs) and meteorological (wind-induced upwelling) conditions contributed to the release of CubANs from the main LC body, which, in tandem with other processes, contributed to the LC evolution during the DwH oil spill incident.

Downstream Suppression of Baroclinic Waves

Baroclinic waves drive both regional variations in weather and large-scale variability in the extratropical general circulation. They generally do not exist in isolation, but rather often form into coherent wave packets that propagate to the east via a mechanism called downstream development. Downstream development has been widely documented and explored. Here we document a novel but also key aspect of baroclinic waves: the downstream suppression of baroclinic activity that occurs in the wake of eastward propagating disturbances. Downstream suppression is apparent not only in the Southern Hemisphere storm track as shown in previous work, but also in the North Pacific and North Atlantic storm tracks. It plays an essential role in driving subseasonal periodicity in extratropical eddy activity in both hemispheres, and gives rise to the observed quiescence of the North Atlantic storm track 1–2 weeks following pronounced eddy activity in the North Pacific sector. It is argued that downstream suppression results from the anomalously low baroclinicity that arises as eastward propagating wave packets convert potential to kinetic energy. In contrast to baroclinic wave packets, which propagate to the east at roughly the group velocity in the upper troposphere, the suppression of baroclinic activity propagates eastward at a slower rate that is comparable to that of the lower to midtropospheric flow. The results have implications for understanding subseasonal variability in the extratropical troposphere of both hemispheres.

Global changes in oceanic mesoscale currents over the satellite altimetry record

Oceanic eddies play a profound role in mixing tracers such as heat, carbon, and nutrients, thereby regulating regional and global climate. Yet, it remains unclear how global oceanic eddy kinetic energy has evolved over the past few decades. Furthermore, coupled climate model predictions generally fail to resolve oceanic mesoscale dynamics, which could limit their accuracy in simulating future climate change. Here we show a global statistically significant increase of the eddy activity using two independent observational datasets of mesoscale variability, one directly measuring currents and the other from sea surface temperature. Regions characterized by different dynamical processes show distinct evolution in the eddy field. For example, eddy-rich regions such as boundary current extensions and the Antarctic Circumpolar Current show a significant increase of 2% and 5% per decade in eddy activity, respectively. In contrast, most of the regions of observed decrease are found in the tropical oceans. Because eddies play a fundamental role in the ocean transport of heat, momentum, and carbon, our results have far-reaching implications for ocean circulation and climate, and the modelling platforms we use to study future climate change.

An Index for Depicting the Long-Term Variability of Mesoscale Eddy Activity over the Kuroshio Extension Region

Using high-resolution satellite-derived sea surface temperature (SST) data from September 1981 to December 2015, the present study develops a new index to detect the long-term variation in mesoscale eddy activity over the Kuroshio Extension (KE) region. This eddy activity index (EAI) highlights the strength of eddy-induced poleward heat transport and has obvious advantages over the other existing KE indices in depicting the low-frequency changes in KE eddy activity. An analysis of the EAI shows that over the long term, the KE eddy activity variability presents a significant spectral peak of about 8 years and is not directly modulated by wind-driven oceanic Rossby waves generated in the central North Pacific. When the EAI is positive, the strengthened KE eddy activity significantly enhances the heat release from ocean to atmosphere over the Kuroshio–Oyashio confluence region (KOCR). This induces an anomalous dipole pattern of near-surface baroclinicity over this region that can persist for up to 6 months, favoring a weakened and northward-moving East Asian jet, and vice versa. It is believed that the new EAI will facilitate future studies focusing on the climatic effects of the KE eddy activity variation.