Hiromichi Igarashi

Simulated Rapid Warming of Abyssal North Pacific Waters

Warming the Deep The coldest ocean waters are located at the bottoms of the major ocean basins, and, because it takes a long time for water to sink from the surface to these regions, they are relatively isolated from the warming trends that are now occurring at shallower depths. However, warming in these deep waters has recently been observed, sooner than anticipated. Masuda et al. (p. 319, published online 24 June) performed computer simulations of ocean circulation and found that internal waves are able to transport heat rapidly from the surface waters around Antarctica to the bottom of the North Pacific, which can occur within four decades, rather than the centuries that conventional mechanisms have suggested. Computer simulations suggest a possible reason for the warming of North Pacific bottom water during the past four decades. Recent observational surveys have shown significant oceanic bottom-water warming. However, the mechanisms causing such warming remain poorly understood, and their time scales are uncertain. Here, we report computer simulations that reveal a fast teleconnection between changes in the surface air-sea heat flux off the Adélie Coast of Antarctica and the bottom-water warming in the North Pacific. In contrast to conventional estimates of a multicentennial time scale, this link is established over only four decades through the action of internal waves. Changes in the heat content of the deep ocean are thus far more sensitive to the air-sea thermal interchanges than previously considered. Our findings require a reassessment of the role of the Southern Ocean in determining the impact of atmospheric warming on deep oceanic waters.

Improved coupled GCM climatologies for summer monsoon onset studies over Southeast Asia

[1] To enhance accurate estimates of Asian monsoon variability by a coupled general circulation model (GCM), the bulk adjustment factors that control latent heat, sensible heat and momentum fluxes are optimized using a 4-dimensional variational data assimilation method. When using the optimized values, a coupled GCM is better able to define the summer monsoon features over Southeast Asia. In particular, the early, rapid onset is realistically simulated around the Indochina Peninsula, which is a key region for the initial stage of the Asian summer monsoon development. The spatial patterns of precipitation rate, wind velocity, and sea surface temperature are successfully reproduced. The optimized values of the bulk adjustment factors for latent heat flux become significantly correlated with the strength of the subgrid-scale disturbances primarily associated with energetic deep convective activities in the tropics, which is one of the major sources of model biases in commonly-used coarse resolution models.

Four-dimensional variational ocean reanalysis: a 30-year high-resolution dataset in the western North Pacific (FORA-WNP30)

We produced a four-dimensional variational ocean re-analysis for the Western North Pacific over 30 years (FORA-WNP30). It is the first-ever dataset covering the western North Pacific over 3 decades at eddy-resolving resolution. The four-dimensional variational analysis scheme version of the Meteorological Research Institute Multivariate Ocean Variational Estimation system (MOVE-4DVAR) is employed to conduct a long-term reanalysis experiment during 1982–2012. After evaluating the basic performance of FORA-WNP30, the interannual to decadal variability is analyzed. Overall, FORA-WNP30 reproduces basic features in the western North Pacific well. One of outstanding features in FORA-WNP30 is that anomalous events such as the Kuroshio large meander and anomalous intrusion of the Oyashio in the 1980s, when there were no altimeter data, are successfully reproduced. FORA-WNP30 is therefore a valuable dataset for a variety of oceanographic research topics and potentially for related fields such as climate study, meteorology and fisheries.

Identifying Pelagic Habitat Hotspots of Neon Flying Squid in the Temperate Waters of the Central North Pacific

We identified the pelagic habitat hotspots of the neon flying squid (Ommastrephes bartramii) in the central North Pacific from May to July and characterized the spatial patterns of squid aggregations in relation to oceanographic features such as mesoscale oceanic eddies and the Transition Zone Chlorophyll-a Front (TZCF). The data used for the habitat model construction and analyses were squid fishery information, remotely-sensed and numerical model-derived environmental data from May to July 1999–2010. Squid habitat hotspots were deduced from the monthly Maximum Entropy (MaxEnt) models and were identified as regions of persistent high suitable habitat across the 12-year period. The distribution of predicted squid habitat hotspots in central North Pacific revealed interesting spatial and temporal patterns likely linked with the presence and dynamics of oceanographic features in squid’s putative foraging grounds from late spring to summer. From May to June, the inferred patches of squid habitat hotspots developed within the Kuroshio-Oyashio transition zone (KOTZ; 37–40°N) and further expanded north towards the subarctic frontal zone (SAFZ; 40–44°N) in July. The squid habitat hotspots within the KOTZ and areas west of the dateline (160°W-180°) were likely influenced and associated with the highly dynamic and transient oceanic eddies and could possibly account for lower squid suitable habitat persistence obtained from these regions. However, predicted squid habitat hotspots located in regions east of the dateline (180°-160°W) from June to July, showed predominantly higher squid habitat persistence presumably due to their proximity to the mean position of the seasonally-shifting TZCF and consequent utilization of the highly productive waters of the SAFZ.

Elucidating the potential squid habitat responses in the central North Pacific to the recent ENSO flavors

The effects of the El Niño Southern Oscillation (ENSO)-mediated environmental changes to marine resources were long recognized; however, species-specific responses were also reported to vary, possibly more so, under the emerging event-to-event diversity of the ENSO conditions. Hence, the objective of this study is to characterize the potential impacts of the ENSO-regulated environmental variability to squid habitat in the central North Pacific under the different ENSO flavors, using habitat models developed from at least a decade of fishery-dependent dataset and environmental parameters. Our findings revealed that the potential squid habitats were largely influenced by ENSO-forced environmental changes during the squid’s spawning and nursery periods, resulting in substantial reduction/enhancement of available habitats in the succeeding summers of Central Pacific El Niño/La Niña, where the latter led to an expansion of favorable spawning and nursery grounds. However, the autumn–winter periods of weaker and short-lived Eastern Pacific El Niño showed elevated potential habitats due to minimal sea surface temperature drop and close proximity of spawning and nursery grounds to optimal feeding environments. The quick, albeit variable, squids’ responses to ENSO flavors accentuate their promising potential as ecological beacons under climate changes, aiding the development of adaptive management strategies for commercially exploited fisheries.

The regional impacts of climate change on coastal environments and the aquaculture of Japanese scallops in northeast Asia: case studies from Dalian, China, and Funka Bay, Japan

Climate changes affect coastal environments and aquaculture, threatening food security and economic growth. Japanese scallop (Mizuhopecten yessoensis) culture is economically important for the coastal communities of Dalian, China, and Funka Bay, Japan. In this study, we combined satellite remote-sensing data, in situ observations, and a suitable aquaculture site selection model to explore the interactions between marine environments and climate variability over a recent 10-year period (2003–2012). Our selection of appropriate zones in these two Far Eastern regions and our analyses of climatic event (Arctic Oscillation (AO), winter East Asian monsoon (EAM), and El Niño/La Niña Southern Oscillation (ENSO)) and meteorological (precipitation, temperature, and wind) data allowed us to determine the impacts of climate change on regional coastal environments and prospects for scallop aquaculture. These analyses showed that AO and EAM strongly influenced the aquaculture areas on the Dalian coast through their effects on temperature during winter. We also determined that wind was the main driving force behind regional environmental changes during spring. Conversely, ocean conditions and suitable areas in Funka Bay changed rapidly relative to oceanic and atmospheric circulation. In Funka Bay, areas appropriate for scallop aquaculture and variations in chlorophyll-a concentration (which reflect the availability of algal food for scallops) were strongly correlated with ENSO, precipitation, and air temperature. These correlations demonstrate the influence of oceanic and atmospheric parameters on the productivity of scallop aquaculture in Funka Bay. Adaptation to oceanic and atmospheric changes should be considered when developing plans and management strategies for coastal scallop aquaculture in northeast Asia.

An attempt of dissemination of potential fishing zones prediction map of Japanese common squid in the coastal water, southwestern Hokkaido, Japan

Accurate prediction of potential fishing zones is regarded as one of the most immediate and effective approaches in operational fisheries. It helps fishermen reduce their cost on fuel and also decrease the uncertainty of their fish catches. To predict potential fishing zones of Japanese common squid, we derived fishing positions from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS), combine with bathymetry and model-derived environmental factors from the 4D-VAR data assimilation system and fitted using habitat suitability index (HSI) model. Validations with an independent DMSP/OLS dataset showed better performance of the model in figuring out the squid aggregations than our previous model established with satellite-derived environmental data. Nighttime visible images during June and early July of 2013 derived from Day/Night band (DNB) of Visible Infrared Imaging Radiometer Suite (VIIRS) sensor with a better resolution and quality compared to DMSP/OLS, were also applied for validation and results showed differences of fitness between actual fishing activities and predictions in Japan Sea and Tsugaru Strait.

Argo data assimilation and its effect on climate state estimation and forecasting in the western North Pacific using a coupled model

In this study, we investigated the effects of ocean subsurface data (Argo data) on climate state estimation and forecasting, focusing on the reproduction of North Pacific subtropical mode water (STMW) using a four-dimensional variational data assimilation system with a coupled model. We produced two reanalysis plus forecast data sets for the ocean and atmosphere in 2010 using a 3 month assimilation period: the first including Argo data (Argo case) and the second did not include Argo data (control case). In the control case, the Kuroshio, Kuroshio Extension front, and recirculation gyres along the front were not adequately reproduced. Consequently, there were large biases in temperature and salinity in the western North Pacific. The assimilation of Argo data effectively corrected these biases and significantly improved reproduction of the Kuroshio fronts and recirculation gyres, resulting in a more realistic reproduction of the winter mixed layer and STMW. The correction of these biases is critical to the 1–3 year predictions of the STMW core properties, and the assimilation of Argo data enables prediction of these properties for more than a year. We showed that assimilation of Argo data affects the surface atmospheric temperature above the STMW formation region.

An improved simulation of the deep Pacific Ocean using optimally estimated vertical diffusivity based on the Green's function method

An improved vertical diffusivity scheme is introduced into an ocean general circulation model to better reproduce the observed features of water property distribution inherent in the deep Pacific Ocean structure. The scheme incorporates (a) a horizontally-uniform background profile, (b) a parameterization depending on the local static stability and (c) a parameterization depending on the bottom topography. Weighting factors for these parameterizations are optimally estimated based on the Greens function method. The optimized values indicate an important role of both the intense vertical diffusivity near rough topography and the background vertical diffusivity. This is consistent with recent reports that indicate the presence of significant vertical mixing associated with finite-amplitude internal wave breaking along the bottom slope and its remote effect. The robust simulation with less artificial trend of water properties in the deep Pacific Ocean illustrates that our approach offers a better modeling analysis for the deep ocean variability.

Variational data assimilation system with nesting model for high resolution ocean circulation

To obtain the high-resolution analysis fields for ocean circulation, a new incremental approach is developed using a four-dimensional variational data assimilation system with nesting models. The results show that there are substantial biases when using a classical method combined with data assimilation and downscaling, caused by different dynamics resulting from the different resolutions of the models used within the nesting models. However, a remarkable reduction in biases of the low-resolution model relative to the high-resolution model was observed using our new approach in narrow strait regions, such as the Tsushima and Tsugaru straits, where the difference in the dynamics represented by the high- and low-resolution models is substantial. In addition, error reductions are demonstrated in the downstream region of these narrow channels associated with the propagation of information through the model dynamics.