Yiyong Luo

On the connection between South Pacific subtropical spiciness anomalies and decadal equatorial variability in an ocean general circulation model

n n Simulations from a 49-year, realistically forced numerical model experiment indicate that decadal variability of temperature and salinity along the equator originates from subsurface spiciness anomalies in the South Pacific. Through western boundary and interior pathways in the thermocline, the subsurface anomalies in the South Pacific are first transferred westward and then northward, eventually appearing along the equator. The large spiciness anomalies in the South Pacific are formed in the eastern subtropics where large unstable salinity gradients are present in conjunction with weak stratification and strong mixing during winters. Our analysis shows that positive anomalies are generated in late winter by diapycnal mixing across isopycnal surfaces that are not exposed to the surface, i.e., through the injection process, in agreement with Yeager and Large ( 2004). In addition, we show that spiciness anomalies can also be created along isopycnals that outcrop to the surface through the subduction process, although this process alone is not enough to explain a significant part of the decadal variability along the equator based upon an active tracer experiment. Both the injection and subduction processes are responsible for forming positive subsurface anomalies in the eastern subtropical South Pacific, while negative anomalies there can be generated by subduction of negative surface anomalies and accumulation via isopycnal advection.

Response of Pacific subtropical-tropical thermocline water pathways and transports to global warming

n n Global warming may change the thermocline water pathways and transports from the subtropics to the tropics in the Pacific Ocean, which are known to have profound implications for the El Nino-Southern Oscillation (ENSO) and thereby global climate. This study investigates the changes by comparing solutions between a present-day climate and a future, warmer climate from a set of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models. As the climate warms, although the total transport from the subtropics to the tropics exhibits no significant change, transport via western boundary pathways increases and via interior pathways decreases. This shift is due to high potential vorticity (PV) zones that extend further westward, thus dynamically guiding thermocline water away from interior pathways to prefer western boundary pathways from the subtropics to the tropics. Additionally, a warmer climate induces a large temperature increase near the sea surface in the eastern tropics and a significantly enhanced Equatorial Undercurrent (EUC) in the western and central Pacific; the former is related to the decreased transport through interior pathways and the latter is linked to the increased transport through western boundary pathways. Implications of the results of this study are also discussed.n

Effects of excessive equatorial cold tongue bias on the projections of tropical Pacific climate change. Part I: the warming pattern in CMIP5 multi-model ensemble

The excessive cold tongue error in the equatorial Pacific has persisted in several generations of climate models. Based on the historical simulations and Representative Concentration Pathway (RCP) 8.5 experiments in the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble (MME), this study finds that models with an excessive westward extension of cold tongue and insufficient equatorial western Pacific precipitation tend to project a weaker east-minus-west gradient of sea surface temperature (SST) warming along the equatorial Pacific under increased greenhouse gas (GHG) forcing. This La Niña-like error of tropical Pacific SST warming is consistent with our understanding of negative SST-convective feedback over the western Pacific warm pool. Based on this relationship between the present simulations and future projections, the present study applies an “observational constraint” of equatorial western Pacific precipitation to calibrate the projections of tropical Pacific climate change. After the corrections, CMIP5 models robustly project an El Niño-like warming pattern, with a MME mean increase by a factor of 2.3 in east-minus-west gradient of equatorial Pacific SST warming and reduced inter-model uncertainty. Corrections in projected changes in tropical precipitation and atmospheric circulation are physically consistent. This study suggests that a realistic cold tongue simulation would lead to a more reliable tropical Pacific climate projection.

The Role of Ocean Dynamical Thermostat in Delaying the El Niño–Like Response over the Equatorial Pacific to Climate Warming

AbstractThe role of ocean dynamics in the response of the equatorial Pacific Ocean to climate warming is investigated using both an atmosphere–ocean coupled climate system and its ocean component. Results show that the initial response (fast pattern) to an uniform heating imposed on the ocean is a warming centered to the west of the date line owing to the conventional ocean dynamical thermostat (ODT) mechanism in the eastern equatorial Pacific—a cooling effect arising from the up-gradient upwelling. In time, the warming pattern gradually propagates eastward, becoming more El Nino–like (slow pattern). The transition from the fast to the slow pattern likely results from 1) the gradual warming of the equatorial thermocline temperature, which is associated with the arrival of the relatively warmer extratropical waters advected along the subsurface branch of the subtropical cells (STCs), and 2) the reduction of the STC strength itself. A mixed layer heat budget analysis finds that it is the total ocean dynamic...

Response of the tropical Pacific Ocean to El Niño versus global warming

Climate models project an El Niño-like SST response in the tropical Pacific Ocean to global warming (GW). By employing the Community Earth System Model and applying an overriding technique to its ocean component, Parallel Ocean Program version 2, this study investigates the similarity and difference of formation mechanism for the changes in the tropical Pacific Ocean under El Niño and GW. Results show that, despite sharing some similarities between the two scenarios, there are many significant distinctions between GW and El Niño: (1) the phase locking of the seasonal cycle reduction is more notable under GW compared with El Niño, implying more extreme El Niño events in the future; (2) in contrast to the penetration of the equatorial subsurface temperature anomaly that appears to propagate in the form of an oceanic equatorial upwelling Kelvin wave during El Niño, the GW-induced subsurface temperature anomaly manifest in the form of off-equatorial upwelling Rossby waves; (3) while significant across-equator northward heat transport (NHT) is induced by the wind stress anomalies associated with El Niño, little NHT is found at the equator due to a symmetric change in the shallow meridional overturning circulation that appears to be weakened in both North and South Pacific under GW; and (4) heat budget analysis shows that the maintaining mechanisms for the eastern equatorial Pacific warming are also substantially different.

Dynamics of the Block Island Sound Estuarine Plume

AbstractBuoyant discharge of freshwater from Long Island Sound (LIS) forms a seasonal buoyant plume outside Block Island Sound (BIS) between the coast of Long Island and the denser shelf waters. The plume’s seasonal variability and its response to tides, winds, and surface heating are investigated through a series of process-oriented experiments using the Regional Ocean Modeling System (ROMS). Results show the importance of river discharge, wind directions, and surface heating in the seasonal variation of the BIS buoyant plume. In winter and spring, the plume is intermediate with a large surface offshore extension detached from the bottom. From winter to spring, the river discharge increases; meanwhile, upwelling-favorable winds keep dominating. They compete with the increase of surface heating and generate a broader buoyant plume in spring than in winter. In summer, the plume is bottom advected with most of its width in contact with the bottom and is featured with the steepest isopycnals and narrowest pl...

Response of the Pacific Ocean Circulation to Climate Change

The response of the Pacific Ocean circulation to climate change is investigated by comparing solutions from a set of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled models for a present-day climate (the 20C3M experiments) and a future, warmer climate (the SRESA1B experiments). Under the warmer climate scenario, the oceanic changes in the tropical Pacific include a relatively weak warming of the western equatorial thermocline, a weakening of the surface current system and a complicated change in the structure of the Equatorial Undercurrent (EUC) with an increased flow in its upper branch but a decreased flow in its lower branch. As the climate warms, the North Pacific Ocean features a basin-scale reduction in mixed layer depth, a weakening of the subtropical countercurrent (STCC), a northward shift of the Kuroshio Extension (KE) and an overall slowdown of the subtropical gyre. In the South Pacific, the warmer climate induces significant changes in the upper ocean of the eastern subtropics including a relatively weak warming, a deepening of mixed layer depth and an anticylonic circulation.

The positive Indian Ocean Dipole–like response in the tropical Indian Ocean to global warming

Climate models project a positive Indian Ocean Dipole (pIOD)–like SST response in the tropical Indian Ocean to global warming. By employing the Community Earth System Model and applying an overriding technique to its ocean component (version 2 of the Parallel Ocean Program), this study investigates the similarities and differences of the formation mechanisms for the changes in the tropical Indian Ocean during the pIOD versus global warming. Results show that their formation processes and related seasonality are quite similar; in particular, wind–thermocline–SST feedback is the leading mechanism in producing the anomalous cooling over the eastern tropics in both cases. Some differences are also found, including the fact that the cooling effect of the vertical advection over the eastern tropical Indian Ocean is dominated by the anomalous vertical velocity during the pIOD but by the anomalous upper-ocean stratification under global warming. These findings are further examined through an analysis of the mixed layer heat budget.

A periodic freshwater patch detachment process from the Block Island Sound estuarine plume

Previous observations suggest periodic freshwater patches separating from the Block Island Sound (BIS) estuarine plume. In this study, the dynamics of the separation process is investigated through a series of numerical experiments using the Regional Ocean Modeling System (ROMS). In addition, we explore the seasonal variability of the freshwater patches and their response to river discharge and ambient current. The model results indicate that episodic freshwater patches are triggered by small changes in tidal currents over the spring-neap tidal cycle. The spring-neap variation in tidal currents causes significant, monthly fluctuations in turbulent mixing and vertical stratification in BIS, modulating the freshwater discharge thereby generating episodic freshwater patches that move both downstream along the southern shore of Long Island and toward Rhode Island Sound (RIS). The realistically configured model shows that the freshwater patches experience strong seasonal variability. They are largest in spring when the river discharge peaks, and smallest in summer due to the weak river discharge and a robust upstream ambient current from RIS. According to the analysis of the freshwater transport out of BIS, we conclude that such detachment occurs at tidal mixing fronts.

Asymmetric Response of the Equatorial Pacific SST to Climate Warming and Cooling

AbstractThe response of the equatorial Pacific Ocean to heat fluxes of equal amplitude but opposite sign is investigated using the Community Earth System Model (CESM). Results show a strong asymmetry in SST changes. In the eastern equatorial Pacific (EEP), the warming responding to the positive forcing exceeds the cooling response to the negative forcing, whereas in the western equatorial Pacific (WEP) it is the other way around and the cooling surpasses the warming. This leads to a zonal dipole asymmetric structure, with positive values in the east and negative values in the west. A surface heat budget analysis suggests that the SST asymmetry mainly results from the oceanic horizontal advection and vertical entrainment, with both of their linear and nonlinear components playing a role. For the linear component, its change appears to be more significant over the EEP (WEP) in the positive (negative) forcing scenario, favoring the seesaw pattern of the SST asymmetry. For the nonlinear component, its change ...