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David S. Gutzler and Tamara M. Wood

Abstract

Geographical variations in the variance and cross-correlation of monthly mean sea surface temperature (SST), outgoing longwave radiation (OLR, a proxy for deep convection and vertical motion), and convergence of winds at the surface and at 850 mb across the tropical Indian and Pacific oceans are examined. Within about 10° of the equator at most longitudes the variance of these quantities associated with the seasonal cycle is less than the variance associated with anomalies from the seasonal cycle. Largest variances in the SST and surface convergence data occur across the eastern near-equatorial Pacific, whereas OLR and 850 mb convergence variances are largest across the western Pacific. OLR anomalies are significantly correlated with collocated SST and surface convergence anomalies from the date line eastward to the South American coast, but are uncorrelated west of the. date line. The OLR and 850 mb convergence anomalies are significantly correlated from about 120°W westward but are uncorrelated east of that longitude. The near-surface convergence field thus contains a complicated vertical structure that may not be adequately represented in models with a single lower layer.

These calculations suggest that the relative effectiveness of different mechanisms for large ocean ocean-atmosphere coupling varies considerably across the near-equatorial oceans. Direct thermodynamic linkage between SST and convection anomalies is consistent with the results only across the eastern near-equatorial Pacific. Surface gradients of SST are most effective at forcing low-level atmospheric circulation anomalies over the eastern Pacific, where SST and surface convergence have large variances and OLR and 850 mb convergence anomalies are small and not well correlated. West of the date line, forcing by midtropospheric latent heating seems most consistent with the observed relationships.

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Emily Shroyer, Amit Tandon, Debasis Sengupta, Harindra J. S. Fernando, Andrew J. Lucas, J. Thomas Farrar, Rajib Chattopadhyay, Simon de Szoeke, Maria Flatau, Adam Rydbeck, Hemantha Wijesekera, Michael McPhaden, Hyodae Seo, Aneesh Subramanian, R Venkatesan, Jossia Joseph, S. Ramsundaram, Arnold L. Gordon, Shannon M. Bohman, Jaynise Pérez, Iury T. Simoes-Sousa, Steven R. Jayne, Robert E. Todd, G. S. Bhat, Matthias Lankhorst, Tamara Schlosser, Katherine Adams, S. U. P Jinadasa, Manikandan Mathur, M. Mohapatra, E. Pattabhi Rama Rao, A. K. Sahai, Rashmi Sharma, Craig Lee, Luc Rainville, Deepak Cherian, Kerstin Cullen, Luca R. Centurioni, Verena Hormann, Jennifer MacKinnon, Uwe Send, Arachaporn Anutaliya, Amy Waterhouse, Garrett S. Black, Jeremy A. Dehart, Kaitlyn M. Woods, Edward Creegan, Gad Levy, Lakshmi H. Kantha, and Bulusu Subrahmanyam

Abstract

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

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