Tong Lee

Increasing intensity of El Niño in the central‐equatorial Pacific

[1] Satellite observations suggest that the intensity of El Niiio events in the central equatorial Pacific (CP) has almost doubled in the past three decades, with the strongest warming occurring in 2009-10. This is related to the increasing intensity as well as occurrence frequency of the so-called CP El Niiio events since the 1990s. While sea surface temperature (SST) in the CP region during El Nino years has been increasing, those during neutral and La Nina years have not. Therefore, the well-documented warming trend of the warm pool in the CP region is primarily a result of more intense El Nino events rather than a general rise of background SST.

Understanding ENSO Diversity

El Nino–Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Nino events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO’s impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions.

Effects of the Indonesian Throughflow on the Pacific and Indian Oceans

Abstract Effects of the Indonesian Throughflow (ITF) on the circulation and thermal structure of the Pacific and Indian Oceans are studied by comparing solutions of a near-global ocean general circulation model with open and closed Indonesian passages from 1981 to 1997. The ITF contributes to the maintenance of the model circulation system around eastern Australia and the southern Indian Ocean. Blockage of the ITF weakens the Indian Ocean South Equatorial Current and Agulhas Current and strengthens the East Australian Current. The ITF does not affect the Mindanao Current, but drains waters carried by this current into the Indian Ocean and thus reduces tropical–subtropical exchange in the North Pacific. Meanwhile, it helps maintain a stronger New Guinea Coastal Undercurrent and thus enhances tropical–subtropical exchange in the south. Water parcels traveling along the western boundary of the South Pacific cross the equator in the presence of the ITF but are confined to the Southern Hemisphere without the I...

Using Green's Functions to Calibrate an Ocean General Circulation Model

Abstract Greens functions provide a simple yet effective method to test and to calibrate general circulation model (GCM) parameterizations, to study and to quantify model and data errors, to correct model biases and trends, and to blend estimates from different solutions and data products. The method is applied to an ocean GCM, resulting in substantial improvements of the solution relative to observations when compared to prior estimates: overall model bias and drift are reduced and there is a 10%–30% increase in explained variance. Within the context of this optimization, the following new estimates for commonly used ocean GCM parameters are obtained. Background vertical diffusivity is (15.1 ± 0.1) × 10−6 m2 s−2. Background vertical viscosity is (18 ± 3) × 10−6 m2 s−2. The critical bulk Richardson number, which sets boundary layer depth, is Ric = 0.354 ± 0.004. The threshold gradient Richardson number for shear instability vertical mixing is Ri0 = 0.699 ± 0.008. The estimated isopycnal diffusivity coeff...

The changing impact of El Niño on US winter temperatures

[1] In this study, evidence is presented from statistical analyses, numerical model experiments, and case studies to show that the impact on US winter temperatures is different for the different types of El Nino. While the conventional Eastern-Pacific El Nino affects winter temperatures primarily over the Great Lakes, Northeast, and Southwest US, the largest impact from Central-Pacific El Nino is on temperatures in the northwestern and southeastern US. The recent shift to a greater frequency of occurrence of the CentralPacific type has made the Northwest and Southeast regions of the US most influenced by El Nino. It is shown that the different impacts result from differing wave train responses in the atmosphere to the sea surface temperature anomalies associated with the two types of El Nino. Citation: Yu, J.-Y., Y. Zou, S. T. Kim, and T. Lee (2012), The changing impact of El Nino on US winter temperatures, Geophys. Res. Lett., 39, L15702, doi:10.1029/2012GL052483.

Seasonal cycles of meridional overturning and heat transport of the Indian Ocean

Abstract A general circulation model of the Indian Ocean is fitted to monthly averaged climatological temperatures, salinities, and surface fluxes using the adjoint method. Interannual variability is minimized by penalizing the temporal drift from one seasonal cycle to another during a two-year integration. The resultant meridional overturning and heat transport display large seasonal variations, with maximum amplitudes of 18 and 22 (× 106 m3 s−1) for the overturning and 1.8 and 1.4 (× 1015 W) for heat transport near 10°S and 10°N, respectively. A dynamical decomposition of the overturning and heat transport shows that the time-varying Ekman flow plus its barotropic compensation can explain a large part of the seasonal variations in overturning and heat transport. The maximum variations at 10°N and 10°S are associated with monsoon reversal over the northern Indian Ocean and changes of the easterlies over the southern Indian Ocean. An external mode with variable topography has a moderate contribution where...

NASA supercomputer improves prospects for ocean climate research

Estimates of ocean circulation constrained by in situ and remotely sensed observations have become routinely available during the past five years, and they are being applied to myriad scientific and operational problems [Stammer et al., 2002]. Under the Global Ocean Data Assimilation Experiment (GODAE), several regional and global estimates have evolved for applications in climate research, seasonal forecasting, naval operations, marine safety, fisheries, the offshore oil industry coastal management, and other areas. This article reports on recent progress by one effort, the consortium for Estimating the Circulation and Climate of the Ocean (ECCO), toward a next-generation synthesis of ocean and sea-ice data that is global, that covers the full ocean depth, and that permits eddies.

Interannual-to-Decadal Variations of Tropical–Subtropical Exchange in the Pacific Ocean: Boundary versus Interior Pycnocline Transports

Abstract Interannual-to-decadal variations of tropical–subtropical mass exchange in the Pacific Ocean are investigated using a near-global ocean general circulation model along with satellite observations of sea level and wind and a data assimilation product. The analysis focuses on the variability of pycnocline transports through the western boundary and interior near 10°N and 10°S. In contrast to time-mean exchange, where boundary and interior pycnocline transports are both equatorward, the variations of boundary and interior pycnocline transports are found to be generally anticorrelated to each other. Moreover, the variation of the boundary pycnocline transport is smaller than that of the interior, again different from time-mean exchange, where the boundary transport at 10°N is substantially larger than that through the interior. Interannual variations of the boundary and interior transports are consistent with near-surface geostrophic flow inferred from sea level data. Interior pycnocline flow into th...

Mechanisms Controlling the Interannual Variation of Mixed Layer Temperature Averaged over the Nino-3 Region

Abstract Processes controlling the interannual variation of mixed layer temperature (MLT) averaged over the Nino-3 domain (5°N–5°S, 150°–90°W) are studied using an ocean data assimilation product that covers the period of 1993–2003. The overall balance is such that surface heat flux opposes the MLT change but horizontal advection and subsurface processes assist the change. Advective tendencies are estimated here as the temperature fluxes through the domain’s boundaries, with the boundary temperature referenced to the domain-averaged temperature to remove the dependence on temperature scale. This allows the authors to characterize external advective processes that warm or cool the water within the domain as a whole. The zonal advective tendency is caused primarily by large-scale advection of warm-pool water through the western boundary of the domain. The meridional advective tendency is contributed to mostly by Ekman current advecting large-scale temperature anomalies through the southern boundary of the d...

Eddy‐induced meridional heat transport in the ocean

[1] A global ocean data synthesis product at eddy-permitting resolution from Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project are used to estimate the oceanic eddy heat transport. We show that in a number of locations the time-mean eddy heat transport constitutes a considerable portion of the total time-mean heat transport, in particular, in the tropics, in the Southern Ocean and in the Kuroshio Current. This research demonstrates that the variability of the eddy heat transport is a significant contributor to the variability of the total heat transport and globally it explains about 1/3 of its variance. Eddies are also found to explain a significant portion of the seasonal-interannual heat transport variance.