Northward Propagation Research Articles

The role of low-frequency intraseasonal oscillations in the anomalous Indian summer monsoon rainfall of 2002

We analyze the dynamical features and responsible factors of the low-frequency intraseasonal time scales which influenced the nature of onset, intensity and duration of active/break phases and withdrawal of the monsoon during the anomalous Indian summer monsoon of 2002 — the most severe drought recorded in recent times. During that season, persistent warm sea surface temperature anomalies over the equatorial Indian Ocean played a significant role in modulating the strength of the monsoon Hadley circulation. This in turn affected the onset and intense break spells especially the long break during the peak monsoon month of July. Strong low-frequency intraseasonal modulations with significant impact on the onset and active/break phases occurred in 2002 which were manifested as a good association between low-frequency intraseasonal oscillations and the onset and active/break spells. Further, SST anomalies over the equatorial Indo-Pacific region on low-frequency intraseasonal time scales were found to affect the equatorial eastward and thereby off-equatorial northward propagations of enhanced convection over the Indian region. These propagations in turn modulated the active/break cycle deciding the consequent severity of the 2002 drought.

Dependence of Simulation of Boreal Summer Tropical Intraseasonal Oscillations on the Simulation of Seasonal Mean

Abstract The link between realism in simulation of the seasonal mean precipitation and summer tropical intraseasonal oscillations and their dependence on cumulus parameterization schemes is investigated using the Florida State University Global Spectral Model (FSUGSM). Forty-member model ensemble simulations of the northern summer season are generated for three different cumulus parameterization schemes [namely, Arakawa–Schubert (Naval Research Laboratory; NRL), Zhang and McFarlane (National Center for Atmospheric Research; NCAR), and Emanuel (Massachusetts Institute of Technology; MIT)]. The MIT scheme simulates the regional pattern of seasonal mean precipitation over the Indian monsoon region well but has large systematic bias in simulating the precipitation over the western Pacific and the Maritime Continent. Although the simulation of details of regional distribution of precipitation over the Indian monsoon region by the NRL and NCAR schemes is not accurate, they simulate the spatial pattern of precipitation over the tropical Indo–Pacific domain closer to observation. The NRL scheme seems to captures the observed northward and eastward propagation of intraseasonal precipitation anomalies realistically. However, the simulations of the NCAR and MIT schemes are dominated by a westward propagating component. The westward propagating mode seen in the model as well as observations is indicated to be an equatorial Rossby wave modified by the northern summer mean flow. An examination of the relationship between simulation of the model climatology and eastward propagating character of monsoon intraseasonal oscillations (ISOs) in a limited sample shows that the scheme that simulates better seasonal mean pattern of rainfall over the tropical Indo–Pacific domain also simulates better intraseasonal variance and more realistic eastward propagation of monsoon ISOs. Among the parameters known to be important for meridional propagation of the summer monsoon ISOs, the meridional gradient of mean humidity in the lower atmosphere seems to be crucial in determining the northward propagation in the equatorial Indian Ocean (between 10°S and 10°N). For better prediction of the seasonal mean Indian monsoon, therefore, the model climatology should have minimum bias not only over the Indian monsoon region but also over the entire Indo–Pacific basin.

Role of SST over the Indian Ocean in Influencing the Intraseasonal Variability of the Indian Summer Monsoon

Intraseasonal variability (10-60 days) of sea surface temperature (SST) over the north Indian Ocean and its influence on regional precipitation variability over the Indian subcontinent are examined. SST, cloud liquid water and precipitation over the Indian Ocean of the Tropical Rainfall Measuring Mission (TRMM), precipitation of Climate Prediction Center Merged Analysis of Precipitation (CMAP), and low-level atmospheric parameters of National Center for Environmental Prediction (NCEP) II reanalysis are utilized for this study. Western Ghats (WG) in the southwest and the Ganges-Mahanadi Basin (GB) in the northeast of the Indian subcontinent are observed to be the regions of maximum precipitation with large standard deviations of the intraseasonal variability. Active (break) phases of precipitation occur in these regions by the northward propagation of positive (negative) precipitation anomalies over the Arabian Sea and the Bay of Bengal. Latitude-time plots during the active phase of the WG region shows that the positive SST anomalies over the Arabian Sea formed by suppressed surface latent heat flux and increased downward shortwave radiation flux lead the positive precipitation anomalies. Surface air temperature anomalies follow the SST anomalies and then destabilize the lower atmosphere between 1000 hPa and 700 hPa. These results indicate that, in the northward propagating dynamical surface convergence, underlying SST anomalies tend to form a favorable condition for convective activity and may sustain enhanced precipitation over the convergence region. This results in enhanced precipitation anomalies over the WG region that move further northeastward and merge with the northward propagating precipitation anomalies from the Bay of Bengal, enhancing the active phase of the GB region.

Did the 2004 Sumatra–Andaman Earthquake Involve a Component of Tsunami Earthquakes?

Abstract Tsunami earthquakes are characterized by (1) slow slip and (2) tsunamis larger than expected from seismic slip. We examine whether the 2004 Sumatra– Andaman earthquake had these features and thus involved a component of tsunami earthquakes. Fitting the effective moment versus frequency curve, we obtain characteristic times τ (Kanamori, 1972) of 70 and 290 sec. The former is on the scaling for normal earthquakes; the latter is longer. In the area off northern Sumatra, by back- projecting the wavefront recorded on the sea surface height (ssh) by the satellite altimetry, we estimate the seaward edge of the fault plane to be located at the deformation front. We then invert the ssh, assuming that the seaward edge of the fault is at this location, and obtain a maximum slip of 40 m, which was estimated to be 15 m previously. This amount of slip, approximately twice the slip estimated seismologically and geodetically, suggests that additional crustal deformation, such as inelastic uplift of the trench sediments, might have occurred near the deformation front. We propose that a similar, possibly slow, slip occurred in the shallow subduction boundary, extending from all over the rupture zone derived by the body-surface waves, with a smaller northward propagation velocity, corresponding to the longer τ. This rupture mode would solve the enigmatic features of this event seen in the seismological and tsunami data. The 2004 Sumatra–Andaman earthquake, having both features (1) and (2) that characterize tsunami eathquakes, likely had a component of tsunami earthquakes.

Two-stage rifting of Zealandia-Australia-Antarctica: Evidence from 40Ar/39Ar thermochronometry of the Sisters shear zone, Stewart Island, New Zealand

The Sisters shear zone is a newly discovered Late Cretaceous detachment fault system exposed for 40 km along the southeast coast of Stewart Island, southernmost New Zealand. Footwall rocks consist of variably deformed ca. 310 and 105 Ma granites that range from undeformed to protomylonite, mylonite, and ultramylonite. The hanging wall includes non-marine conglomerate and brittley deformed granite. K-feldspar thermochronometry of the footwall indicates moderately rapid cooling (20–30 C°/m.y.) due to tectonic denudation over the interval ca. 89–82 Ma. Return to slow cooling at 82 Ma coincides with the age of the oldest seafloor adjacent to the Campbell Plateau and reflects the mechanical transition from continental extension to lithospheric rupture and formation of the Pacific-Antarctic Ridge. Our findings support a two-stage rift model for continental breakup of this part of the Gondwana margin. Stage one (ca. 101–88 Ma) is the northward propagation of continental extension and the Tasman Ridge as recorded in mylonite dredged from the Ross Sea and the Paparoa core complex. Stage two (ca. 89–82 Ma) is extension between the Campbell Plateau and West Antarctica leading to formation of the Pacific-Antarctic Ridge.

Intraseasonal variability of the Tropical Easterly Jet

By analyzing 12-year (1979–1990) 200 hPa wind data from National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis, we demonstrate that the intraseasonal time scale (30–60 days) variability of the Tropical Easterly Jet (TEJ) reported in individual case studies occurs during most years. In the entrance region (east of ∼70° E), axis of the TEJ at 200 hPa is found along the near equatorial latitudes during monsoon onset/monsoon revivals and propagates northward as the monsoon advances over India. This axis is found along ∼5° N and ∼15° N during active monsoon and break monsoon conditions respectively. Examination of the European Centre for Medium Range Weather Forecasts reanalysis wind data also confirms the northward propagation of the TEJ on intraseasonal time scales.

Constraining the timing and migration of collisional tectonics in the Damara Belt, Namibia: U-Pb zircon ages for the syntectonic Salem-type Stinkbank granite

A U-Pb zircon LAM-ICP-MS age of 549 ± 11 Ma from the Stinkbank granite in the south Central Zone of the Damara belt represents the first robust age for the crystallization of a member of the regionally widespread suite of syntectonic Salem-type granites. This age confirms previous assertions that suggest that the main phase of crustal shortening related to the collision between the Congo and Kalahari Cratrons occurred between 550 and ~540 Ma in the south Central Zone, the leading edge of the Congo Craton. Ages from syntectonic granites reported in the literature from the north Central and the Northern Zone record progressively younger ages of ~530 to ~515 Ma and late-stage folding in the Northern Zone is suggested to have occurred between ~510 to ~500 Ma. The younging of ages that date the main collisional phase towards the north is interpreted to track the northward propagation of the deformation and metamorphic front from the main suture zone to the northern foreland of the Damara belt. Geochronological data are sparse, and the lack of detailed structural data precludes the retrodeformation of the fold-and-thrust belt along this 250 km long section. However, the age brackets suggest a migration of the orogenic front at a rate of at least 5 to 7 mm/a, but more likely in the order of several centimetre per year, if the internal strain is removed.

The summer monsoon onset over the tropical eastern Indian Ocean: The earliest onset process of the Asian summer monsoon

The onset process of the tropical eastern Indian Ocean (TEIO) summer monsoon (TEIOSM) and its relationship with the cross-equatorial flows are investigated via climatological analysis. Climatologically, results indicate that the earliest onset process of the Asian summer monsoon occurs over the TEIO at pentad 22 (April 15–20). Unlike the abrupt onset of the South China Sea (SCS) summer monsoon, the TEIOSM onset process displays a stepwise advance. Moreover, a close relationship between the TEIOSM development and the northward push of the cross-equatorial flows over 80°–90°E is revealed. A difference vorticity center, together with the counterpart over the southern Indian Ocean, constitutes a pair of difference cyclonic vortices, which strengthens the southwesterly wind over the TEIO and the northerly wind to the west of the Indian Peninsula from the end of March to late May. Therefore, the occurrence of the southwesterly wind over the TEIO is earlier than its counterpart over the tropical western Indian Ocean, and the cross-equatorial flows emerge firstly over the TEIO rather than over the Somali area. The former increases in intensity during its northward propagation, which provides a precondition for the TEIOSM onset and its northward advance.

Cenozoic tectonic and geodynamic evolution of the Kyrgyz Tien Shan Mountains: A review of geological, thermochronological and geophysical data

The complex crustal structure of the Tien Shan has a strong impact on the distribution of strain induced by the India–Eurasia collision, with intracontinental deformation in Eurasia’s interior as a distant effect. The northward propagation of the India–Eurasia deformation front is suggested by the rejuvenation of mountain ranges and intermittent intramontane basins. The Tien Shan basement is formed by the rigid, heterogeneous Precambrian blocks (microcontinents) of Tarim, Issyk-Kul (or Central Tien Shan) and Aktyuz-Boordin, surrounded by a ‘soft’ matrix of Paleozoic accretion–collision belts. The Kyrgyz Tien Shan Mountains are situated between the active structures of the Tarim Plate and the Pamir indenter (south), and the stable Kazakhstan Shield (north). Underplating by the Tarim Plate and thrusting by the Pamirs are responsible for the building of the Cenozoic Tien Shan, the reactivation of its inherited structural fabric and the tectonic layering of the upper lithosphere underlying the area. Large earthquakes ( M > 6) delineate the northern and southern margins of the Issyk-Kul microcontinent, indicating that crustal heterogeneity influenced the location of active structures in the northern Kyrgyz Tien Shan.

Comments on “Structures and Mechanisms of the Northward Propagating Boreal Summer Intraseasonal Oscillation”

In a recent article, Jiang et al. (2004) analyzed the northward propagations of the tropical convergence zone during the boreal summer monsoon and proposed a simple theory to explain the scale selection of the propagations. Here, we argue that this mechanism of scale selection relies on the choice of two parameters whose values are not justified in their paper. Furthermore, it appears that the effects of the variation of the Coriolis parameter with latitude Can nort be neglected

Characteristics of inertia gravity waves over the South Pacific as revealed by radiosonde observations

Using horizontal wind and temperature data with fine vertical resolution obtained by radiosondes, we investigate the characteristics of parameters and propagation properties of inertia gravity waves in the upper troposphere and lower stratosphere in the subtropics and extratropics over the South Pacific where very few observations have so far been done. Components extracted by a band‐pass filter having cutoff vertical wavelengths of 0.5 and 5 km are analyzed as gravity waves. Gravity wave parameters such as intrinsic frequency , vertical and horizontal wavelengths, and phase and group velocities are estimated by hodograph analysis. The statistics on such parameters are presented for both the stratosphere and troposphere for the three latitudinal ranges 17°–24°S, 25°–33°S, and 34°–48°S. The horizontal wavelengths decrease and increases with increasing latitude south. The dominant values for the Coriolis parameter f divided by are ∼0.5 in the lower stratosphere and ∼0.3 in the troposphere, with little dependence on latitude. The dominant vertical wavelengths for upward propagating waves are longer in the troposphere than in the lower stratosphere. The most dominant direction of horizontal propagation is southward, with a little bias toward southeastward. A case study using ray tracing suggests that the major source of such southeastward waves is located in the upper troposphere. Notable northward propagation of gravity waves is also found in the subtropical lower stratosphere. The source of the gravity waves is considered to be a cutoff cyclone developing in the upper troposphere.

Boreal summer quasi-monthly oscillation in the global tropics

The boreal summer intraseasonal oscillation (ISO) in the global tropics is documented here using a 7-year suite (1998–2004) of satellite measurements. A composite scenario was made of 28 selected events with reference to the oscillation in the eastern equatorial Indian Ocean (EIO), where the oscillation is most regular and its intensity is indicative of the strength of the subsequent northward propagation. The average oscillation period is about 32 days, and this quasi-monthly oscillation (QMO) is primarily confined to the tropical Indian and Pacific Oceans. Topics that were investigated are the partition of convective versus stratiform clouds, the vertical structure of precipitation rates, and the evolution of cloud types during the initial organization and the development of intraseasonal convective anomalies in the central Indian Ocean. During the initiation of the convective anomalies, the stratiform and convective rains have comparable rates; the prevailing cloud type experiences a trimodal evolution from shallow to deep convection, and finally to anvil and extended stratiform clouds. A major northwest/southeast-slanted rainband forms as the equatorial rainfall anomalies reach Sumatra, and the rainband subsequently propagates northeastward into the west Pacific Ocean. The enhanced precipitation in the west Pacific then rapidly traverses the Pacific along the Intertropical Convergence Zone, meanwhile migrating northward to the Philippine Sea. A seesaw teleconnection in rainfall anomalies is found between the southern Bay of Bengal (5–15°N, 80–100°E) and the eastern Pacific (5–15°N, 85–105°W). Local sea-surface temperature (SST)-rainfall anomalies display a negative simultaneous correlation in the off-equatorial regions but a zero correlation (quadrature phase relationship) near the equator. We propose that atmosphere–ocean interaction and the vertical monsoon easterly shear are important contributors to the northeastward propagation component of the intraseasonal rainband. The observed evidence presented here provides critical information for validating the numerical models, and it supports the self-induction mechanism theory for maintenance of the boreal summer ISO.

Seismotectonic, rupture process, and earthquake-hazard aspects of the 2003 December 26 Bam, Iran, earthquake

The catastrophic 2003 Mw 6.6 Bam earthquake in southern Iran attracted much attention, and has been studied with an abundance of observations from synthetic aperture radar, teleseismic seismology, aftershock studies, strong ground motion, geomorphology, remote sensing and surface field work. Many reports have focused on the details of one or other data type, producing interpretations that either conflict with other data or leave questions unanswered. This paper is an attempt to look at all the available data types together, to produce a coherent picture of the coseismic faulting in 2003 and to examine its consequences for active tectonics and continuing seismic hazard in the region. We conclude that more than 80 per cent of the moment release in the main shock occurred on a near-vertical right-lateral strike-slip fault extending from the city of Bam southwards for about 15 km, with slip of up to 2 m but mostly restricted to the depth range 2–7 km. Analysis of the strong ground motion record at Bam is consistent with this view, and indicates that the extreme damage in the city can be attributed, at least in part, to the enhancement of ground motion in Bam because of its position at the end of the northward-propagating rupture. Little of the slip in the main shock reached the Earth's surface and, more importantly, aftershocks reveal that ∼12 km vertical extent of a deeper part of the fault system remained unruptured beneath the coseismic rupture plane, at depths of 8–20 km. This may represent a substantial remaining seismic hazard to the reconstructed city of Bam. We believe that some oblique-reverse slip (up to 2 m, and less than 20 per cent of the released seismic moment) occurred at a restricted depth of 5–7 km on a blind west-dipping fault that projects to the surface at the Bam-Baravat escarpment, an asymmetric anticline ridge that is the most prominent geomorphological feature in the area. This fault did not rupture significantly at shallow levels in 2003, and it may also represent a continuing seismic hazard. Widespread distributed surface ruptures north of the city are apparently unrelated to substantial slip at depth, and may be the result of enhanced ground motion related to northward propagation of the rupture. The faulting at Bam may be in the early stages of a spatial separation (‘partitioning’) between the reverse and strike-slip components of an oblique convergence across the zone. Such a separation is common on the continents, though in this case the slip vectors between the two faults differ only by ∼20° as a substantial strike-slip component remains on the oblique-reverse fault. The Bam earthquake is one in a series of large earthquakes involving faulting along the western edge of the Lut desert. In addition to the unruptured parts of the faults near Bam itself, continuing and substantial hazard is represented by unruptured neighbouring faults, particularly blind thrusts along the Jebel Barez mountains to the south and strike-slip faulting at Sarvestan to the west.

Forcing mechanisms controlling surface and subsurface temperature anomalies along line‐p, North‐East Pacific Ocean

The influence of different mechanisms on surface and subsurface temperature anomalies is considered along Line‐P, an oceanographic line extending from Vancouver Island into the Gulf of Alaska, which has been sampled for almost half a century. The role of a given mechanism is determined by applying canonical correlation analysis (CCA) between the anomalies of a parameter representing the mechanism and the Line‐P temperature anomalies. For each mechanism, it is determined if its direct influence can be detected, and if so, the domain of Line‐P over which it acts. Two areas along Line‐P, characterized by different forcing mechanisms, are identified: (1) offshore, west of 130°W, the main mechanisms influencing Line‐P temperature anomalies are the Ekman transport due to wind stress anomalies (with the zonal wind stress component somewhat more important than the meridional component) and wind mixing anomalies, (2) from the coast to 180 km offshore, coastal upwelling/downwelling anomalies and sea‐surface height anomalies along the coast of North America, resulting in coastal current anomalies and/or northward propagation of coastal waves, are important in determining Line‐P temperature anomalies.

Multiple Regimes and Low-Frequency Oscillations in the Northern Hemisphere’s Zonal-Mean Flow

Abstract This paper studies multiple regimes and low-frequency oscillations in the Northern Hemisphere zonal-mean zonal flow in winter, using 55 yr of daily observational data. The probability density function estimated in the phase space spanned by the two leading empirical orthogonal functions exhibits two distinct, statistically significant maxima. The two regimes associated with these maxima describe persistent zonal-flow states that are characterized by meridional displacements of the midlatitude jet, poleward and equatorward of its time-mean position. The geopotential height anomalies of either regime have a pronounced zonally symmetric component, but largest-amplitude anomalies are located over the Atlantic and Pacific Oceans. High-frequency synoptic transients participate in the maintenance of and transitions between these regimes. Significant oscillatory components with periods of 147 and 72 days are identified by spectral analysis of the zonal-flow time series. These oscillations are described by singular spectrum analysis and the multitaper method. The 147-day oscillation involves zonal-flow anomalies that propagate poleward, while the 72-day oscillation only manifests northward propagation in the Atlantic sector. Both modes mainly describe changes in the midlatitude jet position and intensity. In the horizontal plane though, the two modes exhibit synchronous centers of action located over the Atlantic and Pacific Oceans. The two persistent flow regimes are associated with slow phases of either oscillation.

Seasonal variation in total organic carbon in the northeast Atlantic in 2000–2001

Total organic carbon (TOC) was measured in the midlatitudes of the northeast Atlantic in 2000–2001 during the Programme Océan Multidisciplinaire Méso Echelle (POMME). In spring, mixed layer depth (MLD) was positively correlated with the north latitude, while the surface TOC and chlorophyll‐a (Chl a) were negatively correlated with it. It indicates the northward propagation of TOC accumulation along the onset of spring bloom, which was triggered by shoaling of MLD from the south. Surface TOC was highest in summer, though surface Chl a was lowest. Surface TOC was significantly correlated with surface Chl a during winter and spring but not during summer or fall, indicating the minor influence of total phytoplankton biomass on surface TOC distribution in the oligotrophic season. High surface TOC in summer and fall was related with shallow MLD, which could hold fresh TOC near the surface. Downward TOC flux from the surface to below the seasonal thermocline was estimated for the processes of winter convection (Fc), turbulent diffusion (Fd) and sinking flux (Fp). The sum of these fluxes accounts for 6–75% of annual new production. To our knowledge, this is the first estimation based on the simultaneous measurement of Fc, Fd, Fp, and annual new production.

Bottom‐simulating reflections and geothermal gradients across the western Svalbard margin

AbstractSeismic reflection data reveal prominent bottom‐simulating reflections (BSRs) within the relatively young (<0.78 Ma) sediments along the West Svalbard continental margin. The potential hydrate occurrence zone covers an area of c. 1600 km2. The hydrate accumulation zone is bound by structural/tectonic features (Knipovich Ridge, Molloy Transform Fault, Vestnesa Ridge) and the presence of glacigenic debris lobes inhibiting hydrate formation upslope. The thickness of the gas‐zone underneath the BSR varies laterally, and reaches a maximum of c. 150 ms. Using the BSR as an in‐situ temperature proxy, geothermal gradients increase gradually from 70 to 115 °C km−1 towards the Molloy Transform Fault. Anomalies only occur in the immediate vicinity of normal faults, where the BSR shoals, indicating near‐vertical heat/fluid flow within the fault zones. Amplitude analyses suggest that sub‐horizontal fluid migration also takes place along the stratigraphy. As the faults are related to the northwards propagation of the Knipovich Ridge, long‐term disturbance of hydrate stability appears related to incipient rifting processes.

The 2003 December 26 Bam earthquake (Iran),Mw6.6, aftershock sequence

SUMMARY From 2003 December 29 to 2004 January 30, a dense seismological network of 23 stations was installed in the epicentral area of the 2003 December 26 Bam earthquake to study the aftershock seismicity. We select the 331 earthquakes recorded at a minimum of 10 stations, with rms less than 0.1 s and uncertainties less than 1 km, to infer the precise geometry of the seismicity in the fault region. We also process the data with the Double Difference technique to confirm the results. The aftershock cluster is 25 km long, trends north–south, and is located 5 km west of the Bam-Baravat escarpment, exactly beneath the observed surface breaks. At depth, aftershocks are concentrated between 6 and 20 km, beneath the upper layer of relatively low velocity that experienced the maximum slip, and they dip slightly westward. The southernmost part of the aftershock cluster is narrow and defines the rupture zone that is likely the Bam-Baravat fault at depth. However, it is unlikely that it is connected at surface to the Bam-Baravat escarpment but more likely to the co-seismic ruptures south of Bam. On the contrary, the distribution of the northernmost aftershocks spread into a more complex pattern, which is consistent with a northward propagation of the rupture along the fault plane. The focal mechanisms are consistent with right-lateral strike-slip faulting on N–S trending faults, parallel to the Bam-Baravat escarpment.

Production regimes in the northeast Atlantic: A study based on Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) chlorophyll and ocean general circulation model mixed layer depth

A 5 year time series of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color images (SCHL) is compared with mixed layer depths (MLD) and atmospheric forcings from the Clipper model of the North Atlantic (1998–2002). This comparison is done over the region 16°–22°W, 30°–50°N, where subpolar mode waters are formed and which overlaps the region of the 2001 Programme Océan Multidisciplinaire Méso Echelle (POMME) experiment at sea. Three production regimes are identified on the basis of the seasonal cycling of SCHL and MLD: the well‐known subpolar and subtropical regimes and a midlatitude regime. The midlatitude regime is characterized by a single broad bloom weaker than the subpolar spring bloom and stronger than the subtropical fall bloom, which starts in fall as an entrainment bloom and peaks in spring as a restratification bloom. This specific regime is found between 35°N and 40°N (±2°) in the northeast Atlantic. It corresponds to winter MLDs between Ze (the depth of the euphotic layer) and 2Ze, i.e., it lays between the region where the winter MLD is greater than Sverdrup's critical depth (subpolar regime) and the region where the mixing is never deeper than the well‐lit layer (subtropical regime). The very specific characteristics of the midlatitude regime strengthen the biological carbon pump since production is active in winter within the waters to be subducted. The midlatitude regime also may provide an explanation for the unexpectedly low f ratios sometimes observed during the bloom in the region (North Atlantic Bloom Experiment, POMME). A large interannual variability is observed for the three regimes in terms of the timing and the intensity of the blooms and of the geographical boundaries of the regimes. These variabilities appear to be mainly driven by the synoptic and the low‐frequency atmospheric variabilities. It is also shown that in addition to the northward propagation of the subpolar spring bloom from 41°N (±1.3°) to 50°N, the (fall) entrainment bloom propagates southward over the whole latitudinal range (35°–50°N).

Forecast skill of the tropical intraseasonal oscillation in the NCEP GFS dynamical extended range forecasts

This study examines the forecast performance of tropical intraseasonal oscillation (ISO) in recent dynamical extended range forecast (DERF) experiments conducted with the National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) model. The present study extends earlier work by comparing prediction skill of the northern winter ISO (Madden-Julian Oscillation) between the current and earlier experiments. Prediction skill for the northern summer ISO is also investigated. Since the boreal summer ISO exhibits northward propagation as well as eastward propagation along the equator, forecast skill for both components is computed. For the 5-year period from 1 January, 1998 through 31 December, 2002, 30-day forecasts were made once a day. Compared to the previous DERF experiment, the current model has shown some improvements in forecasting the ISO during winter season so that the skillful forecasts (anomaly correlation>0.6) for upper-level zonal wind anomaly extend from the previous shorter-than 5 days out to 7 days lead-time. A similar level of skill is seen for both northward and eastward propagation components during the summer season as in the winter case. Results also show that forecasts from extreme initial states are more skillful than those from null phases for both seasons, extending the skillful range by 3–6 days. For strong ISO convection phases, the GFS model performs better during the summer season than during the winter season. In summer forecasts, large-scale circulation and convection anomalies exhibit northward propagation during the peak phase. In contrast, the GFS model still has difficulties in sustaining ISO variability during the northern winter as in the previous DERF run. That is, the forecast does not maintain the observed eastward propagating signals associated with large-scale circulation; rather the forecast anomalies appear to be stationary at their initial location and decay with time. The NCEP Coupled Forecast System produces daily operational forecasts and its predication skill of the MJO will be reported in the future.