Yoshimi Kawai

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.

Detection of hot event in the equatorial Indo-Pacific warm pool using advanced satellite sea surface temperature, solar radiation, and wind speed

Using advance satellite-derived sea surface temperature (SST), solar radiation and wind speed products, we investigate hot events (HEs) associated with very high SST of around 30 degrees C. Amplitude of the SST diurnal variation (DSST) is obtained by an empirical formula with the downward surface shortwave radiation (DSSR) and the wind speed. The HE is defined as a connected region with SST > the time-dependent SST threshold of about 30 degrees C having a minimum areal size greater than 3 x 10(6) km(2) and lasting for a period longer than 10 days. Thirty-one hot events were detected from 1993 to 2003 in and around the equatorial Indo-Pacific warm pool. The HE precursor is increase of an area with SST > the time-dependent SST threshold, and the HE starts when the areal size exceeds the areal threshold. It keeps the very high SST for a while, and then its SST decreases with decrease of the HE area toward its ending phase. The HE area and the high-DSST area correspond with each other on timescales of daily and the whole one event period. In the HE period, a clear-sky and low-wind condition becomes the reason for large DSST appearance. Through statistics of the detected 31 HEs, it is indicated that the mean HE period, the mean areal size, and the means of the regional HE areal averaged SST are 24.2 days, 1.57 x 10(7) km(2), and 0.25 degrees C above the time-dependent SST threshold, respectively. The HE area shifts with season though they stay within/around the SST contour of 28 degrees C in the Indo-Pacific oceans.

Study on a Platform Effect in the In Situ Sea Surface Temperature Observations under Weak Wind and Clear Sky Conditions Using Numerical Models

Abstract Estimation of the vertical sea water temperature difference between 0- and 1-m depths is necessary to validate the accuracy of satellite-derived sea surface temperatures (SSTs). The authors have examined diurnal temperature variations in the sea surface layer under weak wind and clear sky conditions by two kinds of numerical models. Under such conditions, a large temperature gradient develops in the vicinity of the sea surface and little turbulence is induced by wind stress mixing. The vertical temperature profiles between 0- and 2-m depths observed with a sea surface temperature profiler buoy (SSTPB) are well simulated by the models. SSTPB was specially designed not to disturb the water column in the vicinity of the surface. However, the 1-m-depth temperatures simulated by the models do not agree with that observed with a buoy under weak wind conditions. The hull of buoy is large and can be inferred to generate turbulence in the near-surface layer through the interaction with the swell, wind wav...

Atmospheric pressure response to mesoscale sea surface temperature variations in the Kuroshio Extension region: In situ evidence

Several research cruises were conducted across the Kuroshio Extension front to examine the low-level atmospheric responses to mesoscale variations in sea surface temperature (SST). Surface meteorological observations, including sea level pressure (SLP) and SST, were collected at two moored buoys that were located on either side of the Kuroshio Extension, and from a research vessel, as it moved between the two buoys during the various cruises. Spatial perturbations in SLP along the ship transects, calculated by subtracting moored-buoy SLP from that of the moving ship, tend to be positive (negative) where SST is lower (higher) on spatial scales of about 100 km, and the magnitude of these SLP perturbations near the SST front can exceed 1.0 hPa. Radiosonde data also show that the atmospheric boundary layer thins (thickens) over lower (higher) SST. Although the contribution of across-track component in the wind cannot be calculated, the along-track component of divergence suggests low-level convergence over higher SST. The thermally induced pressure gradient is important in the momentum budget, suggesting that sea breeze-like local circulations formed over the SST fronts. Indeed, the pressure adjustment mechanism can sometimes dominate even on a scale of 100 km, although it is not always observed. The time scale in which the boundary layer thickness adjusted to SST is estimated to be 1 day or less.

Cloud Response to the Meandering Kuroshio Extension Front

AbstractA unique set of observations on board research vessel (R/V) Mirai in April 2010 captured a striking cloud hole over a cold meander of the Kuroshio Extension (KE) east of Japan as corroborated by atmospheric soundings, ceilometer, shipboard radiation data, and satellite cloud images. Distinct differences were also observed between the warm meander farther to the north and warm water south of the KE. The atmosphere is highly unstable over the warm meander, promoting a well-mixed marine atmospheric boundary layer (MABL) and a layer of solid stratocumulus clouds capped by a strong inversion. Over the warm water south of the KE, MABL deepens and is decoupled from the ocean surface. Scattered cumulus clouds develop as captured by rapid variations in ceilometer-derived cloud base. The results show that the meandering KE front affects the entire MABL and the clouds. Such atmospheric response can potentially intensify the baroclinicity in the lower atmosphere.

Comment on “Importance of pre-existing oceanic conditions to upper ocean response induced by Super Typhoon Hai-Tang” by Z.-W. Zheng, C.-R. Ho, and N.-J. Kuo

[1] Understanding the impact of pre-existing oceanic conditions on the upper ocean response to tropical cyclones (TCs) is one of the important issues for understanding the short-term occurrence of TC-ocean interaction and its impact on TC intensity. Zheng et al. [2008] showed that preexisting cyclonic flow represented by negative sea-surface height anomalies (SSHA) played a crucial role in enhancing sea-surface cooling (SSC) caused by Typhoon Hai-Tang in 2005. On the other hand, the impact of initial oceanic mixedlayer depth and the vertical sea-temperature gradient in the thermocline on the amplitude of SSC has been considered to be small compared with that of the Ekman pumping and entrainment/vertical turbulent mixing [e.g., Wada, 2002]. Therefore, we hardly accept the discussion of Zheng et al. [2008] that the impact of Ekman pumping on SSC is less significant for local SSC occurred in certain areas than that of pre-existing oceanic condition. [2] Zheng et al. [2008] provide no quantitative evidence that SSC caused by the passage of Hai-Tang is irrelevant to the Ekman pumping or that the amplitude of SSC caused by the Ekman pumping is negligibly smaller than that due to relatively low sea surface height (SSH). In this paper, we revisit the relationship between SSC and SSH using a daily oceanic reanalysis dataset [Wada and Usui, 2007] produced by the North Pacific version of the Japan Meteorological Agency/Meteorological Research Institute Multivariate Ocean Variational Estimation system (MOVE) [Usui et al., 2006] in section 2. In section 3, we revisit the impact of the pre-existing oceanic condition on the ocean response to HaiTang using the Argo profiling float data. Section 4 presents our concluding remarks.

The production of the new generation sea surface temperature (NGSST-O ver.1.0) in Tohoku University

Satellite sea surface temperature (SST) observations from infrared radiometers (AVHRR, MODIS) and a microwave radiometer (AMSR-E) are objectively merged to generate the new generation SST product (NGSST-O Ver.1.0). The product is a quality-controlled, cloud-free, high-spatial resolution (0.05 degree-grided), wide-covering (13-63N, 116-166E), and daily SST digital map. The real-time production and distribution of the NGSST-O has started from September 2003 as a part of the GODAE High Resolution Sea Surface Temperature Pilot Project. To examine the accuracy of the NGSST-O product, the NGSST-O SST is compared with the in-situ SST that is obtained by the buoys drifting in the analyzed area for July 2002 to October 2004. The match-ups of 396,254 points show that the bias of the NGSST-O is -0.15 K against the buoy observations and the rms error, 0.85 K.

Detection of cyclone-induced rapid increases in chlorophyll-a with sea surface cooling in the northwestern Pacific Ocean from a MODIS/SeaWiFS merged satellite chlorophyll product

We performed a comprehensive analysis of satellite chlorophyll-a concentration (chl-a) data to detect all events in which chl-a rapidly increased on a time scale of 10 days or less. Our analysis could successfully detect cases in which chl-a increased and sea surface temperature (SST) decreased rapidly after the passages of tropical and extratropical cyclones. The events with large SST decreases tended to occur south of Japan, in the Sea of Okhotsk, and in the regions between 35° and 45° N, especially the Kuroshio–Oyashio Extension (KOE) region. Although the contribution of cyclones on the yearly total chl-a increase was basically small, the cyclone-induced chl-a increases accounted for a few tens of per cent of the total chl-a increase in some areas. In oligotrophic regions, the increases in chl-a tended to become larger as the corresponding SST decrease became larger, although the relationship between them is opposite in mesotrophic and eutrophic regions.

Distortion of near-surface seawater temperature structure by a moored-buoy hull and its effect on skin temperature and heat flux estimates.

Previous studies have suggested that the accuracy of temperature measurements by surface-moored buoys may be affected by distortions of the near-surface temperature structure by the buoy hull on calm, sunny days. We obtained the first definite observational evidence that the temperature near the hull was not horizontally homogeneous at the same nominal depth. We observed large temperature differences of 1.0 K or more between thermometers at 0.2 m depth. The distortion of the surface temperature field yielded an error in estimates of daytime net surface heat flux up to more than 30 Wm−2.

Evidence for SST-Forced Anomalous Winds Revealed from Simultaneous Radiosonde Launches from Three Ships across the Kuroshio Extension Front

AbstractSimultaneous launches of radiosondes were conducted from three research vessels aligned meridionally across a sea surface temperature (SST) front on the flank of the Kuroshio Extension. The soundings carried out every 2 h over 5 days in early July 2012 provided a unique opportunity in capturing unambiguous data on anomalous easterly winds derived from a pronounced meridional SST gradient. The data indicate that a meridional contrast in surface heat fluxes from the underlying ocean enhanced the air temperature anomaly across the SST front, which was observed from the surface up to 300-m altitude. Correspondingly, high and low pressure anomalies that reached 800-m altitude formed on the north and south sides of the SST front, respectively. These temperature and pressure anomalies were maintained even during the passage of synoptic-scale disturbances. Although the free-tropospheric winds are overall westerly, winds below the 1000-m level were easterly due to geostrophic anomalies driven by the northw...