LuAnne Thompson

Role of the Gulf Stream and Kuroshio–Oyashio systems in large-scale atmosphere–ocean interaction : a review

Abstract Ocean–atmosphere interaction over the Northern Hemisphere western boundary current (WBC) regions (i.e., the Gulf Stream, Kuroshio, Oyashio, and their extensions) is reviewed with an emphasis on their role in basin-scale climate variability. SST anomalies exhibit considerable variance on interannual to decadal time scales in these regions. Low-frequency SST variability is primarily driven by basin-scale wind stress curl variability via the oceanic Rossby wave adjustment of the gyre-scale circulation that modulates the latitude and strength of the WBC-related oceanic fronts. Rectification of the variability by mesoscale eddies, reemergence of the anomalies from the preceding winter, and tropical remote forcing also play important roles in driving and maintaining the low-frequency variability in these regions. In the Gulf Stream region, interaction with the deep western boundary current also likely influences the low-frequency variability. Surface heat fluxes damp the low-frequency SST anomalies ove...

Climate-Forced Variability of Ocean Hypoxia

The spatial extent of ocean hypoxic zones, which are uninhabitable by many marine organisms, is very sensitive to dioxygen content. Oxygen (O2) is a critical constraint on marine ecosystems. As oceanic O2 falls to hypoxic concentrations, habitability for aerobic organisms decreases rapidly. We show that the spatial extent of hypoxia is highly sensitive to small changes in the ocean’s O2 content, with maximum responses at suboxic concentrations where anaerobic metabolisms predominate. In model-based reconstructions of historical oxygen changes, the world’s largest suboxic zone, in the Pacific Ocean, varies in size by a factor of 2. This is attributable to climate-driven changes in the depth of the tropical and subtropical thermocline that have multiplicative effects on respiration rates in low-O2 water. The same mechanism yields even larger fluctuations in the rate of nitrogen removal by denitrification, creating a link between decadal climate oscillations and the nutrient limitation of marine photosynthesis.

Contributions of wind forcing, waves, and surface heating to sea surface height observations in the Pacific Ocean

The dominant processes affecting sea surface height (SSH) variability observed by the TOPEX/Poseidon altimeter vary regionally in the Pacific; baroclinic Rossby waves, equatorially trapped Kelvin waves, steric response to seasonal heating, and the response to wind stress curl forcing are all important. The steric response to surface heating dominates seasonal SSH variability in the subpolar gyre and the eastern subtropical gyre. South of the Kuroshio Extension and south of 20°N in the eastern Pacific, the dominant contribution to SSH is from near-annual period Rossby waves. To quantify the wave energy, observed SSH was assimilated into a kinematic model of westward propagating waves. These waves account for >70% of SSH variance between 10°S and 10°N but only ∼30% between 10°N and 30°N. Although wave energy in the eastern Pacific is correlated with SSH anomalies at the equator, the much larger wave energy in the western Pacific is correlated with wind stress curl, suggesting that the Rossby waves there are locally forced. In addition to these planetary waves, the ocean response to wind forcing via Ekman pumping is observed in several places, specifically in the North Equatorial Current. A quasi-steady topographic Sverdrup balance is detectable over most of the North Pacific at latitudes as low as 10–15°N, as well as in the South Pacific, where it is seen north of 50°S. The decomposition of the SSH signal into propagating waves, an Ekman pumping response, and Sverdrup transport is consistent with the results from an isopycnal numerical model.

Heat Budget in the Kuroshio Extension Region: 1993–99

Abstract Processes responsible for the seasonal and interannual variations of the sea surface temperature as well as of the heat content of the upper ocean (0–400 m) in the Kuroshio Extension region are examined from a 3D advection–diffusion model in finite elements, with an embedded bulk mixed layer. The geostrophic velocity is specified externally from TOPEX/Poseidon altimeter data, and Ekman velocity is specified from NCEP wind stress. The thermal field from the model shows good agreement with observations. While both atmospheric and oceanic processes are required to explain observed nonseasonal SST changes, the interannual heat storage rate is dominated by horizontal advection. In particular, the transition between an elongated and a contracted state of the Kuroshio caused by geostrophic advection has a clear signature in the SST. There is an indication that this process is accompanied by consistent changes in nonseasonal entrainment: when the Kuroshio is in an elongated state and warmer waters are pr...

Formation Mechanisms for North Pacific Central and Eastern Subtropical Mode Waters

Abstract The effects of one-dimensional processes on the formation of deep mixed layers in the central mode water (CMW) and eastern subtropical mode water (ESMW) formation regions of the North Pacific have been analyzed using a mixed layer model. By running the model with various combinations of initial (August) background stratification and forcing fields (heat flux, E − P, and wind stress), and comparing the resultant March mixed layer depths, the relative importance of these effects on creating deep mixed layers was diagnosed. Model results suggest that the contributions of evaporation minus precipitation and wind mixing to mixed layer depth in both the CMW and the ESMW formation regions are negligible. In the ESMW formation region (centered at approximately 30°N, 140°W), the initial stratification is very important in determining where deep mixed layers form. Summer heating is quite weak in this region, resulting in a weak (or even nonexistent) seasonal pycnocline at the end of the summer at about 30°...

Sensitivity of Midlatitude Storm Intensification to Perturbations in the Sea Surface Temperature near the Gulf Stream

AbstractThe Gulf Stream region is a primary location for midlatitude storm cyclogenesis and growth. However, the influence of sea surface temperature (SST) on storms in the region is still under question, particularly after a storm has developed. Using the Weather Research and Forecasting (WRF) model, a storm that intensified as it transited northward across the Gulf Stream is simulated multiple times using different SST boundary conditions. These experiments test the storm response to changes in both the absolute value of the SST and the meridional SST gradient. Across the different simulations, the storm strength increases monotonically with the magnitude of the SST perturbations, even when the perturbations weaken the SST gradient. The storm response to the SST perturbations is driven by the latent heat release in the storm warm conveyor belt (WCB). During the late stages of development, the surface fluxes under the storm warm sector regulate the supply of heat and moisture to the WCB. This allows the ...

Water Mass Formation in an Isopycnal Model of the North Pacific

Abstract An isopycnal model coupled with a mixed layer model is used to study transformation and formation rates in the North Pacific. When annual formation rates are averaged over the entire North Pacific, a large peak in water mass formation is found at a density of approximately σθ = 26 kg m−3. This peak in formation rate corresponds to the formation of North Pacific Central Mode Water (CMW) in the model. No corresponding peaks in formation rate are found at the densities of Subtropical Mode Water (STMW; σθ ∼25.4 kg m−3) or Eastern Subtropical Mode Water (ESMW; σθ ∼24–25.4 kg m−3) when averaged over the entire model basin. However, when calculated locally, enhanced formation rates are found at the densities of these mode water masses. The formation of each of the three types of North Pacific mode water in the model occurs because of different circumstances. As expected, STMW formation is dependent on the strong cooling and resultant deep mixed layers over the Kuroshio Current region. However, formation...

Decadal Variability of North Pacific Central Mode Water

An isopycnal model forced with wind stress and heat fluxes from 1965 through 1993 was used to examine the effects of variable atmospheric forcing on the ventilation of the North Pacific. During this time period, a climatic regime shift occurred that had significant impacts on heat fluxes, sea surface temperature (SST), and wind stress patterns. The climate shift, occurring in the winter of 1976/77, affected the formation rates and locations, and properties of the Central Mode Water (CMW) formed in the model. Three model runs were compared: one with variable buoyancy forcing and climatological wind forcing, one with variable wind forcing and climatological buoyancy forcing, and one with variability in both the buoyancy and the wind forcing. The comparison indicates that buoyancy forcing is of primary importance in the variability of mode water formation and properties surrounding the climate shift. One measure of the climate shift is the Pacific decadal oscillation (PDO), an index of SST variability in the North Pacific, which changed sign in 1976/77. A positive state for the PDO is associated with deeper model mixed layers, formation of denser varieties of CMW, and an anticyclonic circulation anomaly in the CMW density range.

The Signature of the Midlatitude Tropospheric Storm Tracks in the Surface Winds

Abstract Storm-track analysis is applied to the meridional winds at 10 m and 850 hPa for the winters of 1999–2006. The analysis is focused on the North Atlantic and North Pacific Ocean basins and the Southern Ocean spanning the region south of the Indian Ocean. The spatial patterns that emerge from the analysis of the 850-hPa winds are the typical free-tropospheric storm tracks. The spatial patterns that emerge from the analysis of the surface winds differ from the free-tropospheric storm tracks. The spatial differences between the surface and free-tropospheric storm tracks can be explained by the influence of the spatial variability in the instability of the atmospheric boundary layer. Strongly unstable boundary layers allow greater downward mixing of free-tropospheric momentum (momentum mixing), and this may be the cause of the stronger surface storm tracks in regions with greater instability in the time mean. Principal component analysis suggests that the basin-scale variability that is reflected in th...

Stability of a potential vorticity front: from quasi-geostrophy to shallow water

The linear stability of a simple two-layer shear flow with an upper-layer potential vorticity front overlying a quiescent lower layer is investigated as a function of Rossby number and layer depths. This flow configuration is a generalization of previously studied flows whose results we reinterpret by considering the possible resonant interaction between waves. We find that instabilities previously referred to as ‘ageostrophic’ are a direct extension of quasi-geostrophic instabilities. Two types of instability are discussed : the classic long-wave quasi-geostrophic baroclinic instability arising from an interaction of two vortical waves, and an ageostrophic short-wave baroclinic instability arising from the interaction of a gravity wave and a vortical wave (vortical waves are defined as those that exist due to the presence of a gradient in potential vorticity, e.g. Rossby waves). Both instabilities are observed in oceanic fronts. The long-wave instability has length scale and growth rate similar to those found in the quasi-geostrophic limit, even when the Rossby number of the flow is O(1). We also demonstrate that in layered shallow-water models, as in continuously stratified quasi-geostrophic models, when a layer intersects the top or bottom boundaries, that layer can sustain vortical waves even though there is no apparent potential vorticity gradient. The potential vorticity gradient needed is provided at the top (or bottom) intersection point, which we interpret as a point that connects a finite layer with a layer of infinitesimal thickness, analogous to a temperature gradient on the boundary in a continuously stratified quasi-geostrophic model.