Publications

Export 17 results:
Sort by: Author Title Type [ Year  (Desc)]
2017
Wolfe, CL, Cessi P, Cornuelle BD.  2017.  An intrinsic mode of interannual variability in the Indian Ocean. Journal of Physical Oceanography. 47:701-719.   10.1175/jpo-d-16-0177.1   AbstractWebsite

An intrinsic mode of self-sustained, interannual variability is identified in a coarse-resolution ocean model forced by an annually repeating atmospheric state. The variability has maximumloading in the Indian Ocean, with a significant projection into the South Atlantic Ocean. It is argued that this intrinsic mode is caused by baroclinic instability of the model's Leeuwin Current, which radiates out to the tropical Indian and South Atlantic Oceans as long Rossby waves at a period of 4 yr. This previously undescribed mode has a remarkably narrowband time series. However, the variability is not synchronized with the annual cycle; the phase of the oscillation varies chaotically on decadal time scales. The presence of this internal mode reduces the predictability of the ocean circulation by obscuring the response to forcing or initial condition perturbations. The signature of this mode can be seen in higher-resolution global ocean models driven by high-frequency atmospheric forcing, but altimeter and assimilation analyses do not show obvious signatures of such a mode, perhaps because of insufficient duration.

2015
Schonau, MC, Rudnick DL, Cerovecki I, Gopalakrishnan G, Cornuelle BD, McClean JL, Qiu B.  2015.  The Mindanao Current mean structure and connectivity. Oceanography. 28:34-45.   10.5670/oceanog.2015.79   AbstractWebsite

The Mindanao Current (MC), a low-latitude western boundary current in the Pacific Ocean, plays an important role in heat and freshwater transport to the western Pacific warm pool and the Indian Ocean. However, there have been relatively few comprehensive studies of the structure and variability of the MC and its connectivity to regional circulation. The Origins of the Kuroshio and Mindanao Current (OKMC) initiative combines four years of glider observations of the MC, a historical conductivity-temperature-depth (CTD)/float climatology, and results from a global strongly eddying forward ocean general circulation model simulation and a regional ocean state estimate. The MC is resolved as a strong southward current primarily within the upper 200 m, approaching 1 m s(-1), and extending roughly 300 km offshore of Mindanao. Observations and model simulations show a persistent northward Mindanao Undercurrent (MUC) below the thermocline. The MC transports water masses of North Pacific origin southward, while the MUC carries water with South Pacific characteristics northward. The subthermocline transport of the MC and the MUC is connected to other undercurrents in the Philippine Sea. The variability of this transport is a topic of continuing research.

Qiu, B, Rudnick DL, Cerovecki I, Cornuelle BD, Chen S, Schonau MC, McClean JL, Gopalakrishnan G.  2015.  The Pacific North Equatorial Current: New insights from the Origins of the Kuroshio and Mindanao Currents (OKMC) Project. Oceanography. 28:24-33.   10.5670/oceanog.2015.78   AbstractWebsite

Located at the crossroads of the tropical and subtropical circulations, the westward-flowing North Equatorial Current (NEC) and its subsequent bifurcation off the Philippine coast near 13 degrees N serve as important pathways for heat and water mass exchanges between the mid- and low-latitude North Pacific Ocean. Because the western Pacific warm pool, with sea surface temperatures > 28 degrees C, extends poleward of 17 degrees N in the western North Pacific, the bifurcation and transport partitioning of the NEC into the Kuroshio and Mindanao Currents are likely to affect the temporal evolution of the warm pool through lateral advection. In addition to its influence on physical conditions, NEC variability is also important to the regional biological properties and the fisheries along the Philippine coast and in the western Pacific Ocean. This article synthesizes our current understandings of the NEC, especially those garnered through the recent Origins of the Kuroshio and Mindanao Current (OKMC) project.

Kim, SY, Cornuelle BD.  2015.  Coastal ocean climatology of temperature and salinity off the Southern California Bight: Seasonal variability, climate index correlation, and linear trend. Progress in Oceanography. 138:136-157.   10.1016/j.pocean.2015.08.001   AbstractWebsite

A coastal ocean climatology of temperature and salinity in the Southern California Bight is estimated from conductivity-temperature-depth (CTD) and bottle sample profiles collected by historical California Cooperative Oceanic Fisheries Investigation (CalCOFI) cruises (1950-2009; quarterly after 1984) off southern California and quarterly/monthly nearshore CTD surveys (within 30 km from the coast except for the surfzone; 1999-2009) off San Diego and Los Angeles. As these fields are sampled regularly in space, but not in time, conventional Fourier analysis may not be possible. The time dependent temperature and salinity fields are modeled as linear combinations of an annual cycle and its five harmonics, as well as three standard climate indices (El Nino-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), North Pacific Gyre Oscillation (NPGO)), the Scripps Pier temperature time series, and a mean and linear trend without time lags. Since several of the predictor indices are correlated, the indices are successively orthogonalized to eliminate ambiguity in the identification of the contributed variance of each component. Regression coefficients are displayed in both vertical transects and horizontal maps to evaluate (1) whether the temporal and spatial scales of the two data sets of nearshore and offshore observations are consistent and (2) how oceanic variability at a regional scale is related to variability in the nearshore waters. The data-derived climatology can be used to identify anomalous events and atypical behaviors in regional-scale oceanic variability and to provide background ocean estimates for mapping or modeling. (C) 2015 Elsevier Ltd. All rights reserved.

Gasparin, F, Roemmich D, Gilson J, Cornuelle B.  2015.  Assessment of the upper-ocean observing system in the equatorial Pacific: The role of Argo in resolving intraseasonal to interannual variability*. Journal of Atmospheric and Oceanic Technology. 32:1668-1688.   10.1175/jtech-d-14-00218.1   AbstractWebsite

Using more than 10 years of Argo temperature and salinity profiles (2004-14), a new optimal interpolation (OI) of the upper ocean in the equatorial Pacific is presented. Following Roemmich and Gilson's procedures, which were formulated for describing monthly large-scale anomalies, here every 5 days anomaly fields are constructed with improvements in the OI spatial covariance function and by including the time domain. The comparison of Argo maps with independent observations, from the TAO/TRITON array, and with satellite sea surface height (SSH), demonstrates that Argo is able to represent around 70%-80% of the variance at intraseasonal time scales (periods of 20-100 days) and more than 90% of the variance for the seasonal-to-longer-term variability. The RMS difference between Argo and TAO/TRITON temperatures is lower than 1 degrees C and is around 1.5 cm when the Argo steric height is compared to SSH. This study also assesses the efficacy of different observing system components and combinations, such as SSH, TAO/TRITON, and Argo, for estimating subsurface temperature. Salinity investigations demonstrate its critical importance for density near the surface in the western Pacific. Objective error estimates from the OI are used to evaluate different sampling strategies, such as the recent deployment of 41 Argo floats along the Pacific equator. Argo's high spatial resolution compared with that of the moored array makes it better suited for studying spatial patterns of variability and propagation on intraseasonal and longer periods, but it is less well suited for studying variability on periods shorter than 20 days at point locations. This work is a step toward better utilization of existing datasets, including Argo, and toward redesigning the Tropical Pacific Observing System.

2014
Verdy, A, Mazloff MR, Cornuelle BD, Kim SY.  2014.  Wind-driven sea level variability on the California coast: An adjoint sensitivity analysis. Journal of Physical Oceanography. 44:297-318.   10.1175/jpo-d-13-018.1   AbstractWebsite

Effects of atmospheric forcing on coastal sea surface height near Port San Luis, central California, are investigated using a regional state estimate and its adjoint. The physical pathways for the propagation of nonlocal [O(100 km)] wind stress effects are identified through adjoint sensitivity analyses, with a cost function that is localized in space so that the adjoint shows details of the propagation of sensitivities. Transfer functions between wind stress and SSH response are calculated and compared to previous work. It is found that (i) the response to local alongshore wind stress dominates on short time scales of O(1 day); (ii) the effect of nonlocal winds dominates on longer time scales and is carried by coastally trapped waves, as well as inertia-gravity waves for offshore wind stress; and (iii) there are significant seasonal variations in the sensitivity of SSH to wind stress due to changes in stratification. In a more stratified ocean, the damping of sensitivities to local and offshore winds is reduced, allowing for a larger and longer-lasting SSH response to wind stress.

2013
Subramanian, AC, Miller AJ, Cornuelle BD, Di Lorenzo E, Weller RA, Straneo F.  2013.  A data assimilative perspective of oceanic mesoscale eddy evolution during VOCALS-REx. Atmospheric Chemistry and Physics. 13:3329-3344.   10.5194/acp-13-3329-2013   AbstractWebsite

Oceanic observations collected during the VOCALS-REx cruise time period, 1-30 November 2008, are assimilated into a regional ocean model (ROMS) using 4DVAR and then analyzed for their dynamics. Nonlinearities in the system prevent a complete 30-day fit, so two 15-day fits for 1-15 November and 16-30 November are executed using the available observations of hydrographic temperature and salinity, along with satellite fields of SST and sea-level height anomaly. The fits converge and reduce the cost function significantly, and the results indicated that ROMS is able to successfully reproduce both large-scale and smaller-scale features of the flows observed during the VOCALS-REx cruise. Particular attention is focused on an intensively studied eddy at 76 degrees W, 19 degrees S. The ROMS fits capture this eddy as an isolated rotating 3-D vortex with a strong subsurface signature in velocity, temperature and anomalously low salinity. The eddy has an average temperature anomaly of approximately -0.5 degrees C over a depth range from 50-600 m and features a cold anomaly of approximately -1 degrees C near 150 m depth. The eddy moves northwestward and elongates during the second 15-day fit. It exhibits a strong signature in the Okubo-Weiss parameter, which indicates significant non-linearity in its evolution. The heat balance for the period of the cruise from the ocean state estimate reveals that the horizontal advection and the vertical mixing processes are the dominant terms that balance the temperature tendency of the upper layer of the ocean locally in time and space. Areal averages around the eddies, for a 15-day period during the cruise, suggest that vertical mixing processes generally balance the surface heating. Although, this indicates only a small role for lateral advective processes in this region during this period, this quasi-instantaneous heat budget analysis cannot be extended to interpret the seasonal or long-term upper ocean heat budget in this region.

2011
Todd, RE, Rudnick DL, Mazloff MR, Davis RE, Cornuelle BD.  2011.  Poleward flows in the southern California Current System: Glider observations and numerical simulation. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006536   AbstractWebsite

Three years of continuous Spray glider observations in the southern California Current System (CCS) are combined with a numerical simulation to describe the mean and variability of poleward flows in the southern CCS. Gliders provide upper ocean observations with good across-shore and temporal resolution along two across-shore survey lines while the numerical simulation provides a dynamically consistent estimate of the ocean state. Persistent poleward flows are observed in three areas: within 100 km of the coast at Point Conception, within the Southern California Bight (SCB), and offshore of the SCB and the Santa Rosa Ridge (SRR). Poleward transport by the flows within the SCB and offshore of the SRR exceeds the poleward transport off Point Conception, suggesting that the poleward flows are not continuous over the 225 km between observation lines. The numerical simulation shows offshore transport between the survey lines that is consistent with some of the poleward flow turning offshore before reaching Point Conception. The poleward current offshore of the SRR is unique in that it is strongest at depths greater than 350 m and it is observed to migrate westward away from the coast. This westward propagation is tied to westward propagating density anomalies originating in the SCB during the spring-summer upwelling season when wind stress curl is most strongly positive. The across-shore wave number, frequency, and phase speed of the westward propagation and the lack of across-shore transport of salinity along isopycnals are consistent with first-mode baroclinic Rossby dynamics.

2010
Kim, SY, Cornuelle BD, Terrill EJ.  2010.  Decomposing observations of high-frequency radar-derived surface currents by their forcing mechanisms: Locally wind-driven surface currents. Journal of Geophysical Research-Oceans. 115   10.1029/2010jc006223   AbstractWebsite

The wind impulse response function and transfer function for high-frequency radar-derived surface currents off southern San Diego are calculated using several local wind observations. The spatial map of the transfer function reflects the influence of the coast on wind-current dynamics. Near the coast (within 20 km from the shoreline), the amplitudes of the transfer function at inertial and diurnal frequencies are reduced due to effects of coastline and bottom bathymetry. Meanwhile, the amplitude of low-frequency currents increases near the coast, which is attributed to the local geostrophic balance between cross-shore pressure gradients against the coast and currents. Locally wind-driven surface currents are estimated from the data-derived response function, and their power spectrum shows a strong diurnal peak superposed on a red spectrum, similar to the spectra of observed winds. Current magnitudes and veering angles to a quasi-steady wind are typically 2-5% of the wind speed and vary 50 degrees-90 degrees to the right of the wind, respectively. A wind skill map is introduced to present the fractional variance of surface currents explained by local winds as a verification tool for wind data quality and relevance. Moreover, the transfer functions in summer and winter are presented to examine the seasonal variation in ocean surface current response to the wind associated with stratification change.

2008
Haidvogel, DB, Arango H, Budgell WP, Cornuelle BD, Curchitser E, Di Lorenzo E, Fennel K, Geyer WR, Hermann AJ, Lanerolle L, Levin J, McWilliams JC, Miller AJ, Moore AM, Powell TM, Shchepetkin AF, Sherwood CR, Signell RP, Warner JC, Wilkin J.  2008.  Ocean forecasting in terrain-following coordinates: Formulation and skill assessment of the Regional Ocean Modeling System. Journal of Computational Physics. 227:3595-3624.   10.1016/j.jcp.2007.06.016   AbstractWebsite

Systematic improvements in algorithmic design of regional ocean circulation models have led to significant enhancement in simulation ability across a wide range of space/time scales and marine system types. As an example, we briefly review the Regional Ocean Modeling System, a member of a general class of three-dimensional, free-surface, terrain-following numerical models. Noteworthy characteristics of the ROMS computational kernel include: consistent temporal averaging of the barotropic mode to guarantee both exact conservation and constancy preservation properties for tracers; redefined barotropic pressure-gradient terms to account for local variations in the density field; vertical interpolation performed using conservative parabolic splines; and higher-order, quasi-monotone advection algorithms. Examples of quantitative skill assessment are shown for a tidally driven estuary, an ice-covered high-latitude sea, a wind- and buoyancy-forced continental shelf, and a mid-latitude ocean basin. The combination of moderate-order spatial approximations, enhanced conservation properties, and quasi-monotone advection produces both more robust and accurate, and less diffusive, solutions than those produced in earlier terrain-following ocean models. Together with advanced methods of data assimilation and novel observing system technologies, these capabilities constitute the necessary ingredients for multi-purpose regional ocean prediction systems. (c) 2007 Elsevier Inc. All rights reserved.

2003
Willis, JK, Roemmich D, Cornuelle B.  2003.  Combining altimetric height with broadscale profile data to estimate steric height, heat storage, subsurface temperature, and sea-surface temperature variability. Journal of Geophysical Research-Oceans. 108   10.1029/2002jc001755   AbstractWebsite

A new technique is demonstrated for combining altimetric height (AH) and sea-surface temperature (SST) with in situ data to produce improved estimates of 0/800 m steric height (SH), heat content, and temperature variability. The technique uses a linear regression onto AH to construct an initial guess for the subsurface quantity. This guess is then corrected toward the in situ data creating an estimate with substantially less error than could be achieved using either data set alone. Inclusion of the SST data further improves the estimates and illustrates how the procedure can be generalized to allow inclusion of additional data sets. The technique is demonstrated over a region in the southwestern Pacific enclosing the Tasman Sea. Nine-year time series of heat storage and temperature variability, averaged over 4degrees latitude and longitude and 1 year in time, are calculated. The estimates have RMS errors of approximately 4.6 W/m(2) in heat storage, 0.10degreesC in subsurface temperature and 0.11degreesC in surface temperature, and fractional errors of 20, 28, and 18%, respectively, relative to the total variance overall spatial and temporal scales considered. These represent significant improvements over previous estimates of these quantities. All the time series show strong interannual variability including the El Nino event of 1997. Application of these techniques on a global scale could provide new insight into the variability of the general circulation and heat budget of the upper ocean.

2001
Roemmich, D, Gilson J, Cornuelle B, Weller R.  2001.  Mean and time-varying meridional transport of heat at the tropical subtropical boundary of the North Pacific Ocean. Journal of Geophysical Research-Oceans. 106:8957-8970.   10.1029/1999jc000150   AbstractWebsite

Ocean heat transport near the tropical/subtropical boundary of the North Pacific during 1993-1999 is described, including its mean and time variability. Twenty-eight trans-Pacific high-resolution expendable bathythermograph (XBT)/expendable conductivity-temperature-depth (XCTD) transects are used together with directly measured and operational wind estimates to calculate the geostrophic and Ekman transports. The mean heat transport across the XBT transect was 0.83 +/- 0.12 pW during the 7 year period. The large number of transects enables a stable estimate of the mean field to be made, with error bars based on the known variability. The North Pacific heat engine is a shallow meridional overturning circulation that includes warm Ekman and western boundary current components flowing northward, balanced by a southward flow of cool thermocline waters (including Subtropical Mode Waters). A near-balance of geostrophic and Ekman transports holds in an interannual sense as well as for the time mean. Interannual variability in geostrophic transport is strikingly similar to the pattern of central North Pacific sea level pressure variability (the North Pacific Index). The interannual range in heat transport was more than 0.4 pW during 1993-1999, with maximum northward values about 1 pW in early 1994 and early 1997. The ocean heat transport time series is similar to that of European Centre for Medium-Range Weather Forecasts air-sea heat flux integrated over the Pacific north of the XBT line. The repeating nature of the XBT/XCTD transects, with direct wind measurements, allows a substantial improvement over previous heat transport estimates based on one-time transects. A global system is envisioned for observing the time-varying ocean heat transport and its role in the Earth's heat budget and climate system.

2000
Cornuelle, BD, Chereskin TK, Niiler PP, Morris MY, Musgrave DL.  2000.  Observations and modeling of a California undercurrent eddy. Journal of Geophysical Research-Oceans. 105:1227-1243.   10.1029/1999jc900284   AbstractWebsite

A deep, nonlinear warm eddy advecting water that was also anomalously saltier, lower in oxygen, and higher in nutrients relative to surrounding waters was observed in moored current and temperature measurements and in hydrographic data obtained at a site similar to 400 km off the coast of northern California. The eddy was reproduced using a nonlinear quasi-geostrophic model, initialized by an iterative procedure using time series of 2-day averaged moored current measurements. The procedure demonstrates how a data assimilative technique synthesizes and enhances the resolution of a relatively sparse data set by incorporating time-dependence and model physics. The model forecast showed significant skill above persistence or climatology for 40 days. Our hypothesis, that the eddy was generated at the coast in winter and subsequently moved 400 km offshore by May, is consistent with the eddy movement diagnosed by the model and with the observations and coastal climatology. The model evolution significantly underpredicted the temperature anomaly in the eddy owing in part to unmodeled salinity compensation in trapped California Undercurrent water. Together, observations and model results show a stable nonlinear eddy in the California Current System that transported water and properties southwestward through the energetic eastern boundary region. Coherent features such as this one may be a mechanism for property transfer between the eddy-rich coastal zone and the eddy desert of the eastern North Pacific Ocean.

1999
Miller, AJ, Cornuelle BD.  1999.  Forecasts from fits of frontal fluctuations. Dynamics of Atmospheres and Oceans. 29:305-333.   10.1016/s0377-0265(99)00009-3   AbstractWebsite

A primitive equation ocean model is fit with strong constraints to non-synoptic hydrographic surveys in an unstable frontal current region, the Iceland-Faeroe Front. The model is first initialized from a time-independent objective analysis of non-synoptic data (spanning 2 to 6 days). A truncated set of eddy-scale basis functions is used to represent the initial error in temperature, salinity, and velocity. A series of model integrations, each perturbed with one basis function for one dependent variable in one layer, is used to determine the sensitivity to the objective-analysis initial state of the match to the non-synoptic hydrographic data. A new initial condition is then determined from a generalized inverse of the sensitivity matrix and the process is repeated to account for non-linearity. The method is first tested in 'identical twin' experiments to demonstrate the adequacy of the basis functions in representing initial condition error and the convergence of the method to the true solution. The approach is then applied to observations gathered in August 1993 in the Iceland-Faeroe Front. Model fits are successful in improving the match to the true data, leading to dynamically consistent evolution scenarios. However, the forecast skill (here defined as the variance of the model-data differences) of the model runs from the optimized initial condition is not superior to less sophisticated methods of initialization, probably due to inadequate initialization data. The limited verification data in the presence of strong frontal slopes may not be sufficient to establish Forecast skill, so that it must be judged subjectively or evaluated by other quantitative measures. (C) 1999 Elsevier Science B.V. All rights reserved.

1996
Morris, M, Roemmich D, Cornuelle B.  1996.  Observations of variability in the South Pacific subtropical gyre. Journal of Physical Oceanography. 26:2359-2380.   10.1175/1520-0485(1996)026<2359:oovits>2.0.co;2   AbstractWebsite

Variability of the subtropical gyre in the South Pacific Ocean was investigated using high-resolution expendable bathythermograph sections along a repeated track between New Zealand and Hawaii. The southern part of the section sampled most of the zonal flow in the subtropical gyre with the eastward flowing branch between New Zealand and Fiji and the westward branch extending north of Fiji to approximately 10 degrees S. The time series began in September 1987 and extended through 1994, averaging four cruises every year. The geostrophic shear field was calculated, relative to 800 m, with the aid of a mean T-S relationship. Variability was present at a broad range of spatial and temporal scales but annual fluctuations were particularly prominent. The authors conclude that 30 snapshots of temperature, measured over a period of seven years, are sufficient to resolve the annual cycle of the gyre scale circulation along the transect. The shape and intensity of the gyre varied seasonally throughout the water column (0-800 m). Geostrophic transport was most intense (15 Sv, where Sv=10(6)m(3)s(-1)) in November. At this time, the northern edges of eastward dow at the surface and in the thermocline were closest together and the ratio of thermocline to surface transport was highest. Most intense flow occurred approximately two to three months after the basinwide seasonal peak in Ekman pumping. Transport was weakest(ll Sv) in May and was associated with an increase in the poleward slant of the gyre center with depth and a decrease in the ratio of thermocline to surface transport. Seasonal wind forcing was considered as a possible mechanism for the observed annual intensification of the gyre-scale circulation. A simple linear model of thermocline response to local changes in wind stress curl explained a significant fraction of the observed annual variability. Conservation of potential vorticity q yielded an estimate for the absolute mean how (-1 cm s(-1) at 800 m), consistent with direct measurements in the region. Interannual variability, possibly related to the El Nino-Southern Oscillation cycle, was observed. The cold event of 1988/89 appeared to be associated with relatively weak gyre-scale transport. After 1991, gyre-scale transport was more intense and a prominent change in the small-scale circulation occurred, with a shift in the alongtrack wavenumber spectral energy to higher wavenumbers.

Morawitz, WML, Sutton PJ, Worcester PF, Cornuelle BD, Lynch JF, Pawlowicz R.  1996.  Three-dimensional observations of a deep convective chimney in the Greenland sea during Winter 1988/89. Journal of Physical Oceanography. 26:2316-2343.   10.1175/1520-0485(1996)026<2316:tdooad>2.0.co;2   AbstractWebsite

All available temperature data, including moored thermistor. hydrographic, and tomographic measurements, have been combined using least-squares inverse methods to study the evolution of the three-dimensional temperature field in the Greenland Sea during winter 1988/89. The data are adequate to resolve features with spatial scares of about 40 km and larger. A chimney structure reaching depths in excess of 1000 m is observed to the southwest of the gyre center during March 1989. The chimney has a spatial scale of about 50 km, near the limit of the spatial resolution of the data, and a timescale of about 10 days, The chimney structure breaks up and disappears in only 3-6 days. A one-dimensional vertical heat balance adequately describes changes in total heat content in the chimney region from autumn 1988 until the time of chimney breakup, when horizontal advection becomes important. A simple one-dimensional mixed layer model is surprisingly successful in reproducing autumn to winter bulk temperature and salinity changes, as well as the observed evolution of the mixed layer to depths in excess of 1000 m. Uncertainties in surface freshwater fluxes make it difficult to determine whether net evaporation minus precipitation, or ice advection, is responsible for the observed depth-averaged salinity increase from autumn to winter in the chimney region. Rough estimates of the potential energy balance In the mixed laver suggest that potential energy changes are reasonably consistent with turbulent kinetic energy (TKE) production terms. Initially the TKE term parameterizing wind forcing and shear production is important, but as the mixed layer deepens the surface buoyancy production term dominates. The estimated average annual deep-water production rate in the Greenland Sea for 1988/89 is about 0.1 Sverdrups, comparable to production rates during the 1980s and early 1990s derived from tracer measurements. The location of the deep convection observed appears to be sensitively linked to the amount of Arctic Intermediate Water (AIW) present from autumn through spring. Although AIW is an important source of salt for the surface waters, too much AIW overstratifies the water column, preventing deep convection from occurring.

1992
Roemmich, D, Cornuelle B.  1992.  The Subtropical Mode Waters of the South-Pacific Ocean. Journal of Physical Oceanography. 22:1178-1187.   10.1175/1520-0485(1992)022<1178:tsmwot>2.0.co;2   AbstractWebsite

The subtropical mode waters (STMW) of the southwestern Pacific Ocean are described, including their physical characteristics, spatial distribution, and temporal variability. STMW is a thermostad, or minimum in stratification, having temperatures of about 15-degrees-19-degrees-C and vertical temperature gradient less than about 2-degrees-C per 100 m. Typical salinity is 35.5 psu at 16.5-degrees-C. The STMW layer is formed by deep mixing and cooling in the eastward-flowing waters of the separated East Australia Current. Surface mixed layers are observed as deep as 300 m north of New Zealand in winter, in the center of a recurring anticyclonic eddy. The STMW thermostad in the South Pacific is considerably weaker than its counterparts in the North Atlantic and North Pacific, a contrast that may help to discriminate between physical processes contributing to its formation. A quarterly time series of expendable bathythermograph transects between New Zealand and Fiji is used to study the temporal variability of STMW. Large fluctuations are observed at both annual and subannual periods. Based on the quarterly census of STMW volume, the lifetime of the thermostad is estimated to be of order 1 year. During the years 1986-91 wintertime sea surface and air temperature minima warmed by about 1.5-degrees-C. The volume of STMW decreased dramatically during that period, with the 1989-91 census showing only a small fraction of the 1986-87 STMW volume. The observed fluctuations may be due either to long-period change in air-sea heat exchange or to fluctuations in heat transport by ocean currents.