Space-derived Parameters of Coastal Upwelling in the SE Baltic Sea

Igor Kozlov1,2, Toma Mingėlaitė2, Inga Dailidienė2

1Satellite Oceanography Laboratory, Russian State Hydrometeorological University Malookhtinsky pr. 98, St. Petersburg, Russia

2Klaipėda University H. Manto 84, Klaipėda, Lithuania

Abstract- In this work we present a basic statistics of coastal upwelling parameters inferred from satellite infrared (IR) Terra/Aqua MODIS sea surface temperature (SST) maps acquired over the South-Eastern (SE) Baltic Sea between 2000 and 2013. The maximum observed SST gradients across the front were up to 1.6 °C/km, temperature drop up to 14 °C with total upwelling-affected area up to 16000 km2. The observed horizontal scale of the upwelling is about 100-400 km along the coast, and 10-20 km (max 70-80 km) cross-shore. The duration of the upwelling in this part of the sea is from several days up to several weeks. It is found that intensive coastal upwelling in the SE Baltic may lead up to 40-km long intrusion of relatively cold and saline sea water into the Curonian Lagoon forming very pronounced property gradients there and affecting nearly a half of the Lithuanian part of the lagoon.

I. INTRODUCTION

Upwelling in the global ocean has a great importance to coastal productivity and regional climate. Smaller water bodies like the Baltic Sea are not an exception as well.

The Baltic Sea is relatively small and shallow sea and due to its coastal configuration the coastal upwelling is a very common feature here [1]. Wind-induced coastal upwelling is important dynamical feature affecting the circulation and the ecosystem of the region.

For a long time the main problem encountered in variety of sea surface temperature (SST) studies was the lack of available temperature data. Insufficient number of coastal monitoring stations and rare daily measurements resolve neither spatial coverage nor temporal variability of upwelling front, thus strongly underestimating all the important upwelling-related parameters and its effect on the total marine environment. Nowadays the above stated monitoring difficulties are effectively solved with the help of satellite remote sensing which allows one to obtain spatially detailed, regularly and continuously repeated datasets for the Baltic Sea and its coastal lagoons.

Concerning the Baltic Sea, satellite-derived sea surface temperature maps have been often exploited during the thermally stratified summer periods to analyse and understand different aspects of the coastal upwelling [1-6, 8-13]. As shown in [3] based on IR satellite data for 1980-1984, in summer time 14 upwelling zones in the Baltic Sea can be distinguished. One of these zones is the SE Baltic Sea (see Fig. 1) where

upwelling events are generally recorded in May - September, when the temperature stratification is the most significant. As will be further shown below, during intensive upwelling events relatively cool and saline upwelling water may rapidly inflow to the shallow Curonian Lagoon, the biggest lagoon in the European seas, and strongly influence the lagoon environment.

The objective of this work is to utilize MODIS sea surface temperature (SST) data for evaluation of some upwelling-induced parameters, such as cross-front SST differences and gradients, alongshore and cross-shore length, and the total affected area in the SE Baltic and in the Curonian Lagoon.

Figure 1. Study area in the SE Baltic Sea with overlaid bathymetry contours.

II. DATA AND APPROACH

For analysis of the coastal upwelling impact on SE Baltic Sea waters MODIS Terra/Aqua SST dataset for the SE Baltic Sea and Curonian Lagoon is examined between 2000 and 2013.

The analysis was performed for the thermally stratified warm period from April till September. In this time, upwelling-induced SST signatures are strong enough to be detectable in infrared (IR) satellite images. As suggested e.g. in [4] here we use the SST difference of 2˚C relative to the ambient waters as a threshold to define the upwelling events.

MODIS Terra/Aqua Level 2 daytime (MODIS thermal bands 31 (11μ) and 32 (12μ) imagery (L2_LAC_SST product) covering the study site with spatial resolution of about 1 km were obtained from the NASA OceanColor website (http://oceancolor.gsfc.nasa.gov/).

III. RESULTS

A. Statistics of upwelling parameters in the SE Baltic Altogether during the warm period from April to September

between 2000 and 2013, 34 distinct upwelling events were detected along the SE Baltic Sea coast. Most frequently upwelling occur from May to July – in about 75% of all cases, whereas 15% of the cases took place in August, two upwelling events were recorded in September and only one event occurred at the end of April (Fig. 2). As seen, the upwelling is more frequently observed during the summer months when higher temperature gradients are present in vertical stratification of the water column.

Satellite images show that the duration of the upwelling in this part of the sea may reach up to several weeks. Three of recorded upwelling events were short and lasted just 1-2 days, in 13 cases they lasted 3-6 days, in 6 cases they lasted 8-9 days, and, finally, 12 out of 34 upwelling events lasted more than 10 days (see Fig. 3).

It was recorded that the horizontal scale of the upwelling can be from 100 to 400 km along the coast, up to 70 km cross-shore and might cover the area up to 16000 km2 (Fig.3).

Figure 2. Monthly distribution of coastal upwelling events detected between 2000-2013.

Figure 3. The total surface area covered by coastal upwelling in the SE Baltic between 2000 and 2013

As already noticed in previous studies [3, 8], intensity of

coastal upwelling along the SE Baltic Sea coast strongly depends on favourable wind conditions and local bottom topography. In the half of all cases upwelling cross-shore length is about 10 km, typically being 10-20 km (Fig. 4). In some rare cases it may extend up to 70 km offshore especially when the long transverse filaments are formed on the upwelling front.

An example of extremely high propagation of upwelling waters offshore was observed during the most intensive upwelling event (recorded between 2000 and 2013) in July 2006 (see Fig. 5). In this case several narrow filaments extending as far as 70-80 km offshore were detected.

Figure 4. The horizontal upwelling cross-shore length (km) in the SE Baltic Sea during the maximum upwelling development phase.

Figure 5. The most intensive coastal upwelling event in the SE Baltic Sea recorded in July-August 2006. © NASA

Maximum horizontal SST gradients (up to 1.6 °C km-1) were also recorded for upwelling event in July 2006, but typically they were about 0.1-0.5 °C km-1 (see Fig. 6).

Figure 6. Maximum observed horizontal SST gradients across

upwelling front in the SE Baltic Sea.

The typical observed SST differences between upwelled and ambient waters were of the order of 2-6 ˚C, but during intensive upwelling events these were up to 10-14 ˚C (Fig. 7).

Figure 7. Maximum SST differences between upwelled and ambient

waters in the SE Baltic Sea.

B. Upwelling influence on the CuroniAnalysis of available MODIS SST m

intensive coastal upwelling events strongly the SE Baltic coastal zone, but also impactregime of the northern part of the Curonianinflow of relatively cold and saline waters ftakes place as also have been reported in [10

Fig. 8 shows an example of the intensivinflow to the Curonian Lagoon recorded onclearly seen, cool upwelling waters enter tthe Klaipėda Strait and a strong temperagradient is formed across the frontal boundadistance inside the lagoon (see Figs. 8-9).

During the whole study period 16 upwellinBaltic were observed to further influence thewater regime (Fig. 10). As observed, the iwater inflow and its spatial extent in the lagoverned by the alongshore wind component

The typical upwelling water propagationKlaipėda Strait down into the lagoon is abototal affected area being about 40-100 km2, bcases when it was up to 40 km covering abolagoon waters (see Fig. 10). However, suchof cool upwelling waters on the Northern larare and is related only to major upwellingBaltic.

Figure 8. MODIS SST map depicting the inflow of coo

the Curonian lagoon on 15 July 2006. ©

an Lagoon maps revealed that

influence not only ts the hydrological

n Lagoon when the from the SE Baltic ].

ve upwelling water n 15 July 2006. As the lagoon through ature (and density) ary at about 40 km

ng events in the SE e Curonian Lagoon intensity of marine agoon are primarily t.

n distance from the out 10-25 km with but there have been out 200 km2 of the

h a strong influence agoon part is rather g events in the SE

ol upwelling waters into © NASA

Figure 9. Chronology of the upwell

Lagoon during the intensive upw Since the Curonian Lagoon i

closed water body the watesummer time is higher here thtemperature in the SE Baltic, sogradients formed by the upweCuronian Lagoon were recordedto 0.8 °C/km.

Water temperature differencupwelling affected Curonian Lalagoon waters were a bit highabout 5-7 °C on average, but inflows it was up to 16 °C.

Figure 10. Upwelling water propagatio

area (km2) in the Curonian Lago

ling water inflow to the Curonian welling on 13-19 July 2006.

s rather shallow and almost er temperature during the han the average sea surface o the horizontal temperature elling water inflow to the d quite high and reached up

ces (Fig. 11) between the agoon part and the ambient her than in the Baltic Sea,

during the most intensive

on distance (km) and total affected oon between 2000 and 2013.

Figure 11. Water temperature differences (°C) between the upwelling-affected waters in the Northern part of the Curonian Lagoon and the ambient

lagoon waters.

IV. CONCLUSIONS AND DISCUSSION

In this study MODIS-derived sea surface temperature data was used for the period between 2000 and 2013 to analyse the coastal upwelling in the SE Baltic Sea and its effects on the Curonian Lagoon.

Altogether during the warm period from April to September between 2000 and 2013, 34 distinct upwelling events were detected along the SE Baltic Sea coast. Most frequently upwelling is detected in the SE Baltic Sea coastal area between May and August. The observed horizontal scale of the upwelling is from 100 to 400 km along the coast, and up to 70 km cross-shore when the long transverse filaments are formed on the upwelling front. The total maximum area of upwelling-affected waters can be as high as 16000 km2.

Maximum recorded horizontal SST gradients across upwelling front were up to 1.6 °C km-1, but typical values are about 0.1-0.5 °C km-1. The typical observed SST differences between upwelled and ambient waters were of the order of 2-6 ˚C, but during intensive upwelling events these could be up to 10-14 ˚C.

As detected, intensive coastal upwelling events strongly influence not only the SE Baltic coastal zone, but also have a significant impact on the hydrological regime of the Northern part of the Curonian Lagoon. During the whole study period 16 upwelling events in the SE Baltic were further observed to influence the Curonian Lagoon waters.

The typical upwelling water propagation distance from the Klaipėda Strait down to the lagoon is about 10-25 km and total affected area being about 40-100 km2. However, in specific cases the propagation distance was observed to reach up to 40 km covering about 200 km2 of the lagoon waters. The corresponding horizontal temperature gradients (difference) between the upwelled and ambient lagoon waters may reach up to 0.8 °C/km (16 °C).

The results presented in this study are in agreement with previously reported information on coastal upwelling in this

part of the Baltic Sea [2, 4, 7, 11, 13]. However, we provide a more detailed records of main upwelling-related parameters for the SE Baltic between 2000 and 2013. We have also documented some basic parameters relevant to the upwelling water intrusion to the shallow Curonian Lagoon which has been recently acknowledged in [10].

All in all, MODIS-based SST maps acquired from space provide an excellent opportunity to evaluate main spatio-temporal upwelling-related parameters in the SE Baltic and in the Curonian Lagoon more completely than it was previously done with sparse conventional observations from coastal stations, enabling to perform further analysis for understanding the dynamical aspects of the coastal upwelling in this part of the Baltic Sea in the near future.

ACKNOWLEDGMENT

This work was supported by European Social Fund Agency project "Development of Technological and Environmental Researches in Lithuanian Marine Sector", Nr. VP1-3.1-ŠMM-08-K-01-019; and by NordForsk funded network “The Nordic Network for Baltic Sea Remote Sensing (NordBaltRemS; 2012-2014)”. IK acknowledge the support by the Mega-Grant of the Russian Federation Government under Grant №11.G34.31.0078.

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