Showing papers on "Monsoon published in 1997"

The North American Monsoon

Abstract: The North American monsoon is an important feature of the atmospheric circulation over the continent, with a research literature that dates back almost 100 years. The authors review the wide range of past and current research dealing with the meteorological and climatological aspects of the North American monsoon, highlighting historical development and major research themes. The domain of the North American monsoon is large, extending over much of the western United States from its region of greatest influence in northwestern Mexico. Regarding the debate over moisture source regions and water vapor advection into southwestern North America, there is general agreement that the bulk of monsoon moisture is advected at low levels from the eastern tropical Pacific Ocean and the Gulf of California, while the Gulf of Mexico may contribute some upper-level moisture (although mixing occurs over the Sierra Madre Occidental). Surges of low-level moisture from the Gulf of California are a significant part of intrase...

Influence of the North American Monsoon System on the U.S. Summer Precipitation Regime

Abstract: Key features of the U.S. summer precipitation regime are examined within the context of the evolving North American monsoon system. The focus is on the antecedent and subsequent atmospheric conditions over the conterminous United States relative to the onset of monsoon precipitation over the southwestern United States, which typically begins in early July. The onset of the monsoon in this region is determined using a precipitation index, based on daily observed precipitation for a 31-yr (1963–94) period. Lagged composites of the observed precipitation and various fields from the NCEP–NCAR reanalysis for the period 1979–94 provide a comprehensive picture of atmospheric conditions during the evolution of the U.S. warm season precipitation regime. The summer precipitation regime is characterized by an out-of-phase relationship between precipitation over the Southwest and the Great Plains–northern tier and an in-phase relationship between precipitation over the Southwest and the East Coast. Changes i...

Response of the African Monsoon to Orbital Forcing and Ocean Feedbacks in the Middle Holocene

Abstract: Simulations with a climate model that asynchronously couples the atmosphere and the ocean showed that the increased amplitude of the seasonal cycle of insolation in the Northern Hemisphere 6000 years ago could have increased tropical Atlantic sea surface temperatures in late summer. The simulated increase in sea surface temperature and associated changes in atmospheric circulation enhanced the summer monsoon precipitation of northern Africa by more than 25 percent, compared with the middle Holocene simulation with prescribed modern sea surface temperatures, and provided better agreement with paleorecords of enhanced monsoons.

A Model for the Boreal Summer Intraseasonal Oscillation

Abstract: The tropical intraseasonal oscillation (ISO) exhibits pronounced seasonality. The boreal summer ISO is more complex than its winter counterpart due to the coexistence of equatorial eastward, off-equatorial westward, and northward propagating, low-frequency modes and their interactions. Based on observational evidence and results obtained from numerical experiments, a mechanism is proposed for the boreal summer ISO in which the Northern Hemisphere summer monsoon (NHSM) circulation and moist static energy distribution play essential roles. With a climatological July mean basic state, the life cycle of model low-frequency waves consists of four processes: an equatorial eastward propagation of a coupled Kelvin–Rossby wave packet, an emanation of moist Rossby waves in the western Pacific, a westward propagation and amplification of the Rossby waves in South Asian monsoon regions, and a reinitiation of the equatorial disturbances over the central Indian Ocean. The life cycle spans about one month and p...

Climatology and Interannual Variability of the Southeast Asian Summer Monsoon

Abstract: In this paper, results from a pilot study for the South China Sea Monsoon Experiment are reported. Based on analyses of 9 years of pentad and monthly mean data, the climatology of subseasonal features and interannual variability of the Southeast Asian monsoon (SEAM) are documented. The present analysis is focused on the sudden onset of the South China Sea monsoon and its relation to the atmospheric and oceanic processes on the entire Asian monsoon region. It is found that the onset of the SEAM occurs around mid-May, signaling the earliest stage of the entire Asian summer monsoon system. The establishment of monsoon rainfall over the South China Sea is abrupt, being accompanied by substantial changes in the large scale atmospheric circulation and sea surface temperature in the adjacent oceans. The onset and fluctuations of SEAM involve the interaction and metamorphosis of the large scale convection over the Indo-China, the South China Sea and the southern Bay of Bengal. Results show that the onset time of the SEAM differs greatly from one year to another. The delayed (advanced) onset of the monsoon may be related to basin-wide warm (cold) events of the Pacific and Indian Oceans. We also present evidence showing that the SEAM fluctuations in May may foreshadow the development of the full-scale Asian summer monsoon during the subsequent months.

Climates of South Asia

Abstract: METEOROLOGICAL BACKGROUND. South Asia: An Introduction to the Region. Tropical General Circulation: South Asian Context. Characteristic Features of South Asian Summer Monsoon. Other Meteorological Features and Systems of South Asia. CLIMATIC CHARACTERISTICS. Mean Climatology of South Asia. Rainfall Variability. Temperature Variability. Regional Climatic Features. The Changing Climate of South Asia: Its Environmental Impacts. Appendices. Bibliography. Index.

The South Asian Monsoon and the Tropospheric Biennial Oscillation

Abstract: A mechanism is described that involves the south Asian monsoon as an active part of the tropospheric biennial oscillation (TBO) described in previous studies. This mechanism depends on coupled land‐atmosphere‐ocean interactions in the Indian sector, large-scale atmospheric east‐west circulations in the Tropics, convective heating anomalies over Africa and the Pacific, and tropical‐midlatitude interactions in the Northern Hemisphere. A key element for the monsoon role in the TBO is land‐sea or meridional tropospheric temperature contrast, with areaaveraged surface temperature anomalies over south Asia that are able to persist on a 1-yr timescale without the heat storage characteristics that contribute to this memory mechanism in the ocean. Results from a global coupled general circulation model show that soil moisture anomalies contribute to land-surface temperature anomalies (through latent heat flux anomalies) for only one season after the summer monsoon. A global atmospheric GCM in perpetual January mode is run with observed SSTs with specified convective heating anomalies to demonstrate that convective heating anomalies elsewhere in the Tropics associated with the coupled ocean‐atmosphere biennial mechanism can contribute to altering seasonal midlatitude circulation. These changes in the midlatitude longwave pattern, forced by a combination of tropical convective heating anomalies over East Africa, Southeast Asia, and the western Pacific (in association with SST anomalies), are then able to maintain temperature anomalies over south Asia via advection through winter and spring to set up the land‐sea meridional tropospheric temperature contrast for the subsequent monsoon. The role of the Indian Ocean, then, is to provide a moisture source and a low-amplitude coupled response component for meridional temperature contrast to help drive the south Asian monsoon. The role of the Pacific is to produce shifts in regionally coupled convection‐SST anomalies. These regions are tied together and mutually interact via the large-scale east‐west circulation in the atmosphere and contribute to altering midlatitude circulations as well. The coupled model results, and experiments with an atmospheric GCM that includes specified convective heating anomalies, suggest that the influence of south Asian snow cover in the monsoon is not a driving force by itself, but is symptomatic of the larger-scale shift in the midlatitude longwave pattern associated with tropical SST and convective heating anomalies.

On the Origin of the Bolivian High and Related Circulation Features of the South American Climate

Abstract: The climatological structure in the upper-tropospheric summertime circulation over South America is diagnosed using a GCM (with and without South American topography), a linear model, and observational data. Emphasis is placed on understanding the origin of observed features such as the Bolivian high and the accompanying “Nordeste low” to the east. Results from the linear model indicate that these two features are generated in response to precipitation over the Amazon basin, central Andes, and South Atlantic convergence zone, with African precipitation also playing a crucial role in the formation of the Nordeste low. The direct mechanical and sensible heating effects of the Andes are minimal, acting only to induce a weak lee trough in midlatitudes and a shallow monsoonal circulation over the central Andes. In the GCM, the effects of the Andes include a strengthening of the Bolivian high and northward shift of the Nordeste low, primarily through changes in the precipitation field. The position of ...

On the Origin of the Bolivian High and Related Circulation Features of the South

Abstract: The climatological structure in the upper-tropospheric summertime circulation over South America is diagnosed using a GCM (with and without South American topography), a linear model, and observational data. Emphasis is placed on understanding the origin of observed features such as the Bolivian high and the accompanying ‘‘Nordeste low’’ to the east. Results from the linear model indicate that these two features are generated in response to precipitation over the Amazon basin, central Andes, and South Atlantic convergence zone, with African precipitation also playing a crucial role in the formation of the Nordeste low. The direct mechanical and sensible heating effects of the Andes are minimal, acting only to induce a weak lee trough in midlatitudes and a shallow monsoonal circulation over the central Andes. In the GCM, the effects of the Andes include a strengthening of the Bolivian high and northward shift of the Nordeste low, primarily through changes in the precipitation field. The position of the Bolivian high is primarily determined by Amazonian precipitation and is little affected by the removal of the Andes. Strong subsidence to the west of the high is found to be important for the maintenance of the high’s warm core, while large-scale convective overshooting to the east is responsible for a layer of cold air above the high.

Interaction Between the ENSO and the Asian Monsoon in a Coral Record of Tropical Climate

Abstract: The oxygen isotopic composition of a banded coral from the western equatorial Indian Ocean provides a 150-year-long history of the relation between the El Nino–Southern Oscillation (ENSO) phenomenon and the Asian monsoon. Interannual cycles in the coral time series were found to correlate with Pacific coral and instrumental climate records, suggesting a consistent linkage across ocean basins, despite the changing frequency and amplitude of the ENSO. However, decadal variability that is characteristic of the monsoon system also dominates the coral record, which implies important interactions between tropical and midlatitude climate variability. One prominent manifestation of this interaction is the strong amplitude modulation of the quasi-biennial cycle.

On the dynamics of the Lakshadweep high and low in the southeastern Arabian Sea

Abstract: Hydrography and altimetry show that a ”high” in surface topography forms off southwest India in January (midnortheast monsoon) and that a ”low” forms during the southwest monsoon; the high and low, called Lakshadweep high and low after the island chain in the vicinity of where they form, propagate westward, extending across the southern Arabian Sea a few months after genesis. We investigate the dynamics of the high and low with an analytic model and with numerical simulations using a dynamical reduced-gravity model for the north Indian Ocean. We conclude that the high and low do not owe their existence to nonlinearity. They are a consequence of westward propagating Rossby waves radiated by Kelvin waves propagating poleward along the western margin of the Indian subcontinent. Most important for the annual cycle of the high and low are the annual and semiannual Kelvin waves off southwest India; these appear to be forced primarily by alongshore winds in the Bay of Bengal and by winds in the equatorial Indian Ocean. Other Kelvin waves, provided their period is greater than about 40 days, can contribute to the high and low; these Kelvin waves are expected to introduce intra-annual and interannual variability in the annual cycle. Below the critical period of 40 days, energy is trapped at the coast. The analysis provides a mechanism for the early onset of upwelling off southwest India, which is known to begin in February, well before the onset of the upwelling-favorable southwest monsoon winds.

Rainfall variability over south-east Asia : Connections with Indian monsoon and enso extremes : New perspectives

Abstract: Seasonal and annual rainfall data for 135 stations for periods varying from 25 to 125 years are utilized to investigate and understand the interannual and short-term (decadal) climate variability over the South-east Asian domain Contemporaneous relations during the summer monsoon period (June to September) reveal that the rainfall variations over central India, north China, northern puts of Thailand, central parts of Brunet and Borneo and the Indonesian region east of 120 E vary in phase However, the rainfall variations over the regions surrounding the South China Sea, in particular the north-west Philippines, vary in the opposite phase. Possible dynamic causes for the spatial correlation structure obtained are discussed Based on the instrumental data available and on an objective criteria, regional rainfall anomaly time series for contiguous regions over Thailand, Malaysia, Singapore, Brunet, Indonesia and Philippines are prepared. Results reveal that although there are year-to-year random fluctuations, there are certain epochs of the above- and below-normal rainfall over each region These epochs are not forced by the El NiA±o La Nina frequencies Near the equatorial regions the epochs tend to last for about a decade, whereas over the tropical regions, away from the Equator, epochs last for about three decades There is no systematic climate change or trend in any of the series. Further, the impact of El NiA±o (La Nina) on the rainfall regimes is more severe during the below (above) normal epochs than during the above (below) normal epochs. Extreme drought flood situations tend to occur when the epochal behaviour and the El Nino La Nina events are phase-locked

Northern Hemisphere Summer Monsoon Singularities and Climatological Intraseasonal Oscillation

Abstract: Using climatological pentad mean outgoing longwave radiation (OLR) and European Centre for Medium-Range Weather Forecasts analysis winds, the authors show that the Northern Hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations (CISOs). The extreme phases of CISO characterize monsoon singularities—monsoon events that occur on a fixed pentad with usual regularity, whereas the transitional phases of CISO represent the largest year-to-year monsoon variations. The CISO results from a phase-locking of transient intraseasonal oscillation to annual cycle. It exhibits a dynamically coherent structure between enhanced convection and low-level convergent (upper-level divergent) cyclonic (anticyclonic) circulation. Its phase propagates primarily northward from the equator to the northern Philippines during early summer (May–July), and westward along 15°N from 170°E to the Bay of Bengal during August and September. The propagation of CISO links monsoon singular...

Rainfall Anomalies in the Source Region of the Nile and Their Connection with the Indian Summer Monsoon

Abstract: In light of the droughts and subsequent food crises that have plagued the Ethiopia–Sudan region in the course of its history, and especially during the last 3 decades, the author examines both the interannual and intraseasonal variabilities of the July–September rains and compares them to the Indian summer monsoon. Regional rainfall indexes for the region stretching from Eritrea to Lake Victoria are computed using seasonal totals for the period 1901–88. Daily data for 1982–88 are also considered. Though all these regions are only partly affected by the Indian monsoon cross-equatorial flow and although they are separated from India by an extensive dry belt (Red Sea, Somalia, west Arabian Sea), there exists a close association between summer rainfall variations in India and in the western parts of East Africa. An even stronger relationship is revealed between these latter regions and Bombay surface pressure, with as much as 79% of the variance in common during 1953–88. This relationship has been vi...

Seasonal transition of summer rainy season over indochina and adjacent monsoon region

Abstract: The mean onset and withdrawal of summer rainy season over the Indochina Peninsula were investigated using 5-day averaged rainfall data (1975-87). The mean seasonal transition process during onset and retreat phases in Indochina, India and the South China Sea is also examined using 5-day mean OLR (1975-87) and 850 hPa wind (1980-88) data. It was found that the onset of summer rainy season begins earlier in the inland region of Indochina (Thailand) in late April to early May than in the coastal region along the Bay of Bengal. This early onset of rainy season is due to pre-monsoon rain under the mid-latitude westerly wind regime. The full summer monsoon circulation begins to establish in mid-May, causing active convective activity both over the west coast of Indochina and the central South China Sea. In case of withdrawal, the earliest retreat of summer rainy season is found in the central northern part of Indochina in late September. The wind field, on the other hand, already changes to easterlies in the northern South China Sea in early September. This easterly wind system covers the eastern part of Indochina where post-monsoon rain is still active. In late October, the wind field turns to winter time situation, but post monsoon rain still continues in the southern part of the Indochina Peninsula until late November.

Sensitivity of the asian summer monsoon to aspects of sea-surface-temperature anomalies in the tropical pacific ocean

Abstract: SUMMARY The response of both the time of onset, and the strength of the Asian summer monsoon, to regional aspects of the sea surface temperature (SST) anomalies in the tropical Pacific Ocean associated with El Nifio/Southern Oscillation (ENSO) has been investigated through a series of sensitivity experiments with the Universities’ Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). This paper builds on the work of Ju and Slingo (1995) and on their hypothesis that the relationship between the Asian summer monsoon and ENSO involved the latitudinal position and strength of the tropical convective maximum (TCM) over Indonesia and the west Pacific in the preceding spring. The inference from their results was that the modulation of the TCM might be associated either with changes in the Walker circulation through the influence of the east Pacific SST anomalies, or with changes in the local Hadley circulation associated with the in situ SST anomalies in the west Pacific. The investigation has focused on the particular contrasting years of 1983 and 1984. The experiments described in this paper are designed to isolate the effects of the principal SST anomalies in the east and central Pacific, associated with El Niiio/La Nifia, from those of opposite sign which develop in the west Pacific as a complementary pattern during the mature phase of El NifioLa NiBa. The results of the experimentation suggest that, at least for the test cases of 1983 and 1984, the modulation of the Walker circulation, with implied additional subsidence over the eastern hemisphere, is the dominant mechanism whereby the Asian summer monsoon is weakened during El Nifio years. However, the late onset during El Nifio years may also be associated with the complementary cold SST anomalies in the west Pacific which delay the northwards transition of the TCM. During La Nifia, the modulation of the Walker circulation appears not to be the controlling factor which determines the stronger monsoons. The UGCM results suggest that the complementary warm SST anomalies in the west Pacific enhance the TCM, and it is this in situ response by the TCM that leads to an early onset and stronger monsoon. The importance of warm anomalies in the west Pacific in the development of a strong monsoon has been investigated further through a case-study of the 1994 season. The year 1994 was an El Nifio year in which the monsoon was unexpectedly active, but which was also marked by wanner than normal SSTs in the west Pacific. The sensitivity experiments have also elucidated the role of El NiBo in influencing the precursory signature of stronger subtropical westerlies over India and south-east Asia during the winter and spring preceding weak monsoons. The results suggested that the equatorwards shift of the subtropical jet is a remote response to the warm SST anomalies in the central and east Pacific associated with El Nifio.

Sea Surface Temperature and Large-Scale Circulation Influences on Tropical Greenhouse Effect and Cloud Radiative Forcing

Abstract: Two independent sets of meteorological reanalyses are used to investigate relationships between the tropical sea surface temperature (SST) and the large-scale vertical motion of the atmosphere for spatial and seasonal variations, as well as for El Nino/La Nina episodes of 1987–88. Supergreenhouse effect (SGE) situations are found to be linked to the occurrence of enhanced large-scale rising motion associated with increasing SST. In regions where the large-scale atmospheric motion is largely decoupled from the local SST due to internal or remote forcings, the SGE occurrence is weak. On seasonal and interannual timescales, such regions are found mainly over equatorial regions of the Indian Ocean and western Pacific, especially for SSTs exceeding 29.5°C. In these regions, the activation of feedback processes that regulate the ocean temperature is thus likely to be more related to the large-scale remote processes, such as those that govern the monsoon circulations and the low-frequency variability of...

Sea surface temperature and productivity records for the past 240 kyr in the Arabian Sea

Abstract: Deep-sea sediments of two cores from the western (TY93-929/P) and the southeastern (MD900963) Arabian Sea were used to study the variations of the Indian monsoon during previous climatic cycles Core TY93-929/P was located between the SW monsoon driven upwelling centres off Somalia and Oman, which are characterized by large seasonal sea surface temperature (SST) and particle flux changes By contrast, core MD900963, was situated near the Maldives platform, an equatorial ocean site with a rather small SST seasonality (less than 2°C) For both cores we have reconstructed SST variations by means of the unsaturation ratio of C37 alkenones, which is compared with the δ18O records established on planktonic foraminifera In general, the SST records follow the δ18O variations, with an SST maximum during oxygen isotope stage 55 (the Last Interglacial at about 120–130 kyr) and a broad SST minimum during isotope stage 4 and 33 (approximately 40–50 kyr) The SST difference between the Holocene and the Last Glacial Maximum (LGM) is of the order of 2°C In both cores the SSTs during isotope stage 6 are distinctly higher by 1–2°C than the cold SST minima during the last glacial cycle (LGM and stage 3) To reconstruct qualitatively the past productivity variations for the two cores, we used the concentrations and fluxes of alkenones and organic carbon, together with a productivity index based on coccolith species (Florisphaera profunda relative abundance) Within each core, there is a general agreement between the different palaeoproductivity proxies In the southeastern Arabian Sea (core MD900963), glacial stages correspond to relatively high productivity, whereas warm interstadials coincide with low productivity All time series of productivity proxies are dominated by a cyclicity of about 21–23 kyr, which corresponds to the insolation precessional cycle A hypothesis could be that the NE monsoon winds were stronger during the glacial stages, which induced deepening of the surface mixed layer and injection of nutrients to the euphotic zone By contrast, the records are more complicated in the upwelling region of the western Arabian Sea (core TY93-929/P) This is partly due to large changes in the sedimentation rates, which were higher during specific periods (isotope stages 6, 54, 52, 3 and 2) Unlike core MD900963, no simple relationship emerges from the comparison between the δ18O stratigraphy and productivity records The greater complexity observed for core TY93-929/P could be the result of the superimposition of different patterns of productivity fluctuations for the two monsoon seasons, the SW monsoon being enhanced during interglacial periods, whereas the NE monsoon was increased during glacial intervals A similar line of reasoning also could help explain the SST records by the superimposition of variations of three components: global atmospheric temperature, and SW and NE monsoon dynamics

Modeling bio-geophysical feedback in the African and Indian monsoon region

Abstract: An asynchronously coupled global atmosphere-biome model is used to assess the dynamics of deserts and drought in the Sahel, Saudi-Arabia and the Indian subcontinent. Under present-day conditions of solar irradiation and sea-surface temperatures, the model finds two solutions: the first solution yields the present-day distribution of vegetation and deserts and the second shows a northward spread of savanna and xerophytic shrub of some 600 km, particularly in the southwest Sahara. Comparison of atmospheric states associated with these solutions corroborates Charney’s theory of a self-induction of deserts through albedo enhancement in the Sahel. Over the Indian subcontinent, changes in vegetation are mainly caused by a positive feedback between increased soil moisture and stronger summer monsoon.

Atmospheric Water Vapor over China.

Abstract: Chinese radiosonde data from 1970 to 1990 are relatively homogeneous in time and are used to examine the climatology, trends, and variability of China’s atmospheric water vapor content. The climatological distribution of precipitable water (PW) is primarily dependent on surface temperature. Influenced by the east Asia monsoon, China’s precipitable water exhibits very large seasonal variations. Station elevation is also a dominant factor affecting water vapor distribution in China. An increase (decrease) in precipitable water over China is associated with an increase (decrease) of precipitation in most regions. Increases in the percentage of PW relative to climatology are greater in winter and spring than in summer and autumn. Interannual variation and trends in precipitable water and surface temperature are closely correlated in China, confirming a positive “greenhouse” feedback. Interannual variations between precipitable water and precipitation are also significantly correlated.

Simulated Changes in the Asian Summer Monsoon at Times of Increased Atmospheric CO2

Abstract: Journal of the Meteorological Society of Japan, Vol. 75, No. 6, pp. 1019-1031, 1997 1019Simulated Changes in the Asian Summer Monsoonat Times of Increased Atmospheric CO2By Akio Kitoh, Seiji Yukimoto, Akira Noda and Tatsuo Motoi Meteorological Research Institute, Tsukuba, Ibaraki 305, Japan (Manuscript received 17 April 1997, in revised form L August 1997)AbstractPossible changes in the Asian summer monsoon due to increased atmospheric CO2 are investigated byan MRI global coupled atmosphere-ocean general circulation model. The summer (June-August) monsoonrainfall in India increases significantly with global warming. On the other hand, the monsoon wind shear index, defined as the difference between 850 hPa and 200 hPa zonal winds over the northern Indian Ocean, decreases. At 850 hPa, the westerly wind shifts northward and intensifies from the Sahel to northwest of India, but the monsoon westerly over the Arabian Sea weakens. It is found that increased moisture content in the warmer air leads to larger moisture flux convergence, contributing to the increased rainfall.Therefore, the monsoon wind shear index is not a good indicator for identifying any change of monsoon accompanying global warming. In contrast to the increased rainfall in India, change in rainfall is little over China where soil moisture becomes drier at times of increased CO2. It is also noted that the northern Eurasian continent becomes wetter in the increased-CO2 climate. The magnitude of the interannual variability of the Asian summer monsoon rainfall becomes larger in the CO2 experiment than in the control experiment, particularly in the later stage of the experiment after CO2 doubling. However it should be noted that the interdecadal variation of this interannual variability is also large both in the control and the CO2 experiments.

Seasonal and spatial variability in phytoplankton biomass, productivity and growth in the northwestern Indian Ocean: the southwest and northeast monsoon, 1992–1993

Abstract: Phytoplankton abundance, composition, primary production and growth rate were measured in the Somali Basin, the Gulf of Aden, and the southern part of the Red Sea during the SW monsoon (May–Aug 1992) and NE monsoon (Jan–Feb 1993). Strong upwelling (SST <20°C) occurred between 7° and 11°N along the Somali coast during July. Diatoms dominated in patchy blooms of phytoplankton north of Ras Hafun at the border of the Somali Current. Picophytoplankton (mainly Synechococcus and pico-eukaryotes) were present, but their contribution to the total chlorophyll a content was less than 30%. Nutrient concentrations in the main upwelling wedge south of Ras Hafun were high (nitrate over 15 μM), but chlorophyll a concentrations remained relatively low (≤0.6 mg.m−3) both in the freshly upwelled water of the Somali Current and downstream around Socotra Island and in the Great Whirl. In the latter areas, nutrients were not depleted (nitrate concentration varied between 3 and 8 μM) but primary production did not exceed 1 g C.m−2.day−1 as mixed layer depth largely exceeded the euphotic zone depth. Primary production for the whole northern Somali Basin showed a mean of 1.25 g C.m−2.day−1 (range 0.8–2.8 g C.m−2. day−1). During the NE monsoon, phytoplankton was dominated by picophytoplankton (up to 80% of total chlorophyll a content; dominated by Synechococcus, pico-eukaryotes and Prochlorococcus). Strong winds resulted in nutrient entrainment from deeper water. Surface values of chlorophyll a were ca 0.3 mg.m−3 and mean primary production of the Somali Basin was 0.8 g C.m−2. day−1 (range 0.5–1.0 g C.m−2. day−1). In the southern Red Sea and Gulf of Aden a reverse seasonal pattern occurred. During the SW monsoon, both areas were oligotrophic, including a deep chlorophyll maximum. Phytoplankton was dominated by picoplankton, namely Synechococcus. Primary production was low (0.5–0.6 g C.m−2. day−1 h). During the NE monsoon winter cooling resulted in deep vertical mixing and surface water replete with nutrients. Chlorophyll a values varied between 0.5 and 1.0 mg.m−3, with the highest values in diatom blooms in the Red Sea. Mean primary production was 1.2 g C.m−2.day−1 (range 0.5–2.2 g C.m−2.day−1). A detailed study, applying flow-cytometry, of the daily net growth rates of individual phytoplankton species/groups during the NE monsoon showed a large spatial variation indicative of active dynamics in the algal composition even on a daily basis. For picophytoplankton the highest growth rates exceeded 1.5 day−1, but larger phytoplankton showed lower maxima for growth rates (ca 1 day−1). Net growth rates decreased often to negative values at the bottom of the euphotic zone, implying a high grazing pressure. Net growth rates were converted to daily variations in plant carbon and compared with 14C-based primary production data (only during the NE monsoon). In nutrient replete regions, the two methods showed comparable values for carbon production, indicating that all newly fixed carbon resulted in an increase in plant biomass. Higher nutrient levels in particular favoured growth of larger phytoplankton (> 3 μm). In more oligotrophic (low surface nitrate) areas, < 35% of the primary production resulted in a net increase in plant carbon. At some stations grazing pressure balanced phytoplankton growth, as plant carbon did not change over a full day incubation period.

Climate Trends Associated with Multidecadal Variability of Atlantic Hurricane Activity

Abstract: Anomalous long-term variations of ocean heat transport offer an attractive and intuitively creditable explanation for many long-term climate trends. Multidecadal variations of intense Atlantic hurricane activity are but one manifestation of an extensive array of regional and global climate trends which appear to be linked to variations of heat transport by the Atlantic thermohaline circulation. In addition to influencing Atlantic tropical cyclones and the closely associated West African monsoon, Atlantic thermohaline variability appears to be linked to global SST variations and related trends occurring throughout the global climate system. Consequently, understanding decadal trends in hurricane activity may be critically dependent on understanding the somewhat broader issue of decadal variations of the major ocean circulations. The net transport of warm surface layer water to high latitudes by the so-called “Atlantic conveyor belt” (i.e., thermohaline) circulation is sensitive to surface-layer salinity anomalies in the “deep water” formation areas of the North Atlantic. A major decrease of surface-layer salinity appeared over portions of these areas during the late 1960s which reduced ocean water density and slowed the surface water sinking process associated with deep water formation. This trend, in turn, lead to diminished northward heat transport by the ocean hence, to cooling of ocean surface temperatures in much of the North Atlantic and warming of SSTs in much of the South Atlantic. These regional Atlantic SST anomalies initiated the atmospheric circulation anomalies associated with the long running Sahel drought and the associated decrease of intense Atlantic hurricane activity in recent decades. At approximately the same time, the ocean surface also cooled in much of the North Pacific while strong SST warming occurred in much of the Southern Hemisphere Atlantic, Indian, and Pacific Ocean areas. This global distribution of altered ocean surface temperatures has been directly linked to altered patterns of Atlantic and West African surface pressure and monsoon circulations. These global climate changes also include the energetics of ENSO and related variables in the tropical Pacific and Indian Oceans and numerous “teleconnected” interactions between the tropical Pacific, the North Pacific, North America and Europe. At present there are few long-term observational data for making reliable direct estimates of trends in the net Atlantic Ocean conveyor transport. Consequently, no such information is available for detecting and further anticipating forthcoming decadal trends in hurricane activity. Needed research is suggested which includes surveys and the synthesis of additional trend data for the specification of plausible and physically consistent global interactions linking the Atlantic conveyor circulation and other decadal trend associations in the global climate system. In this way, some of these global data may yield factors which are potentially useful for forecasting the onset and termination of new decadal trends of hurricane activity. This prospect is examined in data for the most recent 50-year period and in prior realizations of similar concurrent climate trends in earlier historical data.

The response to deforestation and desertification in a model of West African monsoons

Abstract: Since Charney proposed his theory on the dynamics of deserts and droughts in the Sahel [Charney, 1975], there has been significant scientific interest in the interaction between vegetation and climate in this region. The essence of this interaction is that the atmospheric circulation, and therefore rainfall, over this region may be sensitive to changes in vegetation cover near the desert border. Here we describe simulations of the West African monsoons with a simple zonally-symmetric model. The results suggest that the potential impact of human induced change of land cover on regional climate depends critically on the location of the change in vegetation cover. That is, desertification along the border with the Sahara (e.g., in Chad, Niger, Mali and Mauritania) leaves a relatively minor impact on monsoon circulation and regional rainfall; deforestation along the southern coast of West Africa (e.g., in Nigeria, Ghana and Ivory Coast) may result in complete collapse of monsoon circulation, and a significant reduction of regional rainfall.

Surges over the Gulf of California during the Mexican Monsoon

Abstract: Northward surges of relatively cool, moist, maritime air from the tropical Pacific into the southwestern United States occur via the Gulf of California every summer during the Mexican monsoon season. These surges advect large amounts of moisture northward and promote increased convective activity in Arizona. Mesoscale model output from 32 successive 24-h simulations are used to examine both the large-scale and mesoscale features associated with surge events. Model data are compared to high-resolution aircraft measurements on two days, showing clearly that the model is capable of reproducing many of the detailed structures found in the observations. Results from a large-scale analysis indicate that strong surges are produced in the model when a midlatitude trough is in the proper phase relationship with a tropical easterly wave. Weak surges occur when this relationship is missing.

Relief and climate in South Asia: the influence of the western ghats on the current climate pattern of peninsular India

Abstract: The 1500-km-long Western Ghats mountain barrier of peninsular India interacts with the southwest monsoon in a manner which bears heavily on the exceptionally varied climate pattern of the Deccan. Karnataka Province alone concentrates five of the six major climate types of the entire Indian Union. This review explores the interactions between atmospheric structure over South Asia and relief and discusses the efficiency with which the passive margin uplift, second only to the Himalayan barrier, acts as a climatic gatekeeper to the subcontinent. Particular attention is given to rainfall patterns and regimes. These are revealed by a variety of statistical classification and mapping techniques, and the analysis is guided by the steep environmental gradient observed on the immediate backslope of the Ghats, where annual totals can drop from 6000 to 600 mm in ca. 80 km. This is strongly reflected in the landform, soil, vegetation and cropping patterns and raises the question of the relationship between the uplift history of the mountain barrier at geological time-scales, the history of the South Asian monsoon circulation and the stability and diversity of the climatic pattern as seen today. The tightly arranged suite of bioclimatic regions also provides a unique geographical backdrop to the agricultural diversity of South India, rarely found on such a scale in other monsoon contexts of the Tropics. © 1997 The Royal Meteorological Society.

Simulation of the Asian Summer Monsoons of 1987 and 1988 with a Regional Model Nested in a Global GCM

Abstract: State-of-the-art general circulation models have deficiencies in simulating the observed amplitude and phase of the mean patterns of circulation and precipitation over the Asian monsoon region. They are also deficient in simulating the observed seasonal variations of circulation and precipitation averaged over the monsoon region. To improve these simulations, the National Centers for Environmental Prediction regional Eta Model is nested in the Center for Ocean‐Land‐Atmosphere Studies (COLA) GCM. The Eta Model is a gridpoint model with a horizontal resolution of 80 km and 38 layers in the vertical. The Eta Model domain (30 8‐1408E, 308S‐508N) covers the Asian monsoon region, which includes the Indian, Chinese, and Southeast Asian monsoons. The COLA GCM is a sigma coordinate spectral model with rhomboidal truncation at 40 waves and 18 vertical levels. The Eta Model is nested in the GCM such that its lateral boundary conditions and initial conditions are derived