Boreal Fall Research Articles

Impacts of Indian Ocean SST biases on the Indian Monsoon: as simulated in a global coupled model

In this study, the impact of the ocean–atmosphere coupling on the atmospheric mean state over the Indian Ocean and the Indian Summer Monsoon (ISM) is examined in the framework of the SINTEX-F2 coupled model through forced and coupled control simulations and several sensitivity coupled experiments. During boreal winter and spring, most of the Indian Ocean biases are common in forced and coupled simulations, suggesting that the errors originate from the atmospheric model, especially a dry islands bias in the Maritime Continent. During boreal summer, the air-sea coupling decreases the ISM rainfall over South India and the monsoon strength to realistic amplitude, but at the expense of important degradations of the rainfall and Sea Surface Temperature (SST) mean states in the Indian Ocean. Strong SST biases of opposite sign are observed over the western (WIO) and eastern (EIO) tropical Indian Ocean. Rainfall amounts over the ocean (land) are systematically higher (lower) in the northern hemisphere and the south equatorial Indian Ocean rainfall band is missing in the control coupled simulation. During boreal fall, positive dipole-like errors emerge in the mean state of the coupled model, with warm and wet (cold and dry) biases in the WIO (EIO), suggesting again a significant impact of the SST errors. The exact contributions and the distinct roles of these SST errors in the seasonal mean atmospheric state of the coupled model have been further assessed with two sensitivity coupled experiments, in which the SST biases are replaced by observed climatology either in the WIO (warm bias) or EIO (cold bias). The correction of the WIO warm bias leads to a global decrease of rainfall in the monsoon region, which confirms that the WIO is an important source of moisture for the ISM. On the other hand, the correction of the EIO cold bias leads to a global improvement of precipitation and circulation mean state during summer and fall. Nevertheless, all these improvements due to SST corrections seem drastically limited by the atmosphere intrinsic biases, including prominently the unimodal oceanic position of the ITCZ (Inter Tropical Convergence Zone) during summer and the enhanced westward wind stress along the equator during fall.

Influence of Indian Ocean Dipole and Pacific recharge on following year’s El Niño: interdecadal robustness

The Indian Ocean Dipole (IOD) can affect the El Niño–Southern Oscillation (ENSO) state of the following year, in addition to the well-known preconditioning by equatorial Pacific Warm Water Volume (WWV), as suggested by a study based on observations over the recent satellite era (1981–2009). The present paper explores the interdecadal robustness of this result over the 1872–2008 period. To this end, we develop a robust IOD index, which well exploits sparse historical observations in the tropical Indian Ocean, and an efficient proxy of WWV interannual variations based on the temporal integral of Pacific zonal wind stress (of a historical atmospheric reanalysis). A linear regression hindcast model based on these two indices in boreal fall explains 50 % of ENSO peak variance 14 months later, with significant contributions from both the IOD and WWV over most of the historical period and a similar skill for El Niño and La Niña events. Our results further reveal that, when combined with WWV, the IOD index provides a larger ENSO hindcast skill improvement than the Indian Ocean basin-wide mode, the Indian Monsoon or ENSO itself. Based on these results, we propose a revised scheme of Indo-Pacific interactions. In this scheme, the IOD–ENSO interactions favour a biennial timescale and interact with the slower recharge-discharge cycle intrinsic to the Pacific Ocean.

Book review: Look northward to the boreal, the home of many migrating birds

ISSN 1948‐6596 news and update book review Look northward to the boreal, the home of many migrating birds Boreal birds of North America: A hemispheric view of their conservation links and significance, by Jeffrey V. Wells (editor), 2011, Studies in Avian Biology No. 41, University of California Press, 136 pp. £30.95 (Hardback) ISBN: 9780520271005; http://www.ucpress.edu/ To many people, the term “boreal forest” evokes an image of a vast, cold, and inaccessible wilder‐ ness far from home. Most of us live south of the boreal, and when we think of bird diversity we tend to look toward the tropics, where species diversity is high and species range size tends to be small, placing many species under threat of extinc‐ tion due to human‐caused habitat destruction. This book provides an eye‐opening look to the north, strongly demonstrating the importance of North America’s boreal forest to avian biodiversity both there and throughout the Americas via sea‐ sonal migration. The book also shows that, while being one of the largest wilderness areas left on the planet, the boreal forest is under growing threat. Despite its small size for an edited volume (9 chapters totaling 125 pages), this book presents a remarkably balanced and comprehensive over‐ view of the biogeography, ecology, and conserva‐ tion of the boreal birds of North America. Most chapters focus on broad biogeographic patterns; these include contributions on the threats to the boreal forest, the global role of the boreal in sus‐ taining bird populations, the use of survey data in geospatial modeling of bird abundance, and the role of boreal migrants in winter bird communities far to the south. These are supplemented by con‐ tributions that focus on geographically more nar‐ row areas, including chapters on waterfowl con‐ servation in the western boreal forest, boreal landbird migrants in eastern North America, and boreal migrants in the tropical Andes. Finally, two contributions provide examples of in‐depth re‐ search on two species that have experienced alarming declines: surf scoters and rusty black‐ birds. The mix of broad‐scale overviews with fo‐ cused looks at individual species in decline leaves the reader with a remarkably rich understanding of the importance of the boreal forest and the research being undertaken (and needed in the future) to better understand and protect its avi‐ fauna. Some remarkable statistics emerge from this book. Spanning from interior Alaska across the continent to Newfoundland, North America's boreal forest covers 5.9 million km 2 , representing 25% of remaining intact forests on Earth. Over 300 bird species regularly breed there, accounting for 43% of all species that regularly occur in Canada and/or the U.S. The total number of adult boreal breeders is estimated at somewhere between 1.65 and 3 billion, accounting for 30% of all breed‐ ing landbirds of Canada and the U.S., 38% of wa‐ terfowl, and 30% of shorebirds. Between 3 and 5 billion adults and immature birds migrate south from the boreal every fall; during the northern winter, they play important ecological roles in North, Central, and South America, where they can comprise up to 50% of the species composi‐ tion in some feeding guilds. While the above num‐ bers are impressive, the threats to boreal birds are as well. From 1966 to 1994, Saskatchewan’s boreal plain was deforested at a rate of 0.89% per year, three times the global rate of deforestation, and rates of boreal forest disturbance due to oil and gas exploration, hydropower projects, agricul‐ ture, and other industrial development are also alarming. One of the strengths of this book is that it goes beyond the presentation of patterns, and argues for a subtle change in conservation values. While traditional approaches to conservation have emphasized rarity and endemism in prioritizing conservation efforts, Wells and Blancher (p. 8) advance the idea that “some regions have a high stewardship responsibility for maintaining species that are still abundant.” This point is well taken, frontiers of biogeography 4.3, 2012 — © 2012 the authors; journal compilation © 2012 The International Biogeography Society

Observational Study of the Oceanic Surface Parameters in the Eastern Indian Ocean During Two Contrasting Dipole Years 2005 and 2006

The Indian Ocean dipole (IOD) is one of the major interannual climate variation signals in this region through the coupled air–sea interactions. In this letter, the oceanic surface parameters in the eastern Indian Ocean during two consecutive contrasting dipole years 2005 (negative) and 2006 (positive) are examined exclusively from satellite and in situ observations. Results showed that there was sea surface temperature anomaly up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\pm 2\ ^{\circ}\hbox{C}$</tex></formula> in the south eastern equatorial Indian Ocean (EEIO) during the mature phase (September–November) of the dipole. Except for the south EEIO, the south of the equator showed very high positive sea level anomaly (up to 45 cm) during the boreal fall (September–November) in 2006 under the influence of the strong positive IOD event. Similar coherent differences were observed in freshwater flux and sea surface salinity anomalies which show the large impact of these dipole events in the south EEIO box. Also, the seasonal rainfall during the northeast Indian monsoon for these two years shows substantial difference over southern India which reveals the paramount impact of IOD on the northeast monsoon rainfall.

Impact of Indian Ocean Dipole and El Niño/Southern Oscillation wind‐forcing on the Wyrtki jets

Interannual variability of the Wyrtki jets is studied in the context of Indian Ocean Dipole (IOD) and El Niño and Southern Oscillation (ENSO) wind‐forcing using a three dimensional numerical ocean model and observations. The boreal fall (October–November) Wyrtki jet is more significantly affected than the boreal spring (April–May) Wyrtki jet since both the IOD and ENSO tend to peak toward the end of the calendar year. Various statistical methods are used in an attempt to separate the impacts of the IOD and ENSO on these jets, with emphasis on the fall jet. The first two modes of an Empirical Orthogonal Function (EOF) decomposition account for about 90% and 85% of variability in zonal currents and wind stress respectively along the equator in the Indian Ocean, but EOF analysis does not cleanly separate out IOD and ENSO forcing and response. Partial correlation analysis reveals that IOD wind‐forcing and zonal equatorial current response are stronger on average than for ENSO and extend further west across the basin. Composite analysis of IOD only, ENSO only, and combined IOD and ENSO years provides a complementary definition of the relative contributions of these two phenomena on Wyrtki jet variability and in general is consistent with the results of the partial correlation analysis.

Ocean Surface Impacts on the Seasonal-Mean Precipitation over the Tropical Indian Ocean

Abstract This study analyzes factors affecting the predictability of seasonal-mean precipitation over the tropical Indian Ocean. The analysis focuses on the contributions from the local sea surface temperature (SST) forcing in the Indian Ocean, the remote SST forcing related to ENSO in the tropical eastern Pacific, and the role of local air–sea coupling. To understand the impacts of the individual factors, the prediction skill over the tropical Indian Ocean for four model simulations, but with different treatments for the ocean, are compared. The seasonality in precipitation skill, the local precipitation–SST relationship, and prediction skill related to Indian Ocean dipole mode (IODM) are examined. It is found that the importance of the accuracy of local SST and the presence of local air–sea coupling in the Indian Ocean has a strong seasonal dependence. Accurate local SSTs are important during the boreal fall season, whereas the local air–sea coupling is important during the boreal spring. The precipitation skill over the Indian Ocean during boreal winter is primarily from ENSO. However, ENSO impacts are better realized with the inclusion of an interactive ocean. For all four seasons, the simulation without the interannual variations of local SST in the Indian Ocean shows the least precipitation skill and a much weaker seasonality. It is also found that, for the simulation where the global SSTs are relaxed to the observations and hence maintain some level of active air–sea coupling, the observed seasonal cycle of precipitation–SST relationship is reproduced reasonably well. In addition, the analysis also shows that simulations with accurate SST forcing display high precipitation skill during strong IODM events, indicating that IODM SST acts as a forcing for the atmospheric variability.

Understanding the responses of sea surface temperature to the two different types of El Niño in the western North Pacific

In this study, we examine the physical processes of the response of sea surface temperature (SST) in the western North Pacific to the two different types of El Niño, the Central Pacific El Niño and the Eastern Pacific El Niño. While a center of anomalous warm SST is observed in the eastern tropical Pacific in the case of the Eastern Pacific El Niño, it is located in the central tropical Pacific for the Central Pacific El Niño. We investigate the detailed SST response in the Kuroshio Extension region, where the anomalous SST is warm (cool) in the Central Pacific El Niño (Eastern Pacific El Niño) during the boreal fall and winter. We diagnose the SST tendency to identify the roles of the surface net heat fluxes and Ekman transport.During the fall and winter, the atmospheric teleconnections over the North Pacific are quite different between the two types of El Niño, which might be due to changes in atmospheric diabatic forcing in relation to rainfall in the tropical Pacific. Our results suggest that the physical processes, which are responsible for the western North Pacific SST in response to the two different types of El Niño, are basically similar. That is, Ekman heat transport plays a key role in warming and cooling of the SST anomalies in the Kuroshio Extension region in the Central Pacific El Niño and Eastern Pacific El Niño, respectively. In contrast, the net surface heat fluxes act to weaken the SST anomalies. This result indicates that changes in the anomalous atmospheric circulation over the North Pacific, which is directly associated with Ekman currents at the upper levels, are important to determine the responses of SST in the Kuroshio Extension region to the two types of El Niño.

Impact of climate change on mid-twenty-first century growing seasons in Africa

Changes in growing seasons for 2041–2060 across Africa are projected using a regional climate model at 90-km resolution, and confidence in the predictions is evaluated. The response is highly regional over West Africa, with decreases in growing season days up to 20% in the western Guinean coast and some regions to the east experiencing 5–10% increases. A longer growing season up to 30% in the central and eastern Sahel is predicted, with shorter seasons in parts of the western Sahel. In East Africa, the short rains (boreal fall) growing season is extended as the Indian Ocean warms, but anomalous mid-tropospheric moisture divergence and a northward shift of Sahel rainfall severely curtails the long rains (boreal spring) season. Enhanced rainfall in January and February increases the growing season in the Congo basin by 5–15% in association with enhanced southwesterly moisture transport from the tropical Atlantic. In Angola and the southern Congo basin, 40–80% reductions in austral spring growing season days are associated with reduced precipitation and increased evapotranspiration. Large simulated reductions in growing season over southeastern Africa are judged to be inaccurate because they occur due to a reduction in rainfall in winter which is over-produced in the model. Only small decreases in the actual growing season are simulated when evapotranspiration increases in the warmer climate. The continent-wide changes in growing season are primarily the result of increased evapotranspiration over the warmed land, changes in the intensity and seasonal cycle of the thermal low, and warming of the Indian Ocean.

Seasonality of Tropical Instability Waves and Its Feedback to the Seasonal Cycle in the Tropical Eastern Pacific

This study investigated the seasonality of tropical instability waves (TIWs) and its feedback to the seasonal cycle in the tropical eastern Pacific using a high-resolution ocean model covering 1958–2007. The climatological mean of the TIWs featured intraseasonal fluctuations, implying that TIWs are not occurring randomly, but their amplitude is partly in phase from one year to another. This seasonality of TIW activity is attributed to their dependency on the seasonal mean variation of current and temperature. Energy conversion analysis confirmed that the strong variability of TIWs near 4°N was due to the barotropic energy conversion associated with the large meridional shear of NECC and SEC and that at another pole near 2°N was due to the baroclinic energy conversion associated with the temperature front in the mixed layer. The former and latter poles are somehow largely responsible for amplifying the dynamic and thermal eddies of TIWs, respectively. The intensified TIWs during a boreal fall increase the tropical eastern Pacific SST by associating the warm thermal advection by anomalous currents, with a rate of up to 1°C/month in September. Therefore, this leads to interactive feedback between seasonal and intraseasonal variations, that is, TIWs in the tropical eastern Pacific.

Is a global warming signature emerging in the tropical Pacific?

The tropical pacific experienced a hitherto‐unseen anomalous basinwide warming from May 2009 through April 2010 with the maximum warming to the east of the dateline, but for a weak anomalous cooling west of 140°E after early boreal fall. Our observed analysis and model experiments isolate the potential teleconnections from TP during the summer of 2009. Further, we show through an empirical orthogonal function analysis of the tropical Pacific SSTA that the anomalous conditions in TP during this period could have manifested as a canonical El Niño, but for a slowly intensifying background west–east gradient. This zonal SST gradient is subject to an increasing trend associated with global warming. A possible implication is that any further increase in global warming may result in more basinwide warm events in place of canonical El Niños, along with the occurrence of more intense La Niñas and El Niño Modokis.

IOD and ENSO impacts on the extreme stream-flows of Citarum river in Indonesia

Extreme stream-flow events of Citarum River are derived from the daily stream-flows at the Nanjung gauge station. Those events are identified based on their persistently extreme flows for 6 or more days during boreal fall when the seasonal mean stream-flow starts peaking-up from the lowest seasonal flows of June–August. Most of the extreme events of high-streamflows were related to La Nina conditions of tropical Pacific. A few of them were also associated with the negative phases of IOD and the newly identified El Nino Modoki. Unlike the cases of extreme high streamflows, extreme low streamflow events are seen to be associated with the positive IODs. Nevertheless, it was also found that the low-stream-flow events related to positive IOD events were also associated with El Nino events except for one independent event of 1977. Because the occurrence season coincides the peak season of IOD, not only the picked extreme events are seen to fall under the IOD seasons but also there exists a statistically significant correlation of 0.51 between the seasonal IOD index and the seasonal streamflows. There also exists a significant lag correlation when IOD of June–August season leads the streamflows of September–November. A significant but lower correlation coefficient (0.39) is also found between the seasonal streamflow and El Nino for September–November season only.

The NCEP GODAS Ocean Analysis of the Tropical Pacific Mixed Layer Heat Budget on Seasonal to Interannual Time Scales

Abstract The mixed layer heat budget in the tropical Pacific is diagnosed using pentad (5 day) averaged outputs from the Global Ocean Data Assimilation System (GODAS), which is operational at the National Centers for Environmental Prediction (NCEP). The GODAS is currently used by the NCEP Climate Prediction Center (CPC) to monitor and to understand El Niño and La Niña in near real time. The purpose of this study is to assess the feasibility of using an operational ocean data assimilation system to understand SST variability. The climatological mean and seasonal cycle of mixed layer heat budgets derived from GODAS agree reasonably well with previous observational and model-based estimates. However, significant differences and biases were noticed. Large biases were found in GODAS zonal and meridional currents, which contributed to biases in the annual cycle of zonal and meridional advective heat fluxes. The warming due to tropical instability waves in boreal fall is severely underestimated owing to use of a 4-week data assimilation window. On interannual time scales, the GODAS heat budget closure is good for weak-to-moderate El Niños. A composite for weak-to-moderate El Niños suggests that zonal and meridional temperature advection and vertical entrainment/diffusion all contributed to the onset of the event and that zonal advection played the dominant role during decay of the event and the transition to La Niña. The net surface heat flux acts as a damping during the development stage, but plays a critical role in the decay of El Niño and the transition to the following La Niña. The GODAS heat budget closure is generally poor for strong La Niñas. Despite the biases, the GODAS heat budget analysis tool is useful in monitoring and understanding the physical processes controlling SST variability associated with ENSO. Therefore, it has been implemented operationally at CPC in support of NOAA’s ENSO forecasting.

Geochemical and climate modeling evidence for Holocene aridification in Hawaii: dynamic response to a weakening equatorial cold tongue

A 13.5 m sequence of Holocene limnic sediments from a sinkhole on Oahu, Hawaii provides a valuable paleoclimatic record for the central subtropical Pacific. δ 13C analysis of plant leaf waxes ( n-alkanes: n-C 27, n-C 29, n-C 31 and n-C 33) is used to infer vegetative changes. Average δ 13C values of the suite of n-alkanes increase from approximately −31 ± 0.5‰ at 10 ca kyr BP (calibrated thousand years before present) to about −27 ± 0.5‰ by 6 ca kyr BP and then remain roughly constant until the Polynesian arrival (about 1.15 ca kyr BP). The increase in δ 13C values of n-alkanes is interpreted to indicate a shift in the local vegetation from C 3 to C 4-dominated flora. Based on mass-balance calculations, the observed increase in the δ 13C values translates to at least a doubling of the relative abundance of C 4 plants. We argue that the expansion of C 4 plants was a response to decreased overall water availability (aridification) due to reduced wintertime precipitation. Model simulations of an orbitally-induced increase in insolation along the equator during the Holocene provide evidence for a wintertime drying trend in the eastern subtropical North Pacific. This trend is associated with boreal fall to winter warming of the cold tongue in the eastern equatorial Pacific (EEP). These model results provide a conceptual framework to explain a dynamic link between the reconstructed Holocene drying trend in Hawaii and orbitally-forced climate change in the EEP that is analogous to the modern El Niño-Southern Oscillation teleconnection.

Surface Flux Observations on the Southeastern Tropical Pacific Ocean and Attribution of SST Errors in Coupled Ocean–Atmosphere Models

Abstract A new dataset synthesizes in situ and remote sensing observations from research ships deployed to the southeastern tropical Pacific stratocumulus region for 7 years in boreal fall. Surface meteorology, turbulent and radiative fluxes, aerosols, cloud properties, and rawinsonde profiles were measured on nine ship transects along 20°S from 75° to 85°W. Fluxes at the ocean surface are essential to the equilibrium SST. Solar radiation is the only warming net heat flux, with 180–200 W m−2 in boreal fall. The strongest cooling is evaporation (60–100 W m−2), followed by net thermal infrared radiation (30 W m−2) and sensible heat flux (&amp;lt;10 W m−2). The 70 W m−2 imbalance of heating at the surface reflects the seasonal SST tendency and some 30 W m−2 cooling that is mostly due to ocean transport. Coupled models simulate significant SST errors in the eastern tropical Pacific Ocean. Three different observation-based gridded ocean surface flux products agree with ship and buoy observations, while fluxes simulated by 15 Coupled Model Intercomparison Project phase 3 [CMIP3; used for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report] general circulation models have relatively large errors. This suggests the gridded observation-based flux datasets are sufficiently accurate for verifying coupled models. Longwave cooling and solar warming are correlated among model simulations, consistent with cloud radiative forcing and low cloud amount differences. In those simulations with excessive solar heating, elevated SST also results in larger evaporation and longwave cooling to compensate for the solar excess. Excessive turbulent heat fluxes (10–90 W m−2 cooling, mostly evaporation) are the largest errors in simulations once the compensation between solar and longwave radiation is taken into account. In addition to excessive solar warming and evaporation, models simulate too little oceanic residual cooling in the southeastern tropical Pacific Ocean.

The Southeast Pacific Warm Band and Double ITCZ

Abstract The east Pacific double intertropical convergence zone (ITCZ) in austral fall is investigated with particular focus on the growing processes of its Southern Hemisphere branch. Satellite measurements from the Tropical Rainfall Measuring Mission (TRMM) and Quick Scatterometer (QuikSCAT) are analyzed to derive 8-yr climatology from 2000 to 2007. The earliest sign of the south ITCZ emerges in sea surface temperature (SST) by January, followed by the gradual development of surface convergence and water vapor. The shallow cumulus population starts growing to form the south ITCZ in February, a month earlier than vigorous deep convection is organized into the south ITCZ. The key factors that give rise to the initial SST enhancement or the southeast Pacific warm band are diagnosed by simple experiments. The experiments are designed to calculate SST, making use of an ocean mixed layer “model” forced by surface heat fluxes, all of which are derived from satellite observations. It is found that the shortwave flux absorbed into the ocean mixed layer is the primary driver of the southeast Pacific warm band. The warm band does not develop in boreal fall because the shortwave flux is seasonally so small that it is overwhelmed by other negative fluxes, including the latent heat and longwave fluxes. Clouds offset the net radiative flux by 10–15 W m−2, which is large enough for the warm band to develop in boreal fall if it were not for clouds reflecting shortwave radiation. Interannual variability of the double ITCZ is also discussed in brief.

Seasonal Mixed Layer Heat Balance of the Southwestern Tropical Indian Ocean*

Abstract Sea surface temperature (SST) in the southwestern tropical Indian Ocean exerts a significant influence on global climate through its influence on the Indian summer monsoon and Northern Hemisphere atmospheric circulation. In this study, measurements from a long-term moored buoy are used in conjunction with satellite, in situ, and atmospheric reanalysis datasets to analyze the seasonal mixed layer heat balance in the thermocline ridge region of the southwestern tropical Indian Ocean. This region is characterized by a shallow mean thermocline (90 m, as measured by the 20°C isotherm) and pronounced seasonal cycles of Ekman pumping and SST (seasonal ranges of −0.1 to 0.6 m day−1 and 26°–29.5°C, respectively). It is found that surface heat fluxes and horizontal heat advection contribute significantly to the seasonal cycle of mixed layer heat storage. The net surface heat flux tends to warm the mixed layer throughout the year and is strongest during boreal fall and winter, when surface shortwave radiation is highest and latent heat loss is weakest. Horizontal heat advection provides warming during boreal summer and fall, when southwestward surface currents and horizontal SST gradients are strongest, and is close to zero during the remainder of the year. Vertical turbulent mixing, estimated as a residual in the heat balance, also undergoes a significant seasonal cycle. Cooling from this term is strongest in boreal summer, when surface wind and buoyancy forcing are strongest, the thermocline ridge is shallow (&amp;lt;90 m), and the mixed layer is deepening. These empirical results provide a framework for addressing intraseasonal and interannual climate variations, which are dynamically linked to the seasonal cycle, in the southwestern tropical Indian Ocean. They also provide a quantitative basis for assessing the accuracy of numerical ocean model simulations in the region.

Seasonal Influences on Coupled Ocean–Atmosphere Variability in the Tropical Atlantic Ocean

Abstract Numerous studies and observational analyses point to a connection between the annual cycle and tropical Atlantic variability, specifically the influence of the seasons. Although a previous study has shown that the annual cycle is not necessary for the generation of this variability, this study demonstrates that the annual cycle provides particular conditions that modulate this variability. Particular seasons are investigated through the use of a coupled ocean–atmosphere model using anomaly coupling as the coupling strategy in order to control the mean state of the system. To isolate the influence of each season, the model is integrated in perpetuated mean states that simulate perpetual boreal spring, summer, fall, and winter seasonal mean states. These are compared to a control simulation that contains an annual cycle. Evidence is shown that the annual cycle modulates tropical Atlantic variability in the following three ways: 1) the background mean state for some seasons provides favorable conditions for the growth of particular patterns through regional air–sea feedback mechanisms, 2) mechanisms that excite the variability are seasonally dependent, and 3) the progression through the annual cycle is important for certain variabilities to be excited and grow.

Interannual variability of Kelvin wave propagation in the wave guides of the equatorial Indian Ocean, the coastal Bay of Bengal and the southeastern Arabian Sea during 1993–2006

The observed variability of the Kelvin waves and their propagation in the equatorial wave guide of the Indian Ocean and in the coastal wave guides of the Bay of Bengal (BoB) and the southeastern Arabian Sea (AS) on seasonal to interannual time scales during years 1993–2006 is examined utilizing all the available satellite and in-situ measurements. The Kelvin wave regime inferred from the satellite-derived sea surface height anomalies (SSHA) shows a distinct annual cycle composed of two pairs of alternate upwelling (first one occurring during January–March and the second one occurring during August–September) and downwelling (first one occurring during April–June and the second one occurring during October–December) Kelvin waves that propagate eastward along the equator and hit the Sumatra coast and bifurcate. The northern branches propagate counterclockwise over varied distances along the coastal wave guide of the BoB. The potential mechanisms that contribute to the mid-way termination of the first upwelling and the first downwelling Kelvin waves in the wave guide of the BoB are hypothesized. The second downwelling Kelvin wave alone reaches the southeastern AS, and it shows large interannual variability caused primarily by similar variability in the equatorial westerly winds during boreal fall. The westward propagating downwelling Rossby waves triggered by the second downwelling Kelvin wave off the eastern rim of the BoB also shows large interannual variability in the near surface thermal structure derived from SODA analysis. The strength of the equatorial westerlies driven by the east–west gradient of the heat sources in the troposphere appears to be a critical factor in determining the observed interannual variability of the second downwelling Kelvin wave in the wave guides of the equatorial Indian Ocean, the coastal BoB, and the southeastern AS.

Simulating tropical instability waves in the equatorial eastern Pacific with a coupled general circulation model

Satellite observations of SSTs have revealed the existence of unstable waves in the equatorial eastern Pacific and Atlantic oceans. These waves have a 20–40-day periodicity with westward phase speeds of 0.4–0.6 m s−1 and wavelengths of 1000–2000 km during boreal summer and fall. They are generally called tropical instability waves (TIWs). This study investigates TIWs simulated by a high-resolution coupled atmosphere-ocean general circulation model (AOGCM). The horizontal resolution of the model is 120 km in the atmosphere, and 30 km longitude by 20 km latitude in the ocean. Model simulations show good agreement with the observed main features associated with TIWs. The results of energetics analysis reveal that barotropic energy conversion is responsible for providing the main energy source for TIWs by extracting energy from the meridional shear of the climatological-mean equatorial currents in the mixed layer. This deeper and northward-extended wave activity appears to gain its energy through baroclinic conversion via buoyancy work, which further contributes to the asymmetric distribution of TIWs.

Observed Strengthening of the Zonal Sea Surface Temperature Gradient across the Equatorial Pacific Ocean*

Abstract Decadal variations of very small amplitude [∼0.3°C in sea surface temperature (SST)] in the tropical Pacific Ocean, the genesis region of the interannual El Niño–Southern Oscillation (ENSO) phenomenon, have been shown to have powerful impacts on global climate. Future projections from different climate models do not agree on how this critical feature will change under the influence of anthropogenic forcing. A number of attempts have been made to resolve this issue by examining observed trends from the 1880s to the present, a period of rising atmospheric concentrations of greenhouse gases. A recent attempt concluded that the three major datasets disagreed on the trend in the equatorial gradient of SST. Using a corrected version of one of these datasets, and extending the analysis to the seasonal cycle, it is shown here that all agree that the equatorial Pacific zonal SST gradient has strengthened from 1880 to 2005 during the boreal fall when this gradient is normally strongest. This result appears to favor a theory for future changes based on ocean dynamics over one based on atmospheric energy considerations. Both theories incorporate the expectation, based on ENSO theory, that the zonal sea level pressure (SLP) gradient in the tropical Pacific is coupled to SST and should therefore strengthen along with the SST gradient. While the SLP gradient has not strengthened, it is found that it appears to have weakened only during boreal spring, consistent with the SST seasonal trends. Most of the coupled models included in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report underestimate the strengthening SST gradient in boreal fall, and show almost no change in the SLP gradient in any season. The observational analyses herein suggest that both theories are at work but with relative strengths that vary seasonally, and that the two theories need not be inconsistent with each other.