Distinct influence of air–sea interactions mediated by mesoscale sea surface temperature and surface current in the Arabian Sea

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 8061-8080, doi:10.1175/JCLI-D-16-0834.1.During the southwest monsoons, the Arabian Sea (AS) develops highly energetic mesoscale variability associated with the Somali Current (SC), Great Whirl (GW), and cold filaments (CF). The resultant high-amplitude anomalies and gradients of sea surface temperature (SST) and surface currents modify the wind stress, triggering the so-called mesoscale coupled feedbacks. This study uses a high-resolution regional coupled model with a novel coupling procedure that separates spatial scales of the air–sea coupling to show that SST and surface currents are coupled to the atmosphere at distinct spatial scales, exerting distinct dynamic influences. The effect of mesoscale SST–wind interaction is manifested most strongly in wind work and Ekman pumping over the GW, primarily affecting the position of GW and the separation latitude of the SC. If this effect is suppressed, enhanced wind work and a weakened Ekman pumping dipole cause the GW to extend northeastward, delaying the SC separation by 1°. Current–wind interaction, in contrast, is related to the amount of wind energy input. When it is suppressed, especially as a result of background-scale currents, depth-integrated kinetic energy, both the mean and eddy, is significantly enhanced. Ekman pumping velocity over the GW is overly negative because of a lack of vorticity that offsets the wind stress curl, further invigorating the GW. Moreover, significant changes in time-mean SST and evaporation are generated in response to the current–wind interaction, accompanied by a noticeable southward shift in the Findlater Jet. The significant increase in moisture transport in the central AS implies that air–sea interaction mediated by the surface current is a potentially important process for simulation and prediction of the monsoon rainfall.This work is supported by ONR (N00014-15-1-2588 and N00014-17-1-2398), NSF (OCE- 1419235), and NOAA (NA15OAR4310176).2018-03-0

Distinct influence of air–sea interactions mediated by mesoscale sea surface temperature and surface current in the Arabian Sea

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 8061-8080, doi:10.1175/JCLI-D-16-0834.1.During the southwest monsoons, the Arabian Sea (AS) develops highly energetic mesoscale variability associated with the Somali Current (SC), Great Whirl (GW), and cold filaments (CF). The resultant high-amplitude anomalies and gradients of sea surface temperature (SST) and surface currents modify the wind stress, triggering the so-called mesoscale coupled feedbacks. This study uses a high-resolution regional coupled model with a novel coupling procedure that separates spatial scales of the air–sea coupling to show that SST and surface currents are coupled to the atmosphere at distinct spatial scales, exerting distinct dynamic influences. The effect of mesoscale SST–wind interaction is manifested most strongly in wind work and Ekman pumping over the GW, primarily affecting the position of GW and the separation latitude of the SC. If this effect is suppressed, enhanced wind work and a weakened Ekman pumping dipole cause the GW to extend northeastward, delaying the SC separation by 1°. Current–wind interaction, in contrast, is related to the amount of wind energy input. When it is suppressed, especially as a result of background-scale currents, depth-integrated kinetic energy, both the mean and eddy, is significantly enhanced. Ekman pumping velocity over the GW is overly negative because of a lack of vorticity that offsets the wind stress curl, further invigorating the GW. Moreover, significant changes in time-mean SST and evaporation are generated in response to the current–wind interaction, accompanied by a noticeable southward shift in the Findlater Jet. The significant increase in moisture transport in the central AS implies that air–sea interaction mediated by the surface current is a potentially important process for simulation and prediction of the monsoon rainfall.This work is supported by ONR (N00014-15-1-2588 and N00014-17-1-2398), NSF (OCE- 1419235), and NOAA (NA15OAR4310176).2018-03-0

Arabian Sea Response to Monsoon Variations

This study aims to quantify the impact of strong monsoons on the mixed layer heat budget in the Arabian Sea by contrasting forced ocean general circulation model simulations with composite strong and weak monsoon winds. Strong (weak) monsoons are defined as years with zonal component of the Somali Jet being greater (smaller) by more than a standard deviation of the long-term mean of the National Centers for Environmental Prediction reanalysis winds. Coastal upwelling is shown to be demonstrably stronger for strong monsoons leading to significant surface cooling, shallower thermoclines, and deeper mixed layers. A coupled ecosystem model shows that surface chlorophyll, primary, and export production are indeed higher for strong monsoons compared to weak monsoons driven by the supply of colder, nutrient-rich waters from greater than 100 m depths. The surprising result is that a strong monsoon results in stronger negative wind stress curl away from the coasts and drives Ekman pumping that results in a deeper thermocline. The weaker stratification and larger turbulent kinetic energy from the winds drive deeper mixed layers leading entrainment cooling with some contribution from the advection of colder upwelled waters from the coastal upwelling regions. Thus the strong monsoons, in fact, enhance oceanic heat uptake indicating that ocean dynamics are cooling the surface and driving the lower atmosphere which has implications for the interpretation of monsoon variability from paleorecords

A numerical investigation of the influence of monsoonal wind reversals over the East African coastal ocean

Includes abstract.Includes bibliographical references (leaves 81-94).In this dissertation, the variability in the East African coastal ocean (Somali basin) due to the monsoon transition is investigated. The monsoon is characterized by wind reversals and seasonality in the precipitation of a region. The surface circulation of the western Indian Ocean during the summer (JAS) and winter (JFM) monsoon winds is investigated using the Regional Ocean Modelling System (ROMS) ROMS is forced with the Comprehensive Ocean Atmosphere Data Sets (COADS) while the initial and lateral boundary conditions are derived from the World Ocean Atlas The domain area of the model study is constrained by 10•S to 15'N and 3S"E to 55"E, An overview of the surface circulation of the Somali basin is given. discussing the Somali Current East African Coastal Current, South Equatorial Counter Current Southern Gyre, and the Great Whirl The motivation of this dissertation is to improve the understanding of the circulation patterns within the Somali Basin from intra seasonal to seasonal timescales, using the ROMS model The model results suggest a seasonally reversing Somali current with a sub-surface counter current, consistent with observations_ Other prominent features such as the Great Whirl, which occurs during the Southwest monsoon and the Southern Gyre, are also apparent in the simulation The East African Coastal Current (EACC) and the South Equatorial Counter Current (SECCI are also major features of the Somali basin circulation that are equally apparent from the model simulation The model equally reproduces the equatorial jets as expected during the transition period of April/May and October/November with the net result of mass transport from the western end of the basin towards the east

Episodic dust events of Utahs Wasatch Front and adjoining region

pre-printEpisodic dust events cause hazardous air quality along Utah's Wasatch Front and dust loading of the snowpack in the adjacentWasatch Mountains. This paper presents a climatology of episodic dust events of the Wasatch Front and adjoining region that is based on surface weather observations from the Salt Lake City International Airport (KSLC), Geostationary Operational Environmental Satellite (GOES) imagery, and additional meteorological datasets. Dust events at KSLC-defined as any day [mountain standard time (MST)] with at least one report of a dust storm, blowing dust, and/or dust in suspension with a visibility of 10 km or less-average 4.3 per water year (WY: October-September), with considerable in-terannual variability and a general decline in frequency during the 1930-2010 observational record. The distributions of monthly dust-event frequency and total dust flux are bimodal, with primary and secondary maxima in April and September, respectively. Dust reports are most common in the late afternoon and evening. An analysis of the 33most recent (2001-10WY) events at KSLC indicates that 11 were associated with airmass convection, 16 were associated with a cold front or baroclinic trough entering Utah from the west or northwest, 4 were associated with a stationary or slowly moving front or baroclinic trough west of Utah, and 2 were associated with other synoptic patterns. GOES imagery from these 33 events, as well as 61 additional events from the surrounding region, illustrates that emission sources are located primarily in low-elevation Late Pleistocene-Holocene alluvial environments in southern and western Utah and southern and western Nevada

Numerical modelling of the coastal ocean off Tanzania

Includes bibliographical references (pages 71-89).In this model study of the coastal ocean off Tanzania, the Regional Ocean Modelling System (ROMS) was employed to model the coastal ocean off Tanzania over the domain of 5°N-15°S and 38-55°E. It was integrated for ten years with monthly mean Comprehensive Ocean and Atmosphere Data Sets (COADS) winds and heat fluxes. Initial and lateral boundary conditions were derived from the World Ocean Atlas. The model was used to simulate the annual cycle, and the sea surface temperature (SST) output compared with the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) sea surface temperature (SST) measurements for the same region. Although broadly comparable, the model SST was generally warmer than that of TMI data. The high SSTs in the Tanzanian coastal waters (greater than 28°C) occur from December to May while SSTs of less than 28°C occur during the rest of the year. The East African Coastal Current (EACC) experiences its lowest spatial and temporal average speeds (about 0.4ms- 1) in February and its maximum speed (1.7 ms⁻¹) in July. Speeds of greater than 1 ms⁻¹ occur during both transition seasons north of 6°S. The meridional wind stresses appear to be positively correlated with the EACC(r>0.6) in all locations and they are statistically significant (p<0.05). The annual cycle of the model flow in the southern Tanzanian waters seems to be positively correlated with the flow to the north of Madagascar (r=0.57 and p=O.O5). The flow in these regions changes in phase with each other from October to April and June to July with minimum speeds in November. For the other months, the flow in these regions is out of phase with each other. The model currents off southern Tanzania attain their maximum speeds in August when the South West monsoon is fully developed while the flow north of Madagascar attains its maximum speed in September when the South West monsoon fades. However, the flow in the southern Tanzanian waters is more affected by the reversal of winds over the tropical western Indian Ocean (r=0.69, p=0.01) than that north of Madagascar (r=0.51, p=0.09). This difference results in a larger annual speed range in the flow off southern Tanzania (about 0.4 ms⁻¹ ) than that to the north of Madagascar (about 0.3ms⁻¹). The ROMS model realistically simulates the annual cycle of the sea surface temperature and heat flux, the East African Coastal Current and the annual cycle of the flow entering the coastal ocean off the southern part of Tanzania. However, studies which integrate the large scale domain and regional coupled ocean-atmosphere interactions are needed to better understand of the East African climate and ocean variability. Such model results combined with suitable remote sensing and in situ observations will help improve understanding of the circulation and properties of the coastal ocean off Tanzania

THE ROLE OF SEA SURFACE TEMPERATURE IN THE RAINFALL REGIME IN SUB-SAHARAN AFRICA

This study sought to understand the role of the surface temperatures of the ocean and continent in the variability of rains in sub-Saharan Africa, contributing to the improvement of weather forecasting and prevention of extreme events in the region.Through perturbation experiments using a climate model of intermediate complexity,we seek to understand the roles of sea surface temperature (SST) of ocean basins around the sub-Saharan Africa and the surface temperature in Southern Africa, in defining the spatial distribution of Spatial distribution of rainfall in the region of action of Intertropical Convergence Zone (ITCZ) which assumes an approximate shape of an "inverted S" during the quarter December-January-February in this continent, due to connections with other systems of the same scale. By comparing the regional perturbation experiments of SST and the continental surface temperaturewith respect to the climatology we came to the conclusion that the SST in the Atlantic and Indian Oceans have major contribution in the formation of "inverted S" of precipitation than the continent's surface temperature. Comparing these two ocean basins, the Indian SST has predominant role in the climate variability in sub-Saharan Africa, modulating the position and intensity of the ITCZ and therefore the formation of their spatial distribution across the continent.  

Absence of the Great Whirl giant ocean vortex abates productivity in the Somali upwelling region

Somali upwelling is the fifth largest upwelling globally with high productivity, attracting tuna migratory species. A key control on the upwelling productivity is its interaction with one of the world’s largest oceanic eddies, the Great Whirl inducing a strong downwelling signal. Here, we use satellite-derived observations to determine the Great Whirl impact on the extent of the upwelling-driven phytoplankton bloom. We find that following decreases in upwelling intensity, productivity has declined by about 10% over the past two decades. The bloom extent has also been diminishing with an abrupt decrease around 2006–2007, coinciding with an abrupt increase in the downwelling effect. Absent or weak Great Whirl leads to the occurrence of smaller anticyclonic eddies with a resulting downwelling stronger than when the Great Whirl is present. We suggest that 2006–2007 abrupt changes in the bloom and downwelling extents’ regimes, are likely driven by Indian Ocean Dipole abrupt shift in 2006